posted by Christophe MAES
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 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 advects salty 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 detailed structure of the SSS signature at the full scale of the Pacific basin (see also the accompanying figure at different time periods). The SSS in the equatorial cold tongue is typically found to be greater than 35.1 within a narrow 2° band of latitude that is positioned slightly south of the equator and 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.