Interhemispheric influence of Indo-Pacific convection oscillation on Southern Hemisphere rainfall through southward propagation of Rossby waves

Abstract

Tropical Indo-western Pacific convection anomalies influence atmospheric circulation, impacting climate far beyond the tropics. Here we present observational and modelling evidence for an interhemispheric effect of the boreal summer (June–August) Indo-Pacific convection oscillation (IPCO), which is characterized by a zonal seesaw pattern between the north Indian Ocean (NIO) and the western North Pacific (WNP). It is found that the IPCO is significantly correlated with simultaneous rainfall over many parts of the Southern Hemisphere (SH), including western Australia and western Brazil. These interhemispheric connections in rainfall remain significant when then El Nino–Southern Oscillation, the Indian Ocean Dipole, and the SH Annular Mode related signals are excluded simultaneously, which may help to improve the SH seasonal predictions. The physical mechanism that underlies this interhemispheric connection is investigated using observations, Rossby wave diagnostics and a series of atmospheric model experiments. Results suggest that convective heating anomalies associated with the IPCO excite two distinct southward-propagating equivalent barotropic wave trains that propagate into the extratropics in the SH: the South Africa–mid-latitudes wave train excited by the heat sink over the NIO, and the Maritime Continent–subtropical Australia wave train excited by the heat source over the WNP. We further demonstrate that upper tropospheric northerlies over tropical Africa and the Indian Ocean are responsible for the southward propagation of stationary Rossby waves across the easterlies. These equivalent barotropic waves then modulate the moisture transport, baroclinicity, and mid-latitude storminess, and thus affect rainfall anomalies in the SH. The new wave train patterns provide novel insight into the teleconnection pathways of convective heating over the tropical Indian Ocean and western Pacific.