The Observed Agulhas Retroflection Behaviors During 1993–2018

Abstract

The Agulhas Current (AC)-the subtropical western boundary current of the Southern Indian Ocean (e.g., Imawaki et al., 2013;Lutjeharms, 2006)-flows poleward along the eastern coast of South Africa with a mean volume transport up to 84 Sv (1 Sv = 10 6 m s −3 ) and the maximum surface velocity exceeding 2 m s −1 (Beal & Bryden, 1999;Beal et al., 2015). Upon reaching the southern tip of the African continent, the AC departs from the coast, with its main body looping back to feed the eastward-flowing Agulhas Return Current (ARC) (Lutjeharms & Ansorge, 2001). This turn-back point of the AC-known as the Agulhas Retroflection (AR) (Lutjeharms & Van Ballegooyen, 1988)-is a key feature of the Agulhas Current system (Beal & Elipot, 2016;Beal et al., 2011), from which the Agulhas leakage originates and penetrates westward into the Southern Atlantic Ocean (Biastoch et al., 2008;Gordon, 1985). The AC system is also a "hotspot" region for mid-latitude air-sea interaction. The abundant heat and moisture fluxes toward the atmosphere in the AR region destabilize the atmospheric boundary layer and modulate extratropical storm tracks and convective precipitation (

The role of eddy-wind interaction in the eddy kinetic energy budget of the Agulhas retroflection region

“…The CESM-derived W Wind reveals negative centers around the AR regime (figure 4(a)), indicative of pronounced wind stress, responsible for damping mesoscale eddies within the AR region. These zones correlate closely with eddy-shedding positions in the 10 • E-23 • E range (Zhu et al 2021), which are characterized by high EKE levels. Specifically, eddy-wind interaction in the upstream and downstream zones account for ∼12% and ∼36% of the total EKE dissipation, respectively.…”

“…The oceanic mesoscale eddy signals are associated with temporal scales of 30-270 d in the AC system (Zhu et al , 2021. In this study, the cutoff period is set to 260 d between the mean flow and mesoscale eddies, and 30 d between mesoscale eddies and highfrequency processes, respectively.…”

“…Daily satellite observational sea surface height and surface geostrophic velocity data used is the SSALTO/Data Unification and Altimeter Combination System (DUACS) multi-mission altimeter sea level product released in 2018 (DUACS-DT2018) with spatial resolution of 0.25 • for the 1993period (Le Traon et al 1998, Taburet et al 2019.…”

“…Mesoscale eddy activity (quantified by surface EKE) is crucial for the AR dynamics; it regulates zonal displacements of the AR point (Zhu et al 2021) and the formation of Agulhas leakage (Beal et al 2011). Eddy-resolving ocean model simulations are widely utilized in understanding the EKE budget of the AC system , Adeagbo et al 2022, Li et al 2023.…”

Inclination Trend of the Agulhas Return Current Path in Three Decades

“…A large number of previous studies have shown much interest in the Agulhas retroflection position, as it is, on one side, affected by the upstream dynamical processes and, on the other, regulates the allocation ratio of Agulhas leakage to ARC [10,[15][16][17][18]. The variability of the retroflection can also have an impact on the local climate [19]. However, the Agulhas retroflection point itself is restrained by the bottom topography as well as local and large-scale wind patterns [13,20].…”