T. Fischer, A. Kock, D.L. Arévalo-Martínez, M. Dengler, P. Brandt, H.W. Bange
Multidiurnal warm layer
and inhibited gas exchange
in the Peruvian upwelling regime
BMBF-VerbundvorhabenSOPRAN
RD 1 Ocean Circulation and Climate Dynamics Düsternbrooker Weg 20
D-24105 Kiel
contact: tfischer@geomar.de www.geomar.de
Measuring nitrous oxide (N 2 O) in the top 10 meters of the Peruvian upwelling
Vertical N 2 O gradients exist and are associated with a shallow stratified layer
Existence of multidiurnal shallow stratification is verified by glider surveys
References
Calleja, M. Ll., Duarte, C. M., Vaquer-Sunyer, R., Agusti, S., and Herndl, G. J. (2013): Prevalence of strong vertical CO2 and O2 varia- bility in the top meters of the ocean, Global Biogeochem. 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, Biogeo- sciences, 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 up- weling, Global Biogeochem. 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 Wannink- hof, R. (eds.): Gas transfer at water surfaces, AGU Geophysical Monograph Series 127, Washington D.C., 181-185 ---
Acknowledgments
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.
A 1-D model constrained by glider timeseries can reproduce the N 2 O gradients
Hypothesis:
Mixing is inhibited in a shallow stratified layer that is not
eroded over one or more nights. The
subsequent isolation of the surface layer (the ’multidiurnal
warm layer‘) from deeper layers, and the continuing out- gassing cause sur- face depletion.
CTD stations
0 6 12 18 24
10−6 10−5 10−4 10−3
local hour
N2
< 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 )
N
2O conc en tr ation str atifica tion da y / nigh t wat er depth 5000 m 3000 m 150 m 100 m
N2 = 10-6 N2 = 10-4
N2 = 710-4
N2 = 10-3
Stratification of top 10m
vs.
time of day for regions of high and low N
2O concentra-
tions.
profiles with N2O > 15 nmol/kg
at 5m
profiles with N2O < 15 nmol/kg
at 5m
Stronger N 2 O gradients are associated with higher N 2 O concentrations and night time stratification
Ratio of N
2O at 5m
and N
2O at 10m
from 45 CTD
casts
Do we estimate gas emissions from adequate concentrations ? Motivation
Shallow sampling away from ship‘s influence In the Maurita-
nian upwelling regime, N
2O supply from below is too low to explain the measured N
2O emissions.
[Kock et al., 2012]
N 2 O measurements during Meteor cruise M91 in December 2012
0.1 - 1 m pump
1 - 10 m Niskin Ship based profiles
Vertical concentration gradients in top layer exist and vary regionally.
Shape of concentration profiles resembles density profiles at night.
Glider fleet in Jan/Feb 2013 4 regional stratification timeseries with different grades of multidiurnal stratification Depth of homogeneous sur- face layer (HLD) is mainly de- termined by wind speed
Simple 1-D two layer model Region 1 - multidiurnal stratification causes distinct gradient Region 3 - diurnal stratification not sufficient for strong gradient
Exchange
across the stra- tified barrier layer is only via entrainment.
For the vertical movement of the barrier the observed HLD timeseries are used.
Region 1 - strong multidiurnal stratification Region 2 - episodes of MDS Region 3 - frequent mixing Region 4 - diurnal
str atifica tion timeser ies HLD and wind timeser ies
local days
depth in m
5 10 15 20 25 30 35
0 5 10 15 20
local days
5 10 15 20 25 30
0 5 10 15 20
local days
5 10 15 20
0 5 10 15 20
local days
5 10
0 5 10 15
20 −6
−5
−4
−3
5 10 15 20 25 30 35
−10
−5 0 5 10
local days
depth in m wind in m/s 5 10 15 20 25 30−10
−5 0 5 10
local days
5 10 15 20
−10
−5 0 5 10
local days
5 10
−10
−5 0 5 10
local days
log
10N
2log
10N
2log
10N
2log
10N
21 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
78oW 77oW 76oW 14oS
13oS 12oS
1 2
3 4
In total 250 glider days were perfor- med by 8 gliders in 4 main regions, recording hydrography.
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
0.5 m
0 20 40 60 80 100
0 50 100 150 200
nmol/kg
10 m
Resulting concentration distributions
50
77
local days
depth in m
model N2O concentrations in nmol/kg
2 4 6 8 10 12 14 16 18 20 22
2 4 6 8
10 10
20 30 40 50 60 70 80
0 10 20 30 40
0 20 40 60 80 100
nmol/kg
0.5 m
0 10 20 30 40
0 20 40 60 80 100
nmol/kg
10 m
Resulting concentration distributions
24 27
median
median
median
median
