The summertime ‘‘heat’’ low over Pakistan/northwestern India: evolution and origin

A deep low in sea-level pressure is present from May to September over Pakistan and northwestern India (hereafter, the Pak–India low). It is often referred as the ‘‘heat’’ low to convey the significance of surface thermal effects reckoned to be important for its origin. The present analysis, rooted in observations and diagnostic modeling, suggests that the Pak–India low is influenced both by regional and remote forcing. Regionally, the influence of Hindu Kush mountains is found to be stronger than the impact of land-surface heating and attendant sensible heating of the planetary boundary layer, questioning the suitability of the ‘‘heat’’ label in canonical references to this circulation feature. Observational analysis indicates that the notable May-to-June deepening of the Pak–India low and its further deepening in July, however, arises from remote forcing—the development of monsoon deep-convection over the Bay of Bengal and eastern India in June and July. It is hypothesized that the associated upstream descent over Iran–Turkmenistan–Afghanistan (i.e. east of the Caspian Sea) and related low-level northerlies over the Elburz–Zagros–Hindu Kush mountains contribute to the strengthening of the Pak–India low in June (and July) from interaction with regional orography.