Effects of Tropical Land on the Propagation and Intensity of a Boreal Intraseasonal Oscillation Event

The Boreal Summer Intraseasonal Oscillation (BSISO) passes through the Maritime Continent, Indian peninsula and Indochina Peninsula during its northeastward propagation. The effect of these land areas on the BSISO is poorly understood. To address this issue, numerical experiments are performed to simulate an BSISO event observed in the area during the period of August-September, 2020 using the Weather Research& Forecast (WRF) model. Sensitivity experiments are conducted to examine the influence of topography in MC and north peninsulas and land-sea contrast on the propagation and amplitude of BSISO. The WRF control simulation shows low-level wind similar to this in ERA5, as well as a northwest - southeast tilted rain band as observed BSISO, indicating that the control simulation reasonably captures the mean state and propagation characteristics of this BSISO event. Flattening the topography in the sensitivity experiments results in stronger low-level wind and smoother propagation. There is a significant increase in water vapor and a decrease in precipitation around the MC islands. Replacing the islands with ocean eliminates the diurnal cycle of precipitation to a large extent, while column-integrated moisture increases significantly in the MC region. Replacing the lands with ocean significantly increased moisture in the two peninsula regions, but the disappearance of the peninsula makes center of water vapor maximum to move southward. The zonal advection is greatly enhanced without land-sea contrast, strengthening convection over the ocean and promoting propagation of this BSISO event. The results suggest substantial impact of the tropical land on the structure and propagation of the BSISO.