Tropical Cyclones and Western Disturbances

Tropical cyclones, the most destructive of nature’s phenomena, are known to form over all tropical oceans, except the South Atlantic and the South Pacific, during certain seasons. These cyclones have a thermal origin. They are believed to form in the Inter-Tropical Convergence Zone (ITCZ).

The frequency, intensity and coastal impact of cyclones vary from region to region. Interestingly, the frequency of tropical cyclones is the least in the north Indian Ocean regions of the Bay of Bengal and the Arabian Sea; they are also of moderate intensities. But the cyclones are deadliest when they cross the coast bordering North Bay of Bengal (coastal areas of Orissa, West Bengal and Bangladesh). This is mainly due to storm surges (tidal waves) that occur in this region inundating the coastal areas.

Tropical cyclones over the Bay of Bengal occur in two distinct seasons, the pre-monsoon months of April-May and the post-monsoon months of October-November. On an average, in fact, almost half a dozen tropical cyclones form in the Bay of Bengal and the Arabian Sea every year, out of which two or three may be severe. Out of these, the stormiest months are May, June, October and November. Compared to the pre-monsoon season of May-June, when severe storms are rare, the months of October and November are known for severe cyclones.

Tropical cyclones in the Bay of Bengal are more frequent than in the Arabian Sea. There could be three reasons for this—(i) waters in the Bay of Bengal are comparatively shallow; (ii) the coastline along the Bay of Bengal is more complicated; and (iii) more number of rivers drains into the Bay of Bengal than into the Arabian Sea.

With the existing knowledge about cyclones, it is not yet possible to physically prevent the build­up of a massive cyclone.

Accepted technology, therefore, only provides the capability to detect and track cyclones with sophisticated satellite imagery and ground-based radar systems. But here too the limitations are glaring. Atmospheric science, for example, is not yet in a position to unambiguously predict the motion and behaviour of a cyclone more than 24 hours ahead of its arrival. In such a brief span of time, the vulnerable sections of the population can merely be warned of the imminent danger and evacuated to safer/cyclone-withstanding structures.

Western disturbances occur during the cold, dry season. The weather is fine with clear skies, low temperatures and humidity, cool breeze and rainless days. However, these fine weather conditions at intervals get disturbed by shallow cyclonic depressions. These depressions—also known as western disturbances—originate over the east Mediterranean Sea and travel eastwards across West Asia, Iran-Afghanistan and Pakistan before they reach the north-western parts of India. On their way, they pick up moisture from the Caspian Sea in the north and the Persian Gulf in the South.

Although the western disturbances cause meagre rainfall, even this little amount of rainfall is highly beneficial to the rabi crops, especially wheat. The precipitation is in the form of light rains in the plains and heavy snowfall in the western Himalayas, It is this snow that sustains the flow of water in’ the Himalayan rivers during the summer months.