Individual and Combined Impacts of Boreal Summer Intraseasonal Oscillation and ENSO on Tropical Western North Pacific Synoptic-Scale Disturbance Intensity

Abstract

Synoptic-scale disturbances (SSDs) over tropical western North Pacific (TWNP) are subjected to impacts of boreal summer intraseasonal oscillation (BSISO) and El Niño–Southern Oscillation (ENSO). This study investigates individual and combined impacts of BSISO and ENSO on the SSD intensity over TWNP. It is found that the impact of the BSISO wet phase on the SSD intensity is pronouncedly different in El Niño and La Niña years, whereas the SSD intensity in the BSISO dry phase is relatively less sensitive to ENSO. El Niño and La Niña induce different SSD intensity changes in the BSISO wet phase. El Niño leads to an increase and eastward shift of large SSD intensity. This is attributed to the increase and eastward shift of cyclonic lower-level wind fields and consequently the barotropic energy conversion from background lower-level wind fields to SSDs. La Niña leads to an increase and northward shift of large SSD intensity. This results from a shift of cyclonic lower-level wind fields and associated barotropic energy conversion from background lower-level wind fields to SSDs to northwestern TWNP. In the BSISO dry phase, the background lower-level wind fields are anticyclonic and the ENSO effects are small due to unfavorable background field. This accounts for the relatively small ENSO effect on the SSD intensity in the BSISO dry phase.