Oceanographic phenomena caused by interaction of currents and bottom topography -

Oceanographic phenomena caused by interaction of currents and bottom topography -

historical notes and theoretical modelling

In go Henni ngs

93

O,er the last fe" centuries. different structures and colours of the oce:111 ,urface have fa,cinatcd ,a1lor,,. fi,hcrmen. disco,erer,,, and ,e1cnti,ts. Variation, of oceanographic parameter, cau,ed by the interaction of water current, with bouom topography arc summari,cd ba,cd on mea,urements at d1f fcrent location, of the ocean,. Eleven observation area, are pre,ented here which renect, 111 panicu lar. the origin from identification. by just watching oceanographic phenomena 111 relation to the ,ea bed. unul present day theoretical modelling. It is ,hown that marine remote ,cn,mg data arc e"ential for the under,tanding of different imaging mechanism, in the electromagnetic ,pectrum. An up to date theory on the radar imaging mechani,m of ,ubmarine ,and wave, applying qua\1-,pecular ,cat·

tcring i, outlined e'Plaining ,urface current and bottom topography interaction.

Ozcanographische Ph:inomene, hervorgerufcn durch Wech~eh,irkung von Stromung und Uo- dentopographie • historischc Aufzcichnungen und theoretische Modellicrung. W;ihrcnd dcr let- Lten Jahrhundene haben die unter,chiedlichen Strukturen und Farben der O,canobcrnUchc vicle Seefahrer. h,cher. Entdeckcr und Wi'>\cn,chafller fasLinien. fa werden Ver.inderungen , on 01eanographi,chcn Parametern. hervorgcrufcn durch die Wcch,elwirkung von Stromung mit dcr Meerc,bo<lentopographic. anhand von Me,,ungcn an ver,ch1cdencn Stellen de, Wchmccrc, ,u,am- mengefasst. Elf Bcobachtungsgebiete werden hier vorge,telh. die besonder, die Entwicklung nm der Beobachtung und ldentifi,ierung o,canographi,chcr Phiinomene in Verbindung mit dcm Meeresbodcn bis hin ,ur theorcti,chen Modellierung bcinhalten. fa wird ge,cigt. dass marine Fcrn- erkundungsdaten cinen wesentlichen Beitrag Lum Verstiindnis der unter\Ch1edlichen Abbil- dung,mechani<,mcn im elcktromagnetischen Spektrum gelcistet haben. Eine gcgenwartigc Theorie ,um Radarabbildung~mecha111,mu, von ,ubmarinen Sandwcllen wird unler Anwcndung von qua,i ,picgelnder Ruck,treuung 7ur Erkliirung der Wcch\clwirl..ung ,wischcn der Obernlichcn,tromung und Bodentopographie vorge,tclh.

I.

lntroduction

Effects of submarine bottom topography on water properties such as temperature, salinity, and turbidity, as well as ocean dynamics, considering (tidal) current ve- locity of the surrounding sea in deep and coa tal water . respectively, are subject of past and present research activities. Islanders, ailors, fishermen, discoverers, and seasiders have been known as the first people to describe the different struc- ture and colour of the ocean surface. A study over the last 250 years shows that

94 I. Hennings

many oceanographic phenomena we re known, attentively observed, and correctly interpreted when co mpared with present oceanographic in situ measurements.

Un-

der special circumstan ces, ocean surf ace currents trace out the shape of underlying sea bottom topography like ba sins, ridges and plateaus in water depths of the order of hundred and thousand meters. Thi

.

ph enomenon has been referred to as topo- graphic steering whi ch ha. been known for nearly a century. One example of such

striking

feature. i

s

the

large-scale ocean

circulation w ithin the Nordic Seas

and

Arctic Ocean with their clo. ed ba in known as the Canada Basin, Makarov B asin, Eurasian Basin, Green

land Sea, Lofoten Bas

in, Norwegian Sea, and Iceland Plateau (N~st &

Isachsen, 2003). ll

has been observed that the ea bottom topogra- phy has a strong effect on water temperature distributi on in winter in the East Chi- na Sea and Yellow Sea. Observations show that the hallower the water depth

is.

the lower is the sea surface temperature. A s the wa ter depth increases, the surface temperature rises. The reason for this phenomenon is thought to be due to cooling effects by northerly w ind during winter. The shallower the ocean i

, the volume

of water to be cooled is reduced, re ulting in more efficient cooling (Tanimoto, 2003).

Today, the classical way of retrieving valuable information on

submarine

bedforms such as

sand

waves is the exten. i ve u e and analy is of side

scan sonar

records, including sing le- a well a

s multibeam echo sounding data. Sand waver..

on the continental shelf off Goeree at the Dutch southern North Sea coas t seem to have been first recognized

1

934 on echo-sounder profiles ma de from on board the survey vessel Oceaa11 of Rij kswater taat, Ministry of Transport, Pu blic Works and Water Management

, the Netherl

ands (Yan Veen, 1 936; Yan Veen. 1938; Kenyon &

Stride, 1 968; Stalk. 2008). Signature of norm aliled radar cro

s section (NRCS)

modulation of ma rine sand wave were fir t noticed 35 year later in K a-band air- borne radar imagery acq uired on 19 September 1 969, one hour before low tide al-

.

o at the Dutch coas t of the southern orth Sea (De Loar, 1 98 1

).

In this paper the foc

us is on the variati on of oceanographic parameter

caused by

the interaction of

currents with sea

bottom topography.

