in the Peruvian upwelling regime

Multidiurnal warm layer and inhibited gas exchange

in the Peruvian upwelling regime

Tim Fischer , Annette Kock , Damian L. Arévalo-Martínez , Marcus Dengler , Peter Brandt , Sören Thomsen , and Hermann W. Bange ^{*,1 1 1 1 2}

(1) GEOMAR Helmholtz Centre for Ocean Research, Kiel, Germany - Research Division 1 - Physical Oceanography

*contact: tfischer@geomar.de

Abstract ID: EGU2015-11426

BMBF-Verbundvorhaben

SOPRAN

2 2

(2) GEOMAR Helmholtz Centre for Ocean Research, Kiel, Germany - Research Division 2 - Chemical Oceanography

This study was supported by the German Federal Ministry of Education and Research through the joint project SOPRAN (Surface Processes in the Anthropocene) under grant no. SOPRAN II FKZ 03F0611A and SOPRAN III FKZ 03F0662A. The friendly support of all crew members of research vessel METEOR during cruise M91 is highly appreciated.

Particular thanks to Rudi Link for helping to construct the sampling equipment, and to the bravehearted who sampled N2O during long hours in the dinghy, drifting in the waves while sacrificing their health. Thanks to Gerd Krahmann who processed the glider hydrographic data. The daily ASCAT global wind field data were provided by A. Bentamy and D. Croize-Fillon of Ifremer, France.

Acknowledgements

Calleja, M. Ll., Duarte, C. M., Vaquer-Sunyer, R., Agusti, S., and Herndl, G. J. (2013): Prevalence of strong vertical CO2 and O2 variability in the top meters of the ocean, Global Biogeochemical Cycles, 27, 941-949, doi: 10.1002/bgc.20081 --- Kock, A., Schafstall, J., Dengler, M., Brandt, P., and Bange. H.W. (2012): Sea-to-air and diapycnal nitrous oxide fluxes in the eastern tropical North Atlantic Ocean, Biogeosciences, 9, 957-964 --- Naqvi, S.W.A., Bange, H.W., Farías, L., Monteiro, P.M.S., Scranton, M.I., and Zhang, J. (2010): Marine hypoxia/anoxia as a source of CH4 and N2O, Biogeosciences, 7, 2159-2190, doi:10.5194/bg-7-2159-2010 ---- Nevison, C.D., Lueker, T.J., and Weiss, R.F. (2004): Quantifying the nitrous oxide source from coastal upweling, Global Biogeochemical Cycles, 18, GB1018, doi:10.1029/2003GB002110 --- Soloviev, A., Edson, J., McGillis, W., Schluessel, P., and Wanninkhof, R. (2002): Fine thermohaline structure and gas-exchange in the near-surface layer of the ocean during GasEx-98, in: Donelan, M.A., Drennan, W.M., Saltzman, E.S., and Wanninkhof, R. (eds.): Gas transfer at water surfaces, AGU Geophysical Monograph Series 127, Washington D.C., 181-185 ---

References

Multidiurnal shallow stratification exists in the upwelling regime,

observed by glider fleet.

Nitrous oxide (N ₂ O) observations in top 10 m

show vertical concentration gradients. A 1-D model constrained by the glider timeseries reproduces

N ₂ O gradients of the observed kind.

Conclusions

Multidiurnal shallow stratification (MDSS) with persistent mixing inhibition is a plausible cause for substantial surface N 2 O depletion observed.

Just diurnal shallow stratification would not be sufficient.

Bias of N 2 O emission estimates is highest

where strongest MDSS and highest concentra- tions occur, i.e. where impact of bias is highest.

Motivation to look for gas gradients in top 10 meters

Profiles far from ship's influence

allow detection of shallow gradients Night time stratifica- tion, high concentra-

tions and strong gradi- ents are associated

4 regions of dense hydrographic observations

Found different grades of persistent stratification

HLD is mainly deter- mined by windspeed

Model schematic

The stratified layer is

extremely low in mixing

Example run

Modelled gradients

vs. region and supply Resulting emission overestimation when sampling at

a specified depth

model observation

Hypothetic mechanism:

Mixing is inhibited in a thin stratified layer that is not eroded for one or more nights. The surface layer (the ’multidiurnal warm layer‘) is thus isolated from gas supply from below, while the continuing outgassing causes surface depletion.

