Suppressed surface heating over north-western parts of the country and the ensuing monsoon rainfall - A report by India Meteorological Department

During the year 2011, the temperatures have been below normal over north and northwestern parts of India due to the enhanced activity of western disturbances. These conditions had raised many apprehensions about the possible impact on the subsequent monsoon, if any.

In this context, this is an attempt to understand the probabilities of less surface heating over northwest India (due to the higher frequency of western disturbances during April 2011) influencing the southwest monsoon rainfall and its extent.

The report covers the following broad topics -

• The monsoon mechanism - The pre-monsoon thermal field over the Indian landmass has an important bearing on the land-sea heating contrast in the region, consequently influencing the establishment, advance and overall performance of the Indian Summer Monsoon Rainfall. Its further sustanance is provided by the latent heat release due to large scale organised convective activity. A schematic representation of monsoon flow is given in the report.
• Factors influencing the heating up of mainland - With the progress of the hot weather (pre-monsoon) season, a ‘heat low’ starts developing at the surface and in the lower tropospheric levels over central parts of the country. It gradually shifts northwards. However, this normal pattern might show variations under some situations such as widespread precipitation and clouding associated with a series of active western disturbances moving eastwards across the region offsetting the influence of intense solar heating as has been noticed during April 2011.
• About the western disturbances - Western disturbances are eastward moving upper air troughs in the sub-tropical westerlies and are perceived to be the secondaries of extra tropical depressions which move northeastward from the eastern Mediterranean Sea and are confined within the latitudinal belt 25 - 35^oN. These systems influence the weather over the country all through the year, even though the climatological frequency of western disturbances is slightly lower during the pre-monsoon season as compared to that during the winter season.
• Temperature situation over nortwest India so far during the current pre-monsoon season as compared to past 30 year period - There has not been any significant incidence of heat wave conditions in the summer season so far. Lower frequency of heat wave occurrences over the meteorological sub-divisions of nortwest India were observed in April 2011. However, it is not so during the past 30 year period.
• Results of past analysis relevant to the current investigation – The report reviews the findings of several papers that have investigated the lag-relationships between the surface parameters and monsoon rainfall.
• Mean surface temperature anomaly over northwest India and monsoon rainfall for the period 1980-2010 - Analysis of the mean surface temperature anomaly over northwest India and All India/Northwest India Summer Monsoon Rainfall for the period 1980 – 2010 indicate that there is no one to one correspondence between the pre-monsoon heating over the northwestern parts and the ensuing rainfall of the southwest monsoon season during the study period. It may be possible that the relationship established by the previous studies mentioned above does not hold good after the 1980’s.
• 2008 - A recent year with similar cold anomalies - A recent year (2008) also witnessed more than normal western disturbance activity during the pre-monsoon season and the report discusses this in details.

The study concludes that -

• There has been enhanced western disturbance activity n terms of the number of weather producing western disturbances over the nortwestern and northern parts of the country, during April 2011.
• However, the lesser heating up of the region is not un-precedented as there had been similar weather and resultant temperature scenario in the past, such as during the years 1983, 1985, 1986,1991, 1997 and 2008 during the span of past 30 years.
• No one-to-one relationship could be observed between surface heating of nortwest India during the pre-monsoon period and the subsequent monsoon rainfall.

Download the report here -