Low-Frequency Modes of Vorticity and Divergence in Monsoon Intraseasonal
Oscillation
Abstract
The low-frequency intraseasonal oscillations (ISOs) dominate the
subseasonal variability of the Indian summer monsoon and involve
rainbands propagating northward from the equator. These oscillations
modulate the active–break cycle of the monsoons and have two distinct
regimes, one where rainfall maximum is located near the equator and the
second one in which it is located around central India. The interaction
of vorticity and divergence is an important feature for the northward
propagation of ISOs. With a correlation study, we show that the
low-frequency modes in vorticity and divergence are coupled in the
boundary layer and upper troposphere. We use multichannel singular
spectrum analysis to extract the low-frequency oscillatory modes in
vorticity and divergence. The examination of the spatiotemporal
structure of these modes reveals that vorticity has coherent northward
propagation at all pressure levels. In contrast, divergence only shows
propagation in the boundary layer and the upper troposphere. The
vorticity low-frequency mode has a near barotropic structure, while the
divergence mode has a baroclinic structure. We point out the contrasting
features of vorticity and divergence for the two regimes of the ISO. The
barotropic vorticity leads the rainfall for the equatorial region,
whereas, for the central Indian region, the vorticity lags the rainfall.
Meanwhile, the rainfall is in phase with baroclinic divergence for both
regimes. These findings are relevant to understanding the initiation and
propagation of ISOs and can contribute to the further development of
simple models of these phenomena.