In

addition, the direct re-

necti on of the sea bed in shallow water w ithout the pre. ence of current

variations

will also be described using two elected examples. Marine remote

en

ing data

have ma de a speci al contribution t owards the explanation of the different imaging

mechanisms

in

the vi sible, infrared and microwave parts of the electromagnetic

spectrum. The discovery of nonli

near waves in the ocean's near- urface layer par-

ti ally cau<;ed by irregularities of the . ea noor imaged from

spaceborne

platforms

has been summarized by Steven on ( 1999). One of the mo t spectacular and unex-

pected re. ult of the . ynthetic aperture radar (SAR) on board the SEASAT

. atellite

nown in pace in 1 978 wa. the imaging of ea bottom topography ignatures in

coastal water (Alpers & Henning , 1984; Shuchman et al., 1985). Bandlike

pat-

terns appearing to be related to the

large

submarine sand waves and ridgei.

of

Georges B ank at the eastern U.S. North Atlanti c coast have been ob erved by the

coastal zone colour scanner (CZCS) on board the Nimbus-7 atellite also l aunched

in space in

1

978

(Yentsch et al.,

1 994).

It has always

turned out th at a strong co-

herence ex ists between fluctuation of the oceanic

stru

cture and di turbance of

Oceanographic phenomena cau l>Cd by interaction of current , and bottom topography ... 95

flow velocity. The disturbances of flow veloci ty arc often caused by vari ations of underwater bottom topography, like seamount in deep ocean water and sand banks and sand waves in shallow coastal wa ters. The presence of water surf ace roughness changes induced by sea bottom topography structures in coastal water<;

has also be ex pected to influence the water surface temperature and might lead to distinctive patterns in remote sensing infrared images.

In the second section differe nt observation sites of the interaction of cur- rent \\ith the sea bottom topography and direct reflecti on of the sea bed in <,hal- low waters without th e presence of current variations are desc ribed. One example of an actual developed theory on the radar imaging mechanism of undulations at the ea bed such as marine <,and waves us ing quasi-specu lar scattering is presented in the thi rd ection. Finally, the la<,t section contain, a summary.

2. Ob ervation site

An overview of eleven selected observation sites is presented here reflecting in particular the historical development of observing oceanograp hic feature1. in the water column and at the sea '>urface in relation to underw ater bottom topography.

The locations of the observation sites are shown in Figure I.

No. I: Humboldt ( 1 859- 1 860) made a temperature experiment at the Spanish coast off L a Coruiia immediately before hi'> research voyage to outh America ( 1 799- 1 804).

No. 2: Chamis ... o ( 1995) presented a brief description how the transmiuance of sea water was used in the W otjc Atoll for safe navigation in 1 8 1 7.

No. 3: Moller ( 1 93 1 ) <,bowed the influence of the sea bottom relief on the depth positions of boundary layers in the Dardanelles and Bospo rus.

No. 4: Defant ( I 940a) analysed eddie1. of the Scylla and Charybdis in relation to tidal current ve locities in th e Strait of Messina.

No. 5: The interaction of oceanographic parameters with the Altair seamount ha<, been described by Defant and Helland-Han en ( 1 939).

o. 6: Plume'> of sea bottom sedi ment1. associated with crest'> of sand waves obse rved in the sea area between the Sandettie and Outer Ruyti ngen Banks in the Strait of Dover have been reported by Harden Jones and Mitson { 1 982).

No. 7: Siegel and Seifert ( 1 985) investigated the influence of the sea bottom on the spectral reflecti on in the sea area of the Oder Bank in the Baltic Sea.

No. : Y entsch et al. ( 1994) showed colour banding on Georges B ank as viewed by CZCS.

No. 9: Satellite radar and optica l visible data of th e Norfolk sa nd banks have been compared by Robinson and Johannessen ( 1 997).

o. I 0: B ecker ( 1 978) presented airborne infrared scanner data of sea water

surface temperature di stribution off the isle of Heli goland in the Ger-

96 l. Hennings

man Bight of the North Sea. Multi frequency radar images of the north- ern ea area off the isle of Heligoland in the German Bight of the North Sea have been analysed by Hennings et al. ( 1998).

No.

I I:

Finally, Hennings et aJ.

(2002) studied

variations of cold, turbid

,

and chlorophy

ll

-rich water

layers

in

relation

to marine

s

and waves in the southern North Sea.

40'N

30'N 30'N

tllJW 60W 40W 20'W o· 20'E 'O'E

12'N

10'N

S'N

6'N

4'N

155'E 16S'E 171'E 174'E

Figure I: Overview of eleven location, of observation ~itc\ worldwide di,cu~sed in th1, paper.

2.1 Entrance to La Corufia (shoal)

Alexander von Humboldt ( 1769-1859) had already taken much interest on deYel- opments in oceanography

such

as current , ocean temperature and effects of

sand

banks. This has been

noticed

by Carl Bottger (Maury, 1859) in hi introduction to the

second German

edition of Maury's ( 1 855) ,,The physical geography of th

e

sea". On board the Spanish frigate Pizarro,

Humboldt

( 1 859- J 860) described in the first week of

June

1799 an oceanographic phenomenon associated with a shoal at the Spanish Atlantic coast. The location of the observation site is marked as No.

I in Fig.

I. A part of the hypothetical track of

the frigate Pi-:.arro pa sing the shoal

with the indicated profile A-8 is pre ented in Figure 2a. A map of the Spanish At-

lantic coast from La Corufia to El Ferrol is shown in Figure 2b. On the way from

La Corufia to El Ferro( the Pi-:.arro probably crossed the

. hoal

Banco Yacentes

(Basuril)

located

at the entrance of

the

Rea de

la

Coruna (Bundesamt fi.ir

Ol,-eanograph1c phenomena cau<,e<l by interaction of currenL, and bottom topogmphy ... 97

hgurc 2 a) Scc11on of a -,ca chart (Bunde,amt fur See,chifffahn und ll}dmgraph1c. 2005> with the lo- callon ot Banco Yaccn1e, (Ba,uril) at the entrance to La Cmuiia. Spain. A recon,truc1ed parl of the trad, of the frigate P1~11rm pa"mg Banco Yacente, i, indicated hy the pc1,11ion of pmhle A-8 h) O,crv1ew map of the pani,h coa,1 hctwccn La C'oruna and El Fermi with the indicated black frame ,hm\ n in Fig :!a.

