Interdecadal changes of the Indian Ocean subtropical dipole mode

Using observational data and outputs from an ocean general circulation model, the interdecadal changes in the Indian Ocean subtropical dipole (IOSD) are investigated for the first time. It is found that the frequency of the IOSD has become higher because of a decreasing trend in the mixed layer depth (MLD) over the southwestern pole in January and February. Positive (negative) sea surface temperature (SST) anomalies associated with the IOSD are generated when the mixed layer becomes anomalously shallow (deep). The thinner mixed layer in the recent decade amplifies this effect and even weak atmospheric forcing may trigger the IOSD. From a diagnosis of the Monin-Obukhov depth, it is shown that an increasing trend of surface heat flux, which is due to the decrease of wind speed (increase of specific humidity near the sea surface) associated with the poleward shift of westerly jet in January (the strengthening of Mascarene high in February), causes the decreasing trend of the MLD. On the other hand, the smaller amplitude in the recent decades is because the IOSD starts to develop in December, but the deeper mixed layer in December in the recent decade provides an unfavorable condition for its development. In addition, the shallower mixed layer in January and February may also amplify the negative feedback processes that damp the SST anomalies. Since no interdecadal changes in interannual variability of atmospheric forcing corresponding to that in the IOSD are observed, the interdecadal trend in the MLD is essential for that of the IOSD.