Fear over the sea

Cyclonic storms are very rare in the Arabian Sea. However, when they form occasionally they favour Gujarat coast or the Gulf coast in the extreme north of the sea for landfall. The low pressure area formed in the Gulf of Thailand as a remnant of the Cyclone Kirogi that hit Vietnam during November 20 transformed into a depression and crossed the Bay of Bengal, circumvented Srilanka, unleashed devastation in the eastern coast of Sri Lanka as a deep depression and intensified as a cyclone and partially hit Kanyakumari and south Kerala coasts.

Reliable sources say that the deep depression that slowly moved northeast from the Sri Lanka coast intensified into a cyclonic storm by 5.30 am on November 30. Global weather monitoring agencies modelled and identified the formation of a cyclone, its progress touching the tip of South India and Lakshadweep islands and its path to Gujarat coast. 

Tropical cyclone in Kerala is not a ‘very rare’ phenomenon. As per IMD records, at least 23 severe weather events have affected Kerala area during the last 120 years. During the past 35 years, a total of 7 systems with intensity of super cyclonic storm originated over the Bay of Bengal and crossed eastern coast of India and re-emerged into Arabian Sea as depression. Two storms formed over Arabian Sea and struck Kerala coast as severe cyclonic storms. 

Ernakulam and Malappuram districts were most affected by this phenomenon. So IMD is aware of the probabilities of cyclone formation in the southeast tip of Indian peninsula. As per IMD Kerala region in general has lesser probability of getting affected by the occurrence of cyclonic storm or severe cyclonic storm. So Kerala region has been grouped as Category II A (lower vulnerability zone) along with Tamil Nadu, Maharashtra, Goa, Karnataka, Daman & Diu, Pondicherry, Laksha-dweep and Andaman & Nicobar islands. 

However, the situation has changed drastically during the last two decades. The frequency of cyclonic storms has increased. The constitution of the Arabian Sea has changed. The sea surface temperature and relative humidity has increased. The present sea surface temperature of Arabian Sea is above 31ºC whereas the average sea surface temperature of the Arabian Sea is 28ºC.  Studies indicate that the Arabian Sea has been warming at a faster rate. As a result, there is a marked increase in the occurrence of cyclones, warmer winters and deficient rainfall. These events indicate the alarming effects of climate change. Sea surface temperatures have begun to rise at an average of 0.014ºC per year, faster than the recorded value prior to 1995 in the Arabian Sea.

This warming has resulted in the increasing number and severity of tropical cyclones and changing pattern of precipitation. Ocean surface temperatures over many parts of the basin remain above 26.5°C throughout the entire year and most storms form along the eastern boundary of the Arabian, where surface temperatures are 1°–4°C warmer than those of the western part. The formation of Arabian Sea cyclones is related to this increase in sea surface temperature.

Coriolis force is important to tropical cyclone development. Thunderstorms need the Coriolis force to turn them into a rotating mass and ultimately a tropical cyclone. Between the equator and 5°N the Coriolis force is too weak to cause rotation; therefore tropical cyclones do not form here. The Latitude of Kerala is ?: ?10.850516N. So Kerala is prone to tropical cyclones.

Important ingredient for storm development is a warm sea surface temperature. For a tropical cyclone to have enough energy to develop it must pass over the above given sea surface temperatures, and the warmer the better. High relative humidity in the surrounding air is also important because a tropical cyclone. Sea gets warmer towards the surface and towards the equator. Surface temperature in the Bay of Bengal is usually between 22°C and 31ºC. It was cooler by 1-2ºC in the Arabian Sea. This difference had major implications for the atmosphere above the two basins. Now the Arabian Sea temperature has increased and has reached to the tune of 31ºC.

(Author is a popular science author and researcher)