Interannual Eddy Kinetic Energy Modulations in the Agulhas Return Current

Abstract

Interannual variability in the mesoscale eddy field in the Agulhas Return Current (ARC) of 32-42°S and 15-35°E is investigated based on satellite altimeter observations and state estimate from the Estimating the Circulation and Climate of the Ocean, Phase II from 1993 to 2016. It is found that the interannual modulation of eddy kinetic energy in the ARC region is externally mediated by the wind stress forcing that generates the westward propagating sea surface height anomalies across the South Indian Ocean subtropical gyre. The wind-forced sea surface height anomalies influence the upstream Agulhas Current volume transports. By modulating the intensity of barotropic instability of the ARC mean flow centered around the retroflection region, the Agulhas Current inflow variability leads to the downstream interannual eddy kinetic energy fluctuations in the ARC region.Plain Language Summary The interannual eddy kinetic energy modulations in the Agulhas Return Current region are investigated in this study on the basis of the satellite altimeter observations and the Estimating the Circulation and Climate of the Ocean, Phase II state estimate for the period of 1993-2016. The interannual eddy kinetic energy modulations in the Agulhas Return Current region are affected by the upstream Agulhas Current volume transports mainly via the barotropic instability. The wind stress curl forcing in the subtropical Indian Ocean is regarded as the major factor to modulate the Agulhas Current volume transport.

The Observed Agulhas Retroflection Behaviors During 1993–2018

“…Therefore, the AC and the AR are likely decoupled in seasonality. This is in line with the notion that the AR is governed by local turbulent dynamics (Le Bars et al, 2012;Zhu et al, 2018) rather than deterministic large-scale wind forcing that gives rise to a well-defined seasonal cycle in the AC.…”

“…We examine the eddy-mean flow interaction from the perspective of the energy conversion analysis (Storch et al, 2012). As the ocean variability is dominated by mesoscale eddy signals with temporal scale of 50-200 days (Chelton et al, 2011), we apply a 200-day high-pass filter to highlight mesoscale eddy variability (Zhu et al, 2018). The mean kinetic energy (MKE) and mean potential energy (MPE) can transfer to the eddy kinetic energy (EKE) via the barotropic conversion rate (BTR) and baroclinic conversion rate (BCR), respectively.…”

“…Here, the two conversion rates are calculated as 𝐴𝐴 We need to state that in this study, the BCR cannot be properly estimated due to the lack of the subsurface velocity and density information. On the other hand, the previous studies have shown that the mesoscale eddy activity in the Agulhas system is dominated by barotropic instability rather than baroclinic instability Tsugawa & Hasumi, 2010;Zhu et al, 2018). Therefore, the energy transfer between the MKE and EKE via the BTR is examined using GlobCurrent geostrophic velocity data due to the geostrophic velocity is about one-order larger than the Ekman velocity in the Agulhas system (Figure S5 in Supporting Information S1).…”

“…Despite existing studies, the observed spatial-temporal characteristics of the AR variability are still incomplete, and the underlying mechanisms have not been firmly established. The pulse-like behaviors of AR imply the complexity and nonlinearity in the underlying dynamics (Dencausse et al, 2010b;De Ruijter et al, 1999;Zhu et al, 2018). Inertia, β-effect, vortex stretching, coastline geometry, bottom topography, and wind forcing have been suggested to co-determine the modeled AR position by a vast body of numerical studies (Arruda et al, 2014;Boudra & Chassignet, 1988;De Ruijter et al, 1999;Dijkstra & De Ruijter, 2001; Le Bars et al, 2012;Renault et al, 2017), but few observation-based analyses (Dencausse et al, 2010b;Lutjeharms & Van Ballegooyen, 1988) were available to consolidate the model-based understanding.…”

Dynamics of Interannual Eddy Kinetic Energy Modulations in a Western Boundary Current

“…Both barotropic and baroclinic instabilities contribute to the energy conversion from the mean flow to the eddy field in the Gulf Stream (Kang & Curchitser, 2015) and the Kuroshio Current (Yan et al., 2019). In the southern hemisphere, the Brazil Current is baroclinically unstable (Brum et al., 2017; Magalhães et al., 2017), whereas barotropic instability is the primary energy conversion term in the Agulhas Current (Elipot & Beal, 2015; Zhu et al., 2018). Despite the EAC jet having a weaker mean flow and transport, the energetic eddy field is comparable with the other large WBCs (Rykova et al., 2017).…”