With the development of sophisticated scientific techniques and advancement in observing techniques like satellite imageries, our understanding of the dynamics of the monsoon has improved significantly in the last few decades. Nevertheless, the origins and mechanics of the monsoon are yet to be fully understood. While empirical seasonal forecasts of the monsoon have been moderately successful for the past hundred years, attempts to model the basic mean structure of monsoon have proved to be elusive. It is, however, generally accepted that monsoon is a result of three physical processes—uneven heating of land and sea causing pressure differential that drives winds from high pressure to low pressure; the rotation of the earth forcing winds to veer towards the right in the northern hemisphere and to the left in southern hemisphere; and change in the state of water from liquid to vapour, which determines the strength and location of the monsoon rains.
Theories of the causes of monsoon in the eighteenth and nineteenth century were based on an understanding of the effects of non uniform heating of land and sea. The earliest propounder of this line of thought, Edmund Halley, an astronomer influenced by Newtonian physics, showed in his ‘sea breeze-land breeze’ model of 1686 the tendency for the north-easterly trade winds on the northern side of the equator and the south-easterly trades to the south of the equator to converge towards the most strongly heated regions of the globe. This theory, refined by George Hadley, observed that the wind flow between land and ocean was in an oblique direction (southwest in summer, northwest in winter). Hadley explained that moving air is deflected as a result of rotation of the earth on its axis, veering towards the right in the northern hemisphere and forming the monsoon south westerlies, and to the left in the southern hemisphere, creating the monsoon north westerlies.
The most spectacular part of the monsoon climate system is the timing of the commencement of the wet season, which often involves a sudden change in the weather from dry hot conditions to cloudy and rainy. For farmers, it is critical to know when the onset will occur as this affects the timing of crops. Prediction systems are thus central to preventing futile sowing and faulty harvesting.
India has large rainfall variability over both space and time. Areas on the west coast and northeast India receive maximum rainfall. Cherrapunji and Mawsynram, the two small towns of Meghalaya located on the southern slopes of the Khasi hills, record an annual rainfall of over 1000 cm. On the other hand, areas like western Rajasthan and Tamil Nadu, receive a summer rain of less than 20 cm. Also monsoon experiences a temporal variability both within the season and annually. A delay in the start of monsoon can be just as harmful, even if the aggregate for the year is normal. As per an estimate one bad growing season brought on by an abnormal monsoon can lower the season’s agricultural output by as much as 15 per cent.
Despite the known variability of the monsoon, government has been making concerted efforts to improve the ability of scientists to predict rainfall patterns so as to anticipate and pre-empt crisis. The cost in establishing sophisticated weather and climate prediction systems has, however, dissuaded most South Asian governments. The India Meteorological Department (IMD), an exceptional case in point, established in 1875 by H F Blanford, began issuing seasonal forecasts of monsoon rainfall since 1884, following the Indian famine of 1877. With the advancements in Indian space technology and research capability there has been considerable improvement in India’s capacity to monitor and predict monsoon. India is now capable of predicting monsoon rains in almost all scales—short range (upto 48 hours), medium range (3-10 days in advance), extended range (upto one month) and long range (seasonal scale), with varied accuracies. India has acquired most sophisticated computing resources to handle simulation and prediction of weather systems. Besides IMD, other institutions, National Centre for Medium Range Weather Forecasting, Noida and Indian Institute of Tropical Meteorology, Pune, Centre for Atmospheric Sciences in several premier institutions like IIT, Delhi and Kharagpur; and IISc, Bangalore are working on advanced monsoon modelling. India is among few countries in the world which has implemented a weather forecast based district level agro advisory system that provides reliable and timely advice on agricultural operations like sowing, harvesting, irrigation, application of pesticides and insecticides, and drought and flood situations. Several Indian states too have established crop weather watch groups, which monitor the behaviour of the monsoon and plan future course of action for cropping strategies and dealing with pest attacks.
Onset and Advance: In 2015 the southwest monsoon set over the south Andaman Sea on May 16—four days earlier than normal. On May 21, the monsoon advanced further over the remaining parts of the Andaman Sea.
