MONSOON DEPRESSIONS

 

The Indian Meteorology Department's Southwest Monsoon 2005 End-Of-Season Report noted that of eleven low pressure systems that formed over the India during the season, five developed into monsoon depressions. This number is consistent with the average of four to six that are anticipated each year.

 

Most monsoon depressions form to the east of India over the Bay of Bengal and generally last a few days. They are well known for their ability to produce prodigious amounts of precipitation. In fact, monsoon depressions are responsible for the majority of rain that falls during India's summer monsoon. Meteorologists may use signals from several atmospheric variables to identify a monsoon depression. Let's first examine a monsoon depression in the context of sea level pressure.

 

Our subject monsoon depression formed over land in late June and traveled west-northwest until it dissipated over Uttar Pradesh at the end of the month. Uttar Pradesh is located in northern India along the border with Nepal. The series of charts below represent the sea level pressure on first four days of the depression's existence, beginning on June 25, 2005. I should note that lines of equal pressure are referred to as isobars with each line representing a 2 millibar (mb) gradient.

 

Chart of sea level barometric pressure at 12Z for the date appearing on each chart. The early stages of the monsoon depression are seen on June 25. The depression's further development can be identified on the remaining charts. Full chart images may be viewed by clicking on the individual chart. Image provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado.

As is evident on the chart of June 25 (upper left), this monsoon depression (identified by the red arrow) was in the early stages of development and possessed a central barometric pressure of approximately 998mb. On June 26, the central pressure had decreased to between 994mb and 996mb and the barometric pressure over the entire region had dropped slightly.  The depression's pressure remained steady into June 27, however the area enclosed by 998mb isobar expanded considerably. By June 28 (lower right), the area within the 994mb isobar had expanded slightly, while the area within the 998mb isobar remained fairly steady.

 

Chart of sea level pressure at 12Z on June 30, 2005 (full image) shows the monsoon depression in northeastern India. Image provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado.

Infrared satellite image taken on June 30, 2005 showing a monsoon depression in northeastern India (full image). From Meteo France.

 

On June 30, the monsoon depression moved northwest away from the coast and reached Uttar Pradesh where it ultimately dissipated over the next 24 hours. The infrared satellite image from June 30, 2005 presents a very clear picture of the monsoon depression. This monsoon depression's path was hardly unusual; in fact, most depressions travel in a northwesterly direction across India from the coast into the interior. The reason for this fairly predictable path is the existence of the monsoon trough.

 

The monsoon trough is part of the equatorial trough or the ITCZ that encircles the globe in response to maximums in solar heating. The monsoon trough, characterized by relatively low barometric pressure, can be easily identified on the chart (Figure 5.1) of long-term mean sea level pressure during India's summer.

 

Chart of long-term mean sea-level pressure in millibars from 1968-1996 (full image). The monsoon trough is clearly evident by the band of relatively low pressure. Monsoon depressions typically follow the trough across India. Image provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado.

 

Late June's monsoon depression over northeast India delivered considerable rain to the region, and to the city of Balasore in particular (click here for map of India). Balasore is located on India's coast along the northwest corner of the Bay of Bengal. As the annual precipitation chart (Figure 5.2) indicates, Balasore received approximately five inches of rain in a single day from this storm. This would be the maximum rain received in Balasore on any day for the entire year.

 

Precipitation chart for Balasore, India for the year ending October 22, 2005 (full image). The maximum daily precipitation total related to late June's depression is identified. Image provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado.

 

Balasore wasn't the only city that was drenched during late June's monsoon depression. The chart (Figure 5.3) shows the mean daily precipitation rate (in millimeters per day) during the existence of the depression from June 25 to June 30, 2005. The most significant rates, nearly 35 millimeters (shaded in purple) were located over the northwest corner of the Bay of Bengal. An elongated area stretching inland and shaded in yellow approached 20 millimeters per day.

 

The daily mean precipitation rate for the period June 25 through June 30, 2005 in millimeters per day (full image). A monsoon depression existed in this area at the time resulting in higher than normal values. Image provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado.

 

Several other cities in this region, such as Jharsuguda, Jamshedpur and Jabalpur, experienced either the most significant, or one of their most significant, daily rainfall amounts for the entire 2005 summer monsoon during this depression. Below is the annual rainfall chart for each of the above referenced cities with the depression identified.

 

 

Chart of daily precipitation for the year ending October 22, 2005 for selected Indian cities. The timing of the monsoon depression is identified. Image provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado.

 

 

Charts that display anomaly values are useful tools to understand how conditions during a specific period vary from the long-term mean. The chart (Figure 5.4 ) of precipitation anomalies indicates that the precipitation rate for June 25-30, 2005, were 5 to 15 millimeters higher than the mean. Considering the cloudy and rainy conditions that existed during the monsoon depression, it is not surprising that the chart (Figure 5.5) of surface temperatures shows that the temperature was 2-4° C cooler. The most significant temperature variance is identified by the area shaded in purple.

 

Chart of precipitation rate anomalies for June 25-30, 2005 (full image). Higher than normal precipitation rates were observed during this 5 day period. Image provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado.

Chart of surface temperature anomalies for June 25-30, 2005 (full image). Surface temperatures were several degrees (C) below normal due to cloudy and rainy conditions. Image provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado.

 

The final identifier of monsoon depressions that will be discussed is 500mb relative vorticity. 500mb relative vorticity is the measure of the spin of air parcels relative to the ground at a height of approximately 5,000 meters. Positive values of relative vorticity are associated with regions of low pressure. The red bullseye on the chart (Figure 5.6) from June 28, 2005 marks the 500mb center of the monsoon depression. As air parcels move through the region of maximum vorticity, their spin slows resulting in divergence. Divergence at the 500mb level removes mass from the air column causing a drop in surface barometric pressure. In turn, lower pressure at the surface results in convergence promoting rising air parcels in the column. These rising air parcels contribute to the formation of precipitation so prevalent with monsoon depressions.

 
 

Chart of 500mb relative vorticity on June 28, 2005 from Plymouth State University. The red bullseye of high relative vorticity marks the core of the monsoon depression. High relative vorticity helps to maintain low surface pressure.

  

 

Summary

The 2005 Indian summer monsoon was surprisingly average with the total seasonal rainfall 99% of the long-term average. However, rainfall was not equally distributed throughout the country or during the season. The summer monsoon started late and rainfall during early June was below normal. However, precipitation increased dramatically in late June and persisted until the end of July. Rainfall accumulation during August was nearly 30% below average. Fortunately, an active September eliminated the shortfall.

 

Jharkhand, in northeastern India, was the only area that experienced a moderate drought during 2005, with a rainfall deficiency of approximately 35%.