O b s e r vi n g s e a i c e t hi c k n e s s v a ri a bili t y i n
t h e L a p t e v S e a a n d t h e i m pli c a ti o n s
f o r t h e Tr a n s p ol a r D rif t s y s t e m
Dissert ati o n s u b mitte d b y
H a n s J A K O B B el t e r
i n p arti al f ul fil me nt of t he re q uire me nts f or t he de gree ofD o c t o r of N a t u r al S ci e n c e s ( D r. r e r. n a t. )
t o Fac ult y 1P h ysics a n d Electric al E n gi neeri n g U ni versit y of Bre me n
[Alf r e d We g e n e r I n s ti t u t e , Hel m h oltz Ce ntre f or P ol ar a n d M ari ne Rese arc h, Bre mer h a ve n]
1. Re vie wer:
Pr of. Dr. C hristi a n H a as
[Alf r e d We g e n e r I n s ti t u t e , Hel m h oltz Ce ntre f or P ol ar a n d M ari ne Rese arc h, Bre mer h a ve n]
[U ni v e r si t y of B r e m e n ] 2. Re vie wer:
Pr of. Dr. Torste n K a nz o w
[Alf r e d We g e n e r I n s ti t u t e , Hel m h oltz Ce ntre f or P ol ar a n d M ari ne Rese arc h, Bre mer h a ve n]
[U ni v e r si t y of B r e m e n ]
T his dissert ati o n w as writte n at t he Se a Ice P h ysics secti o n of t he Al-fre d We ge ner I nstit ute, Hel m h oltz Ce ntre f or P ol ar a n d M ari ne Rese arc h, Bre mer h a ve n.
A b s t r a c t
T he Arctic s e a ice c o ver is str o n gl y c o n necte d t o t he gl o b al cli m ate s yste m a n d t heref ore n ot o nl y s u bject t o i nter n al vari a bilit y b ut als o i n a p h ase of si g ni fic a nt c h a n ge rel ate d t o t he o n g oi n g i ncre ase i n gl o b al me a n s urf ace te m p er at ures. T he m ost i m p ort a nt p ar a m-eters t o m o nit or a n d descri b e se a ice are its are al e xte nt, t hic k ness, a n d m oti o n. W hile reli a ble, l o n g-ter m s atellite me as ure me nts of se a ice c o nce ntr ati o n, w hic h is use d t o deri ve t he are a c o vere d b y se a ice, e xist si nce t he l ate 1 9 7 0s, se a ice t hic k ness a n d m oti o n d at a sets of c o m p ar a ble q u alit y a n d le n gt h are c urre ntl y n ot a vail a ble.
T he o ver arc hi n g g o al of t his dissert ati o n is t o c o ntri b ute t o t he i m pr o ve me nt of se a ice t hic k ness o bser vati o ns a n d t o u n derst a n d a n d q u a ntif y t he i m p act of o n g oi n g se a ice t hic k ness c h a n ges a n d vari a bilit y i n t he m ost i m p ort a nt re gi o ns of se a ice f or m ati o n o n t he o ver all Arctic se a ice b u d get. To ac hie ve t h at, t he first st u d y prese nte d i n t his dissert ati o n f o c uses o n e xte n di n g t he k n o wle d ge a b o ut se a ice t hic k ness vari a bilit y i n t he L a pte v Se a b y de vel o pi n g a ne w met h o d t o deri ve se a ice t hic k ness ti me series fr o m m o ore d s o n ars. It is s h o w n t h at d ail y me a n se a ice t hic k ness ti me series c a n b e i nferre d fr o m b asic, m o ore d u p w ar d-l o o ki n g Ac o ustic D o p pler C urre nt Pr o filers. T his a d a pti ve a p pr o ac h all o ws t o re visit d at a sets fr o m p ast m o ori n g de pl o y me nts i n t he L a pte v Se a a n d e x pl oit t he m t o e xte n d t he a vail a ble se a ice t hic k ness rec or ds a n d cl ose o bser vati o n al g a ps i n a re gi o n t h at, d ue t o its li mite d accessi bilit y, is vastl y u n der-s a m ple d.
T hese ne w d at a sets are t he b asis f or t he vali d ati o n of m ulti ple s atellite se a ice t hic k ness pr o d ucts, i ncl u di n g t he l o n gest a vail a ble o ne i ntr o d uce d b y t he E ur o p e a n S p ace A ge nc y, w hic h pr o vi des Arctic- wi de se a ice t hic k ness si nce 2 0 0 2. It is s h o w n t h at i n t he first- ye ar ice d o mi n ate d L a pte v Se a t he i n vesti g ate d s atellite pr o d ucts pr o vi de t he m ost fre q ue ntl y o cc urri n g ( m o d al) r at her t h a n t he me a n se a ice t hic k ness. T his i m p ort a nt disc o ver y all o ws f or a b etter i nter pret ati o n of t he a vail a ble s atellite rec or ds, es p eci all y f or t he i n vesti g ati o n of se a ice v ol u me tr a ns p orts, a n d u n derli nes t heir de ficie ncies i n re prese nti n g d y n a mic all y def or me d se a ice.
B ase d o n t he k n o wle d ge g ai ne d i n t he L a pte v Se a, t he fi n al st u d y prese nte d i n t his dissert ati o n f oll o ws t he Arctic se a ice life c ycle fr o m t he re gi o ns of ice f or m ati o n al o n g t he Tr a ns p ol ar Drift t o w ar ds Fr a m Str ait a n d a n al yses w het her se a ice t hic k ness a n o m alies i n d uce d i n t he s o urce re gi o ns of Arctic se a ice pr o p a g ate t o t he ce ntr al Arctic Oce a n a n d b e y o n d. M ore s p eci fic all y, it is i n vesti g ate d w hic h p artic ul ar pr o cesses are p ote nti all y res p o nsi ble f or t he i n d uce d a n o m alies i n t he s o urce re gi o ns a n d w het her t heir si g n als p ersist u ntil t he e n d of t he Tr a ns p ol ar Drift. I n t he a bse nce of a si n gle-s o urce Arctic-wi de, hi g h te m p or al a n d s p ati al res ol uti o n se a ice t hic k ness d at a pr o d uct, t his fi n al p art pr o m otes t he c o m bi n ati o n of di ffere nt tec h ni q ues a n d t o ols f or t he i n vesti g ati o n of t his c o m ple x Arctic cli m ate p ar a meter. At t he ce nter of t he i n vesti g ati o n is a n e xte n de d l o n g-ter m electr o m a g netic i n d ucti o n s o u n di n g- b ase d se a ice t hic k ness ti me series, w hic h s h o ws a ge ner al t hi n ni n g a n d decre asi n g a ge of se a ice at t he e n d of t he Tr a ns p ol ar Drift b et wee n 2 0 0 1 a n d 2 0 2 0. D ue t o its le n gt h, t his u ni q ue ti me series als o p er mits t o p ut ice t hic k ness me as ure me nts c o n d ucte d d uri n g t he M ulti disci pli n ar y drifti n g O bser vat or y f or t he St u d y of Arctic Cli m ate ( M O S Ai C) e x p e diti o n i nt o t he hist oric al c o nte xt. L a gr a n gi a n ice tr ac ki n g a n d m o delli n g of t her m o d y n a mic se a ice gr o wt h al o n g t he p at h w a ys of Arctic
T he prese nte d e ff orts are a n i m p ort a nt c o ntri b uti o n t o t he b etter u n derst a n di n g of Arctic se a ice t hic k ness vari a bilit y a n d c h a n ge a n d c a n b e see n as st arti n g p oi nts f or m ore t ar gete d a n al yses of t he dri vi n g mec h a nis ms b e hi n d t he m. I n a d diti o n, t he ac q ui-siti o n, vali d ati o n, a n d e xte nsi o n of se a ice t hic k ness o bser vati o ns pr o vi de t he b asis f or m ore det aile d se a ice m o delli n g, w hic h will i m pr o ve n ot o nl y t he m o nit ori n g b ut als o t he pre dicti o n of Arctic se a ice t hic k ness c h a n ges i n t he f ut ure.
C o nt e nt s
A b s t r a c t I N o m e n cl a t u r e V A b bre vi ati o ns . . . V S y m b ols . . . VII 1 I nt r o d u c ti o n 11. 1 Arctic se a ice a n d its r ole i n t he E art h’s cli m ate s yste m . . . 1 1. 2 O bjecti ves a n d o utli ne of t his dissert ati o n . . . 4
2 S e a i c e – T h e o r e ti c al b a c k g r o u n d 7
2. 1 Se a ice t hic k ness distri b uti o n . . . 7 2. 1. 1 T her m o d y n a mic c o m p o ne nts . . . 7 2. 1. 2 D y n a mic c o m p o ne nts . . . 1 0 2. 1. 3 Se a ice t hic k ness distri b uti o n i n t he Arctic . . . 1 1 2. 2 Ke y me as ure me nt tec h ni q ues . . . 1 3 2. 2. 1 U p w ar d- L o o ki n g S o n ar . . . 1 4 2. 2. 2 Electr o m a g netic i n d ucti o n s o u n di n g . . . 1 6 2. 2. 3 S atellite re m ote se nsi n g . . . 1 8 2. 2. 4 C o ncl usi o n . . . 2 1 3 S e a i c e d r af t f r o m u p w a r d-l o o ki n g A c o u s ti c D o p pl e r C u r r e nt P r o fil e r s: a n a d a p ti v e a p p r o a c h, v ali d a t e d b y U p w a r d- L o o ki n g S o n a r d a t a 2 3 3. 1 I ntr o d ucti o n . . . 2 4 3. 2 D at a a n d Met h o ds . . . 2 6 3. 2. 1 D at a pr o cessi n g . . . 2 7 3. 3 Res ults . . . 3 3 3. 4 Disc ussi o n . . . 3 6 3. 4. 1 O p e n w ater detecti o n . . . 3 7 3. 4. 2 Tilt a n d b e a m wi dt h bi as . . . 3 7 3. 4. 3 S o u n d s p ee d c orrecti o n . . . 3 8 3. 4. 4 U ncert ai nt y esti m ates . . . 3 9 3. 4. 5 U L S vers us A D C P- deri ve d ice dr aft . . . 4 1 3. 5 C o ncl usi o ns . . . 4 2 4 S a t elli t e- b a s e d s e a i c e t hi c k n e s s c h a n g e s i n t h e L a p t e v S e a f r o m 2 0 0 2 t o
2 0 1 7: c o m p a ri s o n t o m o o ri n g o b s e r v a ti o n s 4 5
4. 1 I ntr o d ucti o n . . . 4 6 4. 2 D at a a n d met h o ds . . . 4 9 4. 2. 1 S o n ar- b ase d ice dr aft me as ure me nts . . . 4 9 4. 2. 2 S atellite d at a . . . 5 0 4. 2. 3 D at a li mit ati o ns . . . 5 2 4. 3 Res ults . . . 5 3
4. 4 Disc ussi o n . . . 5 9 4. 4. 1 C o m p ar a bilit y of s atellite a n d s o n ar me as ure me nts . . . 5 9 4. 4. 2 St a bilit y of t he C CI- 2 SI T C D R . . . 6 0 4. 4. 3 Ta y m yr 2 0 1 3 / 2 0 1 4 c ase . . . 6 1 4. 5 C o ncl usi o n . . . 6 3 5 Fr o m t h e L a p t e v S e a t o t h e Fr a m S t r ai t – Lif e c y cl e of A r c ti c s e a i c e 6 7 6 I nt e r a n n u al v a ri a bili t y i n Tr a n s p ol a r D rif t i c e t hi c k n e s s a n d p o t e nti al