Seeschifffah11 und Hydrographie, 2005). During this experiment, I lumboldt mea- ..,ured a water temperature of 12.5" C - 13.3" C at the sea surface above the bank and of 15.0'' C - 15.3° C at the water surface out<,ide the ban!,, in deeper water, re-

<;pcctively. The air temperature wa<; 12. ''. Figure 3 shows the water depth, the sea surface temperature, and the water temperature distribution a-, a function of water depth acro..,.., profile A-8 (sec Fig. 2a for position) according to the mca,uremenl'i and description made by Humboldt. Data obtained from on board cablc-,hips dur- ing the nineteenth century visuali,ed the oceanic thermal regimes globally and in

98

I. Hennings

detail. The exi tence of cold water above hallow banks was confirmed as a gener- al

rule as reported by McConnell ( 1990)

0

/ / _r"

- ... u ¹⁷

I ) / I _/

.,,,,,.--

^....

___

^{~ .>}

_u

"<:, /

., ... ---- -? ...

^U'o

¹⁶

I ^-10

^/

^.,,.

^{, /} / ^{.._,., ...}

'

_'<'o ^\

¹⁵

^{~ c:i.}

s::.

"o.".,,.

^/ E

a.

^Q)

_-20 --

^{, / /}

' ¹⁴

^{~ Q)}

"O

<ii

13

⁰ _co

ta ^"t:

-30 12

^::>

~ ^(J)

11

^{co Q)} _(J)

-40 10

0 200 400 600 800 1000 1200

A distance (m) B

Figure 3: Recon,tructcd profile A-B across Banco Yacentes (Ba;,uril) (\ee aho Fig. 2a) ~howing the water depth, the ,ea \urface temperature, and the temperature distribution a, a runction or water depth according to measurement, taken during the first week of June 1799 and the de,cription made by I lumboldt. The bold and broken arrow, with the letter U indicate the direction, or Oood and ebb tidal current\.

2.2 Wotje Atoll Marshall Islands (lagoon)

During

the discovery voyage of the Rus ian brig Rurik

from

1815 to 1818 under the command of

Otto von Kotzebue ( 1

787-1846) looking for a passage across the

Arctic Ocean and exploring the lesser known parts of Oceania, the naturalist Adal- bert

von Chamisso ( 1781-1838)

was a member of the crew. As the Rurik sailed on

18 January 1817 in the lagoon of the Wotje Atoll of the Ratak chain of the Mar- shall Islands in the Pacific Ocean from the mo. t western

island to the most north-

ern island named Oromed, the captain u ed

the transmittance of

sea water for safe

navigation

(Kotzebue, 1821 ). ,,The weather wa clear

and the

bright '>Un. which shone on

the

.

hoals made the plummet dispensable" (English translation), was the

statement

made by C

hamisso ( 1995).

The Wotje Atoll

of the Mar hall Islands

is marked as No. 2 in Fig. I. The unforeseen appearance of cora

l reefs arising almost vertically

from depth cannot be detected by repeated

soundings in time so

the cap- tain could not be warned. Its bad

visibility at night made such

routes in the Pacific Ocean particularly dangerous.

2.3 Dardanelles and Bosporus (sea bottom relief)

Alfred Merz (

1880-1925, director of

the

lnstitut und Museum

fUr Meereskunde in

Berlin

before the

directorship of A. Defant)

had

made laborious

studies of tidal currents at all

depths

in the North

Sea

(Merz, 1921) and of

the

under-currents

Oceanographic phenomena caused by inleraclion of currents and bouom topography

... 99

flowing inlo the

Black

Sea

through the Dardanelles and

the Bm,porus, Turkey.

Lotte Moller (

1

893-

1973), the first

female German oceanographer prepared the ob. ervations made by A.

M

er.l in the narrows of the Dardanelles and the Bosporw, and published

the re ults (Moller, 1928). Later on, Moller

(

1

93

1

)

noticed in her

publication on ,,Wa

.

erschichtung und

-

bcwegung in Meerengen" that the

influ-

ence of

.

ea bottom relief on the depth of boundary

layers in the water column

it- elf can be observed along channels in many individual cases. Figure 4 shows a verticaJ

profile of water

stratification (potential density ) and movement (current velocity) with

the

position of the ,,Unterstrommaximum" (under-current maxi

-

mum), the .,Stromgren.le" (current boundary), and the ,,Grcnze der Wasserartcn"

(boundary of water mas.

c ) as a function of water depth through the Dardanelles (Moller,

1

93

1

). The observation site of the Dardanelles is marked as No. 3 in Fig. I. In channels and river beds

the boundary layers

always follow the bottom topography and arc patially extending if numerous eddie with vertical and hori- zontal axis exist within the water column. Therefore, these eddies must have been generated due to morphological perturbations. According to observations made by Merz, such stationary eddie!:> are present within the whole wa

ter column

of the Turkish narrow<;. as the sea bed

morphology

remain

s unaltered. New stationary

Figure 4: Vcnical prolile

o r

water ,tratification (potential tlen~ity ) and movement (current velocity

\Cctor<,) 1,1,ith the l)O\ition of the under-currcm ma,imum (German: Umerstromma,imum). the current houndary (Gem1an: Stromgren/C). and the boundary of water ma,,e~ (German: GrenLe der Wa"er- anen) as a function of water depth through the Dardanelle,. Turkey (Moller, 1931 ).

eddies can develop in deeper water l ayers on

ly

if sea botlom slopes show forma- tions of corresponding undulations. This description presented by Moller is com- parable with recent observations of three-dimensional bed forms such as marine

sand

waves wi

th sinuou.

crest

lines associated with developing eddies (Hennings

& Herbers. 2006).

100

_1.

_Hennings

2.4 Strait of Messina (sea bottom morphology)

,,The

current velocities and their changes along the Strait

of Messina are very in-

teresting to us

with

regard to the turbulent processes which take place especia lly at the northern outle t and within the most narrow sector of the strait and which

are related

to the eddies of Scylla and Charybdis", described the Austrian oceanogra- pher Albe11 Defont

( 1884-1974), director of the Tnstitut und Museum fi.ir

Meereskunde in Berlin 1927-1945, in his publi

cation on

Scylla

and Charybdis (Defant, l 940a). The location of

the Strait of Messina, Italy, is marke d a No. 4 in Fig.

