Understanding the Subseasonal Modulation of Moisture Transport over the Indian Monsoon Domain

The subseasonal modes of integrated water vapor transport (IVT) over the Indian Summer Monsoon (ISM) domain were examined and their association with different modes of ISM precipitation was analyzed during boreal summer seasons from 1979-2018. The IVT over the monsoon domain was found to exhibit significant variability in the intraseasonal (20-60 days), quasi-biweekly (10-20 days), and synoptic (3-10 days) time scales. The intraseasonal IVT mode is dominant between 0-20°N and reflects the fluctuations of the low-level jet stream. The quasi-biweekly and synoptic-scale IVT variability dominates over the Bay of Bengal and the Indo-Gangetic plain. The intraseasonal IVT mode is the most dominant and it is found to influence the higher frequency subseasonal IVT modes. Meanwhile, large-scale factors such as the El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) were found to modulate the intraseasonal IVT mode and negatively impact the monsoon. Lead-lag correlation analysis between the subseasonal precipitation and IVT modes suggests that the IVT anomalies are driven by the subseasonal convective anomalies and associated changes in atmospheric circulation. Since moisture supply from adjoining oceanic regions is fundamental for monsoon precipitation, there is a general tendency to attribute the variability/trends in precipitation to changes in moisture transport. Our analysis of the subseasonal modes of IVT indicates that such inferences may be misrepresentative, as the monsoon diabatic heating in itself is a strong driver of monsoon circulation and moisture transport.