Tropical intraseasonal oscillation at monthly time scale during boreal summer and winter

Abstract

This study describes the space–time structure of an intraseasonal oscillation with the time scale of a month over the South Asian monsoon region and the tropical Pacific for the boreal summer and winter seasons. These nonlinear oscillations were extracted by applying multichannel singular spectrum analysis on daily anomalies of three-dimensional diabatic heating separately during the summer and winter. The monthly oscillations (MOs) are distinct from the leading monsoon intraseasonal oscillation with a period of 45 days during the summer and the Madden–Julian Oscillation during the winter. The summer MO exhibits horizontal quadrupole structure predominantly in the Northern Hemisphere and propagates eastward over the Indian region and westward over the Pacific along with northward movement. The winter MO is confined to the equatorial region in the Southern Hemisphere and propagates eastward and southward. Both the summer and winter MOs consist of propagating deep vertical columns of heating and cooling anomalies. The associated fields of convection, precipitation and three-dimensional circulation show similar patterns and propagation. The corresponding specific humidity has revealed moisture preconditioning for horizontal propagation and co-located vertical advection. However, there is no clear pattern of ocean–atmosphere interaction associated with the MO as seen in the examination of sea surface temperature and latent heat flux.