1. ,,Be connected with the current convergences", Defant continues, ,,are now also eddies with a vertical axis.

Here, there exist three locations,

which show for

their d

evelopment probably

particular

favourable

morphological

sea

bottom

shapes. These are the locations at Peloro, i.e. the Charybdis, at Scilla,

i.e. the Scyl-

la-eddy

and the eddy at Punta San Ranie ri at the harbour bar of Messina.

These e

ddies have normally

a cyclonic

rotation. But there exist also such eddies of anti-

cyclonic characters.

They are recognizable by their upwelling of water within the

central

parts. The sea surface appears he

re as a smooth apparently oily area, why

they

are

also named macchie d'oglio in Italian

lang

uage·'. Oceanographic

phe-

nomena like Scilla and Charybdis have been

also identified

in SEASAT synthetic

apc1ture radar (SAR) imagery (Alpers

& Salusti

,

1983).

·f:-r-f-t-+>'-+-+--+-...,+-1--ii'+t~-::--'--t-~-t-~,~---1-~-t--;1soo

!

I

.

i ..

i

Figure 5: Depth profile of water temperature distribution across the Gulf Stream nonhweMerly of the ALOre~ measured between 3-6 June 1938 during the International Gulf Stream Expedition (Defont &

Helland-Hansen. 1939).

Oceanographic phenomena caused by intcmcLion of currents and bollom topogmphy ...

IO I

2.5 North Atlantic Ocean (A lta ir seamount)

The interaction of oceanographic parameters like water temperature and <,alinity with a eamount due to a current has been observed during the International Gulf Stream Expedition between 31.05.-26.06.1938. This expedition took place in the North Atlantic north-west of the ALores marked ac, No. 5 in Fig. I. Defant was chief scientist on board the research vessel

Altair

which was a general cargo freighter chartered by the German Navy. Figures 5-6 show depth profiles of water temperature and salinity distribution<;, respectively, across the Gulf Stream north we terly of the Azores measured on 3-6 June 1938 (Defant

&

Helland-Hansen, 1939). Temperature and especially the salinity distributions show wavy distur- bances cau ed at 44.60 N in the vicinity of the Altair seamount. A mean depth-av- eraged residual current velocity of 18 cm s-1 with an almost westerly direction has been measured. Defont and Helland-Hansen ( 1939) assumed that at this location the Gulf Stream was disturbed by the sea bottom topography which was correctly interpreted at that time. But the oceanographic relationships were more complicat- ed as Defant showed in his publication on the oceanographic relations during the anchor station of the

Altair

at the northern boundary of the main now line of the Gulf Stream north of the Azores (Defant, I 940b). However, these variation<; in temperature and salinity disLributions above a seamount are an impressive example of localiLed sea bouom topography disturbances in the deep ocean.

r

1gurc 6: Depth prolilc of water ,alinily di,1ributio11 across the Gulf Stream northweMcrly of 1hc A,ore, measured bel\1-cen 3-6 June 1938 during 1he lntcma1ional Gulf S1ream Expedition (Defont & Hclland- Han-.cn. 1939).

102

I.

Hennings

2.6 Strait of Dover (sand

waves)

Harden Jone and Mitson ( 1 982) showed that large marine sand waves between the Sandettie and Outer Ruytingen Banks in the Strait of Dover are

a<;

ociated with enhanced acoustical noise leve ls at their c rest . The position of the obsen a- ti on site is marked as No. 6 in Fig. I. The authors called the e noise levels ,.plume- like traces" which have been recorded by 30 kHz and I 00 kHz echo sounders as we ll as by 300 kH z sector-scanning sonar. Figure 7 hows an echo sounder record obtained from on board R.Y .

Clio11e

at 1 856 UT - 1 909 UT 1 2 D ecember 1 970

Figure 7: Echo ,ounder record (KH MS 29.

30 I.II,) obtained from on board R.V. C/i1111<' at 1856 UT-1909 UT 12 December 1970 ,ur- veying the ~ea area between Sandcuic ,tnd Outer Ruytingcn Banh in the ,outhern North Sea ,howing large ,and wa,c, ,tnd a,,ociatcc.J .. plume, .. of noi,c (llardcn Jone, & Mit,on.

1982).

Oceanographic phenomena causccl by interaction of current'> and bouom topography ...

103

surveying the bedforms between the Sandellie and Outer Ruytingen Bani..., in the . outhem North Sea. Large sand waves associated with such .. plume.,·· are visible.

It

was shown that noise from sand waves increases with current velocity. Harden Jones and Milson ( 1982) concluded that there exists a relation'>hip between the in- tensity of measured 300 kH, bollom noise and water current velocity associated with a group of small sand waves located between 150 m - 160 m to the southwc<,t of the Outer Ruytingcn anchor station (position: 51° 06.2' N, 1° 53.1' E). Bottom noise and source level are presented as a function of tidal current velocity in Fig- ure 8. The current velocity was measured at 3 m above the sea boLLom, water depth below the vessel was 20 m. A wind speed of 6.1 m s ^I and a wind direction from 220° were measured. The tidal current direction wa<, northea\t. Bollom noise was not detected at velocities less than 0.4 m s-1 but wm, pre. ent at higher velocities and increased with the tidal current up to a maximum velocity of 0.91 m s-1. This was an important result because the lower limit of the current velocity of 0.4 m s- 1 coincides with the lower limit of current velocity for observing a significant NRCS modulation due to marine sand waves (Alpers

&

Hennings, 1984). Later on, Stolte ( 1994) reported distinct links between NRCS and sea surface sonar conditions.

