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.

In this study we have examined the modulation of convectively coupled Kelvin waves (CCKW) by different Madden-Julian oscillation (MJO) states over the Indian, Pacific and Atlantic Ocean domains. Convectively active CCKW events associated with the MJO convectively active, convection suppressed and weak amplitude states were derived using wavenumber-frequency filtered outgoing long wave radiation (OLR) indices over the three domains. Composite analysis of CCKW events during different MJO states indicate that the amplitude and phase speed of CCKW are modulated by the MJO state. The amplitude of CCKW are stronger (weaker) and it propagates slower(faster) and more (less) eastward when the MJO amplitude is strong. The phase speed of CCKW is much slower over the Indian Ocean domain, while it propagates relatively faster over the Atlantic Ocean domain. It is hypothesized that the observed difference in CCKW phase speeds is related to the Gross Moist Stability (GMS). The clear linear relationship observed between the GMS and CCKW phase speeds over the different domains, during different MJO states and the observed differences in CCKW vertical structures support this hypothesis. It is found that the CCKW exhibits a baroclinic vertical structure over the Indian and Pacific Ocean domains and a barotropic vertical structure over the Atlantic Ocean. Planetary-scale convection associated with the MJO reduces the static stability allowing for baroclinic modes to prevail, which in turn reduces the GMS and the effective equivalent depth, eventually slowing down the CCKW phase propagation. The results suggest that CCKW may be treated as a mixed-moisture mode.