Seasonal circulation and volume transport of the Bransfield Current

Abstract

We present the first observational-based assessment of the seasonal circulation and volume transport driven by the Bransfield Current (BC), a surface-intensified coastal jet which flows northeastward along the southern slope of the South Shetland Islands (SSI) in the Bransfield Strait (BS). To this aim, we construct a seasonal climatology based on an extensive dataset of direct velocity measurements which were routinely collected along ship tracks from 375 cruises between 1999 and 2014. A major finding is that the BC is a year-round feature of the circulation in the BS, flowing persistently towards the northeast along the southern slope of the SSI. Along its full path, core velocities of the BC deepen down to at least 200–250 m depth. Maximum velocities are found to reach 60 cm s−1 during summer, and 40 cm s−1 during autumn and spring. Available measurements confirm the BC also flows northeastward during wintertime, at least from Nelson Island to King George Island, with maximum core velocities about 40 cm s−1. The spatial variability along the SSI shelf suggests two distinct regions for the BC: a narrower jet of ∼20–30 km upstream of Nelson Strait, and a wider jet of ∼40 km downstream. Upstream of Nelson Strait, the BC transports about 0.93 ± 0.15 Sv in summer, 0.88 ± 0.19 Sv in autumn and 0.65 ± 0.13 Sv in spring. Available observations during winter lead to weaker transports about 0.50 ± 0.04 Sv from Greenwich Island to Nelson Strait. Downstream of Nelson Strait, volume transport estimates show no significant differences among seasons being on average 1.31 ± 0.20 Sv. As compared to upstream of Nelson Strait, the year-round average volume transport is weaker and about 0.74 ± 0.13 Sv. A mass balance analysis supports that this downstream increase in volume transport is likely due to a recurrent inflow passing through Nelson Strait and feeding the BC with source waters coming from the northern slope of the SSI. These results highlight that future observational efforts must focus on assessing the seasonal hydrography of the BC with mesoscale-solving measurements, especially when noting that the BC has been shown in the past to behave as a summertime buoyancy-driven coastal jet driving relatively warm and fresh Transitional Zonal Water with Bellingshausen influence (TBW), whose presence in the BS might be limited during winter.