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Seasonal dynamics of Sea Surface Salinity off Panama : the Far Eastern Pacific Fresh Pool
G. Alory1, C. Maes2, T. Delcroix2, N. Reul3, S. Illig2,4
1LEGOS, OMP, CNAP, Université de Toulouse, Toulouse, France2LEGOS, IRD, Université de Toulouse, Toulouse, France3LOS, IFREMER, Plouzané, France4IMARPE, Callao, Peru
Vienna, Austria, 22-27 April 2012
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Why focus on SSS in this region?
mean SST
mean SSS
SSS seasonal EOF
• Between 2 climate relevant features: Eastern Pacific warm pool and equatorial cold tongue
• Minimum in SSS (<33: Far Eastern Pacific Fresh Pool) and maximum seasonal variability
• Strong air-sea-land interactions in this region: monsoon, gap winds... (e.g. Xie et al. 2005, Fiedler and
Talley 2006, Kessler 2006)
• Potentially active role of salinity stratification on regional climate (de Boyer Montegut et al. 2007)
• Good test ground for new SSS satellite products (SMOS, Aquarius)
Delcroix et al., 2011
°C
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Main SSS data source: Voluntary Observing Ships
Transect snapshots
• Steep SSS fronts (up to 4 pss/1°) at Fresh Pool west/east boundaries with seasonal displacement >1000 km
• Not always related to SST fronts
Well-sampled TSG line from Panama canal to Tahiti
1950-2009 obs. density
per ¼
°b
in
April
Dec.
SSS SST
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Along-track SSS/SST climatology
• Life cycle of the Fresh Pool : -June-Dec: building
- Jan-Mar: westward stretching
- Apr-May: destruction
• SSS<33 generally associated with SST>28°C
• Mar-Apr coastal SSS increase and cooling in phase
°C
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Central American monsoon
Seasonal north-south migration of ITCZ + high coastal topography:
• in summer, south-westerly winds and strong rainfall in Panama Bight
• in winter, strong north-easterly winds through mountain gaps (Papagayo, Panama) and low rainfall
QScat Wind+
TRMM Rainclimatology
20
15
10
5
0m/s
mm/d
ITCZ
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Wind-driven Ekman pumping
D20
m/d
2.5
0
-2.5
• Upward pumping under the ITCZ
• Winter Ekman pumping dipoles driven by gap winds
• Winter upwelling of salty waters
• Barrier layer under Fresh PoolD20
T-mxl
ρ-mxl
J F M A M J J A S O N D
0
40m
80m
35
33
30
S(z)Argo
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Sea Level Anomaliescm
• Winter eddy dipoles
• Slow south-westward propagation
• SLA trough at 10°N = Costa Rica dome
• Associated with geostrophic currents
15
0
-15
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Surface currents
NECC
SEC
0.8
0.6
0.4
0.2
0
m/s
• Eastern end of the equatorial current system: South Equatorial Current (SEC), North Equatorial Counter-Current (NECC)
• NECC closely associated with ITCZ
• Winter gap winds create coastal cyclonic/anticyclonic circulation
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Along track advection
• JJAS: eastward NECC keeps Fresh Pool trapped to the coast and maintains strong SSS front• JFM: westward stretching of Fresh Pool by SEC• FMA: upwelling in Panama Bight propagating S-E
SEC
NECC
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Main Mechanisms
Surface Currents JFM
• Summer rain � building of Fresh Pool, SSS minimum • Winter gap wind � Westward advection of Fresh Pool by SEC
Upwelling � destruction of Fresh Pool
Wind/Rain JAS
Panama gap wind
Upwelling
NECC
SEC
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Alongtrack regressionbetween TSG/SMOS
colocated dataSignal=10*Error
SMOS: seasonal+ intraseasonal+ interannualvariability
- Main seasonal features appear
- Land contamination near coastline
In situ Climatology vs SMOS 2010 (weekly, ¼°)
SMOS validation
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SMOS 2010: monthly maps of Fresh Pool
SMOS captures main seasonal features of Fresh Pool, can monitor itsspatial extension and associated SSS fronts
Jan Feb Mar
Apr May Jun
Jul Aug Sep
NovOct Dec
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Conclusion and Perspectives
• The Far Eastern Pacific Fresh Pool (SSS<33) is a quasi-permanent feature with the lowest mean SSS and maximum seasonal SSS variations in the tropical Pacific
• Its seasonal cycle results from strong regional ocean-atmosphere-land interactions (monsoon)
• New SMOS satellite is able to monitor the Fresh Pool and complement in situdata
JGR paper out now!
• To quantify the relative contribution of different processes onthe SSS seasonal cycle, regional modelling would be useful
• Potential role of vertical salinity stratification on the regional climate has to be explored