Coherence of Antarctic sea levels, Southern Hemisphere Annular Mode, and flow through Drake Passage

Abstract

It is known from small sets of tide gauges that sub‐surface pressure (sea level corrected for the inverse barometer effect) around Antarctica varies coherently around about half of the continent, and that this coherent signal is related to atmospheric forcing in the form of the Antarctic Oscillation, or Southern Hemisphere Annular Mode. We here confirm that this coherence extends to a more extensive network of tide gauges, and to parts of the continental shelf far from the shore, as measured by bottom pressure gauges. We use time series from an eddy‐permitting ocean model with realistic forcing to relate the coherent mode to fluctuations in transport through Drake Passage, and confirm, using a 1° resolution barotropic model, that the fluctuations are predominantly due to barotropic dynamics, although baroclinic dynamics are expected to play an increasing role at interannual timescales.

Changes in the ocean transport through Drake Passage during the 1980s and 1990s, forced by changes in the Southern Annular Mode

“…The background colouring is bottom depth, in metres. 1999; Hughes et al, 2003]. It will be seen below that this extends to seasonal and interannual timescales also.…”

“…Strong circumpolar coherence in transport has been demonstrated using data from various Antarctic tide gauges and bottom pressure recorder (BPR) deployments [Aoki, 2002;Hughes et al, 2003]. It has also been demonstrated that this subseasonal transport variability is forced (with undetectable lag) by the varying circumpolar eastward winds associated with the Southern Annular Mode (SAM; ).…”

“…We have previously made use of an index derived from NCEP-NCAR reanalysis fields [Hughes et al, 2003]. An alternative (used here) is a station data-based index constructed from air pressure records from 12 stations (6 each at 40S and 65S) for the period to 2000 [Marshall, 2003].…”

“…[3] At subseasonal timescales, the circumpolar transport variability is predominantly barotropic (independent of depth), and hence is well-measured by bottom pressure or sea level (corrected for the inverse barometer effect) [Meredith et al, 1996;Hughes et al, 2003]. Strong circumpolar coherence in transport has been demonstrated using data from various Antarctic tide gauges and bottom pressure recorder (BPR) deployments [Aoki, 2002;Hughes et al, 2003].…”

Variability of Antarctic circumpolar transport and the Southern Annular Mode associated with the Madden‐Julian Oscillation

“…[2] Analysis of tide gauge and bottom pressure recorder (BPR) data has shown that oceanic subsurface pressure (SSP; sea level corrected for the ''inverse barometer effect'') and bottom pressure vary coherently around Antarctica on timescales from intraseasonal to interannual, coincident with changes in the oceanic circumpolar transport [Aoki, 2002;Hughes et al, 2003;Meredith et al, 2004]. Surface westerly (easterly) wind anomalies around the Southern Ocean close to Antarctica accelerate (decelerate) the oceanic circumpolar transport, associated with a fall (rise) in SSP and bottom pressure at the edge of the continent itself.…”