(1)‘Monsoon Depression’ is a synoptic scale tropical disturbance which forms in the monsoon circulation regime. It is an important rain producing system of monsoon period. The Low pressure systems are referred to as depressions when two closed isobars can be drawn on surface synoptic weather charts and winds in the cyclonic circulations are between 17 to 33 knots. Weaker systems with only one closed isobar and wind speeds less than 17 knots are called ‘monsoon lows’.
(2)Most Monsoon depressions form over the warm waters of the Northern parts of Bay of Bengal, within the moist environment of monsoon air. They move in a west to Northwest direction along a track, close to and south of the monsoon trough. Monsoon depressions also form in East Arabiansea and over land in the monsoon trough region, but Bay systems predominate amounting to about 80% of the total. Theaverage number of monsoon depressions which form per month, during the monsoon season (June to September) is 2 to 3.
(3)There is wide scattering in the time interval between the formation of successive monsoon depressions. This interval may be as small as 3 days and may even exceed one month. However, a very large number have formed in the interval ranging 3 to 15 days. Climatologically, life expectancy of monsoon depressions is 2to 5days in the majority of cases. Hardly 10 percent of the systems have existed beyond 6 to 7days. The most frequent separation between co-existing monsoon depressions is 1100 to 2000kms.
(4)Individual Depressions during July and August follow atrack which is quite close to the mean track (which runs in a west to Northwest direction). The scatter of the tracks during June and September is larger. The depressions in these two months recurve in a Northerly to North Easterly direction in many cases.
(5)Generally the 24 hour accumulated rainfall in association with depression is 10-20cm. The left forward sector of the depression is the region of heavy rainfall due to large positive vorticity. A second rain belt often develops outside the depression field to the west,fairly distant from its S.W sector. This is due to convergence between Northwesterly winds and Easterly winds caused by the depression. The zone of maximum rainfall is found to shift to North-West and then to the North-East sector with recurving depressions.
(6)Formation of a depression activates the monsoon and in general, rainfall over the country begins to increase 2-3 days prior to the actual intensification of low pressure system into a depression and this increased activity is maintained for 2-3 days after the depression weakens.
(7)A prolonged spell of dry weather during monsoon (monsoon break) is usually broken by the formation of a monsoon depression.
(8)(a)The intensification of the depression is accompanied with strengthening of lower Tropospheric Westerlies over Peninsular India and lower Tropospheric Easterlies over the GangeticValley. At the most intense stage the Westerlies and Easterlies reach maximum speed 50 to 60kts between 900 to 850 mb.
(b) Strengthening of monsoon current over peninsular India and central Bay of Bengal. This increases cyclonic wind shear in the North Bay.
(c) Increase in the moisture content of the air in the layer 700-800 hpa over North Bay.
(d) The depressions extend in the vertical, generally upto 300 hpa. They have very little tilt up to 500 hpa but tilt markedly to the SouthEast between 500 to 300 mb.
(e) The monsoon depression has a cold core in the lower troposphere upto 600mb and a warm core above that. The temperatures are more in NW quadrant as compared to that of SW quadrantin the lower troposphere.
MONSOON DEPRESSION – SYNOPTIC CONDITIONS.
Synoptic factors which generally precede the formation of monsoon depressions in the head Bay of Bengal are as follows:-
(a)Fall of pressure in North Bay of Bengal without any significant change in other parts of the trough. This fall of pressure is in situ or in association with the movement of a low pressure area from the East (pacific ocean).
(b)Strengthening of the monsoon current over Peninsular India and Central Bay of Bengal to over 40 knots. This increases the cyclonic wind shear in the North Bay.
(c)Formation of initial cyclonic vertex in the layer 800-600 hpa and its gradual descent to the sea level.
(d)Increase in the moisture content of the air in the layer 700 to 800 mb over North Bay of Bengal.
(e)Increase in rainfall activity over the areas around head Bay.
(f)Increased divergence in the upper troposphere over North Bay of
MOMENT OF M.D : The monsoon depressions generally move in a WNW-ly direction .The moment is entirely based on the steering current concept. The synoptic scale systems move along with the wind direction of the level which is just above the circulation level. Thus, moment of the system is guided by a flow in which the system is embedded.
Low Pressure areas (wind speed (17knots) form quite frequently during monsoon season and cause substantial rainfall in the manner of monsoon depressions. These lows have a life of 4 to 6 days and give well distributed rainfall over North India. They are of two types:
(a)Lows that over North-West and adjoining central Bay and move in the manner of monsoon depressions and
(b)Lows which develop on the monsoon trough over land areas of North India and move west-ward.
According to one study the about these lows for a 20 year period is as follows-JULY / AUG / SEPT
No. of Lows that developed over Bay. / 14 / 8 / 14
No. of Lows that developed over the land. / 22 / 28 / 1
(1)There are periods during the monsoon season when there is striking decrease of rainfall over most of India, but increase along the foot hills of Himalayas, parts of North-East India and Southern Peninsula. During these periods the monsoon trough is located close to the Himalayas in foot hillsor there is no monsoon trough at all. Such a synoptic situation is called Break Monsoon. This feature of the interruption of the monsoon rains has been found to be so associated with distinctly identifiable synoptic features. These are seen over surface, in lower, middle and upper troposphere in and around India.
(2) The average duration of a break is about 6 days and very long breaks last about 3 weeks. The 10-day period around mid-August is found to have more breaks than the other 10-day periods in July and August.
(3)‘Break Monsoon is found to be associated with following synoptic features:
(a)The Monsoon trough over North India and Central India at the surface and lower tropospheric levels shifts to the Himalayan foot-hills or not seen at all. All over North India winds are westerlies in the lower Troposphere. (In the normal or active monsoon period winds are Easterlies North of the monsoon trough)
(b) The mean sea level pressure are above normal (+4hpa to +6 hpa) over Central India and below normal along foot hills of Himalayas.
(c)In Lower Troposphere (say upto 700mb) there are strong westerlies over North India. Winds around 850mb reach speeds of 30 to 40 knots. Lower Tropospheric Westerlies are weak over Peninsular India. (In active monsoon condition there is LLJ over Peninsular India).
(d)The Cyclonic vorticity over Central India in normal or active monsoon in the troposphere is replaced by anti-cyclonic vorticity during breaks.
(e)In the middle Troposphere (500hpa), during active monsoon conditions there is an East-West trough over Central parts of India but during breaks we see anti-cyclonic flow and a ridge instead over the area.
(f)In the upper troposphere the Easterly Jet over South Peninsula shifts North-wards.
(g) Some time during break period, STJ shifts to latitudes south of Himalayas from its position North of the Himalayas during the monsoon season .
(h)The setting of breaks at times occurs when a monsoon depression moves to the North carrying the monsoon trough to the Himalayan Foot Hills.