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

Abstract

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.

Reduction in Meridional Heat Export Contributes to Recent Indian Ocean Warming

“…The transport per unit depth profile (Figure 6a) shows the subtle balance at all depths between the southward flowing Agulhas Current and the northward flowing interior that make up the gyre circulation. In the mean, the gyre circulation is 50.8 Sv (defined as the upper 2000 m northward flow), in line with previous estimates which range from 41 to 58 Sv (Palmer et al 2004;McMonigal et al 2018). The Agulhas Undercurrent (Beal and Bryden 1997) is not visible in the zonal mean across the boundary layer, as its weak northward transport over the continental slope is swamped by the larger southward transport of the Agulhas Current offshore.…”

“…the Atlantic Meridional Overturning Circulation strength (Willis 2010;Hobbs and Willis 2012), and in the South Indian to estimate the seasonal cycle of the gyre (McMonigal et al 2018).…”

“…However, other long term changes may affect the heat budget. The interior gyre circulation strengthened between 1987 and 2002 (Palmer et al 2004;McDonagh et al 2005;McMonigal et al 2018) while a proxy based on observations shows no trend in Agulhas Current volume transport over 1993-2015. Instead the current is broadening which could reduce southward heat transport even without a reduction in volume transport (Beal and Elipot 2016).…”

Decadal and Intra-Annual Variability of the Indian Ocean Freshwater Budget

“…The volume and salt transports over the upper 2000 m are quantified monthly using geostrophic velocity and salinity fields from Argo float data. The Argo data are optimally mapped and combined with sea surface height measurements to suppress mesoscale variability (Willis and Fu 2008;Willis 2010;McMonigal et al 2018). The absolute geostrophic velocity field is calculated using the thermal wind relation referenced to Argo drift velocities measured at 1000-m depth.…”

“…Although the hydrographic section across the Indian Ocean at 328S has not been reoccupied since 2009, with the maturation of the global Argo array it is possible to synthesize seasonal hydrographic cross sections of the upper 2000 m of the Indian Ocean using Argo profiles (McMonigal et al 2018).…”