Some hypotheses have been discussed that underwater sound sources might be used by fish shoals as indicators of tidal now or as acoustic beacons by migrants on passage. Harden Jones and Milson ( 1982) concluded that the noise was generated by the movement of bottom sediments which might be detected by marine species such as herring and plaice shoals. Stewart and Jordan ( 1964) ob- served suspended sediment in the water column just over the crests of submerged sand ridges on Georges Shoal of Georges Bank, Gulf of Maine, USA. During the observations the current direction was normal to the ridge. Shallow water bottom topography from radar imagery and visual evidence of surface expressions of ba- thymetry at Asia Rip. Phelp Bank, Nantucket Shoals, Massachw,etts. USA, have been published by Valenzuela et al. ( 1983). In aerial photos a pair of light streaks associated with the crests of submerged ridges has been observed also in the sea

Figure 8: Scauer diagram of the in11:nsi1y of mea,urcd 300 kl 1, bouom noi,c and ,ource lc,cl as,ociated with a group of small sand wa,e, lying 150 m to 16() m to the ,outhwest of the Outer Ruytingcn anchor station (po~i- 11on: 51' 06.2· . I' 53.1 · I:.) a, a function of tidal current \elocity (Harden Jones & Mit,011.

1982).

'in

§

>

~

<;; 100

"O

~ ::, II>

0

Cl,

E

~ 50

Cl, II>

5 c

•

• •

• •

. _• ^.. •••

Tidal velocity cm s-1

180 E

•

0

a. 0 :i..

-

~

175 ~

"O

.; >

170 ~

"'

165 ~ 160 0

150 <./)

100

104

I.

Hennings

area of Nantucket Shoals (Smith,

J

986).

It i.

assumed that these streak1:, are caused by

suspe

nded sediment at large tidal current velocities. The 1:,treak1:, often coincide with a boundary between rough and smooth water at the sea surface. Soulsby et al.

( 1991) showed that the near bed region above the trough of an asymmetric sand wave in the tidal estuary of the river Taw, outheast England, has the large. t values of turbulent kinetic energy. Reynolds stress and ediment concentration. The . and wave trough act a a ource of the e parameters. At peak flow no flow separation was visible, but during the decelerating phase, flow reversal has been measured up to 40 % of the time. Until now it has been general

ly

noticed that strong scatter is often detected in region of the water column expected to be turbulent, but it has never been clear if this i becau e there are higher plankton or !-.Uspended matter concentration in turbulent region . Recently, Ros. and Lueck (2003) showed that turbulent mi

cro.

tructure strongly scatters sound at 307 kHz.

2.7 Oder Bank (shoal)

The visibility of

.

ea bottom relief on optical

.

atellite images depends on the varia- tion of water surface roughne s as. ociated with wind and tidal flow, on the water depth and the reflectivity of the bottom material as well as on the transmission of

light through the water column Lo the sea surface. The laller subject being a high!)

variable factor because the

transmittance

may be

reduced

by

suspended material.

All of the influences mentioned above often make an

interpretation

difficult for

the mathematical formulation of the

optical

imaging mechanism. The influence of

sea bottom on spectral reflection in the sea area of the Oder Bank in the Baltic Sea has been

studied by Siegel

and Seifert (1985). The position of the Oder Bank is marked as No. 7 in Fig. I. The authors noticed bottom reflection up to the surface between 450 nm and 650 nm wavelength of the vi. ible part of the electromagnetic spectrum al the most shallow measurement station of 8 m water depth.

Indeed.

the sea area of the Oder Bank has been often detected in CZCS satellite image. by en- hanced radiance.

2.8 Georges Bank (sand waves and ridges)

Satellite observations of Georges Bank located at the eastern North Atlantic coast of Maine (U.S.A.) made by the CZCS sensor showed bandlike pattern in the wa- ter-leaving radiance most visible at 550 nm wavelength (Yentsch et al., 1994

).

These high-reflectance band. appear to be related to

the

large

sand

waves and ridges that dominate the sea bollom topography at that location marked as No. 8 in Fig.

I. It is speculated

that the banding in the CZCS data i caused by the creation of alternating divergent and convergent zones due to

tidal

currents as the water flow!-. more or less perpendicular over the

underwater dune-trough

configuration.

When the flow

is parallel to the dunes, it is

speculated that suspended absorbing

and reflecting materials will be concentrated by helical vortices.

- -

Oceanographic phenomena caused by interaction or currents and bouom topogmphy ...

105

2.9 Norfolk Banks (tidal current ridge )

Robinson and Johannessen ( 1997) described European remote sensing satellite (ERS-1) SAR and ERS-2 along-track scanning radiometer (ATSR-2) images from the southern North Sea, off the north Norfolk coast of the United Kingdom. The position of the satellite imagery is marked as No. 9 in Fig. I but is not shown here.

Acquisition time of the SAR scene was at 1051 UT 21 October 1995 and the AT- SR-2 image was acquired 30 minutes later. The bathymetry consists of a number of linear tidal current ridges approximately parallel Lo the coast. Significant SAR . ignatures have been caused by tidal flow over the ridges modulating the sea 1,ur- face roughness. The corresponding ATSR-2 550 nm wavelength reflectance image shows also an enhanced reflectance over the banks. It i1, well known that North Sea water is far too turbid for this to be reflection from the sea bed, and the mo. t likely explanation is that suspended sediment concentration i1, enhanced over the banks.

2.10 Heligoland, German Bight (reef )

Sea surface temperature distribution off the isle of Heligoland in the German Bight of the orth Sea was acquired by an airborne infrared scanner (Becker, 1978). The investigated area is indicated as No. 10 in Fig.

I.

The image acquired

Figure 9a: Two-<lirncnsional water ~urface temperature di,tribution around Hcligolan<l at I 506 UT 29 August 1976 acquired by an airborne infrare<l scanner (Becker, 1978).

The isle of llcligoland and the shallow island. called Dune. arc marked by black colour. Contour lines of both islands are blurred due to numerical fillering processes.

