Influence of atmospheric intraseasonal oscillations on seasonal and interannual variability in the upper Indian Ocean

Abstract

An ocean general circulation model, the Hybrid Coordinate Ocean Model (HYCOM), is used to examine the rectification of atmospheric intraseasonal oscillations (ISOs) on the mean and seasonal‐to‐interannual sea surface temperature (SST), mixed layer thickness (hm) and upper ocean heat content in the Indian Ocean. Existing studies have shown that ISOs rectify on seasonal and interannual equatorial surface currents and on cross‐equatorial transport, suggesting that they may also have important impacts on upper ocean variability. To evaluate these impacts, a hierarchy of HYCOM experiments isolates the ocean response to forcing by atmospheric ISO events. Other experiments isolate the ocean response to a range of intraseasonal forcing fields including shortwave radiation, precipitation, and winds. Results indicate that rectification of ISOs onto seasonal and interannual upper ocean variability does occur, and that it is important in some regions. The regions displaying maximum rectification vary between SST, hm, and upper ocean heat content, and from seasonal to interannual timescales. Strong seasonal SST rectification occurs in the Arabian Sea and in the Bay of Bengal. Because SSTs in the Arabian Sea are already warm (28°C), the ISO‐forced seasonal cycle peak of 0.6°C in May can affect convection there. Intraseasonal wind speed and stress have a much larger impact on seasonal and interannual SST, hm, and upper ocean heat content than either intraseasonal shortwave radiation or precipitation. The relative importance of entrainment and turbulent heat flux due to intraseasonal wind speed, and of upwelling, horizontal advection, and vertical mixing due to intraseasonal wind stress, varies with region.