An Index to Characterize the South Indian Ocean High-Pressure System and Its Variability

The South Indian Ocean High-Pressure (SIOHP) system strongly influences regional climate and oceanic circulation, yet a comprehensive index of its variability is lacking. We extend an earlier methodology for subtropical highs to develop an index of SIOHP intensity and position and describe its spatial and temporal variability from 1940 to 2023 using ERA5 mean sea level pressure. The SIOHP shows clear inter- and intra-annual variability, with the highest intensities in winter (JJA) and spring (SON) and a pronounced seasonal migration. In the transition seasons, and most clearly in November, the centroid is bimodally distributed: a composite of the raw pressure field resolves two distinct closed anticyclones, a strong, compact southeastern high and a weaker, northwestward-displaced high about 4 hPa weaker, showing that the SIOHP alternates between two physical configurations differing in both position and strength rather than a single migrating feature. We assess the influence of the El Niño Southern Oscillation (ENSO) using a trend-corrected, ENSO-year classification based on the Relative Oceanic Niño Index. Contrary to earlier expectations, ENSO exerts only a weak and largely insignificant influence: neither seasonal position nor intensity is significantly modulated by ENSO phase, and no significant distinction emerges between canonical and Modoki events. The one robust signal is an equatorward displacement of the high during El Niño summer (DJF). Interannual SIOHP variability therefore appears to be governed largely by factors other than ENSO, which may include internal atmospheric variability, regional air-sea coupling, and remote influences from outside the tropical Pacific. The index offers a tool for monitoring, forecasting, and assessing regional climate variability.