Equatorial Monsoon System as Regulation for a Dipole Mode in the Indian Ocean.

In this paper, we studied the equatorial monsoon systems and its association with the dipole mode in the Indian Ocean using NCEP/NCAR reanalysis data, outgoing long-wave radiation (OLR) and sea-surface temperature (SST) data.    The western Indian Ocean is characterized by a prominent semi-annual cycle, in contrast, the eastern Indian Ocean is dominated by an annual cycle. Because of the above phase difference in the tropical Indian Ocean between its western and eastern regions, the east-west (colder-warmer) SST contrast is usually enhanced in autumn. The inter-annual dipole mode, usually seen in autumn, can be understood as a result of the enforced climatological SST contrast. The termination of the dipole mode is closely connected with the appearance of the climatological suppressed convection during February-March over the entire tropical Indian Ocean.    The influence of the reversed Walker Circulation on the Indian Ocean, in response to the ENSO event, exhibits a distinct seasonal difference. If a divergent wind appears in summer over the Indian Ocean, the resultant SST becomes warmer (colder) in the western (eastern) Indian Ocean through air-sea interaction.   Thus, the seasonal differences of the coupling process between the monsoon and the ENSO may be a significant factor for understanding the phase-locking feature of the dipole mode.