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 34S to quantify the seasonal variability of the upper 2,000-m volume transport. A semiannual cycle is revealed, with peak-to-peak amplitude of 6.43.1Sv(1Sv = 10(6) m(3)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. Plain Language Summary To predict climate, we need to understand how the ocean is changing, which requires separating observed changes from variability at shorter time scales. On the basis of only three transoceanic sections, between 1987 and 2009, it was concluded that the circulation of the South Indian Ocean has undergone multidecadal strengthening, but unknown seasonal variability could account for some of this signal. Here we use a decade of data from profiling floats and satellites, together with a new time series of Agulhas Current transport at 34S, to characterize the seasonal variability of the South Indian Ocean circulation for the first time. We find a semiannual cycle, of peak-to-peak amplitude 6.4Sv(1Sv = 10(6) m(3)s(-1)), that is dominated by annual anomalies in quadrature near each boundary. This seasonal variability does not explain the gyre strengthening, which is therefore part of a longer-term change. The observed semiannual cycle corresponds well with mass balance between the Agulhas Current and the Indonesian Throughflow.