i m p a c t of Atl a nti fi c a ti o n 7 1
6. 1 I ntr o d ucti o n . . . 7 2 6. 2 D at a a n d met h o ds . . . 7 5 6. 2. 1 E M se a ice t hic k ness me as ure me nts . . . 7 5 6. 2. 2 Se a ice p at h w a ys a n d s o urce re gi o ns . . . 7 6 6. 2. 3 T her m o d y n a mic se a ice m o del . . . 7 7 6. 2. 4 S hi p b or ne se a ice t hic k ness o bser vati o ns . . . 7 8 6. 3 Res ults a n d Disc ussi o n . . . 7 8 6. 3. 1 Pr o cesses dri vi n g i nter a n n u al SI T vari a bilit y b et wee n 2 0 0 1 a n d 2 0 1 8 7 8 6. 3. 2 P ossi ble i m p act of Atl a nti fic ati o n o n SI T i n 2 0 1 6 . . . 8 1 6. 3. 3 I nter pret ati o n of se a ice s ur ve ys fr o m t he M O S Ai C ye ar . . . 8 5 6. 3. 4 C o m p aris o n t o R ussi a n s hi p b or ne SI T o bser vati o ns . . . 8 6 6. 4 C o ncl usi o n . . . 8 8 7 C o n cl u si o n a n d o u tl o o k 9 3 A p p e n di x 1 0 1 Li s t of Fi g u r e s 1 0 3 Li s t of T a bl e s 1 0 5 R ef e r e n c e s 1 0 7 A c k n o wl e d g e m e nt s 1 2 5 C u r ri c ul u m vi t a e 1 2 7 S t a t e m e nt / E r kl ¨a r u n g 1 3 1
N o m e n cl a t u r e
A b b r e vi a ti o n s
A A RI Arctic a n d A nt arctic Rese arc h I nstit ute A D C P Ac o ustic D o p pler C urre nt Pr o filer
A E M Air b or ne electr o m a g netic i n d ucti o n i nstr u me nt
A OI Are a of i nterest
A R TI S T Arctic r a di ati o n a n d t ur b ule nce i nter acti o n st u d y A S SI S T Arctic S hi p b or ne Se a Ice St a n d ar diz ati o n To ol
A W Atl a ntic Water
A WI Alfre d We ge ner I nstit ute, Hel m h oltz Ce ntre f or P ol ar a n d M ari ne Rese arc h B M B F B u n des mi nisteri u m f ¨ur Bil d u n g u n d Forsc h u n g
B T B ott o m tr ac k
C CI- 2 Cli m ate C h a n ge I niti ati ve P h ase 2
C D R Cli m ate d at a rec or d
C E R S A T Ce nter f or S atellite E x pl oit ati o n a n d Rese arc h
C O M O S Ai C Ce ntr al O bser vat or y
C RI S T A L C o p er nic us P ol ar Ice a n d S n o w To p o gr a p h y Alti meter
C S 2 Cr y o S at- 2
C S 2 S M O S Mer ge d wee kl y C S 2 a n d S M O S se a ice t hic k ness d at a rec or d
D N M O S Ai C Distri b ute d Net w or k
D N R C o m bi ne d C O a n d D N re gi o ns ( wi der are a ar o u n d t he M O S Ai C fl o e) E C M W F E ur o p e a n Ce ntre f or Me di u m- R a n ge We at her Forec asts
E A S E E q u al Are a Sc al a ble E art h
E M Electr o m a g netic i n d ucti o n
E N VI S A T E n vir o n me nt al S atellite
E R A E C M W F Re- A n al ysis
E S A E ur o p e a n S p ace A ge nc y
F D D Freezi n g de gree d a ys
I B C A O I nter n ati o n al B at h y metric C h art of t he Arctic Oce a n I C E S a t- 2 Ice, Cl o u d, a n d l a n d Ele vati o n S atellite 2
I P C C I nter g o ver n me nt al P a nel f or Cli m ate C h a n ge
I P S Ice Pr o fili n g S o n ar
M O S Ai C M ulti disci pli n ar y drifti n g O bser vat or y f or t he St u d y of Arctic Cli m ate
M YI M ulti- ye ar ice
N A O SI M N ort h Atl a ntic Arctic Oce a n Se a Ice M o del N SI D C N ati o n al S n o w a n d Ice D at a Ce nter
O SI S A F Oce a n a n d Se a Ice S atellite A p plic ati o n Facilit y P D F Pr o b a bilit y De nsit y F u ncti o n
Q U A R C C S Q u a ntif yi n g R a pi d Cli m ate C h a n ge i n t he Arctic: re gi o n al fee d b ac ks a n d l ar ge-sc ale i m p acts
R F B R R ussi a n Fo u n d ati o n f or B asic Rese arc h
R M S E R o ot Me a n S q u are Err or
R V Rese arc h vessel
SI C Se a ice c o nce ntr ati o n
SI T Se a ice t hic k ness
S M O S S oil M oist ure a n d Oce a n S ali nit y S S M /I S p eci al Se ns or Micr o w a ve /I m a ger S T K S hi p b or ne tele visi o n c o m ple x
S YI Sec o n d- ye ar ice
U L S U p w ar d- L o o ki n g S o n ar
S y m b ol s
β S o u n d s p ee d c orrecti o n f act or
D Vertic al s p aci n g b et wee n U L S press ure a n d r a n ge se ns or [ m]
d Se a ice dr aft [ m]
η I nstr u me nt de pt h [ m]
F Oce a n he at fl u x [ W m − 2]
f T her m o d y n a mic ice gr o wt h / dec a y f u ncti o n g L o c al gr a vit ati o n al acceler ati o n [ m s − 2]
∆ H
∆ t Ice gr o wt h r ate [ m s− 1]
H i c e Se a ice t hic k ness [ m]
H s n o w S n o w t hic k ness [ m]
hi Dist a nce E M de vice t o air-s n o w /ice i nterf ace [ m]
hw Dist a nce E M de vice t o ice- w ater i nterf ace [ m]
i N u m b er of A D C P b e a ms
I T D Ice t hic k ness distri b uti o n
κi c e T her m al c o n d ucti vit y of ice [ 2 W m − 1K − 1]
κs n o w T her m al c o n d ucti vit y of s n o w [ 0. 3 3 W m − 1K − 1]
L L ate nt he at of f usi o n [ 3 ·1 08J m− 3]
pul s Press ure at U L S [ h P a]
pat m Press ure at se a s urf ace [ h P a]
Φ Re distri b uti o n f u ncti o n φ A D C P i nstr u me nt tilt [ ◦]
ρ De nsit y of se a w ater [ k g m − 3]
r R a n ge [ m]
σd U ncert ai nt y of h o url y me a n se a ice dr aft fr o m A D C P [ m]
σrφ U ncert ai nt y of tilt-c orrecte d A D C P r a n ge [ m]
σz U ncert ai nt y of A D C P- deri ve d i nstr u me nt de pt h [ m]
T0 Te m p er at ure at t he ice- w ater i nterf ace [ − 1. 8 6◦C]
θ U L S i nstr u me nt tilt [ ] #»
v Ice drift vel o cit y
x L o c ati o n
1 I nt r o d u c ti o n
1. 1
A r c ti c s e a i c e a n d i t s r ol e i n t h e E a r t h’ s cli m a t e s y s t e m
T he Arctic is o ne of t he ke y re gi o ns of i nterest i n t he E art h’s c o m ple x a n d vari a ble cli m ate s yste m. Gl o b al c h a n ges i n cli m ate-rele va nt p ar a meters, li ke t he i ncre ase i n gl o b al me a n s urf ace air te m p er at ure, are a m pli fie d i n t he Arctic ( C h a p m a n a n d Wals h, 1 9 9 3). I n t he rece nt dec a des, t he Arctic me a n s urf ace air te m p er at ure a n o m al y h as b ee n ne arl y t w o t o t hree ti mes hi g her t h a n t he gl o b al me a n ( H a nse n et al., 2 0 1 0; Scree n a n d Si m m o n ds, 2 0 1 0; Pit h a n a n d M a uritse n, 2 0 1 4), w hic h c o nti n ues t o h a ve vast i m plic ati o ns f or o ne of t he Arctic’s ke y c o m p o ne nts – se a ice.
Se a ice f or ms w he n se a w ater freezes. D ue t o its l o w de nsit y c o m p are d t o se a w ater, se a ice fl o ats at t he i nterf ace b et wee n o ce a n a n d at m os p here a n d, t o get her wit h t he a b o ve s n o w c o ver, f or ms a n i ns ul ati n g l a yer f or t he e xc h a n ge of he at, m ass, a n d m o me nt u m b et wee n t he t w o cli m ate c o m p o ne nts ( Mc P hee, 2 0 1 7). D ue t o t he re flecti ve s urf aces of se a ice a n d s n o w, se a ice is a n i m p ort a nt c o m p o ne nt of t he s urf ace e ner g y b u d get. T heref ore, c h a n ges i n t he c o m p ositi o n of se a ice a n d its are al e xte nt c a n h a ve m aj or i m plic ati o ns f or t he e xc h a n ge of e ner g y at t he o ce a n s urf ace. O ne e x a m ple is t he ice- al b e d o fee d b ac k first disc usse d b y B u d y k o ( 1 9 6 9), w hic h descri b es a p ositi ve fee d b ac k mec h a nis m i n d uce d b y c h a n ges i n t he fr acti o n of s ol ar r a di ati o n re flecte d b y t he ice-c o vere d o ce a n s urf ace, k n o w n as al b e d o. Si m pli fie d, a re d ucti o n of se a ice are a e x p oses m ore of t he c o m p ar a bl y d ar k o ce a n s urf ace t o t he at m os p here. T he al b e d o of t he s urf ace re d uces a n d m ore s ol ar r a di ati o n is a bs or b e d, w hic h s u bse q ue ntl y le a ds t o m ore w ar mi n g of t he o ce a n s urf ace l a yer. T his i n d uce d w ar mi n g le a ds t o se a ice melt, f urt her re d ucti o n of t he are a c o vere d b y se a ice a n d e ve n m ore e ner g y- u pt a ke b y t he o ce a n ( H all, 2 0 0 4).
Wit h t he u p p er p art of t he ice c o ver pr otr u di n g a b o ve t he w ater le vel (ice free b o ar d) a n d t he l o wer p art b ei n g s u b mer ge d i nt o t he w ater (ice dr aft), se a ice is e x p ose d t o i n fl ue nces fr o m, b ut als o i nter acts wit h, t he at m os p here a b o ve a n d t he o ce a n b el o w. Se a ice i m p acts s urf ace o ce a n pr o p erties a n d str ati fic ati o n t hr o u g h fres h w ater rele ase d uri n g melt a n d bri ne rejecti o n i n t he u p p er o ce a n l a yer d uri n g se a ice f or m ati o n. T he c h a n ges i n u p p er o ce a n de nsit y res ulti n g fr o m se a ice f or m ati o n a n d melt c o ntri b ute t o t he t her m o h ali ne o ce a n circ ul ati o n, w hic h is a n i nte gr al p art of t he gl o b al o ce a n circ ul ati o n ( R a h mst orf, 2 0 0 3). T hese fe w pr o cesses are si n gle e x a m ples of t he m uc h m ore c o m ple x e ffects a n d i nter acti o ns se a ice h as o n a n d wit h t he Arctic cli m ate s yste m a n d s h o w w h y it is c o nsi dere d a val u a ble i n dic at or f or vari a bilit y of t he Arctic cli m ate b ut als o o n g oi n g gl o b al cli m ate c h a n ge.
