Research Information System
EGU General Assembly, 19 - 30 April 2021, pp.1
In this study, a high-resolution eddy resolving regional ocean + sea ice coupled model (MITgcm) is
used to study the effects of increasing westerlies along the Southern Ocean. Previous studies only
focused on increasing wind stress, thus not taking into account of atmosphere-to-ocean heat and
freshwater fluxes. Here, we conduct two concurrent simulations; i) 1.5 times increased wind stress
(i.e. increased only mechanical forcing) ii) 1.2247 times increased wind speed (i.e. both mechanical
and thermal flux forcing). Model domain covers whole Southern Hemisphere with lateral open
boundary conditions from ECCOv2 ocean reanalysis and surface boundary conditions from
ECMWF ERA-5 atmospheric reanalysis. In both sensitivity scenarios, due to the increase in the wind
stress, the Ekman transport towards Equator towards north is increased. This caused increased
upwelling of warmer North Atlantic Deep Water (NADW) near the Antarctic ice sheet. Both
scenarios show reduced sea ice formation with up to 2 million km2 in the austral summer and up
to 4 million km2 during the austral winter. Sea ice extent is reduced more in the mechanical forcing
simulation than the mechanical+thermal forcing one. This is a clear result that increased wind
anomalies should be studied with increased wind rather than increased stress. The reduction in
the sea ice coverage that is attributed to the warmer water mass can also be observed through the
Sea Surface Temperature (SST) values. The first case shows up to 1 – 1.5 °C very close to the
Antarctica, whereas the second case shows a much limited SST change around 0.5 °C.
Both sensitivity scenarios show an increase of the transport along Drake Passage. However, the
mechanical+thermal case shows larger increase in the Drake transport compared to the
mechanical case. This indicates that a change in the Antarctic Circumpolar Circulation also
modifies the meridional density gradient along with the upwelling characteristics. Finally,
overturning transport in the density space shows that Subtropical Cell and ACC upper Cell
strengthen in the mechanical+thermal case, while there are no significant changes in the thermal
case. In both simulations, Subpolar Cell increases and Lower Cell decreases. We conclude that
studying increased westerlies with two different approaches show significant changes in the
surface and deep circulation. Previous studies which taken into only mechanical forcing part are
missing thermal component of the wind effects.