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Low-Frequency Modes of Vorticity and Divergence in Monsoon Intraseasonal Oscillation
  • Rajat Masiwal,
  • Ashwin K Seshadri
Rajat Masiwal
Indian Institute of Science Bangalore

Corresponding Author:rajatmasiwal@iisc.ac.in

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Ashwin K Seshadri
Indian Institute of Science
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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.