Sea surface salinity in the Atlantic ocean from the SMOS mission and its relation to freshwater fluxes

Abstract

Spatially dense sea surface salinity (SSS) measurements have recently begun to be made from space through the ESA SMOS mission. In this PhD, the sub-annual to interannual SSS distribution and variability is characterized, and its relationship with surface freshwater fluxes, i.e. Evaporation minus Precipitation (E-P) and river outflow (R) is investigated. Given its link to the Meridional Overturning Circulation, the focus is on the Tropical/ Subtropical Atlantic, which encompasses the dynamically different E-dominated Subtropics and P-dominated Tropics. The global and Atlantic SSS variability on different time scales and controlling processes are reviewed, including a description of how SSS is remotely sensed by satellites. The research initially examines the Tropical Atlantic SSS variability using the first year (2010) of SMOS data. This reveals that the variability in 2010 is dominated by eastern/western basin SSS regions ("poles") close to the major Amazon/Orinoco and Congo/Niger rivers. The poles show seasonal ranges up to 6.5 pss and out-of-phase by 6 months seasonal cycles that largely compensate each other, playing a key role in the Tropical Atlantic salinity budget. The growing SMOS record also reveals new aspects of the interannual variability of the SSS seasonal cycle during 2010-2012 and its phase-relationship with E, P, and R. It also shows that the E/W poles' seasonal compensation holds at multi-annual time scales. Next, a novel analysis of the spatio-temporal characteristic scales of SSS from SMOS over the Tropical/Subtropical Atlantic basin is presented. By examining how quickly consistent SSS changes evolve, regions with time persistent and, likewise, spatially homogeneous SSS variations, on sub-annual to interannual time scales, are identified. The spatial scales of SSS in the region are anisotropic, and persist for up to 3-4 months over most of the basin. Determination of SSS time and space scales of variability also provides insights into the controlling mechanisms of SSS. Finally, focusing on the freshwater forcing term of the salt budget equation, E-P is estimated from satellite SSS variations to explore whether and where SMOS can capture the main characteristics of E-P distribution in the region.

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