Composite Structure of Monsoon Low Pressure Systems and Its Relation to Indian Rainfall

The tropical disturbances formed in the Bay of Bengal and the Arabian Sea and over land points in central India, known as low pressure systems (LPSs), are shown to contribute significantly to the seasonal monsoon rainfall over India. Analyses of daily rainfall over India and statistics of the LPSs for the period of 1901-2003 show that the rainfall pattern when the LPSs are present captures the most dominant daily rainfall pattern that represents the active monsoon phase. The rainfall pattern when the LPSs are absent is similar to the pattern representing the break monsoon phase. The location, number, and duration of the LPSs are found to be closely related to the phases and propagation of the dominant intraseasonal modes of the Indian rainfall. The LPSs are also associated with the strengthening of the monsoon trough and low-level monsoon winds. The number of LPSs and their total duration and the corresponding rainfall during July and August exceed those in June and September. The LPS tracks reach up to northwest India during flood years, whereas they are confined to central India during drought years. However, the contribution of rainfall during the LPSs to the total seasonal rainfall is same during flood or drought years. Although the LPSs seem to play an important role in the monsoon rainfall, they alone may not determine the interannual variability of the seasonal mean monsoon rainfall.