O ne of t he m ost c o m m o n p ar a meters use d t o descri b e se a ice is its are al c o ver a ge. B ase d o n se a ice c o nce ntr ati o n d at a, re gi o ns c o vere d b y ice are descri b e d usi n g se a ice are a a n d e xte nt. W hile se a ice are a is a me as ure f or t he act u al are a c o vere d b y ice ( k m2),
se a ice e xte nt refers t o gri d de d fiel ds of d at a a n d w het her si n gle gri d cells are c o nsi dere d ice-c o vere d or n ot. C o m m o nl y, gri d cells wit h se a ice c o nce ntr ati o ns of at le ast 1 5 % are
Fi g u r e 1. 1: A M S R- 2 s e a i c e c o n c e ntr ati o n d at a ( S pr e e n et al., 2 0 0 8) fr o m t h e d a ys of m a xi m u m e xt e nt o n M ar c h 4, 2 0 2 0 (l eft) a n d mi ni m u m e xt e nt o n S e pt e m b er 9, 2 0 2 0 (ri g ht) o bt ai n e d fr o m w w w.s e ai c e p ort al. d e ( a c c ess e d O ct o b er 2 0 2 0).
c o nsi dere d ice-c o vere d a n d t he e ntire gri d cell c o ntri b utes t o t he t ot al se a ice e xte nt. T heref ore, t he val ue of se a ice e xte nt is us u all y hi g her t h a n t he are a val ue.
Si nce t he ye ar 2 0 0 0, Arctic se a ice c o vers b et wee n 1 4 a n d 1 6 × 1 06 k m2 at its m a xi m u m
e xte nt i n wi nter ( C o mis o et al., 2 0 1 7) i ncl u di n g t he w h ole ce ntr al Arctic Oce a n a n d m ost p arts of t he a dj ace nt se as (e x a m ple f or M arc h, 2 0 2 0 i n Fi g. 1. 1 (left)). D uri n g t he s u m mer melt se as o n, t he ice e d ge retre ats t o t he ce ntr al Arctic Oce a n a n d se a ice c o vers o nl y b et wee n 3. 5 a n d 8 × 1 06 k m2 ( C o mis o et al., 2 0 1 7) w he n it re ac hes its mi ni m u m e xte nt i n
Se pte m b er (e x a m ple f or Se pte m b er, 2 0 2 0 i n Fi g. 1. 1 (ri g ht)). T he re gi o n c o vere d b y se a ice d uri n g t he e xte nt mi ni m u m i n s u m mer m ar ks t he p er m a ne nt ice z o ne. It c o nsists of se a ice t h at h as s ur vi ve d at le ast o ne melt se as o n a n d is t heref ore c o nsi dere d m ulti- ye ar ice ( M YI). T he se as o n al ice z o ne is de fi ne d as t he are a of o ce a n b et wee n t he p er m a ne nt ice z o ne a n d t he b o u n d ar y of t he se a ice e xte nt m a xi m u m. It is p artl y c o vere d b y ice d uri n g t he ye ar a n d d o mi n ate d b y first- ye ar ice ( F YI, ice t h at h as n ot s ur vi ve d a f ull melt se as o n).
S olel y o bser vi n g se a ice e xte nt o nl y all o ws f or a di ffere nti ati o n b et wee n ice c o vere d a n d o p e n w ater are as. C o m bi ni n g o bser vati o ns of se a ice e xte nt, t hic k ness, a n d m oti o n are re q uire d t o descri b e o ver all c h a n ges of t he Arctic se a ice m ass b al a nce. T he c o nsi der ati o n of t he te m p or al de vel o p me nt of t he m ost i m p ort a nt se a ice p ar a meters, e xte nt a n d t hic k-ness, i n dic ates a c o n necti o n t o o bser ve d tre n ds i n gl o b al s urf ace air te m p er at ures. T he o ver all are a c o vere d b y se a ice is re d uci n g i n all m o nt hs ( C a valieri a n d P ar ki ns o n, 2 0 1 2; C o mis o et al., 2 0 1 7; Str o e ve a n d N otz, 2 0 1 8), w hile se a ice i n ge ner al is t hi n ni n g ( K w o k, 2 0 1 8). D ue t o t he i m p act of se a ice o n t he s urf ace e ner g y a n d m ass b u d gets, c o nti n u o us decli ne i n e xte nt a n d t hi n ni n g will i ncre ase t he direct i nter acti o n b et wee n at m os p here a n d o ce a n wit h m aj or i m plic ati o ns f or t he e ntire Arctic ec os yste m.
1 I nt r o d u c ti o n 1. 1 Arctic se a ice T he pre dicte d t ot al s u m mer se a ice l oss is pr ojecte d t o c h a n ge n ot o nl y t he Arctic, b ut als o t he gl o b al cli m ate s yste m se verel y ( Str o e ve et al., 2 0 1 2; O verl a n d a n d Wa n g, 2 0 1 3; O verl a n d et al., 2 0 1 9). Acc or di n g t o t he I nter g o ver n me nt al P a nel f or Cli m ate C h a n ge (I P C C), t hese cli m atic c h a n ges will als o h a ve e xte nsi ve s o ci o ec o n o mic i m p acts i n t he Arctic ( L arse n et al., 2 0 1 4). Besi des ne w p ossi bilities f or ec o n o mic di versi fic ati o n, s hi p pi n g, f orestr y, a n d t o uris m, w hic h are c o nsi dere d p ositi ve i m p acts, t he l oss i n se a ice c o ver a n d p er m afr ost m a y c a use d a m a ge t o all ki n ds of i nfr astr uct ure use d a n d b uilt i n t he Arctic a n d i m p act t he li veli h o o ds of i n di ge n o us c o m m u nities ( L arse n et al., 2 0 1 4).
I n t he c o nte xt of gl o b al cli m ate c h a n ge, t he disc ussi o ns a b o ut o bser ve d tre n ds i n se a ice e xte nt a n d t hic k ness m ostl y refer t o c h a n ges o n a n Arctic- wi de sc ale. H o we ver, a m ore det aile d a n al ysis of t he Arctic se a ice c o ver re ve als t h at str o n g re gi o n al a n d se as o n al vari a bilit y e xists ( H a as, 2 0 1 7). T heref ore, o bser vati o ns of se a ice h a ve t o meet t he e x p ect ati o n of t he hi g hest p ossi ble res ol uti o n o n b ot h te m p or al a n d s p ati al sc ales t o m o nit or t his vari a bilit y. W hile s atellite o bser vati o ns pr o vi de reli a ble, ye ar-r o u n d, l o n g-ter m se a ice e xte nt a n d c o nce ntr ati o n rec or ds, se a ice t hic k ness rec or ds of c o m p ar a ble le n gt h a n d q u alit y are n ot a vail a ble t o t his d ate. S atellite se a ice t hic k ness rec or ds c o ver a m uc h s h orter p eri o d t h a n se a ice e xte nt a n d c o nce ntr ati o n rec or ds, are a vail a ble o nl y d uri n g t he wi nter se as o n, a n d h a ve m ostl y b ee n vali d ate d i n re gi o ns d o mi n ate d b y M YI. C o nsi deri n g t he tre n d t o w ar ds t hi n ner a n d y o u n ger se a ice d o mi n ati n g t he Arctic ( K w o k, 2 0 1 8), it is e mi ne ntl y i m p ort a nt t h at s atellite se a ice t hic k ness d at a sets are c o nti n ue d, e xte n de d t o t he s u m mer se as o n, a n d vali d ate d f or F YI- d o mi n ate d re gi o ns.
O ne of t hese F YI- d o mi n ate d re gi o ns is f o u n d i n t he e aster n Arctic a n d es p eci all y i n t he L a pte v Se a ( Rei m nitz et al., 1 9 9 4). T he L a pte v Se a is c o nsi dere d o ne of t he m ost i m p ort a nt s o urce re gi o ns of Arctic se a ice ( Ri g or et al., 2 0 0 2; H a nse n et al., 2 0 1 3). T he pre vaili n g o ffs h ore- directe d wi n ds tr a ns p ort ne wl y f or me d ice a w a y fr o m t he s helf se as, e x p osi n g vast are as of o p e n w ater t o t he c ol d at m os p here, w hic h le a ds t o m ore ice f or m ati o n i n t he s h all o w w aters ( Ti m o k h o v, 1 9 9 4; Kr u m p e n et al., 2 0 1 3). T his m a kes t he L a pte v Se a a re gi o n of m aj or i nterest f or t he l o n g-ter m de vel o p me nt of Arctic se a ice. T he ice tr a ns p orte d n ort h w ar d fr o m t he Si b eri a n c o ast is i nc or p or ate d i nt o t he Tr a ns p ol ar Drift s yste m, w hic h acts as a c o n ve y or b elt m o vi n g ice acr oss t he ce ntr al Arctic Oce a n t o w ar ds t he Fr a m Str ait ( Ri g or et al., 2 0 0 2). T his pre vaili n g ice drift re gi me acr oss t he Arctic Oce a n w as alre a d y o bser ve d a n d use d b y Fri dtj of N a nse n d uri n g his Fr a m drift e x p e diti o n fr o m 1 8 9 3 t o 1 8 9 6 ( N a nse n, 1 8 9 7). At t he e n d of t he Tr a ns p ol ar Drift, m ost of t he ice e xits t he Arctic Oce a n t hr o u g h t he Fr a m Str ait a n d melts as it pr o gresses s o ut h w ar d. T he Fr a m Str ait t heref ore p oses a n ot her are a of m aj or i nterest f or se a ice t hic k ness o bser vati o ns i n t he Arctic. Ice re ac hi n g t he Fr a m Str ait a n d its vici nit y c arries i nte gr ate d si g n als of t he mec h a nis ms acti n g o n t he ice al o n g its j o ur ne y t hr o u g h t he Arctic Oce a n ( H a nse n et al., 2 0 1 3) a n d a n al ysi n g a n d u n derst a n di n g its vari a bilit y i m pr o ves t he u n derst a n di n g of t he c o m ple x i nter acti o ns b et wee n t he ice a n d t he at m os p here as well as t he o ce a n.
I n li g ht of t he o n g oi n g re d ucti o n of se a ice e xte nt a n d t hic k ness, i n vesti g ati n g c h a n ges i n t hese re gi o ns of i nterest m a y gi ve i nsi g ht i nt o t he re as o ns f or t he o bser ve d Arctic-wi de c h a n ge. D ue t o its l o c ati o n a n d c o m p ar a bl y g o o d accessi bilit y, t he Fr a m Str ait h as b ee n t he desti n ati o n f or n u mer o us rese arc h e x p e diti o ns a n d m ulti ple l o n g-ter m se a
ice t hic k ness d at a sets e xist ( H a nse n et al., 2 0 1 3; Re n ner et al., 2 0 1 4; Kr u m p e n et al., 2 0 1 6). U nf ort u n atel y, o bser vati o ns of se a ice t hic k ness i n t he L a pte v Se a are s p arse a n d c o nti n u o us l o n g-ter m rec or ds use d f or t he a n al ysis of i nter a n n u al vari a bilit y a n d s atellite d at a vali d ati o n are n o n-e xiste nt. H o we ver, i n fl ue nces alteri n g t he t hic k ness of se a ice are f ar-re ac hi n g o n b ot h te m p or al a n d s p ati al sc ales a n d i n or der t o f ull y c o m pre he n d t heir i m p act, it is i m p ort a nt t o f oll o w t he f ull life c ycle of se a ice, fr o m ice f or m ati o n u ntil its disi nte gr ati o n a n d melt. D ue t o t he li mit ati o ns of se a ice t hic k ness me as ure me nt tec h ni q ues a n d te m p or ar y i n accessi bilit y of re gi o ns of m aj or i nterest, o bser vi n g se a ice ste p b y ste p al o n g its p at h w a y t hr o u g h t he Arctic re m ai ns a c h alle n ge. It is t heref ore vit all y i m p ort a nt t o c o m bi ne di ffere nt o bser vati o n al met h o ds a n d a d diti o n al t o ols t o m o nit or a n d u n derst a n d t he vari a bilit y of Arctic se a ice as acc ur atel y a n d det aile d as p ossi ble.
