The heavy downpour during the winter chill has been attributed to western disturbances coupled with changes in climate owing to global warming. Post-monsoon rain, winter rain in particular, has been witnessing a gradual increase over the decades. This December, it beat the earlier record of winter rain registered in the city on December 22, 1942.
Scientists at the Indian Institute of Tropical Meteorology (IITM) in Pashan, Pune, in a research paper published in Climate Dynamics last May, had dwelt upon the increase in frequency of winter precipitation (snow or rain) owing to the western disturbances (WDs). The international journal publishes high-quality research on changes in the global climate system.
The WDs that bring snowfall in some northern regions of India, had an impact on the overall climate of Maharashtra and other regions in December. Several parts of Maharashtra recorded heavy rain and Pune city recorded the highest December rainfall of 53.1 mm in 24 hours exceeding the monthly average of 4.8 mm. On December 22, 1942, the highest previous winter rainfall was recorded to be 42.4 mm in 24 hours. Post that, from December 18-22, the all-India weather bulletin had been mentioning the impact of WDs in local weather conditions, in weather bulletins.
Scientists at IITM’s Centre for Climate Change and Research R K Madhura, R Krishnan, Jayashree Revadekar, M Mujumdar and B N Goswami published their paper, ‘Changes in western disturbances over western Himalayas in a warming environment’ which talks of the increasing frequency of WDs – a low pressure system originating over the eastern Mediterranean sea and moves eastward. This causes rainfall in Iran, Pakistan and India and snowfall in some parts of India, according to the scientists.
From January to February, WDs bring heavy bursts of rain and snow. “The low-pressure cyclonic systems are carried towards India by subtropical westerlies, which are prevailing winds. The winds stopped by the Himalayas are unable to proceed further and lead to significant precipitation over southern Himalayas,” says Revadekar.
The scientists stated that weather in winter is mostly cool, dry and pleasant over south Pakistan northern and central India, and south peninsular India and Bangladesh. It is occasionally disturbed by passing western disturbances associated with clouds and rainfall followed by cold waves and fog.
Trying to figure out the increasing frequency of heavy precipitation over western Himalayas, the scientists said their analysis suggested that pronounced warming over the Tibetan plateau in recent decades due to elevation dependency of climatic warming signal favoured enhancement of meridianal temperature gradients.
The studies indicate that the observed pattern of mid-tropospheric warming in recent decades over west central Asia led to an increase in instability of the western winds thereby increasing WDs leading to a higher propensity for heavy precipitation over the western Himalayas.
Winter and early spring precipitation over western Himalayas primarily from WDs is vital to replenishment of water resources, winter crops, and flora and fauna of the region. While surface temperature show a significant warming over western Himalayas in the last few decades, the observed regional precipitation changes are irregular and not spatially coherent.
Though the study focussed on WDs and precipitation from December to April, there can at times be considerable rain in summer as well, particularly over northwest India.