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Vendredi 21 avril - S. Bulusu - Decadal changes in sea surface salinity

Par SEMSOU Dernière modification 09/03/2017 15:48
Quand ? Le 21/04/2017,
de 11:00 à 12:00
Participants Subrahmanyam Bulusu, Professor, University of South Carolina (USA)
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Subrahmanyam Bulusu,

Professor of Satellite Oceanography & Physical Oceanography,

Satellite Oceanography Laboratory, School of the Earth and Ocean Sciences

University of South Carolina, Columbia (USA)


Title : Investigating Interannual and Decadal Changes in Sea Surface Salinity in the Oceanic Subtropical Gyres and their connection to the Global Water Cycle


Abstract :

There is evidence that the global water cycle has been undergoing an intensification over several decades as a response to increasing atmospheric temperatures, particularly in regions with skewed evaporation – precipitation (E-P) patterns such as the oceanic subtropical gyres. Moreover, observational data (rain gauges, etc.) are quite sparse over such areas due to the inaccessibility of open ocean regions. We analyzed spatial and temporal salinity trends in five subtropical gyre regions over the past six decades using Simple Ocean Data Assimilation (SODA) reanalysis. It reveals that a positive rising trend in sea surface salinity in the subtropical gyres emphasizing evidence for decadal intensification in the surface forcing in these regions. Zonal drift in the location of the salinity maximum of the south Pacific, north Atlantic, and south Indian regions implies a change in the mean near-surface currents responsible for advecting high salinity waters into the region. Our results indicate an overall salinity increase within the mixed layer, and a salinity decrease at depths greater than 200m in the global subtropical gyres over 61 years, of which each individual gyre was analyzed in further detail. We determine that freshwater fluxes at the air-sea interface are the primary drivers of the sea surface salinity (SSS) signature over these open ocean regions by quantifying the advective contribution within the surface layer. This was demonstrated through a mixed layer salinity budget in each subtropical gyre based on the vertically integrated advection and entrainment of salt. Our analysis of decadal variability of fluxes into and out of the gyres reveals little change in the strength of the mean currents through this region despite an increase in the annual export of salt in all subtropical gyres, with the meridional component dominating the zonal. This study reveals that the salt content of E-P maximum waters advected into the subtropical gyres is increasing over time. A combination of increasing direct evaporation over the regions with increasing remote evaporation over nearby E-P maxima is believed to be the main driver in increasing salinity of the subtropical oceans, suggesting an intensification of the global water cycle over decadal timescales.

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