SOME NOTES ON THE 2006 MONSOON at INDIA                                     


 The portion of the ITCZ lying over India and nearby countries in summer is known as the monsoon trough. The southeasterly winds that originally blew on the southern side of this trough start to blow more from the south and southwest due to the coriolis force (which turns winds towards the right in the northern hemisphere), bringing moisture in from the Arabian sea over most of the subcontinent. In the case of northeastern India, moisture also comes in from the Bay of Bengal (BOB). 


2006 MONSOON TROUGH: Figure 1a below shows the average position of the monsoon trough during July and August. Notice the trough lies over land, thus reducing the chances of many strong depressions or actual tropical cyclones developing.


              Figure 1a. The mean position of the monsoon trough in July and August from climatology, annotated with red line. Courtesy ESRL.


Now note, in Figure 1b, the difference in the orientation of the monsoon trough during July-August in 2006. Notice the southward dip in the monsoon trough over the northern Bay of Bengal. I believe that this was a major contributor to the unusually high number of monsoon depressions that formed during the 2006 season, since the base of the trough was quite close to the very warm Bay water rather than over the land masses further north that it is usually located over. Also, note in this chart of sea surface temperature anomalies from 06-01 to 10-01 2006 that there was a pool of above normal water temperatures in the northern Bay of Bengal during the 2006 monsoon season. This coincidence of low pressure during July-August 2006 and high theta-e (very unstable) air probably allowed more low pressure areas to form more often over the northern Bay of Bengal than we would normally see. 



      Figure 1b. Mean vector winds during July and August 2006, with the mean position of the Monsoon Trough annotated by the red line. Compare to   figure 1a and note the southward dip over the Bay of Bengal. Courtesy ESRL.




2006 MONSOON DEPRESSIONS:  A critical feature of the Indian monsoon at the east coast is the monsoon depression. These synoptic-scale low pressure areas are similar in size, intensity and appearance to warm core tropical depressions that form into tropical cyclones all over the world, but because they are mainly cold core systems, forming through a combination of primarily horizontal temperature differences (cold core) and secondarily from surface heat and moisture energy fluxes (warm core), they are more similar in structure to subtropical cyclones. Monsoon depressions also have closed height centers up to about 400mb, and a signature vorticity maximum (vort max) at 500mb. These depressions usually form over the Bay of Bengal and then move westward along the monsoon trough, often originating from low pressure areas within the monsoon trough that get an energizing boost from the divergence of an upper level 500mb shortwave trough or even the right entrance or left exit region of a jet streak. They also can develop from tropical lows such as decaying typhoons that have migrated overland from the western Pacific. Although they can form into a tropical cyclone, it is a rare event, since they don't have a very large expanse of water to travel over before they hit the coast of eastern India. But one did form in the 2006 season, crossing Orissa in early July, and is shown in Figure 2 on the right (more). An average of four to six monsoon depressions form over the Bay of Bengal each season, with one or two forming over the Arabian Sea or inland as well. It is also important to note that because these depressions usually exhibit a tilt in 500mb heights towards the south and southwest in the upper levels, their heaviest rainfall usually falls to the south and southwest of the center, where the colder upper air will encourage stronger convection.




Figure 2. Tropical Cyclone 03B, which formed east of Orissa and moved inland on July 2nd, 2006. Courtesy MODIS TERRA.


  I have included figure 2 below, which contains an excerpt from the 2006 Monsoon season summary issued by the India Meteorological Department (IMD):


The season as a whole had been quite active in terms of the number of low pressure systems. In all, 16 systems (1 severe cyclonic storm, 8 depressions/ deep depressions and 7 low pressure areas/ well marked low pressure areas) formed during the season. All the systems formed over the Bay of Bengal except one land depression and one severe cyclonic storm over Arabian Sea. The systems formed over the Bay of Bengal generally had a west-northwesterly track causing heavy rainfall over central India, especially over Orissa, West Madhya Pradesh, Maharashtra, Gujarat, and West Rajasthan. 

          The only low pressure area in June which formed over the North Bay and adjoining Gangetic West Bengal (68 June) was short lived and dissipated over Jharkhand and neighborhood. In July, one depression, 3 low pressure areas and one well marked low pressure area formed. All these systems moved west-northwestwards, except one which moved northwestwards. In August, one deep depression, 3 depressions and one low pressure area formed. All of them formed over the north Bay and crossed Orissa coast. They also had long tracks mostly in a westerly/west-northwesterly direction across central India and moved up to west Rajasthan as remnants. In September, one severe cyclonic storm formed over the Arabian Sea. It dissipated over the Sea itself due to large vertical wind shear and cold air advection. In addition, 3 depressions, including one land depression and one low pressure area formed. The last depression of the season formed over the Bay of Bengal in the afternoon of 28 September and crossed Orissa coast close to Gopalpur on 29 evening. It then moved westwards and weakened gradually.

                          Figure 2. Excerpt from the IMD's 2006 Monsoon Season Summary. Courtesy India Meteorological Department.


  So 14 of the season's 16 organized low pressure systems moved over Orissa, including 7 depressions. Note that four depressions crossed the Orissa coast in August alone, with 2 crossing the region in the one-week period we covered, when the average for the entire season is 4-6. It's easy to see why this season, and the very wet period in August in particular, ran high rainfall surpluses at Bhubaneshwar and Orissa state. Finally, once again, it appears at this point that the major reason for Orissa's much above normal rainfall was the aberrant orientation of the monsoon trough this season, which allowed a large number of monsoon depressions to form over the northern Bay of Bengal and then move in directly over Orissa state. It is important to mention that this pattern continued unabated all the way to the end of the season in late October.