On the weakening of northward propagation of intraseasonal oscillations during positive Indian Ocean Dipole events

Abstract

The northward propagation of intraseasonal oscillations (ISO) is one of the major modes of variability in the tropics during boreal summers, associated with active and break spells of monsoon rainfall over the Indian region. These northward propagations modulate the Indian summer monsoon rainfall. The northward march starts close to the equator over warm waters of the Indian Ocean and continues till the foothills of the Himalayas. In this study, we investigate the influence of Indian Ocean Dipole (IOD) on northward propagations. We show that northward propagations tend to be weaker during positive Indian Ocean Dipole (pIOD) events. The “moisture mode” framework is used to understand the processes responsible for the weakening of northward propagations during pIOD years. Our analyses show that moistening caused by the horizontal advection is the major contributor for the northward propagations during negative IOD (nIOD) years, and its amplitude is much smaller during pIOD years. Further analyses revealed that the reduction in the advection of the background entropy/moisture by zonal wind perturbations during pIOD is primarily responsible for the reduction in the horizontal advection. The mean structure of entropy between 925 and 500 hpa levels remained similar over most of the Asian monsoon region across the contrasting IOD years, and the reason for weaker northward propagations can be attributed to the weaker zonal wind perturbations at intraseasonal timescales. These weaker zonal wind perturbations during ISO events in pIOD years results from weak Rossby Vortex lobes. The weakening of Rossby wave response owing to cooler than average sea surface temperatures in the South-East Equatorial Indian Ocean and warmer than average West Equatorial Indian Ocean is proposed to be the possible reason for the weakening of northward propagations during pIOD years.