Indo-Pacific Tripole: An Intrinsic Mode of Tropical Climate Variability

Dake Chen
Lamont-Doherty Earth Observatory


Because of the huge warm water pool that straddles across the western Pacific and the eastern Indian Oceans, the climate variations in these two oceans are closely related and need to be studied as a whole. Similar to the El Niño-Southern Oscillation (ENSO) phenomenon in the tropical Pacific, there is a zonal mode of climate variability in the tropical Indian region. Observations reveal a striking out-of-phase relationship between the zonal sea level gradients in these two oceans and, to a lesser extent, between the corresponding sea surface temperature (SST) gradients. This can be explained in simple terms as follows. The Walker circulation ascends above the warm pool, with easterly surface winds on the Pacific side and westerly on the Indian side, which piles waters up in the warm pool and lowers sea level and SST in the eastern Pacific and western Indian Oceans. This produces a tripole structure with opposite zonal gradients of sea level and SST in the two oceans. When the double-cell Walker circulation weakens or strengthens, these gradients decrease or increase together, and positive feedbacks between the gradients and the Walker circulation may take place. This mode of variability, which we call Indo-Pacific Tripole (IPT), is very robust and can be excited from either Pacific or Indian basin. Here we examine the structure, excitation and evolution of IPT using both data analysis and model results, with emphasis on local ocean-atmosphere interaction as well as inter-basin connection, thus providing a unified framework for the tropical Indo-Pacific climate variability.