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- Title
The Response of the Subtropical Front to Changes in the Southern Hemisphere Westerly Winds—Evidence From Models and Observations.
- Authors
Behrens, Erik; Bostock, Helen
- Abstract
The location of the Subtropical Front (STF), the boundary between Subtropical and Subantarctic Water in the Southern Ocean is proposed to be influenced by the strength and location of the Southern Hemisphere westerly winds. We use a hydrodynamic hindcast model and recent observations to test if changes in the westerly winds can cause meridional shifts in the STF over interannual to decadal timescales by modulating local Ekman transport. We find that increased, or northward, shifted westerly winds lead to an enhanced northward Ekman transport over large parts of the Southern Ocean, resulting in a northward shift in the STF. Conversely for weaker or southward shifted westerly winds where the STF migrates south. However, this relationship breaks down in regions with strong eddy variability and western boundary current systems. In these regions an increase in westerly winds lead to a southward shift in the STF. Observation data from 2004 to 2019 display a southward shift of STF associated with the positive Southern Annular Mode. However, the shift is smaller than the latitudinal shifts in the location of the zero wind stress curl and maximum westerly winds (−0.4° latitude/decade). This discrepancy is due to positive Ekman trends resulting from the intensification of the westerly winds, which oppose the southward migration. Changes in the Ekman transport and the overall southward shift of the STF have also resulted in an observed positive trend in chlorophyll‐a concentrations south of the STF, which could have ramifications for marine ecosystems and the biological pump in the Southern Ocean. Plain Language Summary: The Subtropical Front (STF) is an important water mass barrier, between warm, salty, and nutrient‐depleted Subtropical Waters of the subtropical gyre to its north, and cold, fresh, but nutrient‐rich Subantarctic Waters of the Southern Ocean to its south. The position of the STF is thought to be influenced by the westerly winds. In this study we investigate if the STF shifts in relation to changes in these westerly winds. Our model experiments show that over large parts of the Southern Ocean, the changes in the location of the STF follow changes in the westerly winds, except in regions of strong oceanic currents. The observations show that between 2004 and 2019 a small southward trend of the STF has been detected over most parts of the Southern Ocean as a consequence of southward shift of the westerly winds due to positive Southern Annular Mode (SAM). However, the shift in the STF has not been as large as the shift in the winds, as it has been opposed by the strengthening of the westerly winds. The recent southward shift in the STF has led to an increase in plankton growth south of the STF due to increased mixing of the Subtropical and Subantarctic Waters. Key Points: Stronger westerly winds shift the Subtropical Front (STF), due to enhanced Ekman transports, northward, except in regions of strong currentsThe shift of the STF (2004–2019) is smaller than the shift in the westerly winds, possibly due to increased Ekman transportsSouthward shifts of the STF triggers a positive chlorophyll‐a response south of the front, and a negative response north of the front
- Subjects
ANTARCTIC oscillation; WESTERLIES; OCEAN currents; WATER masses; MARINE ecology; SEAWATER
- Publication
Journal of Geophysical Research. Oceans, 2023, Vol 128, Issue 2, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2022JC019139