Figure: Signature of the equatorial
upwelling conditions in the Pacific Ocean in terms of SSS (as shown larger than 34.8) in conjunction of SST (black thick line represents the 28°C isotherm,
ci=1°C) and of density (blue thick line represents the 22.5 kg/m3 isolign,
ci=0.5) as derived from the SMOS-ESA satellite mission.
A salient feature of the present-day
climate is the equatorial gradient of sea surface temperatures in the Pacific
Ocean, characterized by a warm pool in the west (>28°C) and a cold tongue in
the east (<20°C). The upwelling conditions caused by the local divergence of
currents in the cold tongue also advectssalty water upward along the equatorial
thermocline. If the climatological evidence broadly depicts such conditions,
the space-borne measurements of the SMOS mission reveal for the first time the detailedstructureof
the SSS signature at the full scale of the Pacific basin (see also the accompanyingfigure
at different time periods). The SSS in the equatorial cold tongue is typically found
to be greaterthan 35.1 within a narrow 2° band of
latitude that is positioned slightly south of the equatorand that stretches
across the eastern Pacific basin up to the Galapagos Islands. On the northern
edge of the eastern equatorial Pacific this signature results in a very strong
horizontal gradient (larger than 2 units over 100 km) with the fresh waters of
the Panama warm pool. By considering a water density criterion (a computation
based on SST and SSS fields, both from the satellite mission), it can be shown
that the cold tongue is characterized by a strong seasonal cycle with a 3°C
amplitude in SST where the warm season of February-March contrasts with the
cold season extending from September to November. As the representation of
surface salinity in ocean models improves, the present analyses of SSS should
prove to be a useful means for investigating the variability of the cold tongue
on ENSO and longer interannual time scales.
Reference: The salinity signature of the equatorial Pacific cold tongue
as revealed by the satellite SMOS mission, by Christophe Maes (IRD/LPO), Nicolas
Reul (IFREMER/LOS), David
Behringer (NOAA/NWS/NCEP) and Terence O’Kane (CSIRO); accepted for
publication in Geoscience Letters, 2014.
Contact: Christophe.Maes@ird.fr
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