1. 2
O bj e c ti v e s a n d o u tli n e of t hi s di s s e r t a ti o n
T he o ver arc hi n g g o al of t his dissert ati o n is t o deter mi ne t he i m p act o n g oi n g c h a n ges a n d vari ati o ns i n se a ice t hic k ness i n t he re gi o ns of se a ice f or m ati o n h a ve o n t he o ver all Arctic se a ice b u d get. To ac hie ve t h at, it is necess ar y n ot o nl y t o o bser ve se a ice t hic k ness reli a bl y a n d wit h hi g h te m p or al a n d s p ati al res ol uti o n b ut als o i n t he re gi o ns m ostl y a ffecte d b y t hese c h a n ges. T his dissert ati o n s u p p orts t he de vel o p me nt of c o nti n u o us, l o n g-ter m, a n d l ar ge-sc ale se a ice t hic k ness d at a rec or ds t hr o u g h t hree m ai n f o ci: t he de vel o p me nt a n d a d a pt ati o n of k n o w n me as ure me nts pri nci ples t o ge ner ate ne w se a ice t hic k ness d at a sets; t he a p plic ati o n of t hese d at a sets t o vali d ate e xisti n g se a ice t hic k ness rec or ds fr o m s atellites; a n d t he a n al ysis of a vail a ble a n d ne wl y ac q uire d d at a rec or ds t o i m pr o ve t he u n derst a n di n g of o bser ve d ice t hic k ness vari a bilit y i n re gi o ns re prese nt ati ve f or di ffere nt st a ges of se a ice de vel o p me nt.
C h a pter 2 gi ves a ge ner al s u m m ar y of se a ice i n t he Arctic a n d t he mec h a nis ms t h at le a d t o c h a n ges i n its t hic k ness. It als o i ntr o d uces di ffere nt met h o ds of me as uri n g se a ice t hic k ness, t heir a d va nt a ges a n d dis a d va nt a ges, a n d f o c uses o n t he t hree met h o ds t h at are esse nti al f or t he st u dies prese nte d i n t his dissert ati o n.
T his dissert ati o n i ncl u des t hree se p ar ate st u dies t h at were c o n d ucte d t o f ul fil t hree m ai n o bjecti ves. T he fi r s t o bj e c ti v e of t he prese nte d dissert ati o n is:
To de vel o p a ne w met h o d t o deri ve se a ice t hic k ness d at a sets t h at e xte n d b ac k i n ti me l o n g e n o u g h t o b e use d f or t he vali d ati o n s atellite se a ice t hic k ness pr o d ucts a n d t o all o w t he i n vesti g ati o n of i nter a n n u al se a ice t hic k ness vari a bilit y i n t he F YI- d o mi n ate d L a pte v Se a.
C h a pter 3 prese nts a st u d y descri bi n g t he de vel o p me nt a n d vali d ati o n of a n a d a pti ve met h o d t o e xte n d m o ori n g- b ase d o bser vati o ns of se a ice dr aft i n t he L a pte v Se a. I niti all y, me as ure me nts fr o m t w o m o ore d U p w ar d- L o o ki n g S o n ars ( U L Ss), s p eci fic all y desi g ne d f or me as uri n g se a ice, are pr o cesse d t o pr o vi de se a ice dr aft ti me series fr o m 2 0 1 3 t o 2 0 1 5. D ue t o t he te m p or al li mit ati o ns of t hese ti me series, di ffere nt a p pr o ac hes are a n al yse d t o e xte n d t he a vail a ble t w o- ye ar d at a set. M ulti ple st u dies h a ve s h o w n t h at u p w ar d-l o o ki n g Ac o ustic D o p pler C urre nt Pr o filers ( A D C Ps), usi n g s o n ar- b ase d met h o ds si mil ar t o U L Ss t o deri ve o ce a n c urre nts a n d se a ice drift vel o cities, c a n als o b e use d t o deri ve se a ice dr aft ( S hc her bi n a et al., 2 0 0 5; B a n ks et al., 2 0 0 6; Bj o er k et al., 2 0 0 8; H y att et al., 2 0 0 8). T hese
1 I nt r o d u c ti o n 1. 2 O bjecti ves a n d o utli ne of t his dissert ati o n pre vi o us a p pr o ac hes relie d o n i nte gr ate d or e xter n al press ure se ns ors t o deri ve i nstr u me nt de pt h of t he A D C P, w hic h is o ne of t he m ost i m p ort a nt c o m p o ne nts of t he pr o cessi n g c h ai n fr o m s o n ar- b ase d me as ure me nts of r a n ge ( dist a nce fr o m t he i nstr u me nt t o t he ice-w ater i nterf ace) t o se a ice dr aft val ues. T he a p pr o ac h descri b e d i n C h a pter 3 s h o ice-ws t h at i nstr u me nt de pt h c a n b e i nferre d fr o m def a ult me as ure me nts of A D C Ps o p er ate d i n b ott o m tr ac k m o de. B ase d o n t his a p pr o ac h, d ail y me a n se a ice dr aft ti me series c a n b e ge ner ate d fr o m A D C Ps e ve n w he n t he y are n ot e q ui p p e d wit h a reli a ble press ure se ns or. W hile U L Ss h a ve o nl y b ee n de pl o ye d fr o m 2 0 1 3 t o 2 0 1 5, A D C Ps were de pl o ye d o ver m uc h l o n ger ti me p eri o ds i n t he L a pte v Se a. Foll o wi n g t he met h o d de vel o p e d i n C h a p-ter 3, ol d A D C P d at a arc hi ves fr o m t he L a pte v Se a are i n vesti g ate d t o e x pl oit d at a sets p ote nti all y usef ul f or t he deri vati o n of se a ice dr aft. B uil di n g u p o n t his ne wl y ac q uire d d at a arc hi ve, t he s e c o n d o bj e c ti v e is:
To vali d ate s atellite se a ice t hic k ness d at a i n t he L a pte v Se a. B ase d o n t he reli a-bilit y of t hese s atellite rec or ds, t he g o al is t o i n vesti g ate i nter a n n u al ice t hic k ness vari a bilit y i n t he L a pte v Se a.
C h a pter 4 prese nts t he c o m p aris o n of se a ice dr aft ti me series deri ve d fr o m A D C P a n d U L S me as ure me nts fr o m m o ori n gs distri b ute d o ver t he e ntire L a pte v Se a wit h di ffere nt s atellite se a ice t hic k ness pr o d ucts b ase d o n t he E ur o p e a n S p ace A ge nc y’s ( E S A) Cli m ate C h a n ge I niti ati ve P h ase 2 ( C CI- 2) se a ice t hic k ness cli m ate d at a rec or d ( C D R). T he ac q uire d s o n ar- b ase d vali d ati o n d at a rec or d pr o vi des d at a fr o m 2 0 0 3 t o 2 0 1 6 a n d is c o m p are d t o gri d de d a n d or bit E S A C CI- 2 se a ice t hic k ness d at a a n d t he mer ge d Cr y o S at- 2 ( C S 2) a n d S oil M oist ure a n d Oce a n S ali nit y ( S M O S) s atellite d at a fr o m 2 0 0 2 t o 2 0 1 7. Foll o wi n g t his vali d ati o n, t he ne wl y ac q uire d i nsi g hts are use d t o i nter pret s atellite- deri ve d se a ice t hic k ness c h a n ges i n t he L a pte v Se a.
C h a pter 5 gi ves a s h ort s u m m ar y of t he res ults a n d m ost i m p ort a nt i nsi g hts fr o m t he d at a i n vesti g ati o n a n d vali d ati o n of C h a pters 3 a n d 4. It als o s hifts t he f o c us fr o m t he L a pte v Se a t o t he Fr a m Str ait, w here se a ice t hic k ness o bser vati o ns were c o n d ucte d usi n g electr o m a g netic i n d ucti o n ( E M) s o u n di n g t o i n vesti g ate s e a ice t hic k ness vari a bilit y at t he e n d of t he Tr a ns p ol ar Drift. T he a n al ysis of t his a d diti o n al l o n g-ter m d at a set, b ase d o n a di ffere nt me as ure me nt tec h ni q ue, is c arrie d o ut t o f ul fil t he t hi r d o bj e c ti v e of t his dissert ati o n, w hic h is:
To i n vesti g ate t he prec o n diti o ni n g e ffect of se a ice t hic k ness vari a bilit y i n t he s o urce re gi o ns of Arctic se a ice o n Arctic- wi de ice t hic k ness a n d es p eci all y o n t he t hic k ness of se a ice e xiti n g t he Arctic t hr o u g h t he Fr a m Str ait.
C h a pter 6 prese nts t he st u d y o n p at h w a ys of Arctic se a ice. Se a ice t hic k ness vari a bilit y is i n vesti g ate d at t he m ai n e xit g ate of Arctic se a ice – t he Fr a m Str ait. L a gr a n gi a n tr ac ki n g re ve als t h at a b o ut 6 5 % of se a ice re ac hi n g Fr a m Str ait ori gi n ates fr o m t he L a pte v Se a. T he st u d y f urt her i n vesti g ates t he re as o ns f or t he o bser ve d i nter a n n u al vari a bilit y a n d c o n nects o bser ve d t hi n ni n g n ort h of t he Fr a m Str ait t o o ce a nic i n fl ue nces e x p ose d t o t he ice alre a d y i n t he L a pte v Se a. C o n necti n g t he m ai n re gi o n of Arctic se a ice f or m ati o n t o se a ice s h ortl y b ef ore it le a ves t he Arctic Oce a n a n d melts, hel ps rec o nstr uct t he life c ycle of Arctic se a ice, t he mec h a nis ms f or mi n g a n d c h a n gi n g it, a n d s u p p orts t he pre dicti o n of
f ut ure c h a n ges i n a gl o b al cli m ate s yste m t h at is s u bject t o a c o nti n u o us tr a nsf or m ati o n pr o cess.
T he c o ncl u di n g C h a pter 7 s u m m arises t he ke y fi n di n gs of t his dissert ati o n a n d pr o-vi des a n o utl o o k t o w ar ds f ut ure scie nti fic st u dies t h at c a n b uil d o n a n d c o m ple me nt t he prese nte d w or k.
R e m a r k
C h a pters 3, 4 a n d 6 prese nt p u blis he d a n d s u b mitte d p a p ers w hic h were c o m pile d wit h c o ntri b uti o ns fr o m t he me nti o ne d c o- a ut h ors. All t hree p a p ers are i ncl u de d i n a n u n al-tere d f or m w hic h le a ds t o mi n or vari ati o ns i n st yle, l a n g u a ge, te nses, a n d a b bre vi ati o ns t hr o u g h o ut t his dissert ati o n. S u m m aries of t he c o ntri b uti o ns of t he res p ecti ve a ut h ors are gi ve n at t he b e gi n ni n g of e ac h of t hese c h a pters.
