Aquarius Sea Surface Salinity in the South Indian Ocean: Revealing Annual-Period Planetary Waves

Abstract:

Increased understanding of the dynamics of the real ocean depends crucially on global-scale routine observations of oceanic and surface atmospheric variables. A new milestone has been reached with the launch of two dedicated satellite missions to routinely measure the sea surface salinity (SSS) fields from space. In the present work, a thorough analysis of the first 2 years of Aquarius SSS data in the South Indian Ocean is performed together with the Argo and RAMA data. This analysis is focused into 3 questions: How accurately is Aquarius observing the fresh Indonesian Throughflow and the salty subtropical waters? Can Aquarius give a spatial context for interpreting the data measured by the RAMA mooring system, which are highly resolved in time, but very sparse in space? Can westward propagating seasonal signals be observed in the Aquarius SSS fields as recently described in model simulations? We find Aquarius observations to be highly correlated with those obtained from Argo and RAMA, with lowest RMS differences found in the salty subtropical waters. In the tropical region, Aquarius gives slightly fresher SSS than in situ data mainly due to rainfall effects, except in the eastern basin where the freshening seems to be related to sharp localized leakages of very fresh waters from the Indonesian seas that the Aquarius L3 product is not able to properly resolve. Aquarius data are shown to reproduce quite well the annual cycle obtained from RAMA and Argo gridded data sets. The annual cycle is characterized by SSS propagating features with different characteristics west and east of the Ninety East Ridge, with the observed patterns better interpreted as a superposition of planetary wave modes. The SSS features are strikingly different from altimetry-derived sea surface height waves. Our results suggest that SSS annual propagation might be reflecting coupled ocean-atmosphere dynamics and surface-subsurface processes operating over the entire South Indian Ocean.