Dominant Process for Northward Propagation of Boreal Summer Intraseasonal Oscillation Over the Western North Pacific

Northward propagation of boreal summer intraseasonal oscillation (BSISO) over the Western North Pacific (WNP) has significant impacts on extreme events over Asia and Europe. Here we test hypotheses that northward propagation mechanisms over the WNP may differ from those over the Indian Ocean (IO) by performing numerical experiments with changing mean states through lowering the Tibetan Plateau (TP). Our results suggest that air-sea interaction plays a dominant role in the propagation over the WNP, whereas the mean vertical wind shear mechanism is the major driver over the IO. Lowering TP significantly reduces sea surface temperature (SST) anomalies to the north of BSISO center due to the enhanced surface wind and latent heat flux anomalies. This air-sea interaction reduces upward transport of heat and moisture from surface to lower troposphere, weakening the northward propagation over the WNP. This study implies changes in SST patterns under global warming may influence BSISO propagation.