2
S e a i c e – T h e o r e ti c al b a c k g r o u n d
T he si m pli fie d descri pti o n of t he ice- al b e d o fee d b ac k ( C h a pter 1) s h o ws t he rele va nce of ice-c o vere d a n d ice-free are as f or t he s urf ace e ner g y b u d get i n t he Arctic Oce a n ( Fi g. 2. 1). H o we ver, es p eci all y w he n it c o mes t o t he al b e d o of t he ice s urf ace, it is t o o si m plistic t o o nl y c o nsi der t he e xiste nce or a bse nce of ice. S n o w-c o vere d ice pr o vi des t he hi g hest al b e d o ( 0. 8 t o 0. 9), w hile s n o w-free ice is sli g htl y d ar ker ( 0. 5 t o 0. 7), a n d fres hl y f or me d y o u n g ice is al m ost tr a ns p are nt, w hic h res ults i n a s urf ace al b e d o cl ose t o t h at of t he d ar k o ce a n ( C urr y et al., 1 9 9 5). I n t his c o nte xt, se a ice t hic k ness is rele va nt as well. T hi n ner ice all o ws a l ar ger fr acti o n of s ol ar e ner g y t o b e a bs or b e d b y t he ice or p e netr ate t hr o u g h t o t he u n derl yi n g o ce a n ( Nic ol a us et al., 2 0 1 2; K atlei n et al., 2 0 1 9). To pre dict a n d m o nit or w h at t his i ncre ase d e ner g y i n p ut t o t he o ce a n me a ns f or pri m ar y pr o d ucti vit y ( Ass m y et al., 2 0 1 7), o ce a n he at de p ositi o n ( Per o vic h et al., 2 0 0 7; Pi n ker et al., 2 0 1 4), a n d f ut ure de vel o p me nt of se a ice i n ge ner al, it is vit al t o o bser ve se a ice t hic k ness reli a bl y. To c o m pre he n d t he r ole a n d distri b uti o n of se a ice t hic k ness i n t he Arctic, it is esse nti al t o u n derst a n d h o w se a ice f or ms a n d h o w its t hic k ness c h a n ges.
T his c h a pter s u m m arises h o w se a ice gr o ws, e val u ates pr o cesses c h a n gi n g its t hic k ness, a n d descri b es its ge ner al distri b uti o n. It als o pr o vi des i nf or m ati o n a b o ut di ffere nt ice t hic k ness me as ure me nt tec h ni q ues, w h at t he y me as ure, a n d h o w t o i nter pret a n d utilise t he a vail a ble d at a t o i n vesti g ate p ast, prese nt, a n d f ut ure c h a n ges a n d vari a bilit y i n Arctic se a ice t hic k ness.
2. 1
S e a i c e t hi c k n e s s di s t ri b u ti o n
T he te m p or al de vel o p me nt of se a ice t hic k ness distri b uti o n, ∂ I T D / ∂t , is g o ver ne d b y t hree pr o cesses t h at are gi ve n b y ( T h or n di ke et al., 1 9 7 5):
∂ I T D ∂t = − ∂ H∂i c e(f · I T D ) I − di v ( #»v · I T D ) I I + Φ I I I , ( 2. 1)
wit h f b ei n g t he f u ncti o n f or t her m o d y n a mic i ncre ase / decre ase i n ice t hic k ness ( H i c e),
w hic h de p e n ds o n t he l o c ati o n ( x ) a n d t he ti me (t), re prese nte d i n ter m I . C o m bi ni n g f or t he d y n a mic c o m p o ne nt of se a ice gr o wt h are t he di ver ge nce i n t he ice drift vel o c-it y (#»v (x, t )), w hic h descri b es t he a d vecti o n of se a ice (ter m I I ), a n d t he re distri b uti o n f u ncti o n ( Φ), w hic h descri b es mec h a nic al def or m ati o n (ter m I I I ).
2. 1. 1 T h e r m o d y n a mi c c o m p o n e nt s
Se a w ater freezes at a p pr o xi m atel y − 1. 9◦C. T he c ol d at m os p here c o ols t he o ce a n s urf ace,
w hic h gr a d u all y i ncre ases t he de nsit y of t he s urf ace w aters. As se a w ater a p pr o ac hes its freezi n g p oi nt, it p e netr ates d o w n w ar ds i n t he w ater c ol u m n a n d is re pl ace d b y less de nse, w ar mer w ater t h at is s u bse q ue ntl y c o ole d b y t he at m os p here. T his vertic al mi xi n g
Fi g u r e 2. 1: S c h e m ati c s h o wi n g r el e v a nt m e c h a nis ms of s e a i c e t h er m o d y n a mi cs ( a d a pt e d fr o m Per o vi c h a n d Ri c ht er- M e n g e ( 2 0 0 9); L e p p ¨ar a nt a ( 2 0 1 1)). S e a i c e gr o ws a n d m elts f or c e d b y h e at e x c h a n g e wit h t h e at m os p h er e a n d t h e o c e a n a n d b y r a di ati o n. Gr o wt h a n d m elt o c c ur at t h e u p p er a n d l o wer b o u n d ari es a n d i n t h e i c e i nt eri or ( L e p p ¨ar a nt a, 2 0 1 1).
re ac hes d o w n t o t he h al o cli ne, w hic h is t he b o u n d ar y l a yer b et wee n l o wer de nsit y s urf ace a n d w ar m a n d s ali ne dee p er w aters ( C ottier et al., 2 0 1 7). De p e n di n g o n t he at m os p heric c o n diti o ns, se a ice b e gi ns t o gr o w as a t hi n l a yer i n c al m c o n diti o ns, or as l o ose ice cr yst als (fr azil ice) m o vi n g i n t he t ur b ule nt s urf ace l a yer i n wi n d y c o n diti o ns. O nce t he s urf ace l a yer c al ms, fr azil ice cr yst als c o ns oli d ate t o a s oli d l a yer. As se a ice c o nti n ues t o gr o w, c o n gel ati o n ice cr yst als f or m at t he ice- w ater i nterf ace ( Le p p ¨ar a nt a, 2 0 1 1).
D uri n g t he ice f or m ati o n pr o cess, se a s alt i o ns are rejecte d fr o m t he cr yst als a n d f or m bri ne at t he ice- w ater i nterf ace ( Petric h a n d Eic ke n, 2 0 1 7). W he n ice gr o wt h c o nti n ues, bri ne p o c kets c a n b ec o me e ncl ose d i nt o t he ne wl y f or me d ice, si g ni fic a ntl y alteri n g t he p h ysic al pr o p erties of t he ice c o ver ( M a y k ut a n d U nterstei ner, 1 9 7 1). A d diti o n al ice i m p urities i n fl ue nci n g t he p h ysic al pr o p erties of se a ice i ncl u de i nc or p or ati o ns of g ases, or g a nic m atter, se di me nts, a n d p oll ut a nts ( Ri g or a n d C o n ol y, 1 9 9 7; Det hle ff et al., 2 0 0 0; D a m m et al., 2 0 1 8).
T he gr o wt h r ate of se a ice is deter mi ne d b y t he e ner g y b al a nces at t he se a ice b ott o m a n d s urf ace ( Fi g. 2. 1), w hic h are c o u ple d b y c o n d ucti ve he at fl u xes t hr o u g h t he s n o w a n d ice l a yers ( Petric h a n d Eic ke n, 2 0 1 7). Se a ice f or ms a n i ns ul ati n g l a yer at t he o ce a n s urf ace, li miti n g t he e xc h a n ge of he at b et wee n o ce a n a n d at m os p here. I n ge ner al, se a ice gr o wt h at t he b ott o m re q uires t he s urf ace air te m p er at ure t o b e b el o w t he freezi n g p oi nt of se a w ater a n d t he c o n d ucti ve he at fl u x fr o m t he w ar mer o ce a n t hr o u g h t he ice a n d t o t he at m os p here t o b e l ar ger t h a n t he o ce a n he at i n p ut fr o m b el o w ( M a y k ut a n d
2 S e a i c e – T h e o r e ti c al b a c k g r o u n d 2. 1 Se a ice t hic k ness distri b uti o n U nterstei ner, 1 9 7 1; Petric h a n d Eic ke n, 2 0 1 7). As se a ice gr o ws t hic ker, t he i ns ul ati n g e ffect of t he ice c o ver i ncre ases. It t a kes l o n ger t o tr a ns p ort he at fr o m t he o ce a n t o t he at m os p here, w hic h is re q uire d t o m ai nt ai n t he se a w ater freezi n g p oi nt a n d f or m ice at t he ice- w ater i nterf ace ( Le p p ¨ar a nt a, 2 0 1 1). He nce, t he r ate of gr o wt h at t he ice b ott o m is n ot o nl y de p e n de nt o n t he p h ysic al pr o p erties of t he i n h o m o ge ne o us ice c o ver b ut als o o n its t hic k ness.
T he acc u m ul ati o n of s n o w o n se a ice dr astic all y i ncre ases t he i ns ul ati n g e ffect of t he ice c o ver. T he t her m al c o n d ucti vit y of s n o w is a p pr o xi m atel y o ne or der of m a g nit u de l o wer t h a n t h at of se a ice a n d alt h o u g h s n o w o nl y acc o u nts f or a s m all fr acti o n of t he t ot al m ass of se a ice, it f urt her re d uces its gr o wt h r ate ( Petric h a n d Eic ke n, 2 0 1 7). S n o w c a n als o c o ntri b ute t o se a ice gr o wt h at t he ice s urf ace. Di ffere nt t y p es of ice f or m at t he s urf ace w he n s n o w is i n filtr ate d b y li q ui d w ater fr o m preci pit ati o n, s urf ace melt, or fl o o di n g a n d refreezes. Si nce fl o o di n g is t he m ost c o m m o n mec h a nis m t o f or m ice fr o m s n o w, t hic k s n o w c o vers are re q uire d t o s u b mer ge t he ice i nt o t he o ce a n. S n o w c o vers s u fficie ntl y t hic k t o i niti ate t his pr o cess are u nc o m m o n i n t he ce ntr al Arctic a n d m ostl y o cc ur i n l o w-l atit u de se as o n t he n ort her n he mis p here a n d i n t he A nt arctic ( Le p p ¨ar a nt a, 2 0 1 1). H o we ver, rece nt st u dies h a ve f o u n d t h at t he i m p act of s n o w-ice o n se a ice gr o wt h i n t he ce ntr al Arctic m a y b e i ncre asi n g as m ore fre q ue nt st or ms bri n g he a v y preci pit ati o n t o t he t hi n ni n g ce ntr al Arctic ice c o ver ( Mer k o uri a di et al., 2 0 1 7; Pr o v ost et al., 2 0 1 7).
Ulti m atel y, t her m o d y n a mic se a ice gr o wt h is li mite d. B ase d o n t he t hic k ness a n d t he c o m p ositi o n of t he s n o w a n d ice c o ver, t he c o n d ucti ve he at fl u xes fr o m t he o ce a n t o t he
Fi g u r e 2. 2: E x a m pl e of s e a i c e t hi c k n ess distri b uti o n m e as ur e d i n t h e B e a uf ort S e a i n A pril 2 0 0 8 ( H e n dri c ks, 2 0 0 9). T h e t hr e e a p p ar e nt m o d es i n di c at e t hi c k n ess es of t h er m o d y n a m-i c all y gr o w n s e a m-i c e: first- ye ar m-i c e ( F YI, 0. 5 m), s e c o n d- ye ar m-i c e ( S YI, a p pr o xm-i m at el y 2 m) a n d m ulti- ye ar i c e ( M YI, 3. 3 m).
at m os p here are re d uce d, w hic h sl o ws t he t her m o d y n a mic ice gr o wt h u ntil a n e q uili bri u m t hic k ness is re ac he d ( M a y k ut a n d U nterstei ner, 1 9 7 1; Le p p ¨ar a nt a, 2 0 1 1). T he oretic al esti m ates b y M a y k ut a n d U nterstei ner ( 1 9 7 1) i n dic ate t h at t he e q uili bri u m t hic k ness of u n def or me d M YI c a n re ac h val ues b et wee n 3 a n d 4 m.
