Near-surface salinity in the tropical Atlantic
DFG Forschergruppe 1740 Atlantic Freshwater Cycle
T. Fischer
1, P. Brandt
1,2, J. Karstensen
1
[1] GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany
[2] Kiel University, Kiel, Germany
Vertical/diapycnal mixing is important for heat and salt budgets.
Sources of strong mixing can be permanent shear (equator) or sporadic shear from near-inertial waves (off-equator, Hummels et al., 2019 under review)
Local budgets at PIRATA sites
Seasonal mixed layer budgets. Storage = sum of fluxes
PIRATA sites (during Atlantic Cold Tongue)
heat fluxes
heat storage
salt fluxes
salt storage
Hummels et al., 2014Schlundt et al., 2014
Local budgets at PIRATA sites
PIRATA sensors also document heavy rain events:
Density gradient in top 10m (with contribution from S and T)
Rain rate in mm/h
Wind speed at 10 m Local noon
100 mm of freshwater and weak wind
7-day near-surface stratification, suppressing turbulent mixing and trapping heat and freshwater.
Near-surface S and T gradients.
The impact of rain is motivation for two interests:
HOAPS 4.0 rain (0.5° x 0.5)°.
The yellow frame encompasses the region where ITCZ related rain is found during the year.
Local budgets at PIRATA sites
Define regions of strong rain on larger scales (space and time)
and explore ML reaction (obviously has a lot to do with migrating ITCZ)
Explore frequency and
strength of multi-day events and near-surface gradients
6
Define persistence of a patch as overlapping percentage with previous month + overlapping
percentage with next month
95% interval of mean persistence of patches of respective size class
Persistence of a random patch
100,000 km2
Defining regions of strong rain on month timescale
Defining regions of strong rain on month timescale
%
Based on HOAPS4.0 precipitation (1987-2014)
Seasonal cycle of probability to be in a wet region that lasts for a month or longer
There is no all-year wet region.
The record keeper 1987-2014 is position 4.75°N 16.5°W with 10 wet months per year.
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2
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5
6
7
8
9
10
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12
a quarter half half
During migration, the moving budget volume „incorporates“ salinity, which has to be accounted for.
salinity
On average, the surface layer below the wet region keeps its salinity,
while water is incorporated that is worth a
salinity increase of 0.08 mth-1 (+-0.03), and net atmospheric freshwater flux is worth a
salinity decrease of -0.15 mth-1 (+-0.004).
Of the 0.07 mth-1 salinity increase by oceanic transport, diapycnal mixing contributes 0.015 mth-1 (+-0.01).
Seasonally migrating budget volume
On seasonal timescale:
oceanic advection and mixing
diapycnal mixing atmospheric freshwater flux
decreasing salinity
Criteria for N2 and wind are based on experiences from a multi-day NSS study in the Eastern Boundary Upwelling System off Peru (Fischer et al., 2019)
Map of glider data
Exploring near-surface gradients
>1300 glider hydrographic survey days in the tropical Atlantic (2008 to 2014)
Conditions to be looked for:
N2>10-4 in top 8m
(near-surface stratification NSS is strong enough to dampen turbulence, and cause surface trapping and near-surface gradients) Wind < 6 m/s
(condition for multi-day NSS) HOAPS4 based classification as month-scale wet region
No NSS
NSS
Multi-day NSS
days with
0
>0
all
profiles with N2>10-4 in top 8m
Exploring near-surface gradients
Definitions:
Windy: wind > 6 m/s Calm: wind <= 6 m/s Wet/dry: inside/outside HOAPS4 classified region of month- scale rainy conditions
No NSS
NSS
Multi-day NSS
dry &
windy
dry &
calm
wet &
calm
40%
78%
5%
87%
11%
Multi-day NSS amplifies diurnal T cycle by factor 3
Wet conditions have half the T cycle (probably due to lower insolation), but the S gradient makes up for the density gradient.
Near-surface temperature and salinity
NSS, dry NSS, wet
Multi-day NSS, dry Multi-day NSS, wet
DT DS
2m
4m
6m
8m
10m
2
4
6
8
10
2
4
6
8
10
DT
DT
DT
DS
DS DS
2m
4m
6m
8m
10m
2m
4m
6m
8m
10m
2m
4m
6m
8m
10m 1
0.5
0
1
0.5
0
1
0.5
0
1
0.5
0
0h 6h 12h 18h 24h 0h 6h 12h 18h 24h
0h 6h 12h 18h 24h 0h 6h 12h 18h 24h
-0.2 -0.1 0 -0.2 -0.1 0
-0.2 -0.1 0 -0.2 -0.1 0
Percentage of S gradients < - 0.015 m-1 (corresponds to N2>10-4):
0.1 % 0.7 % 13 %
Near-surface S gradients to be expected
Lübbecke et al. 2019
Also river runoff can cause multi-day near-surface stratification
SST off Angola and Namibia in 2016 had the
appearance of a Benguela Nino event.
SSS anomaly in Feb 2016
SST anomaly in 2016
An extreme Argo profile in Feb 2016
In fact, it was a very shallow freshwater lense which trapped the heat.
Diapycnal mixing is an important process for mixed layer budgets, with different drivers.
Near-surface stratification events can be longer than just one afternoon.
This is not rare during weak wind, and is driven by insolation or freshwater.
Near-surface S gradients can be considerable in the tropical Atlantic.
