Pakistan summer monsoon rainfall consists of a large portion of the local annual total rainfall, and in the recent monsoon seasons, prolonged periods of anomalous rainfall and excessive flooding have appeared in Pakistan. A full understanding of the monsoon rainfall variability is important for the sustainable development of the country. Based on multiple data analyses and the weather research and forecasting model, the potential impact of Tibetan Plateau (TP) heating on the interannual variability of Pakistan monsoon rainfall is investigated. It is observed that a significant negative relationship exists between the thermal forcing over the southeastern TP and Pakistan monsoon rainfall in July–August. Both the data analyses and model sensitivity experiments identify that the TP heating drives a Rossby wave response in the upper atmosphere characterized with an anticyclonic anomaly over the southern TP but a cyclonic anomaly to the north. This dipole pattern of anomalous circulation induces an evident upper-level convergence over Pakistan, corresponding with remarkable vertical sinking motion. Meanwhile, in the lower troposphere, the TP heating causes anomalous westerly wind along the Himalayas over the northern India continent. Such westerly anomaly further induces less water vapor transport into Pakistan from the Bay of Bengal. Therefore, both the dynamic and thermodynamic processes regulated by positive TP heating are not beneficial for the occurrence of monsoon rainfall in Pakistan. This study proposes a new potential mechanism in which TP heating acts as a driver of Pakistan monsoon rainfall variability on interannual time scales.

The climatology of precipitation and drought are analyzed by using different indices in the region of south central Asia (SCA). The spatial precipitation pattern is delineated by using principal component analysis (PCA) over the period of 1951–2010, which identifies six subregions in the SCA. The monthly and annual trends of precipitation were analyzed by applying the five statistical tests: Student’s t, Mann–Kendall, and Spearman’s rho tests for linear trend and turning point analysis and Sen’s slope for randomness and slope magnitude, respectively, at the α = 0.05 significance level. The time series analysis shows data similarity between Global Precipitation Climatology Centre (GPCC) and area-weighted precipitation of 52 meteorological stations in Pakistan, which results in a high correlation (R2 = 0.93). Two main drought periods were identified (1971 and 2000–02); also, 2001 was an extremely dry year in the SCA region. The drought in 1952 was the most severe in Pakistan; the longest drought period was 2000–02. Intense droughts were reported in the whole SCA region when the annual percent of precipitation was below 80%. It is noted that the A-5 region (northeast SCA), where 19 droughts were reported, is the most vulnerable. The monthly precipitation analysis shows a significant increasing trend in the months of September and June in the A-3 (northwest SCA) and A-5 regions, respectively, while a decreasing trend is observed in January and August in the A-4 region (east SCA). The decadal analysis shows significant decreasing trend (−21.5 mm decade−1) in region A-4, while the highest increasing trends (17.1 and 7.5 mm decade−1) are observed in Pakistan and the A-5 region respectively.

Pakistan lies in the region over the globe where effects of climate change due to global warming are more likely to occur. These climatic changes are reflected in significant gradual fluctuations in measurements of meteorological parameters. Agricultural activity in any region depends upon the role of components of hydrological cycle over the region. An important component of hydrological cycle is evapotranspiration (ET), which is closely linked with changes in meteorological parameters. This study investigates impact of probable variation in four important meteorological parameters i.e. temperature (T), net solar radiation (R_n), vapour pressure (e_a) and wind speed (U) on ET to estimate possible changes in reference ET in a humid zone of Pakistan by employing observed climatological records for 30 years (1981–2010). Analyses are done on the basis of possible ±20% changes in climatological mean values of the observed meteorological parameters. The analyses indicate that 20% rise in observed values of T, R_n and U increases value of total (annual) ET demand in the zone up to 11.93%, 16.37% and 2.83% respectively, and 20% rise in the observed values of vapour pressure decreases value of total (annual) ET till −2.5%. Moreover, the analyses also showed that ET is more sensitive to T and R_n in monsoon, vapour pressure in winter, and to wind speed in summer. Hence, this study might be useful in planning and formulating future strategies to meet expected future water requirements due to global warming in humid zone of Pakistan and its surrounding region.