The dependence of Indian winter precipitation extreme on the North Atlantic Oscillation

Teleconnection patterns that span long spatial distances influence extreme rainfall events over a region. Therefore, comprehending  these atmospheric patterns and their underlying mechanism is crucial for weather forecasting and for predicting extreme rainfall events. The study of extreme precipitation events over north India, during the winter season, is crucial, as the region’s agriculture heavily depends on winter precipitation. The present study analyses the extreme rainfall days over north India during the winter season, and its association between different phases of North Atlantic Oscillation (NAO). Daily NAO index data from 1950 to 2020 are used in this study to determine the number of positive/negative NAO days. Ninety-five percentile thresholds of precipitation criteria are used to identify extreme rainfall events over states of Punjab, Himachal Pradesh, and Uttarakhand. India Meteorological Department gridded rainfall data at 0.25° resolution is used in this study. Although the 95-percentile threshold is highest over Himachal Pradesh (HP), the number of extreme rainfall days in HP is less than Uttarakhand and Punjab during the study period. A significant positive correlation is found between the number of positive NAO days and the number of extreme rainfall days for all these states in winter season. Further, the frequency of extreme rainfall events during positive NAO phase is almost twice than the negative NAO phase over all these states. The meteorological conditions during extreme rainfall events are analysed separately for both, positive and negative NAO days. It is noted that the subtropical westerly jet streams and Rossby waves play a significant role in the precipitation associated with extreme rainfall events. The path of westerlies and Rossby waves is different in two phases of NAO. Added moisture transport from the Arabian Sea raises the likelihood of extreme precipitation days during the positive NAO phase. During the positive NAO phase, the atmospheric conditions are more favourable for many extreme rainfall days.