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.
| Original language | English (US) |
|---|---|
| Journal | Journal of Geophysical Research: Oceans |
| DOIs | |
| State | Accepted/In press - Jan 1 2018 |
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Keywords
- Agulhas Current
- Indian Ocean
- reduced gravity model
- Rossby waves
- seasonal cycle
- wind stress curl
ASJC Scopus subject areas
- Geophysics
- Forestry
- Oceanography
- Aquatic Science
- Ecology
- Water Science and Technology
- Soil Science
- Geochemistry and Petrology
- Earth-Surface Processes
- Atmospheric Science
- Space and Planetary Science
- Earth and Planetary Sciences (miscellaneous)
- Palaeontology
Cite this
Seasonal Phasing of Agulhas Current Transport Tied to a Baroclinic Adjustment of Near-Field Winds. / Hutchinson, Katherine; Beal, Lisa; Penven, Pierrick; Ansorge, Isabelle; Hermes, Juliet.
In: Journal of Geophysical Research: Oceans, 01.01.2018.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Seasonal Phasing of Agulhas Current Transport Tied to a Baroclinic Adjustment of Near-Field Winds
AU - Hutchinson, Katherine
AU - Beal, Lisa
AU - Penven, Pierrick
AU - Ansorge, Isabelle
AU - Hermes, Juliet
PY - 2018/1/1
Y1 - 2018/1/1
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 - reduced gravity model
KW - Rossby waves
KW - seasonal cycle
KW - wind stress curl
UR - http://www.scopus.com/inward/record.url?scp=85054559591&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85054559591&partnerID=8YFLogxK
U2 - 10.1029/2018JC014319
DO - 10.1029/2018JC014319
M3 - Article
AN - SCOPUS:85054559591
JO - Journal of Geophysical Research: Oceans
JF - Journal of Geophysical Research: Oceans
SN - 2169-9275
ER -