— Dr Manasmani Dev Goswami There has been no rain for a long period over Indian sphere. Everybody expects at least a few showers during the brink of winter to ease the dry spell before the onset of monsoon. The phenomenon indicates a shift or change in weather happenings and a time-space variation of the entities in the hydrologic cycle. This type of climate-behaviour for some years is not only prevailing in Indian sub-continent, it is vivid over the globe. This is a warning for a probable atmosphere of desertification in seasons to come.
Assam, which always receives a good amount of rainfall over the years, is also under the influence of unusual weather condition during the last two-three years. Assam receives sufficient rainfall every year out of South-West Monsoon (SWM) of the country. It is worth-mentioning that the success of agriculture in this region still depends on the timely onset, proper amount and even distribution of rains. During the last three years, Assam faced drought-like situation due to delayed and insufficient rainfall.
There is a strong linkage between the dry spell during pre-monsoon and monsoon season. If the onset of the monsoon and the duration with the output of the monsoon in the previous years deviates from the usual time, space and amount, there is possibility of prevalence of unusual dry spell before onset of monsoon.
With the exception of Jammu and Kashmir in the extreme north of India and in Tamil Nadu in the south, 80 to 90 per cent of the rainfall over the country occurs mostly during the SWM season. The dates of the onset of the monsoon in different parts of the country, the intensity and the distribution of rain, however, display variations in time and space and from year to year. Normally, the SWM reaches the Kerala coast by the end of May, advances along the Konkan coast in early June and extends over the entire country by the end of July. The rains continue up to the end of September, when the SWM recedes. In November and December, the north-east monsoon is the main contributor to the amount of rainfall over the south eastern portion of the peninsula.
The activity of the SWM is not uniform in time and space during the whole season. The intensity and the distribution of rainfall are controlled by a series of tropical depressions or low-pressure systems which originate near the head of the Bay of Bengal and travel across the country in a west-north-westerly direction. Heavy rainfall occurs mainly to the south of the tracks of these depressions. Three or four depressions form in a month during the monsoon. When in some years they are scarce, the rainfall will be confined to the Western Ghats and the mountain ranges of Assam and the foothills of the Himalayas, with the interior parts of the country not getting their usual share. And there is drought in these areas. Even when the depressions are of normal frequency and intensity, their tracks determine the distribution of rainfall over northern and central India. When they follow a north-westerly track across the plains of the Ganga, there will be floods in northern India and drought in the Peninsula. The reverse is the case when they take a westerly track across central India. The conditions in the States, where the tracks of depressions terminate, e.g., in Gujarat and Rajasthan, tend to be erratic. When a depression reaches these States, they get abundant rains; otherwise, they are subject to a prolonged drought. The monsoon depressions are, therefore, said to be the single factor that control the distribution of rainfall over the region.
For many years, scientists have been trying to uncover other influential factors that govern the SWM and gain the capability to forecast the outcome of the season. It is essential to know the outcome and even control the adverse situations arisen in the region. But the monsoon is too complex to be predicted so easily. The study on monsoon is still going on for definite identification of structure of the situation.
Sir Gilbert Walker, a renowned meteorologist on Indian weather conditions stated that when atmospheric pressure was low over South American coast and high over the Indian Ocean region, rainfall was likely to be deficient. When the reverse happened, the monsoon was usually good. But, even with Gilbert’s foremost procedure of prediction, droughts could not be successfully predicted. The ability to predict drought successfully has even remained elusive to this day. Scientist DR Sikka and his associates showed that ENSO events (the oceanic and the atmospheric components are now together termed the El Nino Southern Oscillation) strongly influence the Indian monsoon with poor rains in years when an El Nino (atmospheric component associated with warming of the surface water of the Pacific Ocean) occurred and a good monsoon during a La Nina (atmospheric component associated with cooling of the surface water of the Pacific Ocean). There is a strong link between monsoon and the oceans that surround India.
There is still a demand for accurate estimation and prediction for the Indian weather and monsoon phenomena. Though some models have been used recently as a technique for predicting monsoon by way of simulation of the ocean and the atmospheric interactions, there is sufficient scope of development in the prediction procedure. As a region of smart monsoon and congenial weather condition, it is time to identify the weather happenings justifiably so that common people are aware of the possible weather chaos. Assam Tribune
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