Observed changes in the South Indian Ocean gyre circulation, 1987-2002

We use hydrographic data at 32°S from 1987, 1995 and 2002 to investigate changes in the strength of the subtropical gyre circulation in the Indian Ocean. Relative geostrophic transports are computed for the ocean interior using geopotential anomalies and a zero-velocity surface at 2230 dbar and then filtered with an 8° Gaussian to remove the high wavenumbers. Our estimates of the relative gyre transports are: 41 ± 5.1 Sv (1 Sv = 10⁶ m³s^-1) for 1987, 42 ± 7.0 Sv for 1995 and 58 ± 7.0 Sv for 2002. This represents a 40% increase from 1987 to 2002. The main areas of change in the geostrophic transports are just east of Madagascar Ridge and around Broken Plateau, which is consistent with differences we observe in the isopycnal depths in these areas. Maps of contoured velocity suggest that most of the change happened between 1995 and 2002, which supports our transport estimates.