T he sec o n d t her m o d y n a mic i n fl ue nce o n ice t hic k ness is melt, w hic h, i n ge ner al, o cc urs d ue t o f o ur di ffere nt pr o cesses ( Fi g. 2. 1). S ol ar a n d at m os p heric he at fl u xes le a d t o melti n g of s n o w a n d ice at t he s urf ace. I n c ases w here t he s ol ar r a di ati o n is a bs or b e d i n t he ice, it c a uses i nter n al melti n g pr o cesses. W he n o ce a n he at fl u xes e xcee d t he c o n d ucti ve fl u xes fr o m t he o ce a n t o t he at m os p here, se a ice st arts t o melt at t he ice- w ater i nterf ace ( b ott o m melt) or l ater all y at t he ice e d ges i n le a ds ( Le p p ¨ar a nt a, 2 0 1 1).
Fi g ure 2. 2 s h o ws a n e x a m ple of t he se a ice t hic k ness distri b uti o n me as ure d i n t he Be a uf ort Se a i n A pril 2 0 0 8 ( He n dric ks, 2 0 0 9). T he distri b uti o n dis pl a ys t he fre q ue nc y of o cc urre nce of di ffere nt ice t hic k ness val ues. T he m ost fre q ue ntl y o cc urri n g t hic k ness val ue ( m o de) is c o nsi dere d t o b e a me as ure f or t her m o d y n a mic all y gr o w n ice ( R a b e nstei n et al., 2 0 1 0; H a as, 2 0 1 7). T hree disti nct m o des are visi ble, i n dic ati n g t her m o d y n a mic all y gr o w n F YI ( 0. 5 m), sec o n d- ye ar ice ( S YI, ice t h at h as s ur vi ve d o ne s u m mer melt se as o n, a p pr o xi m atel y 2 m), a n d M YI ( 3. 3 m). H o we ver, t he e x a m ple als o s h o ws t he o cc urre nce of ice t hic k ness val ues c o nsi der a bl y l ar ger t h a n t he t he oretic al e q uili bri u m t hic k ness, w hic h are t he res ult of d y n a mic, a n d m ore s p eci fic all y ice def or m ati o n pr o cesses.
2. 1. 2 D y n a mi c c o m p o n e nt s
D y n a mic e ffects o n se a ice t hic k ness c a n b e di vi de d i nt o t w o m ai n mec h a nis ms ( Fi g. 2. 3). W hile di ver ge nce a n d a d vecti o n are descri b e d b y ter m I I i n E q u ati o n 2. 1, t he mec h a nic al def or m ati o n of se a ice f oll o wi n g c o n ver ge nce is gi ve n b y t he re distri b uti o n f u ncti o n, Φ (ter m I I I ).
T he di ver ge nce of t he ice c o ver is t he res ult of t he pre vaili n g wi n d fiel ds a n d, t o a lesser de gree, o ce a n c urre nts ( S pree n et al., 2 0 1 1). As l o n g as ice m oti o n is n ot pre ve nte d b y o bst acles or c o astli nes, se a ice drifts at a p pr o xi m atel y 1 t o 2 % of t he me a n wi n d s p ee d ( S pree n et al., 2 0 1 1). H o we ver, m ulti ple st u dies h a ve s h o w n t h at Arctic se a ice drift vel o cities are i ncre asi n g ( S pree n et al., 2 0 1 1; It ki n a n d Kr u m p e n, 2 0 1 7), w hic h is attri b ute d t o t he ge ner al t hi n ni n g of Arctic se a ice ( R a m p al et al., 2 0 0 9), b ut als o t o t he p ositi ve tre n d i n wi n d stress c a use d b y a s hift i n st or m tr ac ks ( H a k ki ne n et al., 2 0 0 8). Di ver ge nce i n t he ice c o ver ge ner ates s m all o p e ni n gs, le a ds, a n d p ol y n y as. O p e n w ater is e x p ose d t o t he c ol d at m os p here a n d ne w ice f or ms. T he re m o val of se a ice of a cert ai n t hic k ness b y di ver ge nce res ults i n zer o t hic k ness or a t hi n ice si g n al i n t he o ver all ice distri b uti o n ( H a as, 2 0 1 7). Re gi o ns w here t he c o nti n u o us ge ner ati o n of o p e n w ater are as le a ds t o m ost of t he Arctic’s se a ice f or m ati o n are f o u n d o n t he s h all o w s hel ves of t he R ussi a n Arctic a n d s p eci fic all y i n t he L a pte v a n d E ast Si b eri a n Se a. T he pre vaili n g wi n d fiel ds tr a ns p ort ne wl y f or me d ice n ort h w ar d a n d a w a y fr o m t he c o ast, w hic h o p e ns l ar ge are as of o p e n w ater w here ne w ice c a n b e f or me d ( Ti m o k h o v, 1 9 9 4; Kr u m p e n et al., 2 0 1 3). T he re distri b uti o n f u ncti o n, Φ, descri b es t he def or m ati o n of se a ice i n res p o nse t o c o n ver ge nce ( ne g ati ve di ver ge nce) i n t he se a ice c o ver. Es p eci all y t hi n ice is s usce pti ble t o b ei n g tr a nsf or me d i nt o t hic ker ice b y def or m ati o n ( H a as, 2 0 1 7). De p e n di n g o n t he i niti al t hic k ness a n d p h ysic al pr o p erties of t he ice t h at is def or me d, fr act ure mec h a nics,
2 S e a i c e – T h e o r e ti c al b a c k g r o u n d 2. 1 Se a ice t hic k ness distri b uti o n
Fi g u r e 2. 3: S c h e m ati c of t h e d y n a mi c i n fl u e n c es o n t h e d e vel o p m e nt of t h e s e a i c e t hi c k n ess distri b uti o n ( a d a pt e d fr o m H a as a n d Dr u c ke n mill er, 2 0 0 9). S e a i c e t hi c k n ess v ari es c o nsi d er a bl y a n d d e p e n ds o n v ari o us at m os p h eri c a n d o c e a ni c f a ct ors i n cl u di n g wi n d, o c e a n c urr e nts, a n d s e a a n d air s urf a c e t e m p er at ur es ( M ei er a n d H a as, 2 0 1 2).
t he s n o w a n d ice i nterf aces, t he e ner g y of t he def or m ati o n, a n d t he sc ales o n w hic h t hese pr o cesses o cc ur, se a ice def or m ati o n ge ner ates a wi de r a n ge of t hic k nesses ( H a as, 2 0 1 7). Mec h a nic al c h a n ges i n ice t hic k ness are as y m metric, w hic h me a ns ice t hic k ness i ncre ases mec h a nic all y b ut decre ases o nl y t her m o d y n a mic all y b y melt ( Le p p ¨ar a nt a, 2 0 1 1), or b y t he a d vecti o n of t hi n ner ice t o t he s p eci fic re gi o n. T he i n fl ue nce of def or m ati o n o n t he te m p or al de vel o p me nt of t he ice t hic k ness distri b uti o n is t he m ost c h alle n gi n g t o q u a ntif y. W hile t her m o d y n a mic ice gr o wt h is re prese nte d b y t he m o de, t he gr a de of def or m ati o n deter mi nes t he t ail of t he o ver all t hic k ness distri b uti o n a n d t heref ore g o ver ns its me a n. 2. 1. 3 S e a i c e t hi c k n e s s di s t ri b u ti o n i n t h e A r c ti c
C h a n ges i n se a ice t hic k ness at a n y gi ve n l o c ati o n a n d ti me are t he res ult of a c o m bi n ati o n of t he a b o ve- me nti o ne d t her m o d y n a mic a n d d y n a mic pr o cesses. H o we ver, d ue t o t he l ar ge-sc ale at m os p heric a n d o ce a nic circ ul ati o n p atter ns i n t he Arctic, i n di vi d u al re gi o ns are d o mi n ate d b y eit her def or me d M YI or t her m o d y n a mic all y gr o w n y o u n ger ice ( Z h a n g et al., 2 0 0 0). T he p ersiste nt at m os p heric hi g h press ure s yste m o ver t he Be a uf ort Se a is t he m ai n dri ver of t he a ntic ycl o nic (cl o c k wise) Be a uf ort G yre a n d t he Tr a ns p ol ar Drift s yste m ( Ri g or et al., 2 0 0 2, Fi g. 2. 4). O ffs h ore- directe d wi n ds fr o m t he Si b eri a n c o ast p us h ne wl y f or me d, t hi n ice n ort h w ar d w here it is i nc or p or ate d i nt o t he Tr a ns p ol ar Drift. T his pr o cess le a ds t o c o nti n u o us ice f or m ati o n o n t he s h all o w R ussi a n s hel ves a n d a d o mi n a nce of t her m o d y n a mic all y gr o w n y o u n g se a ice i n t his re gi o n ( Kr u m p e n et al., 2 0 1 3). Ice n ot
Fi g u r e 2. 4: J a n u ar y t o A pril 2 0 2 0 m e a n s e a i c e t hi c k n ess fr o m t h e E S A C CI- 2 cli m at e d at a r e c or d ( H e n dri c ks et al., 2 0 1 8 a), s u p eri m p os e d b y s c h e m ati cs of t h e g e n er al Ar cti c s e a i c e drift p att er ns.
e xiti n g t he Arctic t hr o u g h Fr a m Str ait is p us he d t o w ar ds t he n ort her n c o ast of Gree nl a n d a n d t he C a n a di a n Arc hi p el a g o w here it piles u p a n d str o n gl y i ncre ases its t hic k ness m ai nl y t hr o u g h def or m ati o n pr o cesses ( K w o k a n d C u n ni n g h a m, 2 0 1 5). M ost of t his ice c o nti n ues t o circ ul ate wit hi n t he Be a uf ort G yre a n d is us u all y m uc h ol der a n d t hic ker t h a n t he ice p assi n g al o n g t he Tr a ns p ol ar Drift ( Ri g or et al., 2 0 0 2, Fi g. 2. 4).
T he stre n gt h of b ot h l ar ge-sc ale circ ul ati o n re gi mes, t he Be a uf ort G yre a n d t he Tr a ns-p ol ar Drift, is g o ver ne d b y t he ge ner al wi n d- dri ve n Arctic Oce a n circ ul ati o n a n d c a n b e li n ke d t o t he Arctic Oscill ati o n ( A O), w hic h is descri b e d b y s urf ace le vel press ure a n o m a-lies o ver t he N ort her n He mis p here a n d es p eci all y t he ce ntr al Arctic Oce a n ( T h o m ps o n a n d Wall ace, 1 9 9 8). A p ositi ve A O p h ase is c h ar acterise d b y a ne g ati ve s urf ace press ure a n o m al y o ver t he ce ntr al Arctic Oce a n, w hile a ne g ati ve A O p h ase s h o ws a p ositi ve pres-s ure a n o m al y. T he m ore c ycl o nic (c o u nter-cl o c k wipres-se) m oti o n of pres-se a ice d uri n g a p opres-siti ve A O p h ase le a ds t o i ncre ase d tr a ns p orts of se a ice fr o m t he R ussi a n s hel ves a n d a f ast Tr a ns p ol ar Drift ( Ri g or et al., 2 0 0 2). Acc or di n g t o Ri g or et al. ( 2 0 0 2), t he i ncre asi n g ice f or m ati o n i n t he R ussi a n Arctic, a f aster Tr a ns p ol ar Drift, a n d a c oi nci de nt al sl o wi n g of se a ice m oti o n i n t he Be a uf ort G yre res ult i n less ri d gi n g a n d recirc ul ati o n of se a ice a n d s h o ul d c o ntri b ute t o t he t hi n ni n g of se a ice d uri n g a p ositi ve A O p h ase. D uri n g a ne g a-ti ve A O p h ase, ice fr o m t he Be a uf ort G yre gets tr a ns p orte d t o w ar ds t he e aster n Arca-tic
2 S e a i c e – T h e o r e ti c al b a c k g r o u n d 2. 2 Ke y me as ure me nt tec h ni q ues m uc h f aster t h a n d uri n g a p ositi ve A O p h ase. D ue t o t he stre n gt he ne d Be a uf ort G yre circ ul ati o n t he ice e xiti n g Fr a m Str ait te n ds t o b e t hic ker t h a n d uri n g a p ositi ve A O p h ase ( Ri g or et al., 2 0 0 2). H o we ver, t here is a n o n g oi n g de b ate w het her t he A O is t he c orrect mec h a nis m t o des cri b e vari ati o ns i n t he ge ner al wi n d- dri ve n Arctic Oce a n circ ul ati o n a n d t he e ns ui n g vari ati o ns i n se a ice drift s p ee ds ( H a k ki ne n et al., 2 0 0 8; R a m p al et al., 2 0 0 9; S pree n et al., 2 0 1 1; Vi h m a et al., 2 0 1 2).
