Investigating bi-decadal precipitation changes over the Northwest Himalayas during the pre-monsoon: role of Pacific decadal oscillations

During the pre-monsoon season, the Northwest Himalayas region receives a significantly larger proportion of precipitation than the pan Indian average. This heightened precipitation makes the region vulnerable to the repercussions of extreme weather events. Therefore, this study delves into the variations in precipitation during the pre-monsoon period over the Northwest Himalayas spanning from 1979 to 2018. It has been observed that since 1998, the mean baseline for precipitation has shifted downward, signifying a bi-decadal transition. Notably, the seasonal average precipitation has dropped to 50% of the levels recorded before 1998, with March emerging as the primary contributor to this overall decline. Subsequent analysis revealed that western disturbances (WDs) predominantly account for the precipitation in this region during this season. This dominance arises due to the lack of conducive local atmospheric conditions for any other significant 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 Pacific decadal oscillation (PDO) cycle. As a result, the role of global teleconnections is investigated, which concluded that negative PDO conditions after 1998 are responsible for changes in the atmospheric circulation pattern. Such changes resulted in air subsidence over the study region which increased the atmospheric stability while weakening the lower-level convergence and the westerly component of moisture transport. Consequently, the pre-monsoon precipitation has experienced a steep decline in the recent decades. To further examine this hypothesis, a threefold process is employed, which encompasses the study of changes in circulation patterns, cause-effect cluster analysis, and case study comparisons.