The Structure of Borneo Vortices and their Relationship with Cold Surges and Equatorial Waves

The Borneo vortex 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 Borneo vortices found from 41-years October-March tracked ERA5 relative vorticity into five clusters based on their location. We find these Borneo vortices are more likely to occur in southern Borneo on cross-equatorial cold surge days, and closer to Thailand, Vietnam, and Peninsula Malaysia on no surge days especially during October. During easterly surge days, vortices on the west side of Borneo are more prevalent. Kelvin waves strongly modulate rainfall from all Borneo vortices. WMRG and Rossby n=1 (R1) waves modify frequency, rainfall and vorticity through modification of environmental vorticity and north-easterly flow. These properties are highest when the Borneo vortex is within or on the leading edge of the positive vorticity phase of Rossby n=1 (R1) waves (in the Northern Hemisphere) or mixed Rossby gravity (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 vortices 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) which may reduce rainfall close to the vortex. The term “Borneo vortex” covers several distinct phenomena including westward-moving disturbances across the South China Sea, which always rotate cyclonically, as well as the vortices that cross the equator carrying their positive potential vorticity (and anticlockwise circulation) with them.