Equatorial Rossby waves on cold surge days and their impact on rainfall

Abstract

This study aims to understand the process that determines the regional rainfall distribution in the equatorial South China Sea (SCS) and its surrounding during the cold surge (CS) days by looking at the different vorticity phases of active equatorial Rossby (R1) waves and the dynamics of the CSs related to the R1 wave. The CS can be thought of as an anticyclonic outflow from midlatitude and behaves like a dispersive group of R1 meridional modes. From the study, it shows the vorticity of the R1 wave arranges itself in such a way that the anticyclonic phase is located to the west and the cyclonic phase is located to the east of the cold CS axis. The CS winds in the equatorial SCS occur early, remain longer, and possess a larger zonal component in the active cyclonic phase compared with the active anticyclonic phase. During the active cyclonic vorticity phase, the rainfall anomalies pattern in the equatorial SCS changes from a “tick‐mark‐like” pattern to a “V‐shape” pattern as a consequence of the westward propagation of the wave. The probability of rainfall increases near the centre of cyclonic vorticity, but for anticyclonic vorticity the probability increases in the southern part of the vorticity centre. There is both increased (decreased) intensity and probability of extreme rainfall events during the active cyclonic (anticyclonic) vorticity phase on CS days. Overall, the rainfall anomalies are largely dominated by changes in the intensity on wet days rather than the number of wet days. These changes indicate the roles of the R1 wave vorticity phase in the rainfall distribution during CS days. The timing of enhancement and suppression of rain will provide additional valuable information to help improve forecast extreme weather events.

Cross‐Equatorial Surges Boost MJO's Southward Detour Over the Maritime Continent

“…Furthermore, the diagnostics developed in this study can be useful for investigating the processes that lead to the moisture and circulation biases in simulating MJO in climate models or operational forecast models (e.g., Li et al., 2017), with an emphasis on the role of extratropical‐tropical interaction through the CES. While our study does not consider other important processes over the MC, such as the diurnal cycle of convection over the islands (Hagos et al., 2016), the Borneo vortex (Chang et al., 2005), or the potential role of more rotational types of convectively coupled equatorial waves (Diong et al., 2023; Latos et al., 2021; Lubis & Jacobi, 2015), further investigations into these processes may also be necessary to fully explain the dynamics of the MJO and CES interaction. These topics can be fruitful directions for future research.…”