Bi-decadal Changes in Pre-monsoon Precipitation over Northwest Himalayas and its Large-scale Teleconnections

Abstract

Abstract During pre-monsoon season, the Northwest Himalayas region receives significantly larger seasonal proportion of precipitation than the pan Indian average which makes it vulnerable to ramifications of extreme precipitation. Hence, this study investigates the precipitation variations during pre-monsoon over the Northwest Himalayas for the period 1979–2018. It has been observed that, since 1998, the mean baseline for precipitation has shifted downwards, indicating a bi-decadal transition. Notably, since 1998, the seasonal average precipitation has fallen to 50% of pre-1998 levels with the month of March being the dominant contributor towards this overall decline. Consequent analysis revealed that the western disturbances (WDs) are primarily responsible for precipitation in this area during this season as the local atmospheric conditions are not conducive for any other source of moisture and precipitation. However, the strength of zonal moisture transport and WDs aided vorticities are seen to be weakening post 1998 which also interestingly coincides with the strongest El Nino phase on record and the start of a negative PDO cycle. As a result, the role of global teleconnections is investigated, which concluded that negative PDO conditions after 1998 have changed the atmospheric circulation pattern, causing air subsidence (stronger stability) over the study region and weakening the lower-level convergence and the westerly component of moisture transport; and this leads to the observed decline in pre-monsoon precipitation in the recent decades. Following that, the hypothesis was further verified using a threefold process involving a study of the change in circulation patterns, cause-effect cluster analysis and finally by case study comparisons.