Rains decline: Two cheers for monsoon

Kerala has been blessed with a 20 per cent increase in summer showers this time, but that poses the not-so-happy question, whether it will result in a deficient monsoon. The Indian Southwest Monsoon is the severest among all monsoons. Monsoon winds bring along rains with them and that assumes greater importance. Studies based on the last several years’ data indicate that during June and July rainfall shows a significant decreasing trend and an increasing trend in January, February and April. Even though rainfall during winter and summer seasons shows an increasing trend,  hydel power generation and water availability are the concern of the State due to deficient rainfall in June and July, which are considered to be the rainiest months. Summer months are the scariest.
Atmosphere is a connected system. Any minor change in elements of the system may result in significant outcome elsewhere. Monsoon rains have been the base of India’s food security and of the very survival of this land and its people. The typical geographic features of the Indian subcontinent and its atmospheric and oceanic factors influence the pattern of the monsoon.

The monsoon has shown a weakening trend specifically since 1950, but the reasons are not yet well elucidated. Climate models fail to simulate this trend and this raises a major question on the reliability of climate models. This is due to poor understanding of reasons behind the weakening trend. We have little knowledge of all factors that contribute to the pattern of monsoon. Many other factors contribute to the strengthening and weakening of monsoons in India. A physical system cannot be determined by a finite number of observations. Nature is complicated. El Nino, the warming of surface of the Pacific, results in deficient rain in India. During La Nina, the western part of Pacific becomes warmer and the reverse happens.  El Nino indicates drought and La Nina, rains in India. Another phenomenon, Indian Ocean Dipole, which is the see-saw in the Indian Ocean surface temperature, also influences the quantum of precipitation.

There is an alarming reduction in the tree cover and orographic (mountainous) features of Kerala. Quarrying, deforestation, concrete structures and significant changes in the mountainous terrain have resulted in the reduction of moisture reaching the atmosphere. This affects processes between the earth/atmosphere systems and influences the distribution of local rainfall. This is the reason behind deficient monsoon in the state even when the other parts of India receive normal or above normal rainfall. Data show that during the summer months this year, the rainfall in the state recorded a 20 per cent increase (359.1 mm) above normal (298.4mm). Lakshadweeep recorded an 88 per cent increase (296.1mm) above normal (157.1mm).

Last year Kerala received 9 per cent less rainfall (1855.9mm) below the normal (2029.7mm) during the Southwest Monsoon season. But in Lakshadweep it was 11 per cent above (1108.3mm) normal (998.5). There was an 8 per cent decline in the Northwest Monsoon (441.8mm) below normal (480.7mm) during October-December 2017 in Kerala while in Lakshadweep it was 8 per cent above (360.7mm) normal (333.6mm). Such deviations from the normal pattern of rainfall are evident from data recorded since the beginning of the last century. In 2016, the Southwest Monsoon was 33.7 percent below normal and during the North-east Monsoon season it was 61.7 per cent below normal.

This anomalous variation calls for in-depth studies using a variety of land-based meteorological observatories and satellites. Studies show an 8 per cent reduction in the quantum of contribution to the annual rainfall during Southwest monsoon and 6 per cent increase in the quantum of rainfall in the post-monsoon season (India meteorological dept). Even when rest of India receives normal rainfall, there has been a significant decrease in the rainfall received in the state. There has been a major change in rainfall pattern during recent years as seasonal rainfall during the Southwest Monsoon declined and increased in the post-monsoon season.

The number of depressions formed during the Southwest Monsoon season, the strength of the monsoon current and the strength of the tropical easterly jet stream are important rain-bearing systems during Southwest Monsoon. The decline of the above weather systems in recent years over the peninsula may be an important reason for the decrease in Southwest Monsoon rainfall over Kerala. (K.N Krishnakumar et al / Atmospheric Environment 43 (2009) The annual oscillation in the apparent position of the Sun causes the annual shift in the position of the thermal equator, where maximum surface heating is recorded. This is the main reason behind the annual oscillation of temperature, pressure, wind, cloudiness and rain and the cause of monsoons.

The Indian Ocean is hotting up. The narrowing difference between land and ocean temperatures results in the weakening of monsoon. The monsoon is weakening significantly owing to increasing temperatures in the Indian Ocean and relatively restrained warming over land in the central Indian subcontinent.
Rainfall over the central, east and northern regions of India, along the Ganga-Brahmaputra-Meghna basins and the Himalayan foothills, had also declined. The intense heating during late spring and summer around the equator leads to south-westerly winds. Due to higher ocean temperatures, the normal movement of cooler air from the sea to hotter land, with clouds bearing rain, is weakening. This reduction in disparity between sea and land temperatures is responsible for the declining monsoon.

In summer, the northern part of India acts as an intense and extensive heat source that supplies heat directly to the middle troposphere. In winter, they act as a heat sink. The resulting annual oscillation in pressure, temperature and wind is unique to this region, and makes the Southwest Monsoon the strongest. Monsoon drivers– the difference between land and sea temperatures – in the era of global warming are getting stronger. Land in summer in the northern hemisphere is warming faster than the oceans, which would imply that the monsoon would also be stronger. The rising ocean temperatures could raise the moisture in the atmosphere over the sea due to higher evaporation and induce a stronger monsoon, but things are different in India.

Several other factors such as low level and upper level winds, moisture, land and sea surface temperature are involved in the onset process. The chain of events that culminates in the onset of the monsoon over Kerala is initiated by the low level cross-equatorial flow off the coast of Sumatra in early May. This results in an increased moisture flux over the Arabian Sea, deep cumulus convection, latent heat release and a rise in tropospheric temperatures. The western Indian Ocean and northwestern Arabian Sea have warmed by more than 1-1.5 degrees Celsius. It is possible that weakened air currents amplify the sea surface temperatures due to lower evaporation. Increasing particulate matter (aerosols) alter monsoon patterns. Anomalous occurrence of tropical cyclones like Ocki and Mekunu and continuing depressions in the Arabian Sea are attributed to the increase in sea surface temperature. 

Gateways of GREEK GREAT Hippalus

Hippalus, the Greek traveller, is credited with the discovery of the peculiarities of monsoon winds in the first century AD. This led to the opening up of a trade route between India and countries to the west, across the Arabian Sea. Traders would come to Kerala, sail along with Southwest Monsoon winds and sail back when winds reversed, with the retreating monsoon. Although the author is unknown, the Periplus of the Erythraean Sea (Red Sea) mentions the Hippalus’ finding. Pliny the Elder, the Roman writer, claimed that Hippalus discovered the Southwest Monsoon wind, also called the Hippalus . The seasonal reversal of winds and the associated rainfall are termed, the monsoon. This word is derived from the Arabic word, Mausim.

(The author is a popular science writer and freelance researcher)