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