Seasonal variations of the Bransfield Current: dynamic
horizontal/vertical structure and transport
Abstract
The Bransfield Strait is a semi enclosed region located between the
Antarctic Peninsula and the South Shetland Islands (SSI), where the
Bransfield Current (BC) is a relatively warm baroclinic jet (0-250 m)
flowing northeastward along the southern slope of the SSI. Recent
studies have shown that the BC propagates as a buoyant gravity current,
recirculating around the north-eastern tip of the islands while shedding
an anticyclonic eddy. However, most previous works are based on
summertime measurements and a more comprehensive spatio-temporal view of
the regional circulation is still lacking. In this study we provide the
first seasonal description of the BC based on an extensive dataset of
direct velocity measurements. These measurements were routinely
collected along ship tracks from 275 cruises between 1999 and 2014.
Seasonally, the horizontal structure of the BC flowing as a coastal jet
appears more prominent during spring and summer, when its pathway
extends along the island slopes over 180 km at velocities up to 45 cm/s,
from Deception Island towards King George Island. During these seasons,
the BC is consistently narrower at the beginning of its path (15-20 km
wide), and wider before starting its recirculation around the SSI (30 km
wide). Through autumn, this pattern is also apparent but less intense,
especially south off Livingston-Robert Islands, where subsurface
velocities (150 m) are lower at about 10-15 cm/s. The scarcity of winter
data confirms the existence of a relatively strong coastal jet with mean
velocities about 34 cm/s at least south off Nelson Island. Vertically,
the BC also displays a distinctive pattern. At the wake of the islands,
the core of the current flows hugging the island slopes with velocities
about 35 cm/s at 150 m. Differently, when located at channels between
islands, the core of the current migrates offshore flowing at 30-45
cm/s. In terms of zonal volume transport (0-250 m), our estimates
indicate that the BC transports about 0.85, 0.93, 0.90 and 1.01 Sv from
spring to winter, respectively, at the location where all seasons are
covered by data (south off Nelson Island). These results support the BC
is a recurrent feature of the circulation in Bransfield Strait, flowing
northeastward all along the southern slope of the SSI not only during
summer but also during spring and autumn.