Seasonal Phasing of Agulhas Current Transport Tied to a Baroclinic Adjustment of Near-Field Winds

Katherine Hutchinson, Lisa Beal, Pierrick Penven, Isabelle Ansorge, Juliet Hermes

Research output: Contribution to journalArticle

1 Citation (Scopus)

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 languageEnglish (US)
JournalJournal of Geophysical Research: Oceans
DOIs
StateAccepted/In press - Jan 1 2018

Fingerprint

wind field
near fields
adjusting
Rossby wave
baroclinic mode
volume transport
satellite altimetry
oceans
wind forcing
global ocean
planetary waves
Mozambique
communication (human)
Indian Ocean
gravity
communication
Mooring
mooring
ocean models
winter

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 journalArticle

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