Anatomy of Salinity Anomalies Associated With the Positive Indian Ocean Dipole

Abstract

Mechanisms of salinity anomalies associated with the positive Indian Ocean Dipole (pIOD) are investigated through a series of sensitivity experiments and an online budget analysis using a regional ocean model. Special emphasis is placed on the contribution from the rectified effects due to high-frequency variability, which was not quantitatively discussed in previous studies. The results from sensitivity experiments show that positive sea surface salinity (SSS) anomalies in the southeastern tropical Indian Ocean are primarily caused by reduction in precipitation and partly by enhanced evaporation due to increased wind speed, while negative SSS anomalies in the central-eastern equatorial Indian Ocean are generated by zonal advection anomalies induced by anomalous wind stress, consistent with previous studies. Completely new results are that the modulation of nonlinear salinity advection associated with mesoscale eddies also plays an important role in determining the spatial pattern of SSS anomalies, especially in the southeastern tropical Indian Ocean. On the other hand, subsurface salinity anomalies are almost entirely caused by wind stress effects mediated by ocean dynamical processes. Further decomposition of advective anomalies suggests that they are mainly explained by the pIOD-related current anomalies governed by equatorial wave dynamics. However, a vertical shift of nonlinear freshening due to high-frequency variability also substantially contributes to the generation of positive subsurface salinity anomalies in the eastern equatorial Indian Ocean. Our results show that large-scale oceanic changes in response to the pIOD-related atmospheric anomalies are the key drivers of the observed salinity anomalies, while some nonlinear effects also seem to be at work. Plain Language SummaryThe positive Indian Ocean Dipole (pIOD) is characterized by anomalous cooling in the eastern tropical Indian Ocean and warming in the western tropical Indian Ocean and exerts significant impacts on the local and global climate. The signatures of the pIOD have been detected not only in the upper ocean temperature but also in salinity, which is a fundamental parameter along with temperature. Accompanied by the anomalous atmospheric circulation and precipitation pattern, surface and subsurface salinity in the tropical Indian Ocean is known to undergo significant variations associated with the pIOD. In this study, the relative importance of various factors in the generation of these salinity variation and physical processes behind them are quantitatively assessed using a regional ocean model. The results demonstrate that the anomalous large-scale ocean circulation induced by wind anomalies is the dominant factor that causes significant salinity variation, while anomalous precipitation and evaporation also play a secondary role. More comprehensive analyses reveal that contributions from mesoscale eddies and short timescale variations, which have been overlooked in past studies, are also important for the generation of salinity anomalie...

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