I n de p e n de nt of t he ori gi n of o bser ve d vari ati o ns i n se a ice drift s p ee ds, t he d o mi n a nt pr o cesses c h a n gi n g se a ice t hic k ness, a n d t he vari ati o ns i n t he ge ner al circ ul ati o n of se a ice i n t he Arctic, i n di vi d u al re gi o ns c a n b e d o mi n ate d b y t her m o d y n a mic all y gr o w n or d y n a mic all y def or me d se a ice. Si nce t he pr o cesses c h a n gi n g se a ice t hic k ness str o n gl y i m p act t he p h ysic al pr o p erties of t he res p ecti ve ice, di ffere nt me as ure me nt tec h ni q ues are m ore s uit a ble i n di ffere nt re gi o ns a n d f or cert ai n ice t y p es t h a n ot hers.
2. 2
K e y m e a s u r e m e nt t e c h ni q u e s
T he l ar ge-sc ale distri b uti o n of se a ice t hic k ness is o ne of t he i nte gr al p ar a meters f or m o nit ori n g c h a n ges i n t he Arctic se a ice c o ver. Se a ice t hic k ness varies str o n gl y o n l o c al sc ales a n d its vertic al e xte nt is c o m p ar a bl y s m all, w hic h re q uires hi g hl y se nsiti ve a n d acc ur ate me as ure me nt tec h ni q ues ( Eic ke n et al., 2 0 1 4). Besi de t he p ossi bilit y of m a n u all y drilli n g se a ice t o me as ure its t hic k ness, m ost of t he me as ure me nt tec h ni q ues a p plie d are i n direct met h o ds. T his me a ns t h at p ar a meters rel ate d t o t he ice t hic k ness are me as ure d a n d se a ice t hic k ness is i nferre d fr o m t hese me as ure me nts ( H a as, 2 0 1 7). T he m ost c o m m o n met h o ds use d t o me as ure or i nfer se a ice t hic k ness, t heir li mit ati o ns, a n d t heir m ai n are as of a p plic ati o n are prese nte d i n gre ater det ail i n t he f oll o wi n g. T he m ost rele va nt met h o ds f or t he st u dies prese nte d i n t his dissert ati o n are gi ve n a m ore det aile d descri pti o n i n se p ar ate s u bsecti o ns.
T he m ost acc ur ate a n d direct w a y of me as uri n g ice t hic k ness is m a n u all y drilli n g t he ice fr o m t he s urf ace a n d me as uri n g its t hic k ness wit h a g a u ge ( H a as a n d Dr uc ke n miller, 2 0 0 9). T his met h o d all o ws f or t he si m ult a ne o us o bser vati o n of all t hic k ness c o m p o ne nts: ice free b o ar d a n d dr aft (c o m bi ni n g f or ice t hic k ness), as well as s n o w t hic k ness ( H a as, 2 0 1 7). W hile t he acc ur ac y of me as uri n g ice t hic k ness b y drilli n g h oles is u n m atc he d, it is o nl y p ossi ble i n re gi o ns t he o bser ver c a n access. O nce t he o bser ver is o n t he ice, it is te di o us w or k a n d t a kes a l ot of ti me. T he e n or m o us e ff ort re q uire d li mits t he s p ati al e xte nt of t he me as ure me nts a n d restricts t he vali dit y of t he res ults t o a s m all are a. A c ollecti o n of ice t hic k ness val ues me as ure d wit h a drill is us u all y bi ase d t o w ar ds t hic ker ice, as ver y t hi n ice is h ar dl y accessi ble o n f o ot. He nce, m a n u al drilli n g is m ai nl y d o ne t o vali d ate l ar ger-sc ale me as ure me nts ( H a as, 2 0 1 7).
D uri n g s hi p b or ne e x p e diti o ns, Arctic se a ice t hic k ness is us u all y o bser ve d vis u all y d uri n g t he tr a nsit t hr o u g h t he ice (rece ntl y b ase d o n t he Arctic S hi p b or ne Se a Ice St a n-d ar n-diz ati o n To ol, A S SI S T /Ice Watc h pr ot o c ol i ntr o n-d uce n-d b y H utc hi n gs, 2 0 1 8). Ice fr a g-me nts br o ke n b y a p assi n g s hi p us u all y t ur n si de w a ys a g ai nst t he h ull a n d t heir t hic k ness c a n b e esti m ate d vis u all y or e ve n wit h c a mer a s yste ms ( C h a pter 6). H o we ver, vis u al ice t hic k ness me as ure me nts d ate b ac k t o t he e arl y 2 0t h ce nt ur y a n d c o nti n ue t o b e a n i nte-gr al p art of R ussi a n ice c h arti n g e ff orts m a n a ge d b y t he Arctic a n d A nt arctic Rese arc h
I nstit ute ( A A RI). Se a ice t hic k ness i nf or m ati o n is li mite d t o t he r o ute of t he s hi p a n d bi ase d t o w ar ds t hi n ner ice t h at is e asier t o n a vi g ate.
Di ffere nt c o n fi g ur ati o ns of a ut o n o m o us me as ure me nt st ati o ns e q ui p p e d wit h t her mis-t or c h ai ns c a n i nfer se a ice mis-t hic k ness w hile drifmis-ti n g wimis-t h mis-t he ice. O nce mis-t he c h ai n a n d its cl osel y s p ace d t her mist ors are s oli dl y fr oze n i nt o t he ice, vertic al te m p er at ure pr o files are me as ure d. T he vertic al te m p er at ure gr a die nts are ver y di ffere nt i n s n o w a n d ice a n d a bse nt i n t he w ater a n d air c ol u m ns cl ose t o t he res p ecti ve i nterf aces d ue t o t he di fferi n g t her m al c o n d ucti vities of t he rele va nt l a yers. Usi n g a s atellite li n k t o tr a nsfer t he d at a, t his tec h ni q ue all o ws f or ver y acc ur ate disti ncti o n of t he i nterf aces b et wee n air, s n o w, ice, a n d w ater i n q u asi re al-ti me. H o we ver, it is ver y li mite d s p ati all y as it pr o vi des a p oi nt me as ure me nt a n d f oll o ws t he drift p at h of a si n gle ice fl o e. I n a d diti o n, t hese t her mist or b u o ys pr o vi de o nl y t her m o d y n a mic c h a n ges of se a ice t hic k ness a n d are pr o ne t o bre a k d uri n g def or m ati o n e ve nts ( Eic ke n et al., 2 0 1 4).
2. 2. 1 U p w a r d- L o o ki n g S o n a r
U p w ar d- L o o ki n g S o n ars ( U L Ss) are t he pri m ar y s o urce of l o n g- d ur ati o n se a ice dr aft d at a wit h hi g h te m p or al res ol uti o n ( R oss et al., 2 0 1 6). T he y e mit s h ort p ulses of ac o ustic e n-er g y i n n arr o w b e a ms a n d at fre q ue ncies of u p t o 2 Hz ( R oss et al., 2 0 1 6). T he ret ur n si g n als are detecte d a n d t he del a y ti mes b et wee n e mitte d a n d detecte d si g n al are precisel y me as ure d a n d c o n verte d i nt o t he dist a nce b et wee n t he i nstr u me nt a n d t he re flecti n g ice s urf ace (r a n ge, Fi g. 2. 5). A d diti o n al i nf or m ati o n a b o ut t he de pt h of t he i nstr u me nt u n-der ne at h t he w ater s urf ace are re q uire d t o n-deri ve se a ice dr aft. Us u all y U L S m o ori n gs are e q ui p p e d wit h press ure se ns ors t o rec or d c o nte m p or ar y press ure d at a at t he i nstr u me nt. I n c o m bi n ati o n wit h s urf ace press ure i nf or m ati o n, t hese d at a rec or ds pr o vi de acc ur ate dis-t a nces b edis-t wee n dis-t he i nsdis-tr u me ndis-t a n d dis-t he w adis-ter s urf ace. Ta ki n g s o u n d vel o cidis-t y i n se a w adis-ter, p ossi ble c urre nt a n d ti de-i n d uce d i nstr u me nt tilts, a n d b e a m wi dt h bi ases i nt o acc o u nt, r a n ge is s u btr acte d fr o m t he i nstr u me nt de pt h t o o bt ai n se a ice dr aft ( R oss et al., 2 0 1 6, t he rele va nt e q u ati o ns are pr o vi de d i n C h a pter 3). Se a ice t hic k ness c a n b e i nferre d fr o m t he dr aft ti me series usi n g a c o nst a nt r ati o b et wee n t hic k ness a n d dr aft deter mi ne d fr o m drilli n g ( Vi nje a n d Fi n ne kas a, 1 9 8 6) or b y ass u mi n g h y dr ost atic e q uili bri u m. Foll o wi n g Arc hi me des’ pri nci ple, h y dr ost atic e q uili bri u m ass u mes t h at t he f orces acti n g d o w n w ar d ( gr a vit y) a n d u p w ar d ( b u o y a nc y) o n t he o bser ve d ice are e q u all y str o n g, w hic h kee ps t he ice i n a b al a nce d p ositi o n at t he o ce a n s urf ace. B ase d o n t his b al a nce a n d i nf or m ati o n a b o ut s n o w de pt h a n d t he de nsities of ice a n d s n o w, se a ice t hic k ness c a n b e deri ve d ( Eic ke n et al., 2 0 1 4).
W hile all t y p es of s o n ars are tec h nic all y us a ble t o deri ve se a ice dr aft, t he Ice Pr o-fili n g S o n ar (I P S) w as s p eci fic all y desi g ne d f or t he a p plic ati o n of deri vi n g se a ice dr aft fr o m ac o ustic d at a. I P Ss h a ve b ee n m o u nte d t o s u b m ari nes ( B o ur ke a n d G arret, 1 9 8 7; R ot hr o c k a n d We ns n a h a n, 2 0 0 7) or o ce a n o gr a p hic m o ori n gs ( F u ka m ac hi et al., 2 0 0 3; H a nse n et al., 2 0 1 3; Kris h fiel d et al., 2 0 1 4; Be hre n dt et al., 2 0 1 5) i n b ot h t he Arctic a n d A nt arctic. Ot her s o n ar- b ase d i nstr u me nts, s uc h as Ac o ustic D o p pler C urre nt Pr o filers ( A D C Ps), h a ve als o b ee n use d t o deri ve se a ice dr aft ti me series ( S hc her bi n a et al., 2 0 0 5; B a n ks et al., 2 0 0 6; H y att et al., 2 0 0 8).