The southwest monsoon set in over Kerala on 5 June—4 days later than normal. By June 15, the monsoon was all over the central Arabian Sea, some parts of the north Arabian Sea, the entire southern Peninsula, and most parts of central and northeast India. The formation of a couple of intense pressure systems, one each over the Arabian Sea (deep depression) and the Bay of Bengal (depression) towards the end of the third week helped the monsoon advance rapidly resulting in the entire country getting covered by June 26.
Chief synoptic features and rainfall
The season saw 11 low pressure systems (low pressure areas and more intense systems) form. Out of these, 8 intensified into depressions, against a normal four to six depressions every season. Two of these intensified into cyclonic storms ‘Ashobaa’ (7-12 June) and ‘Komen’ (26 July – 2 August), over Arabian Sea and Bay of Bengal respectively and three became deep depressions with two over land (27-30 July and 16-19 September) and one over the Arabian Sea (22- 24 June). Of the three depressions, two formed into land depression over and around Jharkhand (10-12 July) and over east Madhya Pradesh and adjoining Chhattisgarh (4 August). The remaining formed over the Bay of Bengal (20 -21 June). Of the three low pressure areas, one intensified into a well-marked low pressure area.
The setting of southwest monsoon over Kerala on 5th June was associated with the eastward movement and strengthening of the Madden Julian Oscillation (MJO) over the Indian Ocean. This also helped intensification of the onset vortex over the east central Arabian Sea into cyclonic storm ‘Ashobaa’ (7-12 June).
The north-eastward movement of the deep depression formed over Arabian Sea (22-24 June) towards Gujarat brought widespread rainfall over Konkan, Goa and Maharashtra and scattered to isolated rainfall over Saurashtra and Kutch with isolated extremely heavy rainfall for a couple of days. The presence of trough and cyclonic vortex in mid latitude westerlies caused active monsoon conditions over extreme north and western Himalayan region. Consequently, the southwest monsoon advanced further and covered major parts of central India, northern plains and western Himalayan region by 24th June and the entire country by 26th June. Thereafter, the rainfall activity reduced substantially over most of south peninsular and central India, in the wake of unfavourable phase of MJO, which moved eastward into the western Pacific region.
Subsequent MJO activity in the western Pacific directed significant part of cross equatorial flow towards the typhoons developed over the west Pacific Ocean, leading a to weak monsoon circulation pattern. This caused subdued rainfall activity over major parts of central and peninsular India during the first week of July. With the formation of a low pressure area over north Bay of Bengal on 8th July and its movement northwest along the axis of the monsoon trough plus abundant moisture present in the lower levels over the Indo-Gangetic plains, aided rapid intensification into a land depression over and around Jharkhand on 10th July. In addition, the presence of cyclonic circulation over southwest Uttar Pradesh and adjoining areas and couple of western disturbances led to increase in rainfall activity all along the Indo-Gangetic plains and northwest India during the second week of July.
The third and fourth week of July witnessed rapid movement of number of disturbances in mid latitude westerlies, in the form of cyclonic vortex and, the active monsoon trough caused active to vigorous monsoon conditions over central India and western Himalayan region. Further strengthening of the cross equatorial flow in the lower troposphere led to enhanced rainfall along the west coast. However, rainfall activity over peninsular India remained subdued. Towards the end of the July, two intense low pressure systems (deep depressions) formed along the axis of the monsoon trough at both the ends, one over northeast Bay of Bengal and the other, over southwest Rajasthan. As the monsoon trough lacked the characteristic southwards tilt with height, the rainfall associated with the deep depression over Rajasthan was confined to the core area surrounding the system. The deep depression over Bay of Bengal intensified further into a cyclonic storm ‘Komen’. As it moved north-westwards, it brought heavy to very heavy rainfall with extremely heavy rainfall at isolated places over eastern and central parts of the country. Its subsequent movement inland after weakening caused vigorous to active monsoon conditions along the track.
