Wet spells over the core monsoon domain of northern Pakistan during the summer season

Flooding events or wet spells have increased over Pakistan in recent years. However, a long-term classification of large-scale and synoptic-scale configuration for these events has been lacking. In this study, a total of 53 wet spells during the period of 1951–2015 over the core monsoon domain of northern Pakistan are identified using rainfall data. Based on daily geopotential height fields from NCEP/NCAR re-analysis, the dominant synoptic-scale systems, displaying distinct low-level circulation and moisture transport, are found during these wet spells over Pakistan. They are categorized as trough with low pressure system (LPS, 30 cases), trough without LPS (19 cases) and LPS only (4 cases) wet spells. Without the accompanying LPS over India, the trough tends to be deep and intrudes to southern Pakistan with moisture transport mainly from the Arabian Sea. In contrast, the trough is relatively shallow and interacts with presence of the LPS to steer moisture from the Bay of Bengal towards Pakistan. We found that subtropical trough is an essential ingredient of wet spells over Pakistan. This is different from wet spells over the core monsoon domain of India, which is mainly dominated by LPS. The ridge development over Siberia is a precursor to wet spells over Pakistan and provides guidance for prediction.