TY - JOUR
T1 - Seasonal Phasing of Agulhas Current Transport Tied to a Baroclinic Adjustment of Near-Field Winds
AU - Hutchinson, Katherine
AU - Beal, Lisa M.
AU - Penven, Pierrick
AU - Ansorge, Isabelle
AU - Hermes, Juliet
N1 - Funding Information:
The authors would like to acknowledge the financial support necessary to carry out this research provided by the South African National Research Fund (DST-NRF PDP Professional Development Grant 88741) and the South African Environmental Observations Network (SAEON). The authors would furthermore like to thank the following sources for the mobility grants provided: the South African National Antarctic Programme (SANAP; grant 466311), the French Institut de recherche pour le developpement (IRD with travel support provided through ICEMASA; grant 426213 CMS 1039), the South African National Research fund KIC award (grant 111529), and the U.S. National Science Foundation (U.S. NSF award 1459543). The modeling tools ROMS-AGRIF and ROMS-TOOLS used here were provided by IRD. The data from the idealized model simulations are available online at the Zenodo repository with DOI:10.5281/zenodo.1211515. The altimeter products distributed by AVISO were produced by Ssalto/Duacs, with support from Cnes (http://www. aviso.altimetry.fr/duacs/). The in situ mooring data from the ACT experiment are archived with the NOAA National Centers for Environmental Information (https://www.ncei.noaa.gov), with access numbers 0156669 and 0156605.
Funding Information:
The authors would like to acknowledge the financial support necessary to carry out this research provided by the South African National Research Fund (DST-NRF PDP Professional Development Grant 88741) and the South African Environmental Observations Network (SAEON). The authors would furthermore like to thank the following sources for the mobility grants provided: the South African National Antarctic Programme (SANAP; grant 466311), the French Institut de recherche pour le developpement (IRD with travel support provided through ICEMASA; grant 426213 CMS 1039), the South African National Research fund KIC award (grant 111529), and the U.S. National Science Foundation (U.S. NSF award 1459543). The modeling tools ROMS-AGRIF and ROMS-TOOLS used here were provided by IRD. The data from the idealized model simulations are available online at the Zenodo repository with DOI:10.5281/ zenodo.1211515. The altimeter products distributed by AVISO were produced by Ssalto/Duacs, with support from Cnes (http://www. aviso.altimetry.fr/duacs/). The in situ mooring data from the ACT experiment are archived with the NOAA National Centers for Environmental Information (https://www.ncei.noaa.gov), with access numbers 0156669 and 0156605.
Publisher Copyright:
©2018. American Geophysical Union. All Rights Reserved.
Copyright:
Copyright 2019 Elsevier B.V., All rights reserved.
PY - 2018/10
Y1 - 2018/10
N2 - 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.
AB - 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.
KW - Agulhas Current
KW - Indian Ocean
KW - Rossby waves
KW - reduced gravity model
KW - seasonal cycle
KW - wind stress curl
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U2 - 10.1029/2018JC014319
DO - 10.1029/2018JC014319
M3 - Article
AN - SCOPUS:85054559591
VL - 123
SP - 7067
EP - 7083
JO - Journal of Geophysical Research: Oceans
JF - Journal of Geophysical Research: Oceans
SN - 2169-9291
IS - 10
ER -