Early summer surface air temperature variability over Pakistan and the role of El Niño–Southern Oscillation teleconnections

Abstract

Early summer (May–June) is the season where the surface air temperature (SAT) variability is largest and may result in extreme temperature conditions over Pakistan. Therefore, we analysed the early summer interannual SAT variability over Pakistan for the period 1981–2018 using observational dataset and model simulations. We noted that upper‐level anticyclonic circulation anomalies over Pakistan, which are associated with upper to the middle tropospheric descending motion, favour clear skies with an increase in net shortwave radiation that leads to extreme surface warming over the region. Moreover, the El Niño–Southern Oscillation (ENSO) based on Niño3.4 SST index is negatively associated with SAT anomalies over Pakistan. The cold ENSO phase favours the positive geopotential height anomalies due to the strengthening of the Walker circulation that enhance the sinking motion and result in less cloudiness leading to the extreme higher surface temperature conditions over Pakistan, while the opposite happens in the warm ENSO phase. Moreover, the Saudi‐KAU Atmospheric Global Climate Model (AGCM) simulated large‐scale patterns that are in good agreement with the observations. Sensitivity experiments with the AGCM confirm that cold SST anomalies in the ENSO region significantly favour above‐normal SAT anomalies over Pakistan, while the opposite happens in the warm ENSO phase. These results are important to understand and potentially predict regional heatwaves over the South Asian region, particularly over Pakistan.

Predictability of the Early Summer Surface Air Temperature over Western South Asia

“…3b), similar to observations (Fig. 3a), and this is consistent with earlier findings (Rashid et al 2022). A significant SST anomaly signal also appears over the western Indian Ocean and tropical Atlantic region but in this study, we focus on ENSO forced predictability for the regional temperature.…”

“…Moreover, ERA5 SAT reasonably well compares with the observational SAT from CRU (Correlation Coefficient; CC=0.94) and CPC (CC=0.94). This is consistent with earlier findings where it was shown that ERA5 reanalysis performs well compared to the other reanalysis over the region (Arshad et al 2021;Rashid et al 2022). We adopted European Centre for Medium-Range Weather Forecasts (ECWMF) fifth generation prediction system (ECMWF-SEAS5) reforecast dataset (Johnson et al 2019) to analyse the early summer season (May-June; MJ) SAT prediction skill over WSA using the April initial condition (Lead-1 season).…”

“…The region has a semiarid to arid climate that also includes the western half of the high mountains of South Asia with many glaciers, which are the freshwater and hydropower energy sources for the downstream regions (Chaturvedi et al 2014;Pritchard 2019). Annually, the temperature peaks over the WSA region during the spring and early summer (May-June) seasons (Rashid et al 2022), also known as the pre-monsoon season (Latif and Syed 2016). Any anomalous temperature variability over the WSA may strongly impact energy demands, agriculture resources such as wheat crops, and regional water availability through glacier melting that may lead to river flooding (Mannig et al 2018;Zhongming et al 2021).…”

“…These upper-level positive 200-hPa geopotential height anomalies leads to middle tropospheric warming, low cloud cover, and increased surface shortwave radiation, and thus favours surface warming over the region (Saeed et al 2013;Rashid et al 2022). During the early summer (May-June; MJ) season, ENSO in its negative phase reinforces the positive geopotential height anomalies through the strengthening of the upper-level divergence over the western Pacific, favours the upper-level maxima over the WSA region, that enhances the sinking motion and favours the clear sky condition leading to the extremely higher surface temperature over WSA region, including Pakistan, while the opposite happens in the warm ENSO phase (Rashid et al 2022). But how well models predict the surface temperatures and its connections with the upper-level circulation anomalies and ENSO has yet to be known, which is discussed in the current study.…”

Modes of summer temperature intraseasonal oscillations and heatwaves over the Indian region

“…An additional mechanism for the south-eastward propagation was attributed to the upper-tropospheric Rossby wave forcing (Zhao et al 2017). Some studies have also reported modes of interannual variability of temperature over Pakistan (Rashid et al 2022). The interannual variability of surface air temperature in Pakistan during early summer is associated with upper-level divergence and associated sinking motion in the atmosphere over South Asia which are modulated by the strengthening of Hadley and Walker circulation.…”