The strengthening of winds at lower levels and circulation features during the first week of August led to formation of a low pressure area (11-15 Aug.) over west central and adjoining northwest Bay of Bengal off north Andhra Pradesh–south Odisha coasts. This revived active monsoon conditions over major parts of the country and gave heavy to very heavy rainfall at isolated places over eastern, northern and central parts of India. Thereafter, weakening of the low pressure area and subsequent shifting of the monsoon trough northwards to the foot hills of Himalayas, led to a weak monsoon flow pattern, thereby keeping rainfall activity confined to eastern and north-eastern India in the second week.
The first week of September, witnessed reduced rainfall activity over north-western India thereby indicating favourable conditions for withdrawal of the southwest monsoon from west Rajasthan. The change in circulation pattern in the lower tropospheric levels led to withdrawal of monsoon from western Rajasthan on September 4. However, the presence of a trough in lower tropospheric westerlies, couple of cyclonic vortex revived rainfall activity over parts of south peninsular India. The low pressure area formed over west central and adjoining northwest Bay of Bengal on September 12 intensified into a depression over and around south Odisha on September 16 and into a deep depression over Vidarbha and south Chhattisgarh on September 17 resulting in strengthened monsoon activity over northern peninsular India, thereby delaying withdrawal.
Fig. 1 shows the cumulative for the period June 2015 to September 2015. In June, except for 3 subdivisions (Bihar, Nagaland, Manipur, Mizoram, Tripura and Andaman and Nicobar Islands), which received deficient rainfall, all the other subdivisions received normal (20 subdivisions) or excess (13 subdivisions). In July, most subdivisions from peninsular and northern India (along the Himalayas) received deficient or scanty rainfall. The scanty rainfall subdivisions were Marathwada, north interior Karnataka, Telangana and Rayalaseema.
In August, majority of the subdivisions from northwest India, central India and neighbouring south Peninsula received deficient/scanty rainfall. On the other hand, most subdivisions in northeast India received normal/excess rainfall. The excess rainfall was in sub Himalayan West Bengal and Sikkim, Assam, Meghalaya, and Arunachal Pradesh and scanty rainfall was in Saurashtra and Kutch, Gujarat and Madhya Maharashtra.
In September, majority of the subdivisions from northwest India, west central India and east India received deficient/scanty rainfall. On the other hand, most subdivisions from south peninsula and neighbouring central India, and northeast India received normal/excess rainfall. The excess rainfall was in subdivisions of Jammu and Kashmir, Saurashtra and Kutch, Kerala and Andaman & Nicobar Islands.
From the monthly distribution, it can be seen that all subdivisions received deficient/ scantly monthly rainfall during at least one of four months. However, none of the subdivisions were deficient/scanty during all the four months of the season. Except in June, during each of the other three months, at least 14 out of the 36 subdivisions had received deficient/ scanty rainfall. During the peak rainfall months of July and August, 23 subdivisions each had received deficient/scanty rainfall. Saurashtra and Kutch received scanty rainfall during August but received excess rainfall during all the other three months.
Withdrawal of southwest monsoon
Rainfall activity over north-western Rajasthan remained subdued since last week of August. A change in the lower tropospheric circulation pattern over the region from cyclonic to anti- cyclonic resulted in the withdrawal of southwest monsoon from the north-western Rajasthan on September 4. Monsoon withdrew from some more parts of Rajasthan and some parts of Punjab and Haryana on September 9. On September 29, the monsoon withdrew from the remaining parts of Rajasthan, Punjab, Haryana, Chandigarh and Delhi, entire Jammu & Kashmir, Himachal Pradesh, Uttarakhand, most of western Uttar Pradesh and parts of western Madhya Pradesh, Gujarat and north Arabian Sea. On October 6, the monsoon further withdrew from some more parts of Bihar; remaining parts of Madhya Pradesh; parts of Jharkhand, Chhattisgarh, Vidarbha, Madhya Maharashtra; some more parts of Gujarat and the northern Arabian sea.