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DFG Forschergruppe 1740 Atlantic Freshwater Cycle
Surface layer salinity under the influence of the Atlantic ITCZ
T. Fischer
1, P. Brandt
1,2, J. Karstensen
1
[1] GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany [2] Kiel University, Kiel, Germany
oreilly.com
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Surface layer salinity under the influence of the Atlantic ITCZ
‚wet‘ conditions ‚dry‘ conditions
Do atmospheric or oceanic processes
dominate the SSS development in wet conditions?
Favourable conditions for long-term (multi-day) isolated surface layer
salinity
baedermax.de
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Surface layer salinity under the influence of the Atlantic ITCZ
FOR1740: Atlantic Freshwater Cycle
Here we explore the wet regions (>1000 mm/yr) of the Intertropical Convergence Zone
Wet regions
on a timescale of 1 month
have a spatial scale of 100,000 km
2(8 deg
2).
The migrating wet region uses half the rain to freshen the surface layer, half is spread vertically and horizontally.
Freshwater input and low wind foster isolation of the surface layer.
7% of the year the isolation
extends through all night.
Outline
Find size-dependent timescale
Define wet regions on a seasonal timescale (> 1 month)
Monthly average rain fields
HOAPS4.0 precipitation (for 1987-2014)
Outline all patches of strong rain
Filter by characteristic size threshold
Seasonal cycle of probability
to be in a wet region that lasts for a month or longer
5
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HOAPS 4.0 rain (0.5° x 0.5)°.
The yellow frame encompasses the region where ITCZ related rain is found.
Patches of connected grid cells of strong monthly average rain, equivalent to > 1000 mm/yr.
Patches larger than 100,000 km2. They are expected to last at least for one month and allow the
classification of field observational data into ‚wet‘ or ‚dry‘ conditions.
Define wet regions on a seasonal timescale (> 1 month)
HOAPS4.0: Andersson, Axel; Graw, Kathrin; Schröder, Marc; Fennig, Karsten; Liman, Julian; Bakan, Stephan; Hollmann, Rainer; Klepp, Christian (2017): Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite Data - HOAPS 4.0, Satellite Application Facility on Climate Monitoring, DOI:10.5676/EUM_SAF_CM/HOAPS/V002, https://doi.org/10.5676/EUM_SAF_CM/HOAPS/V002.
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To isolate patches of strong rain which represent wet conditions on a timescale of one month or longer:
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 km
27
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Seasonal cycle of probability to be in a wet region that lasts for a month or longer
Based on HOAPS4.0 precipitation (1987-2014)
%
Outline
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.
That means, for studying
processes under the wet region, we have to follow it.
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).
About half the
atmospheric freshwater flux to the wet regions‘
surface layer is decreasing salinity, and half leaves by oceanic transport and mixing
Use SMOS SSS, and HOAPS4 E-P (overlap 2010 to 2014):
How much of the freshwater input is imprinted in surface salinity?
During migration, the moving budget volume incorporates salinity, which has to be accounted for.
Average salinity change
salinity
Outline
Near-surface stratification and isolation of the surface layer
Near-surface stratification
Strong stratification in the top meters of the ocean inhibits mixing and leads to a surface layer that is isolated from the waters below. Evolving differences in water properties pose a measurement issue
Favourable conditions in wet regions
Wind below 6m/s and stratification stronger than 10-4 s-2 is favourable for isolated surface layer that last for more than one day and night. The ITCZ wet regions show high
probabilities for multi-day isolated surface layers.
Example of long-term near-surface stratification
Examples from PIRATA buoy at 11.5N 23W:
- An isolated surface layer of 7 days
- Enhanced SST diurnal cycle in wet conditions
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Near-surface stratification and isolation of the surface layer
Examples of diurnal and multi-day near-surface stratification (from Peruvian upwelling regime in austral summer 2012/2013)
Periods of diurnal and multi-day near-surface stratification alternate…
… and cause near-surface gradients (e.g. N
2O) and measurement issues
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Near-surface stratification and isolation of the surface layer
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The probability to find favourable wind conditions for multi-day near-surface stratification
is high in the wet regions (about 60%)
probability of wind speed below 6
m/s
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Near-surface stratification and isolation of the surface layer
Near-surface N
2in the region is indicative for multi-day near-surface stratification
> 1300 days of near-surface hydrographic data probability to find strong near-surface stratification (layer of N
2>10
-4s
-2in depth < 8m)
7% of nights, near-surface
stratification persists
13
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Outline
Near-surface stratification and isolation of the surface layer
PIRATA mooring at 11.5°N 23°W shows a 7-day near-surface stratification event
wet conditions low wind July
2012
August 2012
Mooring (yellow dot) is in favourable conditions for near-surface stratification (dark blue – wet)
(white – wind below 6 m/s)
Difference of the densities at 1m and 10m depth (s1m, s10m) has been divided into the density difference that is caused by salinity (DsS) and the density difference that is caused by temperature (DsT).
Between dashed lines stratification stronger than N2=10-4 s-2. Yellow triangles: local noon of July 21st to August 2nd 2012.
The event starts with a strong rainfall of 100mm.
Then the freshwater causes a long-lasting near- surface stratification trapping heat and
freshwater in the top few meters
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Near-surface stratification and isolation of the surface layer
wet conditions low wind July
2012
August 2012
