There has been an increasing trend of dry spells during the peak monsoon months of July and August in recent years. Earlier studies have shown that there has been a weakening of the monsoon current through peninsular India during the past 50 years.According to a paper published in the online issue of IEEE Geo science and Remote Sensing Letters, the frequency of dry spells for a continuous period (break-monsoon spells) and the number of the number of dry days (break-monsoon days) during the entire period (1951-2007) studied showed an increasing trend.Clear increase
Particularly, there was a clear increase in the percentage of long breaks compared to the total number of breaks.
The study conducted by Dr. M.R. Ramesh Kumar of National Institute of Oceanography (NIO), Goa, and his team looked into the trends of dry spells for a continuous period (break-monsoon spells) and the number of dry days (break- monsoon days) during July-August over two epochs — 1951-1976 and 1977-2008.
Dr. M.R. Ramesh Kumar is Scientist and Deputy Director, Physical Oceanography Division, NIO. The study reported a weakening of the moisture transport through peninsular India during the past 50 years, based on the National Centers for Environmental Prediction (NCEP)/ National Center for Atmospheric research (NCAR) reanalysis data.Statistical summary
A decade-wise statistical summary of break-monsoon days and break-spells for July-August months was obtained.
Also, a decade-wise distribution of short (type I — less than or equal to 7 days) and long (type II — greater than 7days) monsoon breaks was compiled.
It was observed that the frequency of break-spells and the number of break-days during the period (1951-2007) showed an increasing trend.
Particularly there was a clear increase in the percentage of long breaks to the total number of breaks.
Under normal conditions, convection over the Bay of Bengal is strong and this results in active monsoon condition in peninsular India.
“But the warming of the ocean, particularly in the eastern equatorial Indian Ocean has led to convection being strong there,” said Dr. Ramesh. “This has led to break in the monsoon condition during the peak monsoon period of July-August.”
July-August period accounts for about 61 per cent of rainfall during the monsoon period.
The shift in the convection from the Bay of Bengal to the eastern Indian Ocean due to one degree rise in ocean temperature has led to the diversion of the moisture carrying winds away from the peninsular India.
Moisture transport vectors(of columns of air from surface upto about 9 km height) show a decreasing trend of about -14.2 kg per metre per second per year during the period 1951 to 2008.
By computing the map of the difference in the moisture carrying wind magnitude and direction (vectors) between the two epochs (pre-mid 1970s and post mid 1970s) a graphical image of moisture transport trend in comparison between the two epochs in the region was obtained.Columns of moisture
The map indicated that a large number of the columns of moisture from the surface to about 10 km height were moving out of the Indian landmass into adjoining seas. This indicates a weakening of monsoon activity in recent years. According to Dr. Kumar, one of the reasons for less rainfall in an area is due to divergence of moisture carrying winds from that area. During the post mid-1970s there was less rainfall over Indian subcontinent due to divergence.
“It could be seen that the eastern equatorial Indian Ocean region had convergence of moisture carrying winds while the Indian landmass by and large has divergence of winds, indicative of transport of moisture away from the region,” he said. Convection increased
Also, the enhancement of convection over the equatorial Indian Ocean can induce anomalous subsidence, as was the case in 2002 — one of the worst drought years.
Anomalous subsidence happens due to a weakening Hadley cell that inhibits monsoon convection activity over the Indian subcontinent.
The sea surface temperature rise during 1951-2008 in the eastern Indian Ocean has resulted in intensifying the near equatorial trough and moisture convergence over the eastern Indian Ocean.
Courtesy: The Hindu
Posts
