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.

The regional characteristics of the boreal summer intraseasonal oscillations (BSISO) over southeast Asia are presented. The northeastward transition of the BSISO is characterised by 4 phases, such that convection is enhanced over the Philippines and Indochina in phase 1 and suppressed over Peninsular Malaysia, Borneo and Java. The opposite is true in phase 3. The role of BSISO in modulating extreme precipitation is highlighted, showcasing how its phases impact both the frequency and intensity of extreme precipitation events across the region. Using a method to detect and characterise precipitation features in terms of precipitating areas and their associated object properties, this study shows distinct shifts in precipitation regimes during different phases of the BSISO. Phase 1 exhibits increased large-scale convective activity, particularly affecting regions like the South China Sea and northern Philippines, linked to increased tropical storm frequency but reduced localised extreme precipitation events. In contrast, Phase 3, with active convection over Peninsular Malaysia, Borneo and Sumatra, shows intensified extreme precipitation from smaller to medium-sized areas. BSISO phases also modify the distribution of small and large precipitation objects over land, ocean, and coastal regions. This classification of precipitation regimes provides detailed insights into how the BSISO’s large-scale envelope modifies regional precipitation extremes through various precipitation properties. This information could benefit probabilistic predictions of regional extreme precipitation events at subseasonal time scales.

The Borneo vortex (BV) is a synoptic‐scale vorticity feature found in the South China Sea near Borneo during extended Boreal winter, which can bring heavy rain to the region. Predicting this rainfall is difficult. Therefore, a better understanding of the structure of these vortices and their interaction with equatorial waves could aid forecasters. Here we divide the BVs found from 41‐years of October–March ERA5 data into five clusters based on their tracks identified using relative vorticity maxima. These clusters capture distinct phenomena: vortices moving westwards across the South China Sea, vortices tracking along the north and northwest sides of Borneo, vortices sitting on the west side of Borneo, and vortices that initiate on the northwest side of Borneo, cross the equator and track eastwards along the south coast of Borneo. These clusters have a strong seasonal dependence related to the strength and southward propagation of the northeasterly flow and therefore cold‐surge type. The Madden–Julian oscillation (MJO) is considerably less important than the cold surge for modulating vortex frequency but has a similar order of magnitude impact on vortex rainfall. Kelvin waves strongly modulate rainfall from all BVs. Westward‐moving mixed Rossby–gravity (WMRG) and Rossby n = 1 (R1) waves modify frequency, rainfall, and vorticity through modification of environmental vorticity and northeasterly flow. These properties are highest when the BV is within or on the leading edge of the positive vorticity phase of R1 waves (in the northern hemisphere) or WMRG waves. Westward‐moving vortices north of 4° N are often embedded in and move with R1 or WMRG waves. Examining case studies in detail, we find BVs typically extend upward to 500–400 hPa but can reach to 300 hPa, and those near the equator may not always have closed streamlines. Under vertical wind shear they may tilt, usually to the west.

An extreme rainfall event that occurred from 16 to 18 December 2021 along the coastal regions of Peninsular Malaysia (PM) caused widespread flooding and substantial socioeconomic impacts. This study investigates the mechanisms leading to this event, focusing on the roles of climatic phenomena and local terrains. Two atmospheric interactions play key roles in triggering the event. Firstly, a strong cold surge (CS) associated with the East Asian winter monsoon (EAWM) interacted with the easterly surge over the southern South China Sea, leading to the formation of Borneo vortex. Secondly, a strong northeasterly and CS largely contributed to enhancing and transporting the vortex towards the PM and across the Titiwangsa mountain ranges. The phase change of the Indian Ocean Dipole (IOD) facilitated the eastward propagation of the vortex. Sumatra and PM terrains significantly modulated vortex evolution and moisture convergence over the Strait of Malacca. These findings are analyzed to shed light on interactions between large-scale climate drivers and localized terrain in generating extreme rainfall, emphasizing the necessity of multi-scale analysis for model accuracy.

The Maritime Continent experiences active convective activity all year round, with strong diurnal rainfall variation. Squall lines are frequent in this region, which is surrounded by the warm sea, and coastlines mostly run perpendicular or parallel to the monsoon winds. This study investigates the characteristics of squall lines using Integrated Multi‐satellite Retrievals for the Global Precipitation Measurement (IMERG GPM) half‐hourly precipitation data from 2001 to 2020 during the extended boreal summer monsoon. The squall lines are identified using the area‐overlapping technique and based on the size and rainfall thresholds. A total of 173,831 squall lines are identified and classified into four regional clusters using K ‐means clustering. The clusters are Peninsular Malaysia (PM), northwestern Borneo (NwB), northern Borneo (NB) and South China Sea (SCS). The analysis reveals notable regional differences in propagation speed and direction, size, intensity and lifespan. Squall lines in the PM cluster are smaller and intense but have shorter lifespans, while those in the Borneo clusters are larger, of moderate intensity, and have longer lifespans. The squall‐line propagation speeds are only weakly linked to the background large‐scale flow, indicating that the internal storm dynamics and local circulations play an important role. Composite case studies further reveal that nighttime squall lines are driven by the convergence of offshore winds and monsoonal winds. Along coastal regions, land‐breeze fronts, which act as density currents, may also interact with storm‐generated outflows. Overall, these findings provide important statistical insights into the structure and evolution of squall lines. The findings can support operational forecasters in anticipating squall lines' behaviour and serve as a foundation for developing regionally tailored nowcasting tools.