Seasonal phasing of Agulhas Current transport tied to a baroclinic adjustment of near-field winds

Type Article
Date 2018-10
Language English
Author(s) Hutchinson KatherineORCID1, 2, Beal Lisa M.3, Penven PierrickORCID4, Ansorge Isabelle1, Hermes Juliet1, 2
Affiliation(s) 1 : Univ Cape Town, Dept Oceanog, Cape Town, South Africa.
2 : SAEON Egagasini Node, South African Environm Observat Network, Cape Town, South Africa.
3 : Univ Miami, Rosenstiel Sch Marine & Atmospher Sci, 4600 Rickenbacker Causeway, Miami, FL 33149 USA.
4 : Univ Brest, Lab Oceanog Phys & Spatiale, CNRS, IFREMER,IRD,IUEM, Brest, France.
Source Journal Of Geophysical Research-oceans (2169-9275) (Amer Geophysical Union), 2018-10 , Vol. 123 , N. 10 , P. 7067-7083
DOI 10.1029/2018JC014319
WOS© Times Cited 16
Keyword(s) Agulhas Current, seasonal cycle, wind stress curl, Rossby waves, Indian Ocean, reduced gravity model
Abstract

The Agulhas Current plays a significant role in both local and global ocean circulation and climate regulation, yet the mechanisms that determine the seasonal cycle of the current remain unclear, with discrepancies between ocean models and observations. Observations from moorings across the current and a 22 year proxy of Agulhas Current volume transport reveal that the current is over 25% stronger in austral summer than in winter. We hypothesize that winds over the Southern Indian Ocean play a critical role in determining this seasonal phasing through barotropic and first baroclinic mode adjustments and communication to the western boundary via Rossby waves. Our hypothesis is explored using single layer and one‐and‐a‐half layer models. We find that the barotropic contribution to seasonal phasing is small, with the majority of the seasonal signal deflected offshore and along the Mozambique Ridge. The summertime maximum and wintertime minimum can, however, be reproduced by a one‐and‐a‐half layer reduced gravity model in which adjustment time to wind forcing via Rossby waves is in line with observations from satellite altimetry. Additionally, near‐field winds (to the west of 35° E) are shown to have a controlling influence on the seasonal phasing, as signals from farther afield dissipate through destructive interference with overlying winds before reaching the western boundary. These results suggest a critical role for a baroclinic adjustment to near‐field winds in setting the summertime maximum in Agulhas Current transport.

Plain Language Summary

The Agulhas Current flows along the east coast of South Africa, transporting warm Indian Ocean water southwards, acting as a vital limb of the global ocean conveyor‐belt and influencing local rainfall and climate. This study looks at the seasonal cycle of the Agulhas Current and uses idealized models to explore how winds over the Southern Indian Ocean may influence this seasonality. The current is 25% stronger in summertime, yet we do not have existing knowledge regarding the principal drivers that set this observed seasonality. In this study, we find that baroclinic processes communicating the wind stress curl variability from near‐field winds have a dominant contribution to the seasonality. Wind signals from further afield are found to die out during their journey west and so have little effect on the seasonal cycle of the Agulhas Current. Furthermore, correctly capturing the adjustment time to wind forcing is found to be very important when endeavoring to simulate the Agulhas Current seasonal phasing using a reduced gravity model. This study exposes a link between the seasonality of Agulhas Current and propagation of first baroclinic mode Rossby waves communicating the near‐field wind stress signal over across the western portion of the Southern Indian Ocean.

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How to cite 

Hutchinson Katherine, Beal Lisa M., Penven Pierrick, Ansorge Isabelle, Hermes Juliet (2018). Seasonal phasing of Agulhas Current transport tied to a baroclinic adjustment of near-field winds. Journal Of Geophysical Research-oceans, 123(10), 7067-7083. Publisher's official version : https://doi.org/10.1029/2018JC014319 , Open Access version : https://archimer.ifremer.fr/doc/00454/56607/