Diversity of the Boreal Summer Intraseasonal Oscillation

Abstract

Boreal summer intraseasonal oscillation (BSISO) profoundly impacts Northern Hemisphere monsoon onsets and breaks, tropical cyclones, and many climate extremes. BSISO exhibits more complex propagation patterns than the dominant eastward propagation of the Madden-Julian Oscillation. Previous studies have extensively examined the dominant northeastward propagating BSISO mode and its northward component, but this mode only accounts for about half of the total cases. We conducted an objective cluster analysis of the two-dimensional BSISO propagation and revealed two new forms of BSISO propagation besides the northeastward propagation: the dipolar northward propagation and the eastward expansion. We investigate processes governing the different propagation forms using moisture tendency analysis. We show that the propagation diversity is related to BSISO’s circulation structural asymmetries and the associated moistening processes. The Rossby-wave component in the background zonal wind shear favors northward propagation while the Kelvin-wave component favors eastward propagation. The circulation differences are affected by the variation of background states, especially those season-dependent variations. The results provide insights into the BSISO diversity and potential precursors for foreseeing BSISO propagation.