Mintu Sarma
(Meteorologist Regional Meteorological Centre Can be reached at Sarmamintu9@gmail.com.)
The Indian monsoon refers to inherent fluctuations and changes that occur over time due to various factors and phenomena. The South-West-West monsoon in India is a four-month season from June to September, considered the prime rainy season across the country, though technically dry spells also persist during the season. A monsoon is a planetary phenomenon that occurs over Africa, Asia, Australia, and North and South America. It is characterised by a seasonal reversing of wind accompanied by a variety of precipitation. India receives almost 80 percent of its annual rainfall from the SW monsoon, although the monsoon wind and rainfall are affected by different factors such as El Nino, La Nina, Madden-Julian Oscillation (MJO), Indian Ocean Dipole (IOD), etc.
El Nino and La Nina are natural phenomena that generally occur every five to six years. El Nino is a periodic warming in sea surface temperature (SST) across the central and east-central equatorial Pacific, and La Nina is the antithesis to it. These two are the most influential phenomena that affect SW-monsoon wind as well as weather from South America to India. During El Nino, warm water accumulates near the Peru coast, and the trade wind near the equator, which is the lifeline of the SW-Monsoon, becomes weak, whereas La Nina is just the opposite. La Nina is associated with the strong equatorial moister-laden trade wind, a boon for SW monsoons as it is associated with good monsoon rain across the country. Madden Julian Oscillation (MJO) is also an important factor that can influence the monsoon wind. It is a large-scale atmospheric disturbance originating in tropical Africa and travelling eastward. According to a study, MJO wind can counter the effects of El Nino. It influences the weather not only in tropical regions but also in extra-tropical and polar regions. Though a brief conception of MJO has been documented in research papers, total understanding remains a holy grail. The Indian Ocean Dipole (IOD) is another factor that influences SW-Monsoon remarkably. IOD is the difference in SST between the Arabian Sea, the Western Indian Sea (considered the west pole), and the Indian Ocean south of Indonesia (considered the east pole). When SST is greater than normal in the west pole and SST becomes lower than normal in the east pole, the phenomenon is known as a positive IOD, and the reverse condition is known as a negative IOD. Positive IOD is favourable for good rainfall and active monsoons over the Indian subcontinent.
First June is the normal onset day of SW-Monsoon over Kerala, but this year SW-Monsoon wind hit the Kerala coast on 8th June, delayed 7 days but covered the entire country on 2nd July, 6 days prior to the normal date (8th July). According to the India Meteorological Department (IMD), though this year was an El Nino year, the SW monsoon’23 has ended with India receiving “near normal” rainfall, countering the effect of the El Nino condition. During the SW monsoon, India received a cumulative average rainfall of 820 mm against a long-period average of 868.6 mm. Out of 36 meteorological sub-divisions, 3 received excess rainfall, 26 received normal rainfall, and 7 received deficit rainfall. The sub-division constituting Nagaland, Manipur, Mizoram, and Tripura (NMMT), Gangetic West Bengal, Jharkhand, Bihar, East UP, South Interior Karnataka, and Kerala received deficit rainfall. The month of June was comparatively dry, but excessive rainfall was recorded in the month of July due to consecutive western disturbances (WD) over northwest India and a favourable phase of MJO. The month of August 2023 was the century’s driest month since 1901 and the hottest ever recorded in India. However, the country received some excessive rainfall in the month of September due to multiple low-pressure systems that gave some respite from the sweltering heat.
The SW monsoon rainfall distribution seems to have deviated from the normal pattern, which is a matter of concern for NE India as we believe that monsoon rain elevates its beauty. All seven meteorological sub-divisions that received deficit rainfall belong to the country’s eastern and southern sectors. The sub-divisions under the western and northern sectors of India receive normal rainfall, which was quite uncommon until a few years ago. Furthermore, during this monsoon season, 15 low-pressure systems were developed against a normal average of 13, but their temporal distribution remained “skewed,” as revealed by the IMD source. The root causes of such monsoon doldrums need to be analysed with probable advisories since monsoon rain is the lifeline of Indian cultivators.