Frontogenesis in the Agulhas Return Current Region Simulated by a High-Resolution CGCM

Abstract

Detailed mechanisms for frontogenesis/frontolysis of the sea surface temperature (SST) front in the Agulhas Return Current (ARC) region are investigated using outputs from a high-resolution coupled general circulation model. The SST front is maintained throughout the year through an approximate balance between frontolysis by surface heat flux and frontogenesis by horizontal advection. Although a southward (northward) cross-isotherm flow on the northern (southern) side of the front is weaker than a strong eastward along-isotherm current in the frontal region, this cross-isotherm confluent flow advects warmer (cooler) temperature toward the SST front north (south) of the front and acts as the dominant frontogenesis mechanism. In addition, stronger (weaker) frontogenesis in austral summer (winter) is attributed to the stronger (weaker) cross-isotherm confluence, which may be linked to seasonal variations of the Agulhas Current, ARC, and Antarctic Circumpolar Current. On the other hand, the contribution from entrainment is relatively small, because frontolysis by larger (smaller) entrainment velocity on the northern (southern) side opposes frontogenesis by less (more) effective cooling associated with a thicker (thinner) mixed layer and smaller (larger) temperature difference between the mixed layer and entrained water in the northern (southern) region. To gain further insight into the time-mean cross-isotherm confluent flow in the frontal region, the vorticity balance is examined. It is shown that anticyclonic (cyclonic) vorticity advection north (south) of the front by the mean cross-isotherm confluence is in balance with the sum of cyclonic (anticyclonic) vorticity advection by the mean along-isotherm flow and cross-isotherm eddy–mean interaction.

Eddy‐Induced Meridional Salt Flux and Its Impacts on the Sea Surface Salinity Maxima in the Southern Subtropical Oceans

“…Among the three southern subtropical oceans, eddies are most active in the south Indian Ocean. They reach their maximum strength along the east coast of Africa and further southeastward along the subtropical front, where EKE exceeds 0.2 m 2 /s 2 , due to the vacillation of the Agulhas Current and the warm/cold core rings associated with the Agulhas retroflection (e.g., Gordon, 1985;Ohishi et al, 2017;Qu et al, 2019;van Sebille et al, 2010). EKE is also high (>0.1 m 2 /s 2 ) along the west coast of Australia, which is likely linked to the Leeuwin Current and its eddies (e.g., Feng et al, 2005;Furue et al, 2017;Waite et al, 2007).…”

“…High northward salt flux is seen to extend westward from the west coast of Australia, in response to the high eddy activities discussed above (Figure 2a). In the south, large southward salt flux with a magnitude exceeding 20 × 10 −2 psu m 2 /s appears south of about 35°S, presumably as a result of the warm/cold core rings associated the Agulhas Return Current near the subtropical front (e.g., Gordon, 1985;Ohishi et al, 2017;Qu et al, 2019;van Sebille et al, 2010). Large eddy-induced meridional salt flux (>20 × 10 −2 psu m 2 /s) is also seen in the South Pacific and South Atlantic but is generally confined to the western part of the two oceans.…”

Spin‐Up of the Southern Hemisphere Super Gyre

“…The connection between the South Indian and Atlantic Oceans is through the Agulhas leakage. As much as 12–15 Sv of Indian Ocean water, representing an average transport associated with the Agulhas Current and eddies, leaks into the South Atlantic through this Agulhas pathway (e.g., de Ruijter et al, ; Durgadoo et al, ; Gordon, ; Ohishi et al, ; Ridgway & Dunn, ; van Sebille et al, ). No apparent Atlantic‐Pacific subtropical water exchange is observed through the Drake Passage (e.g., Ridgway & Dunn, ; Speich et al, ).…”

Wind–Mixed Layer–SST Feedbacks in a Tropical Air–Sea Coupled System: Application to the Atlantic

“…Although the WES concept implicitly assumes that the oceanic mixed layer depth (MLD) is constant, it is known that variations in the MLD play a role in SST variability (e.g., Alexander et al 2000;Morioka et al 2010;Doi et al 2010;Graham et al 2014;Yamagami and Tozuka 2015;Kataoka et al 2017;Ohishi et al 2017;Tozuka et al 2018). For example, the shallower (deeper) MLD contributes to the warmer mixed layer temperature (approximately equal to SST) by changing the sensitivity to the positive climatological surface heat flux (i.e., the shortwave radiation; heat flux into the ocean is defined as positive) if everything else remains the same.…”