Diagnosing the column-integrated moist static energy budget associated with the northward-propagating boreal summer intraseasonal oscillation

Abstract

Column-integrated moist static energy (MSE) budgets associated with the northward-propagating boreal summer intraseasonal oscillation (BSISO) are diagnosed for the Bay of Bangel (BoB) and western North Pacific (WNP) regions. While an active BSISO convection is in phase with a positive column-integrated MSE perturbation, the MSE tendency exhibits a marked north–south asymmetry about the convection, with positive (negative) anomaly to its north (south). An MSE budget diagnosis reveals that the advection of anomalous MSE by the background southwesterly plays a dominant role in causing such an asymmetry and thus promotes the northward propagation, as the anomalous rainband associated with the BSISO has a northwest-southeast tilted structure. The anomalous BSISO wind plays a negative role, because the low-level easterly anomaly to the north of the BSISO convection tends to suppress the surface evaporation through reduced near-surface wind speed, inhibiting the north–south asymmetry. In addition, as maximum background MSE appears over the Asian land region, the advection by the low-level easterly anomaly contributes negatively over the WNP while it has little impact over the BoB. When the BSISO convection is near the equator, a much stronger subsidence anomaly appears to the north than the south due to the equatorial asymmetry of the summer mean state. This leads to a greater MSE tendency north of the equator due to greater anomalous vertical MSE advection, supporting the initiation of the northward propagation.