Role of Westerly Jet in Torrential Rainfall During Monsoon Over Northern Pakistan

Abstract

Pakistan experiences several torrential rainfall events for many years due to interactive role of midlatitude atmospheric and warm moist monsoon circulations. Deadly events such as flash flooding, runoff, and mudslides may trigger by torrential rainfall events which pose significant threat to human assets. This study was undertaken based on three severe weather events (1988, 2010, and 2013) by using percentage departure to identify wet and dry years during the period 1985–2016, which occurred over northern Pakistan. Rainfall data were obtained from 14 synoptic stations of Pakistan Meteorological Department, which were analyzed at daily, monthly, and annual bases. The data sets of Climate Research Unit, Global Precipitation Climatology Centre, and Pakistan Meteorological Department are used to justify the Weather Research and Forecasting model simulation and for analyzing the various atmospheric and diagnostic variables. Results showed teleconnection between large‐scale circulation pattern and South Asian summer monsoon during the selected events. Detailed analysis revealed that during all three events, a westerly trough was moving in the north of Pakistan and well‐marked monsoon circulations were present at the lower levels. It was observed that the interaction of two weather systems triggered heavy rainfall over area of interest. In addition, high vorticity at all levels and extension of westerly trough to the surface resulted in heavy rainfall and the penetration of upper‐level circulations anomalies consists of blocking signature over Eurasia along with Tibetan anticyclone. Hence, the monsoon low‐pressure system provides moisture convergence and upper‐level westerly trough results in upper‐level divergence to enhance the extreme rainfall events.

The influence of natural variability on extreme monsoons in Pakistan

“…This implies that other contributing factors, such as co-occurrences of other modes of variability, compensate for or compound the impact of ENSO. Evidence suggests that jet stream meandering in the Northern Hemisphere may be one of the contributors (10)(11)(12)(13). On average, the subtropical westerly jet lies north of South Asia during the monsoon season (Fig.…”

Climate warming contributes to the record-shattering 2022 Pakistan rainfall

“…The causes of the historical oods over Pakistan have been investigated in the literature [8][9][10][11][12][13][14][15][16][17][18][19] . On the synoptic scale, it is typically the South Asian monsoon low-pressure systems (LPSs) that often trigger deep convection and bring copious rainfall ahead and south of the storm center [8][9][10] .…”

“…A large fraction of LPSs originate near the Bay of Bengal and penetrate deeper inland, where they play a crucial role in determining the amount and distribution of the summer rainfall over South Asia 11,12 . It has also been suggested that the upper-level divergence resulting from the extratropical troughs lying aloft northern Pakistan contributed to the extreme rainfall in 1988, 2010, and 2013 [13][14][15][16] . On the planetary scale, both Pakistan rainfall and its synoptic triggers are in uenced by the large-scale South Asian monsoon circulation, which is further regulated by the sea surface temperature (SST) anomalies over the tropical Paci c and Indian Oceans [e.g., El Niño-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD)] [17][18][19] .…”

Dynamics of Unusual Extreme Rainfall Events Over Different Altitudes Using Richardson Number

“…The synoptic conditions before the torrential rainfall of both cases indicated clear cyclonic circulations at 850 hPa as well as 500 hPa over the areas of heavy rain. These large‐scale circulations present over the study area contribute a lot to enhancing the convection (Bibi et al., 2020; Hassan et al., 2010; Ullah & Gao, 2012, 2013). Further, both events were simulated by using the WRF model to investigate the instability over rainfall areas.…”