2 S e a i c e – T h e o r e ti c al b a c k g r o u n d 2. 2 Ke y me as ure me nt tec h ni q ues
Fi g u r e 2. 5: S c h e m ati c of t h e m e as ur e m e nt pri n ci pl e of u p w ar d-l o o ki n g, or i c e pr o fili n g s o n ars ( a d a pt e d fr o m ( R oss et al., 2 0 1 6)). T h e t w o- w a y tr a vel ti m e of t h e a c o usti c si g n al e mitt e d fr o m t h e U L S a n d r e fl e ct e d at t h e i c e- w at er i nt erf a c e is c al c ul at e d i nt o dist a n c e b et we e n U L S a n d i c e (r a n g e) a n d s u btr a ct e d fr o m t h e dist a n c e of t h e i nstr u m e nt t o t h e air- w at er i nt erf a c e (i nstr u m e nt d e pt h) t o d et er mi n e s e a i c e dr aft.
S u b m ari ne- m o u nte d U L Ss all o w f or l o n g-r a n ge se a ice t hic k ness tr a nsects i n t he re-gi o ns w here t he s u b m ari nes are o p er ati n g a n d us u all y h a ve t o b e c orrecte d f or se as o n al vari a bilit y i n or der t o b e c o m p are d t o ot her d at a sets ( R ot hr o c k a n d We ns n a h a n, 2 0 0 7). M o ore d U L Ss o n t he ot her h a n d pr o vi de c o nti n u o us ti me series f or si n gle l o c ati o ns a n d f or se ver al ye ars ( R oss et al., 2 0 1 6). St ate of t he art I P Ss are e ve n de pl o ye d wit h a sec-o n d m sec-o sec-ori n g e q ui p p e d wit h a n u p w ar d-l sec-o sec-o ki n g A D C P. C sec-o m bi ni n g ice dr aft ti me series fr o m t he U L S a n d drift d at a fr o m t he A D C P all o ws f or t he deri vati o n of s p ati al se a ice t hic k ness series a n d det aile d i n vesti g ati o ns of se a ice v ol u me tr a ns p orts, es p eci all y w he n m ulti ple U L S / A D C P m o ori n g p airs are de pl o ye d al o n g tr a nsects acr oss k n o w n se a ice p at h w a ys ( H a nse n et al., 2 0 1 3). H o we ver, t he use of m o ori n gs is de p e n de nt o n ice-free c o n diti o ns d uri n g de pl o y me nt a n d rec o ver y, w hic h li mits t heir a p plic ati o n t o t he se as o n al ice z o nes, a n d d at a c a n n ot b e o bt ai ne d i n re al-ti me (see s u m m ar y b o x f or a n o ver vie w of pr os a n d c o ns of m o ore d s o n ars).
Pr os a n d c o ns: m o ore d U p w ar d- L o o ki n g S o n ar M e t h o d: i n direct
A c c u r a c y: u p t o 0. 0 5 m
C o m p o n e nt m e a s u r e d: dr aft
A d v a nt a g e s: Di s a d v a nt a g e s: - hi g h te m p or al res ol uti o n - s p ati all y li mite d - ye ar-r o u n d c o ver a ge - b atter y- p o were d
- t hic k ness distri b uti o n - d at a access o nl y after rec o ver y of p assi n g ice - de pl o y me nt /rec o ver y o nl y
i n ice-free c o n diti o ns 2. 2. 2 El e c t r o m a g n e ti c i n d u c ti o n s o u n di n g
I n c o ntr ast t o U L S me as ure me nts of se a ice dr aft fr o m b el o w t he ice, electr o m a g netic i n d ucti o n ( E M) s o u n di n g is a p plie d fr o m a b o ve t he se a ice a n d s n o w s urf ace. T he E M met h o d t a kes a d va nt a ge of di ffere nt electric al c o n d ucti vities of t he i n vesti g ate d l a yers a n d w as first a p plie d f or ge o p h ysic al e x pl or ati o n ( K o vacs et al., 1 9 8 7). T he E M de vice is e q ui p p e d wit h a tr a ns mitter c oil, w hic h ge ner ates a pri m ar y electr o m a g netic fiel d t h at p e netr ates t hr o u g h t he l o w-c o n d ucti vit y ice a n d s n o w l a yers al m ost u n a ffecte dl y ( K o vacs et al., 1 9 8 7). W hile t he c o n d ucti vit y of ice a n d s n o w r a n ges fr o m 0 t o 5 0 milli- Sie me ns p er metre ( m S m− 1), se a w ater c o n d ucti vities t y pic all y re ac h val ues b et wee n 2 4 0 0 a n d
2 7 0 0 m S m− 1 ( H a as et al., 1 9 9 7). T hese val ues are hi g hl y de p e n de nt o n t he se as o n a n d,
i n t he c ase of t he c o n d ucti vit y of se a ice, o n t he p h ysic al pr o p erties of t he s a m ple d ice ( H a as et al., 1 9 9 7). T he p e netr ati n g pri m ar y electr o m a g netic fiel d i n d uces electric e d d y c urre nts i n t he se a w ater. T he i n d uce d e d d y c urre nts ge ner ate a sec o n d ar y fiel d w hic h p e netr ates u p w ar ds. T he recei ver c oil i n t he E M de vice rec or ds t he t ot al electr o m a g-netic fiel d ( pri m ar y a n d sec o n d ar y fiel d) a n d t heir di ffere nces i n p h ase a n d a m plit u de ( K o vacs et al., 1 9 8 7; He n dric ks, 2 0 0 9). T he pr o p orti o n alit y b et wee n t he stre n gt h of t he sec o n d ar y fiel d a n d t he dist a nce fr o m t he c oils t o t he c o n d ucti ve se a w ater s urf ace is use d t o c alc ul ate t he dist a nce b et wee n t he E M de vice a n d t he ice- w ater i nterf ace, hw ( Keller
a n d Frisc h k nec ht, 1 9 6 6, K o vacs et al., 1 9 8 7, H a as et al., 1 9 9 7, Fi g. 2. 6 a)). A d diti o n al f act ors, s uc h as si g n al fre q ue nc y a n d c oil orie nt ati o n a n d s p aci n g, als o a ffect t he recei ve d sec o n d ar y E M si g n al ( H a as, 2 0 1 7). D ue t o t he f act t h at t he a m plit u de of t he sec o n d ar y fiel d decre ases e x p o ne nti all y wit h i ncre asi n g dist a nce b et wee n t he c o n d ucti ve se a w ater s urf ace a n d t he recei ver c oil, st ate of t he art E M de vices use d f or ice t hic k ness s a m pli n g are c o m m o nl y o p er ate d at hei g hts less t h a n 2 0 m a b o ve t he se a ice s urf ace ( H a as et al., 2 0 0 9). T hese restricti o ns all o w f or t w o p ossi ble me as ure me nt set u ps t h at re q uire di ffere nt a p pr o ac hes t o deri ve ice t hic k ness.
T he first set u p uses gr o u n d- b ase d E M de vices ( G E M), c o m m o nl y b uilt i nt o a li g ht-wei g ht sle d ge, t h at are l o c ate d directl y o n t he ice or s n o w c o ver ( o ffset b et wee n E M de vice
2 S e a i c e – T h e o r e ti c al b a c k g r o u n d 2. 2 Ke y me as ure me nt tec h ni q ues a n d s urf ace h as t o b e k n o w n) a n d e ffecti vel y i nfer t he dist a nce b et wee n t he air-ice a n d ice-w ater i nterf ace, t he t ot al ice t hic k ness ( H a as et al., 1 9 9 7). T he sec o n d set u p uses air b or ne E M de vices ( A E M), t h at are t o we d fr o m eit her a helic o pter, fi xe d- wi n g aircr aft ( K o vacs et al., 1 9 8 7; M ult al a et al., 1 9 9 6), or e ve n m o u nte d t o t he b o w of a s hi p ( H a as, 1 9 9 8) a n d i nfer t he dist a nce fr o m t heir p ositi o n t o t he ice- w ater i nterf ace ( hw ). R a d ar or l aser
alti meters i nte gr ate d i nt o E M de vices me as ure t he dist a nce fr o m t he i nstr u me nt t o t he air-ice i nterf ace, hi ( K o vacs et al., 1 9 8 7; H a as et al., 2 0 0 9). T his dist a nce is s u btr acte d
fr o m t he E M- deri ve d dist a nce b et wee n t he A E M a n d t he ice- w ater i nterf ace t o deri ve se a ice t hic k ness ( Fi g. 2. 6). B ot h t he gr o u n d- b ase d a n d t he air b or ne a p pr o ac h t heref ore pr o vi de t ot al se a ice t hic k ness, i ncl u di n g ice t hic k ness, H i c e, a n d t he t hic k ness of t he s n o w
c o ver, H s n o w.
G E M me as ure me nts are o bt ai ne d o n f o ot or usi n g s n o w m o biles, w hic h li mits t he are al e xte nt of t he me as ure me nts si g ni fic a ntl y. H o we ver, p ar allel gr o u n d me as ure me nts of s n o w de pt h al o n g t he G E M s ur ve ys, usi n g i nstr u me nts s uc h as t he M a g n a Pr o b e ( St ur m a n d H ol m gre n, 2 0 1 8), all o w f or reli a ble ice t hic k ness d at a fr o m t hese gr o u n d- b ase d me as ure-me nts. T he s p aci n g b et wee n i n di vi d u al ure-me as ure ure-me nts is de p e n de nt o n t he s ur ve y s p ee d a n d t he i nstr u me nt c o n fi g ur ati o n ( H a as et al., 1 9 9 7). For b ot h of t he a p pr o ac hes, it is i m p ort a nt t o e ns ure a l ar ge-e n o u g h dist a nce b et wee n t he E M de vice a n d a n y met allic p arts t o a v oi d dist ur b a nce of t he E M fiel d b y ot her hi g hl y c o n d ucti ve me di a (c o m m o nl y, a c o u ple of meters f or t he G E M a n d a p pr o xi m atel y 6 0 m f or t he A E M). I n a d diti o n t o t he s p ati al li mit ati o ns, ice t hic k ness distri b uti o ns i nferre d fr o m G E M me as ure me nts us u all y l ac k i nf or m ati o n a b o ut t hi n ice d ue t o t he li mit ati o ns of s a m pli n g it o n f o ot ( H a as, 2 0 0 4).
Fi g u r e 2. 6: a) S c h e m ati c of t h e m e as ur e m e nt pri n ci pl e of a n air b or n e el e ctr o m a g n eti c i n d u cti o n s o u n di n g ( A E M) i nstr u m e nt ( a d a pt e d fr o m ( H a as et al., 2 0 0 9)). Tot al s e a i c e t hi c k n ess (s n o w + i c e t hi c k n ess) is c al c ul at e d fr o m E M m e as ur e m e nt i nf err e d dist a n c e b et we e n t h e E M d e vi c e a n d t h e i c e- w at er i nt erf a c e, hw , a n d t h e l as er alti m et er m e as ur e d
dist a n c e b et we e n t h e E M d e vi c e a n d t h e i c e /s n o w s urf a c e, hi. b) Air b or n e E M d e vi c e
( E M Bir d) t o we d b y a R V P ol arst er n h eli c o pt er i n t h e Ar cti c ( p h ot o b y St ef a n H e n dri c ks, Alfr e d We g e n er I nstit ut e).