106

I. Hennings

- - -- ---,

fradar ,mage I

I

L _ ..1

-+ "-IJ-l.W'-'-lllloiiio~~ · ~ ~

I

I I

I

Figure 9b: Bathymetric chart of the sea area around Heligoland (Kuratorium fur For;chung im Kii\tcningcnieurwe~en. 1977a. b). Depth contour~ are in meter~. The larger broken frame b the area covered by the infrared scanner image presented in Fig. 9a and the ,mailer broken frame indicate, the coverage of the SAR image shown in Fig. 9c.

at 1506 UT 29 August 1976 is hown in Figure 9a and indicates remarkable two- dimensional temperature gradients in north-south as well as in east-west direction.

Original spatial resolution of the image was I

O

m but has been reduced to about 67 m due to numerical filtering processes. The coverage of the infrared canner image i. indicated by the larger broken frame in Fig. 9b. The data were taken one hour after high water at Heligoland during ebb tidal pha e. The tidal current speed varied between I

O

cm s-1 and 30 cm s-1 and northerly tidal current directions. The wind speed was 3 m s-1 and the wind direction wa from 900. Above the shal-

Oceanographic phenomena caused by interaction of currenL<; and bottom topogmphy ...

J 07

low ea area north-west of the island called Dune the

lowe t water temperatures

<

1

5.7° C were

recorded. It

is assumed

that

bottom water from the westerly

sea

area of the

hoal

was almost transported unmixed above the shallow parts

of

the

sea area.

Becker ( 1 978) concluded

that this phenomenon may

be allributed to

cur-

rent conditions depending on

tidal

pha e and

local

bathymctry of the sea area.

A

bathymetric chart of the sea area around

Heligoland is shown in Figure

9b (Kura- torium fUr Forschung im KUsteningenieurwesen,

I

977a, I 977b).

Multi frequency

X-, C-, and L-band SAR

images north

of the sea area off the isle of Heligoland were analysed by

Hennings

et al. ( 1 998). The coverage of the SAR image is indicated by the smaller broken frame in Fig. 9b. The data have been collected

during the SAR and

X-band Ocean Nonlinearities Research Plat- form North Sea Experiment (SAXON-FPN) at 0644 UT

1

4 November

1

990. Fig- ure 9c is the re ult of the superposition of th e X- and L-band SAR image

.. The

X-band VY polariLed SAR image

is processed in

red

colour and the

L-band VY polari,ed SA

R image is proccs ed

in blue colour, respectively. Features appearing in black and white have not

changed

their characteristics in their X- and L

-band

frequency domain and coloured

signatures show differences.

Elongated streaks of predominantly low radar return are related

to near-shore reefs (see Fig. 9b) and are

Figure 9c: Multi-frequency SAR image of the ,ea area of Heligoland, German Bight. south eastern onh Sea at 0644 UT 14 ovembcr 1990. The X-band VV polari1ed SAR image i, processed in red colour and the L-band VV polari,ed SAR image i, processed in blue colour, respectively. Features ap- pearing in blad.. and white have not changed their characteristics in the X-and L-band frequency do- main. Coloured signatures show changes of the environment.

108

I. Hennings

imaged on all multifrequency radar scenes. Ground resolution in range and az- imuthal direction of the SAR image is 5 m. The data were taken I hour and

56

minutes before high water at Heligoland during flood tidal phase with a mean tidal current speed of

0.6

m s-1 and a mean tidal current direction of 980. The wind speed was 9 m s-1 and the wind direction was from

2000.

2.11 outhem Bight of the North ea ( a nd waves)

A similar effect of cold water flowing acros<, shallow bank<, and reefs as described in <,ection

2. 1

i.., here associated with marine sand waves and has been measured in the sea area of the Southern Bight of the North Sea (Hennings et al.,

2002).

The location of the observation site is marked as No.

11

in Fig. I. Quasi-periodic \'ari-

.;; eoo a )

~ 550 :::, 500

.ci 450

ni •oo

'i£

³⁵⁰

April 24, 1991

~ 300..l...~~~~~~~~~~..._~~~~~-..~~~~~~~

9.5

6 0

:;-,

90 85

20

g

22 J:;

24 -0 ~ ci

26

j

11.36 1148 1200 12:12 12:24 1236 12;48 13;00 13:12 13:24 1336 Time UTC

Figure 10: Time ,cric, of a) bet1m tran,minance TR3 (arbitrary uniL~). b) fluore,cencc FL3 (arbitrary unit~). and c) w,ller temperature Tw 3 (OC) at 3 m water depth in relation to d) water depth profile be- tween 1141 UT-1336 UT 24 April 1991. Each tick marl- indicate, one minute. The direct10n of the tidal current U i, indicated by a white arrow in Fig. IOd. The inve~Ligation, have been camed out in the ,outh- ern onh Sea \\here the ,ea bed i, cmered by ,and wave, numbered by S 1-S8 (Henning, et al. 2002).

Oceanogrnphic phen om ena cau sed b y interaction of cu1ren Ls and bottom topography ... I 09

ation s in beam transmittance TR3, nuorescence FL3, and water temperature Tw 3 at 3 m water depth in relati on to the water depth profi le o f the dri ft path A- B at 1141 UT - 1 336 UT 24 April 1 99 1 are shown in Figure I 0. Cold , turbid, and chl orophyll-rich water from greater water depths below the thermoclin e arrived prior to the marine and wave crests and j ust before the current velocity was al a max imum. Thi s mechanism has been ex plained a quasi resonant internal waves with marine sand waves (Hennings et al. , 2002).

The reaso n why Humboldt measured lower water temperature on the bank than in deeper surrounding waters (see sec tion 2. 1 ) could be due to the presence of a tidal current now. If thi is a umed, then a similar effect as has been observed above the marine sand waves at the Dutch coast in the so uthern North Sea can arise at the B anco Y acentes. In that sea area there can also be an ove rnow of cold- er water from deeper water depths above a shoaling sea bed induced by tidal cur- rents (sec also section 2. 1 0).

