Interdecadal changes of the Indian Ocean subtropical dipole mode
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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.
KeywordsSurface Heat Flux Mixed Layer Depth Southern Annular Mode Ocean General Circulation Model Empirical Orthogonal Function Mode
This study is benefited from discussions with Prof. Yukio Masumoto, Dr. Takeshi Doi, and Mr. Takahito Kataoka, and constructive comments provided by two anonymous reviewers. The OGCM was run on SR11000 system of Information Technology Center, the University of Tokyo under the cooperative research with Center for Climate System Research, the University of Tokyo. Wavelet software was provided by C. Torrence and G. Compo, and is available online (http://paos.colorado.edu/research/wavelets/).
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