In the Maurita- nian upwelling regime, N

₂

O supply from below is much lower than the calculated N

₂

O emissions.

[Kock et al., 2012]

Do we estimate gas emissions from adequate concentrations ?

0.1 - 1 m pump

1 - 10 m Niskin

N2O concentrationstratification day / nightwater depth _{5000 m 3000 m 150 m 100 m}

N² = 10^-6 N² = 10^-4

N² = 710^-4

N² = 10^-3

Vertical concentration gradients in top layer exist and vary regionally.

Shape of concentration profiles resembles density profiles at night.

< 15 15 − 60 > 60

−0.6

−0.4

−0.2 0 0.2 0.4 0.6

nmol/kg log 10 conc ( 5 m ) / conc ( 10 m )

Ratio of N2O at 5m

and N2O at 10m from 45 CTD

casts

0 6 12 18 24

10⁻⁶ 10⁻⁵ 10⁻⁴ 10⁻³

local hour

N2

Stratification of top 10m

vs.

time of day for regions of high and low N2O concentra-

tions.

profiles with N2O > 15 nmol/kg at 5m

profiles with N2O < 15 nmol/kg at 5m

78^oW 77^oW 76^oW 14^oS

13^oS 12^oS

I II

III IV

Extracted composite

hydrographic timeseries:

I II III IV

37 days 31 days 22 days 10 days

1 2 3 5 10

100 300 1000 3000 10000

ASCAT windspeed [m/s]

waterdepth [m]

HLD [m]

0.5 1 2 3 5 10 20 local days

depth in m

5 10 15 20 25 30 35

0 5 10 15

20

log

₁₀

N

²

5 10 15 20 25 30 35

−10

−5 0 5 10

local days

depth in m wind in m/s

local days

5 10 15 20 25 30

0 5 10 15 20

log

₁₀

N

²

depth in m

5 10 15 20 25 30

−10

−5 0 5 10

local days

depth in m wind in m/s

local days

5 10 15 20

0 5 10 15 20

local days

5 10

0 5 10 15 20

log

₁₀

N

²

log

₁₀

N

²

depth in m

5 10 15 20

−10

−5 0 5 10

local days

5 10

−10

−5 0 5 10

local days

depth in m wind in m/s

Region III Region IV Region III Region IV

Region II Region II

Region I Region I

stratification timeseries HLD and ASCAT wind timeseries

(Homogeneous Layer Depth: N

²

<10

^-4

)

Exchange across the stratified barrier layer is only via entrainment. For the vertical movement of the barrier the observed HLD timeseries are used.

outgassing

continuous supply

stratified barrier moving vertically

local days

depth in m

model N₂O concentrations in nmol/kg

5 10 15 20 25 30 35

2 4 6 8

10 10

20 30 40 50 60 70 80

0 20 40 60 80 100

0 50 100 150 200

nmol/kg

concentrations at

0.5 m

0 20 40 60 80 100

0 50 100 150 200

nmol/kg

conc. at

51 10 m

median

78

median

−20 0 20 40 60 80 0

2 4 6 8

10

percent

depth in m

I II

III

−20 0 20 40 60 80 0

2 4 6 8

10

percent

depth in m

B A

C D

−6

IV

−5

−4

−3

stratification and dissipation rate, from glider with microstructure probe

N

²

of O(10

^-3

) and dissipation rate of O(10

^-8

) result in a vertical exchange coefficient K of O(10

^-6

).

7 gliders, 250 gliderdays in total, in Jan. and Feb. 2013

80^oW 79^oW 78^oW 77^oW 76^oW 15^oS

14^oS 13^oS 12^oS 11^oS 10^oS 9^oS

A

B C

D

B D A C

Region I, supply flux 0.4 nmol/m2/s

Webb

x1 x2

conc. at 0.5 m conc. at 10 m

best match

to concentrations observed

in the region

constant supply flux in nmol/m²/s

Region

I II III IV

0.4 0.8 1.5 0.2

0.1 0.05

0.025

10 12 13

16 18

25 29

43 51

78 95

149 171 272 8.9 9.5 11

12 15

17 23

27 128

160 8.2 8.5 9.5

10 12

13 18

20 86

105 9.3 9.4 10.6

11 13

14 19

20 29

32 51

56 89 97 39 48 72

88

28 33 49

59