3. Quasi-specula r scattering theory

The creation of alternating di vergent and convergent zones caused by (ti dal) cur- rents as the water nows over a wavy sea bed configuration and its radar imag ing mechanism belonging to the microwave part of the electromagnetic spectru m will be outlined here as an example. The formulated theory is applicable to the X-band SAR of T erraS AR-X, Germany's lirst civil nat ional remote sensi ng sate llite real- iLed by a public-private paitnership. Radar signatures of sea bottom topography are dominated by Bragg scattering since m ost of the imagin g radars operate at in- cidence angles between 20° and 70° (Valenzuela, 1 978). At low radar incidence angles, < 20°, quasi-specular scattering dominates. In addition, quas i-specul ar scattering becomes dominant at higher radar freque n c ies. According to Bragg scattering theory, th e NRCS for small water surface waves is proportional to the wave height spectral dens ity at the Bragg bac kscatter wave numbers. For quas i- specul ar scattering from a rou gh ocean surface, the NRCS is proportional to the total variance of slopes created by ocean surface waves. The radar imaging mecha- ni sm of sea bottom topography depends strongly on radar incidence angle, radar frequency. radar polari zati on, current !-.peed and -di rection. as well as wind <;peed and -direction. The most import ant assumption for the radar im aging mechanism of submarine bedforms is the presence of strong currents, preferably tidal c urrents

~

0.5 m s-• at wind speeds$ 8 m !-.-• . Specul ar renec tion occ urs when rad iation

is scattered into a given directi on from surface regions with slopes such that the

local specular directi on coincides with the scatterin g direction. The quas i-specular

scattering theory can be appli ed if the wavelengths o r waves in the ocean con-

tri buting to the mean quare surface slop e a re greater than the wavele ngth of th e

microwave. In ge neral, the mean squared lope o f such waves is small. But this is

considerabl y different if waves are influe nced by a surface c urrent gradient. V ery

sleep dislurbed slopes of the order of I 0° or more can arise in the co nvergence

Lo ne of the current correlated with the slope regions of marin e sand waves. Water

HO I. Hennings

waves especially of Lrochoidal shape, can be generated due Lo such wave-current interaction at low to moderate w

ind

speed . These trochoidal shaped wave pro- duce an ensemble average of facets wh

ich create

quasi perpendicular planes rela-

tive

to the transmitted

radar

beam. There ex

i

st also

teep small gravity waves

in this zone which tend

to

become sharp wedges just

before they

break, and al o breaking waves

themselves. Improvements of the quasi-specular scallering theory have

to be made because

the

radar imaging mechanism of the ea bed depend strongly also on

the up- and cros wind wave slope.

, the angle between the upwind and perpendicular curren

t direction

to the and wave crest, and the angle between the radar range direction and

the upwind direction.

The di turbed NRCS 8a ,u, ^{cau ed by}

the dislurbance

of the

.

urface cur- rent 8U(x) due to marine sand waves based on quasi-specular scatlering and obeying a Gram-Charlier eries is given by (Hennings &

Herbers, 2007)

where a is the local NRCS influenced by the di. turbance of 8U(. r), a

₀

is the back- ground NRCS,

R (0)

is the Fresnel reflection coefficient at normal incidence,

8 0

is the angle of incidence, 88 is the time-dependent perturbation

term of the

inci- dence ang

le,

s,,.

^=ds0ldx and

s >.

^=ds0ldy

are the slopes of the rough sea sur- face

in two orthogonal

directions, x and

y,

s O

i

s the verti cal elevation of the sea surface, 81; and 81; are

the time-dependent perturbation terms

of ( and ( in

h x

d. '' t· d .

I

xo yo

two ort ogonal 1ret ions, x an y

respective y.

The

local

joint-probability density function of slope expres. ed in equa- tion ( l) is defined by

(2)

w

here

a

₁₁

and

<Ic

are

the local

standard deviations of the upwind and crosswind wave slopes, st~ and sf are the squared

local

normalized upwind and crosswind wave slopes, c21 and co3 arc

the skewness coefficients as a function

of the wind speed "w· ^and c40, c22· and c04 are con

tant

peakedness coefficients (Cox &

Munk.

1

954). The probability distribution function for sea surface slopes as a

function of wind direction is skewed, and displaced from zero in the upwind-

downwind direction. The peak of distribution

in wind direction i

sh

i

fted toward

Oceanographic phenomena caused by inlcraclion of cun-ents and bollom lopography ...

111

the downwind side by several degrees. This is due to the fact that the longer gravi- ty waves tend to have a higher spectral wave energy den. ity of short waves on their downwind faces than in their troughs or upwind faces (Apel, 1987).

The square tangent of the di. turbed incidence angle in equation (I) is de- rived by

(3a)

and

2

~ ~

^{2 (}

~

²

~

^{2) auperp}

-tan

uO=-ua =- ua" +uac , - - >0

axre'l' ^(3b)

with

80"~

=

J ^{k\f)8F({~ cosadk}

'·

(4a) and

, ' J - 2

^r.

^{(rr.) r}

oo-:= ^{k (x)8F x,k}

^sinadk

'·

(4b)

\~here xperp is the space variable defined perpendicular to the sand wave crest,

k

is the wave number vector of short gravity waves,

ko

i the lower limit of the wave number producing quasi-specular scattering modulation, kc is the maximum wave number neglecting the effect of surface tension, du rcrr t dxr,crr, is the gradient or strain rate of the current velocity perpendicular to the sand wave cresl,

a

is the angle between the upwind direction and the current velocity component perpen- dicular Lo the sand wave crest, and

8F(x.k)

is the perturbation term of the wave energy density spectrum in the short gravity wave regime caused by wave-current interaction applying weak hydrodynamic interaction theory (Alpers

&

Hassel-

mann, 1978). .L .L

The relationship between

1/f( k ). F( k)

and the wave action density spec- Lrum

N(k) =F(k)(w'(k))

¹ isdefinedby(Hollidayetal., 1986)

F(k) =w'(k)N(k)= w'~)i v,(k) (5)

with the wave height spectrum

'l'(k) = a/ -1 ₍₆₎

where ap is known as the Phillips con tant. Based on measurements by Stolle ( 1990), Lhe empirical relation for ap m, a function of wind speed $: 8 m s-1 is used

(7)

112

_I.

