Abstract
Indian Ocean dipole (IOD) is one of the two leading interannual variability modes in the tropical Indian Ocean. It has been known that the growth of IOD is associated with the Bjerknes positive feedback. However, detailed physical processes in the feedback, especially whether the thermocline feedback or the Ekman feedback plays the central role, are still ambiguous. Based on the refined oceanic mixed layer heat budget and atmospheric moisture and moist static energy budget analyses, we show that the growth of IOD is dominated by the coastal thermocline feedback incorporating surface offshore ocean advection. For positive IOD (warm west and cold east), during boreal summer, the seasonal coastal upwelling along the Sumatra-Java coast brings subsurface cold anomalies generated by the shallowing of the thermocline to the surface. The anomalous cold water is further transported by the mean offshore zonal currents, which shapes the spatial distribution of sea surface temperature anomalies of the eastern pole of IOD to form a zonally extended pattern. The zonally extended cold SST anomalies have a strong second-order meridional differential, which drives in-situ anomalous boundary layer divergence through the Lindzen-Nigam mechanism. The anomalous divergence suppresses local convection and thus excites an anomalous anticyclone over the southeastern Indian Ocean. The corresponding equatorial easterly and alongshore southeasterly anomalies over the Sumatra coast further shallow the thermocline off the Sumatra-Java coast through exciting equatorial and coastal oceanic upwelling Kelvin waves. This mechanism applies to various types of IOD, including negative IOD, early IOD, and IOD co-occurring with ENSO. Compared with the conventional thermocline feedback, the offshore advection processes are incorporated, which connects the anomalous cold water from the subsurface to the atmospheric boundary-layer divergence and thus plays a key role in the positive feedback loop.
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