<p>The Borneo vortex (BV) is a cyclonic circulation that strongly influences convection and rainfall over the South China Sea and Maritime Continent. This study investigated the frequency, intensity and dynamical characteristics of BVs using 40-year boreal winter (1980–2020) reanalysis data to clarify the respective and combined influences of MJO phases and cold surges (CS) on BV occurrence. BVs were objectively identified using a tropical vortex detection scheme based on a modified mid-latitude cutoff-low detection method. BV frequency was highest in phase 5, with nearly half accompanied by CS. BV intensity was strongest in phases 4, 5, and 8 for the CS cases, and in phases 5 and 6, with phase 8 also relatively strong, for the no-CS cases, indicating strong MJO influence during phases 4–6, while relatively strong BVs still occurred in phase 8 despite suppressed convection. In phases 5 and 8, strong BV-related vorticity was associated with CS northerlies (or easterlies in the no-CS cases) together with MJO-related westerlies in phase 5 and southerly flow from the Southern Hemisphere in phase 8. BV locations closely followed the latitude of the westerly wind band in phase 5. Vorticity budget analysis showed that strong BVs were associated with positive planetary vorticity advection and enhanced vertical stretching. Positive planetary vorticity advection extended farther south in southern-type BVs, whereas relative vorticity stretching dominated, particularly in phase 8 under CS conditions. These results highlight the important roles of phase-dependent MJO circulation, cold surges, and associated low-level flow structures in controlling BV occurrence.</p>