The Seasonal Cycle of the South Indian Ocean Subtropical Gyre Circulation as Revealed by Argo and Satellite Data

The seasonal variability in volume transport of the South Indian Ocean subtropical gyre is characterized for the first time. Only three complete hydrographic crossings of the gyre have been conducted over a 22-year period, with an upcoming repeat in 2019. Changes to geostrophic transport and thermocline properties imply a strengthening of the gyre from 1987 to 2002. However, some of this strengthening could result from aliasing of seasonal variability. We use data from Argo, satellite altimetry, and an Agulhas Current transport proxy at 34^∘S to quantify the seasonal variability of the upper 2,000-m volume transport. A semiannual cycle is revealed, with peak-to-peak amplitude of 6.4 ± 3.1 Sv(1Sv = 10⁶ m³ s⁻¹) and dominated by annual anomalies in quadrature near the eastern and western boundaries. Seasonal aliasing does not account for the observed gyre strengthening.