The response of the Subtropical Front to changes in the Southern
Hemisphere Westerly Winds - Evidence from models and observations.
Abstract
The location of the Subtropical Front (STF), the boundary between
Subtropical and Subantarctic Water in the Southern Ocean is proposed to
be controlled 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 time scales 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.
Regions with strong eddy variability, such as western boundary current
systems of the Agulhas and East Australian Current behave differently,
as the Sverdrup balance causes an opposite shift. In these regions an
increase in westerly winds lead to a southward shift in the STF. A
southward shift of STF has been observed between 2004-2019. 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 the biological pump and carbon uptake in the Southern
Ocean.