11.9.1.2 Indian Ocean

The Indian Ocean region refers to the area between 35°S to 17.5°N and 50°E to 100°E. The climate of the region is influenced primarily by the Asian monsoons (see Section 11.4.1 for processes influencing monsoons). During January, the ITCZ is located primarily in the SH. The region north of the ITCZ then experiences north-easterly trade winds (northeast monsoons) and the region to the south, the south-easterly trades. During northern summer, the ITCZ is located in the north and virtually covers the entire Bay of Bengal, the surrounding lands and the eastern Arabian Sea. The winds in the north turn into strong south-westerlies (southwest monsoons), while the south-easterlies persist in the south. Precipitation and wind stress bring about a response that is distinctly different in the northern and southern parts of the Indian Ocean (International CLIVAR Project Office, 2006). The wet (dry) season in the Maldives occurs during the southwest (northeast) monsoons. From May to October, the southeast trades dominate in the Seychelles and the climate is relatively cool and dry, and December to March is the principal wet season with winds mainly from west to northwest.

While the monsoons recur each year, their irregularity at a range of time scales from weeks to years depends on feedback from the ocean in ways that are not fully understood. Intra-seasonal variability is associated with the Monsoon Intra-Seasonal Oscillation (MISO) and the Madden-Julian Oscillation (MJO), which are long-lasting weather patterns that evolve in a systematic way for periods of four to eight weeks. On an interannual and decadal scale, statistical methods have shown that while there are periods of high correlation between ENSO and monsoon variation, there are decades where there appears to be little or no association (International CLIVAR Project Office, 2006; see also Section 10.3.5.4). A modulating factor is the Indian Ocean Dipole or Indian Ocean Zonal Mode (IOZM), a large interannual variation in zonal SST gradient (see Section 3.6). The magnitude of the secondary rainfall maximum from October to December in East Africa is strongly correlated with IOZM events, and the positive phase of IOZM, with higher SSTs in the west, counters the drying effect that ENSO has on monsoon rainfall (Ashok et al., 2001).