Section 1.41.3.1 January
The ITCZ and monsoon trough of the South Indian Ocean are very pronounced near 10°S (Fig. 2.6). In the eastern portion of the region, the monsoon trough extends southeastward to the heat lows of the Australian continent. The North Indian Ocean is dominated by the northeast monsoonal flow that crosses the equator and converges into the ITCZ. One significant feature is the flow pattern in the Mozambique Channel. The southeasterly trades in this region are met by cross-equatorial northerlies creating a convergence zone in the channel. Tropical cyclones (TCs) often form within the Mozambique Channel during December, January and February. Fett (1990) presents one episode of TC formation in the Mozambique Channel and states that the 850- hPa wind and height charts (not the satellite cloud imagery) provide best early clues for identifying the TC.
1.3.2 April
During the April transition season the ITCZ and near equatorial trough exist on both sides of the equator near 5° north and south latitudes (Fig. 2.7). Twin cyclones are not uncommon during this season in the southern Bay of Bengal. Cross equatorial flow from the north essentially ceases. A complete reversal of the January pattern takes place in May (not shown) as the southwesterly monsoon begins.
1.3.3 July
During July, the southwest monsoon is the dominant pattern (Fig. 2.8). Unlike the January chart, however, there is no ITCZ nor trough in the North Indian Ocean. In reality, the trough has migrated northward and is located over the Indian subcontinent in the form of heat lows. The sharp turning of the winds near the equator is not a trough but rather a buffer zone. Typically it is convectively inactive. In the South China Sea region, the winter northeasterlies have been replaced by summer southwesterlies.
1.3.4 October
By October, the southwest monsoon has nearly ceased (Fig. 2.9). The beginnings of the South Indian Ocean ITCZ are evident near 5°S as the buffer zone has moved south from the July position. The heat lows over India have also moved south to become a trough over southern India and the open water. A month later (not shown), the two troughs are well defined at 5° north and south latitudes of the equator as in April. The northeast monsoon is once again present over the South China Sea.
Chapter 2
