An Investigation of Reduced Western Disturbance Activity Over Northwest India in November - December 2015 Compared to 2014-A Case Study

摘要

In November-December of 2015, Northwestern India received very low precipitation due to anomalously low Western Disturbances (WDs) activity. The resulting lack of sufficient precipitation and soil moisture hampered the growth of winter crops leading to significant agricultural losses. Relatively stable weather in the absence of precipitation and WDs contributed to extremely high air pollution in New Delhi and also significantly degraded the air quality in many cities of Northwestern India leading to severe health issues. Despite the fact that WDs play a very important role in India's winter weather, limited research has been done to investigate the causes of their inter-annual variability. A case study using NCEP/ NCAR Reanalysis, CMAP precipitation and NOAA Extended Reconstructed Sea Surface Temperature data is evaluated in this paper to better understand the atmospheric drivers of WDs in order to help fill the gap in knowledge. Results show that elevated Sea Surface Temperatures over the North Indian Ocean likely lead to atmospheric circulation anomalies that led to branching and weakening of the subtropical jet stream and weakening of vertical wind shear over Northwestern India. These conditions created an unfavorable environment for the propagation of WDs. However, there was an intensification of vertical wind shear over mid-latitude Eurasia along with increased storm activity. This weakened the Eurasian anticyclone resulting in warmer surface air temperatures over the midlatitudes that led to a redistribution of the meridional temperature gradient.