Abstract/Summary

The Borneo vortex (BV) is a synoptic-scale vorticity feature found in the SouthChina Sea near Borneo during extended Boreal winter, which can bring heavyrain to the region. Predicting this rainfall is difficult. Therefore, a better under-standing of the structure of these vortices and their interaction with equatorialwaves could aid forecasters. Here we divide the BVs found from 41-years ofOctober–March ERA5 data into five clusters based on their tracks identifiedusing relative vorticity maxima. These clusters capture distinct phenomena: vor-tices moving westwards across the South China Sea, vortices tracking along thenorth 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 andtrack eastwards along the south coast of Borneo. These clusters have a strongseasonal dependence related to the strength and southward propagation of thenortheasterly flow and therefore cold-surge type. The Madden–Julian oscilla-tion (MJO) is considerably less important than the cold surge for modulatingvortex frequency but has a similar order of magnitude impact on vortex rainfall.Kelvin waves strongly modulate rainfall from all BVs. Westward-moving mixedRossby–gravity (WMRG) and Rossby n = 1 (R1) waves modify frequency, rainfall,and vorticity through modification of environmental vorticity and northeast-erly flow. These properties are highest when the BV is within or on the leadingedge of the positive vorticity phase of R1 waves (in the northern hemisphere) orWMRG waves. Westward-moving vortices north of 4◦ N are often embedded inand move with R1 or WMRG waves. Examining case studies in detail, we findBVs typically extend upward to 500–400 hPa but can reach to 300 hPa, and thosenear the equator may not always have closed streamlines. Under vertical windshear they may tilt, usually to the west.