Hennings

The intrinsic angular wave-frequency for gravity wave. in equation (5) is defined by

m' = (gk) ,n (8)

For the modulation of the first order perturbed wave-energy density spectrum oFlf"o=(F-F;,)IF;

1

(with F0 ^{as the} unperturbed wave-energy density spectrum)

the expression derived by A lpers and Hennings ( 1984) is used

8F dll ((- -) I J'

- =-4.5 ~ ci +u

₀

- + µ

F;, axpc

₁₁,

L (9)

w ith the absolute value of the group veloc ity for gravity waves

(JO)

where

1i O

is the mean current velocity of the undi sturbed sea area, µ is the rel a- xation rate parameter, g i s the acceleration of gravity. L = ^{LssL i} s the length scale of the steep slope. and L = LGSL is the length scale of the gen1le slope of the and wave, respectively.

Relati on (9) shows that the first-order perturbed wavenumber spectrum of short gravity waves is proporti onal to the current gradient caused by marine sand waves. This is an i mportant result. A s a first approximation, using the continuity equation, this, in turn, i s proportional to the sea bottom slope divided by the

.

quare of the water depth. Consistently, the spectral wave energy density of the scattered waves, and thus the intensity of the radar signal , i s l arge t on the down- stream side of the sand wave or ridge and smallest on the upstream side. As a con- sequence, when the c urrent nows in opposite directions during th e ebb and flood of the tide, a sign rever al of the image intensity is ex pected. w hich is indeed ob- served in radar images (Zimmerman, 1 985). A detailed background of the weak hydrodynam ic interaction theory for the radar imaging mechan ism of the sea bed is given in Alpers a nd Hennings ( 1984) and Hennings and Herbers (2006) .

The same hydrodynamic interaction mechanism described above for the radar imaging theory is al o respon ible for surface manifestations of oceano- graphic phenomena in the optical visible part of the electromagnetic spectrum.

Variations in water temperature and chlorophyll distributi ons renect also the influ-

ence of sea bottom topography undulations because tidal now over wavy sea bed

configurations produces alternating divergent and convergent regions. It can often

be observed that the tidal now i. not of barotropic character near the crests and

troughs of marine sand waves. This implies that mass is not onl y conserved by an

acceleration or deceleration of the now but that up- and down welling of th e three-

dimensional current field can also play a significant role. Yentsch et al. ( 1 99-l)

considered that the biological importance of these feature-. concerns the process of

new production and the off-bank tran sport of inorganic and organic materials (s ee

also section 2.8).

Oceanogmph1c phenomena caust.'<.I

by

intemction

ot

currcnL,

and

bottom topogmphy ... 113

4. ummary

Observation sites concerning variations of oceanographic parameters cau..,ed by the interaction of currenh ""ith sea bottom topography over the period of the la..,t 250 years are described. The development from simple ob..,erving and mea..,uring tools of ph}sical parameter, ""ithin the water column to its mathematical formula- tion for understanding the dynamical processes has been shown. Oceanographic parameters like sea surface temperature, ocean colour, slope, and ..,ea ..,urface roughnes. with their specific variation<, have often been used to inve..,tigate and de- scribe the ocean dynamics innuenced by characteristic feature.., of the ..,ea bed. Sea .,urface temperature variations associated with a shallow submarine bank at the en- trance to La Coruiia at the Spanish coast of the Atlantic Ocean wm, mea..,ured by Humboldt in 1799. These measurements of Humboldt have been confirmed for ex- ample by observing similar effect!-. 1976 above !-.hallow reefs off the isle of He- ligoland imaging \Ca '>Urface temperature and 1991 in the '>Outhem North Sea measuring water quality parameter variations at a water depth of 3 m above ma- rine <,and waves. For most of these observations a strong (tidal) c urrent is required as the dominant source to observe any variation or modulation of oceanographic parameter ....

It has been <,hown that shore- and ship-based a<, well as air- and space- bome marine remote sen<,ing mea..,urement configurations promised considerable improvements in quality and synupsb of field data. I ligh spatial and radiometric resolution data have been acquired and have opened ne.,, horizon<, in ba<,ic re- search. Thermal inhomogeneities developing at calm or low wind speed of 2-3 m s-1 have been referred as ,.calm weather thermal inhomogeneities·· (Fe- dorov, 1986) where sea surface signatures arc often imaged in the vi,ible part of the electromagnetic ,pectrum. The mechanism of its formation can easily be vi..,u- ali7ed in the ca<,e of a monochromatic internal wave. Orbital motion'>, which be- come horiwntal alternating convergent-divergent nows at the sea surface, form accumulations of pa<,sive constituents, ,olar-heated water and surface-active mate- rial in the convergence 70nes.

Nowaday,, the investigation of oceanographic phenomena is coupled with its mathematical formulations. In <,ummary, if the microwave part of the electro- magnetic ..,pectrum is conc;idered here, then the radar imaging mechani-,m of c;ub- marine c;and waves can be physically and mathematically described as a process consisting of at least three steps (sec also section 3): I) The interaction between the current and the sea bottom topography produces variations in the current ve- locity at the sea surface. 2) The variation of the surface current modulate'> the

<,hort-scale \Ca surface roughness which can be described by the weak hydrody-

namic interaction theory in the relaxation time approximation. 3) The disturbed

variance of !'llopes of short ocean ),urfacc waves via the modulation of the spectral

wave energy density at the water-air boundary layer gives rise to changes in radar

backscatter.

114

I. Hennings

Acknowledgement

M. Plettendorff of the library at the Bunde amt fiir See chifffahrt und Hydrogra- phie, Hamburg, Germany, is gratefully acknowledged fo r the scanning of the air- borne infrared scanner image published by Becker ( 1 978). I wou ld like to thank

J.

Evans-Morgis of the Naval Air Development Center for providing the SAR im- agery, C. Wackennan , Environmental Research Institute of Michi gan. for process- ing the SAR images, and M. Metzner for technical support.

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