Rainfall in summer in Pakistan is extremely scarce, and more than three quarters of the country experiences annual precipitation of less than 250 mm. On the other hand, agriculture, for which summer rainfall is particularly important, contributes greatly to Pakistan's GDP. Indeed, those working in the agricultural sector accounts for around 66.5% of the total population in Pakistan. Therefore, Pakistan is particularly sensitive to rainfall variability.

Previous studies have recognized the importance of large-scale atmospheric circulation systems in both the lower and upper troposphere for synoptic-scale rainfall disturbances in Pakistan. Summer rainfall in Pakistan features strong year-to-year variability, which inspires many to try to understand why.

Numerous studies have demonstrated that the interannual summer rainfall variability of South Asia is closely related to the upper-tropospheric anticyclonic/cyclonic anomalies over West Asia or the lower-tropospheric summer monsoon trough.

According to a study recently published in Advances in Atmospheric Sciences, the interannual variability of summer rainfall in Pakistan is a result of these two vertically coupled processes. "This interaction may also be manifested as a tropical-extratropical interaction, which is characterized by the interaction between the tropical monsoon troughs and extratropical disturbances," said Prof. LU Riyu, the lead author of the study.

Flash flooding in Neelam valley after heavy rain on 19 July 2015 (Image by Saadia HINA) 

Prof. LU and his team at the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences found more summer rainfall in Pakistan to be significantly associated with an upper-tropospheric anticyclonic anomaly to the northwest of Pakistan and lower-tropospheric easterly anomalies along the foothills of the Himalayas, including Pakistan.

On the other hand, the upper- and lower-tropospheric anomalies also show separate features. The upper-tropospheric anticyclonic anomaly is closely related to the Silk Road Pattern (SRP), which is a teleconnection pattern along the upper-tropospheric Asian jet, and the easterly anomalies along the foothills of the Himalayas, but not to the lower-tropospheric cyclonic anomaly to the south of Pakistan. By contrast, the lower-tropospheric easterly anomalies are closely related to the cyclonic anomaly to the south of Pakistan, but do not correspond well to the SRP.

"Therefore, particular attention should be paid to different physical mechanisms responsible for these upper- and lower-tropospheric circulation anomalies when investigating the interannual variation of rainfall in Pakistan," said LU.