Antarctic Circumpolar Current transport through Drake Passage: What can
we learn from comparing high-resolution model results to observations?
Abstract
Uncertainty exists in the time-mean total transport of the Antarctic
Circumpolar Current (ACC), the world’s strongest ocean current. The two
most recent observational programs in Drake Passage, DRAKE and cDrake,
yielded transports of 141 and 173.3 Sv, respectively. In this paper, we
use a realistic 1/12 global ocean simulation to interpret these
observational estimates and reconcile their differences. We first show
that the modeled ACC transport in the upper 1000 m is in excellent
agreement with repeat shipboard acoustic Doppler current profiler
(SADCP) transects and that the exponentially decaying transport profile
in the model is consistent with the profile derived from repeat
hydrographic data. By further comparing the model results to the cDrake
and DRAKE observations, we argue that the modeled 157.3 Sv transport,
i.e. approximately the average of the cDrake and DRAKE estimates, is
actually representative of the time-mean ACC transport through the Drake
Passage. The cDrake experiment overestimated the barotropic contribution
in part because the array undersampled the deep recirculation southwest
of the Shackleton Fracture Zone, whereas the surface geostrophic
currents used in the DRAKE estimate yielded a weaker near-surface
transport than implied by the SADCP data. We also find that the modeled
baroclinic and barotropic transports are not correlated, thus monitoring
either baroclinic or barotropic transport alone may be insufficient to
assess the temporal variability of the total ACC transport.