Variability of the Antarctic Coastal Current in the Amundsen Sea
Introduction
The ice sheet around West Antarctica has experienced widespread loss over recent decades (Bindschadler, 2006, Rignot et al., 2008, Dutrieux et al., 2014); a change that could contribute to global sea level rise (Scambos et al., 2004, Pritchard and Vaughan, 2007, Joughin et al., 2010). The Amundsen Sea is one region of West Antarctica that has changed most rapidly. Several studies have suggested that oceanic heat transport to the ice shelves by intrusion of warm Circumpolar Deep Water can contribute to the increase in basal melt rate (Walker et al., 2007, Jenkins et al., 2010, Wåhlin et al., 2010, Jacobs et al., 2011, Jacobs et al., 2012, Arneborg et al., 2012, Nakayama et al., 2013, Dutrieux et al., 2014, Ha et al., 2014).
The Antarctic Coastal Current (AACC; ACoC) is one of the interesting features of the Southern Ocean. Globally, it is the southernmost current, representing the subpolar regime, the entire region south of the Antarctic Circumpolar Current and flows in a westward direction parallel with the Antarctic continent (Orsi et al., 1995, Whitworth et al., 1998, Mathiot et al., 2011). Sverdrup (1953) reported the first direct observations of the current along the Antarctic continental slope, which revealed a westward flow in the Weddell Sea. Whitworth et al. (1998) suggested the characteristics of the AACC change from a broad flow far from the coast to a narrow flow close to the coast. In the narrow continental shelf area, it is difficult to distinguish the locations of the AACC and Antarctic Slope Front (Heywood et al., 1998, Heywood et al., 2004). The AACC is a fast and shallow flow in the continental shelf area and it is often associated with the front of the ice shelf (Jacobs, 1991, Heywood et al., 2004).
The strong westward flows of the AACC and Antarctic Shelf Front affect the dynamics of the ocean environment, such as the water masses and circulations in the area of the Antarctic continental shelf (Whitworth et al., 1998, Mathiot et al., 2011). Strong coastal currents flowing along the edge of the ice shelf increase the exchange of heat and mass at the seawater–ice shelf interface, accelerating the rate of ice shelf melt (Hellmer et al., 2012). Nakayama et al. (2014) established that the consequent dispersion of ice shelf meltwater affects the ocean surface circulation and the formation of water masses. Their numerical simulations suggested that a slight increase in the rate of basal mass loss of the ice shelves of the Amundsen and Bellingshausen seas could substantially increase the transport of meltwater into the Ross Sea, by strengthening the melt-driven shelf circulation and westward coastal current.
The westward AACC is driven by wind stress and buoyancy forcing (Tchernia and Jeannin, 1980, Tchernia, 1981, Núñez-Riboni and Fahrbach, 2009, Combes and Matano, 2014). Núñez-Riboni and Fahrbach (2009) revealed four driving mechanisms that may potentially determine the seasonal variability of the AACC's barotropic and baroclinic components in the Weddell Sea. Wind-driven Ekman transport accounts for 58% of the total barotropic variation of the coastal current (Núñez-Riboni and Fahrbach, 2009), and the density gradient due to the presence of fresh and cool meltwater near the ice shelf between the Antarctic Surface Water (AASW) and Shelf Water represents its baroclinic component (Fahrbach et al., 1992). Ekman transport is related to wind stress, however, sea ice concentration affects the momentum transfer between the wind and the current by modifying the surface drag coefficient (Fennel and Johannessen, 1998, Lüpkes and Birnbaum, 2005).
The variability of the AACC in the Amundsen Sea is understood only superficially because of the relative dearth of measurements in this region. Moreover, the AACC in the Amundsen Sea advances in the non-slope region and it appears nearby the ice shelf. Quantifying the effect of each forcing on the variability of the AACC and on its barotropic and baroclinic components is necessary for fuller understanding of the dynamics of the AACC (Núñez-Riboni and Fahrbach, 2009). We undertook a summertime cruise in 2012 and 2014 onboard the icebreaker R/V Araon and obtained hydrographic data on a transect along the Dotson Trough, which included the Amundsen Sea polynya and sea ice region (Fig. 1). We also collated two years' data recorded by instruments moored near the Dotson Ice Shelf. Using these observational datasets, the objective of this study was to quantify the contributions of the driving mechanisms to the seasonality and non-seasonal (short-term) fluctuation of the coastal current in the Amundsen Sea. Specifically, we investigated the forces that contribute to the seasonal and non-seasonal variability of the AACC, and we considered how the hydrographic state affects the intensity of the AACC and what causes the hydrographic state to change.
This remainder of this paper is organized as follows. Section 2 describes information relevant to the observations and method used in the investigation. Section 3.1 describes the hydrographic condition of the westward AACC in austral summer and its long-term variation. Section 3.2 describes the seasonal variation of the AACC and its possible forcings. Section 3.3 describes the relationship between the AACC and non-seasonal variation of wind and density. Section 4.1 introduces a more specific description of the dynamics of the ocean circulation in the coastal area of Antarctica. Section 4.2 describes the weakening of the westward AACC by the baroclinic effect. Section 4.3 describes the relationship between the coastal current and Ekman pumping velocity. Our findings are summarized in Section 5.
Section snippets
Data
Oceanic data were collected during hydrographic surveys and from moored stations (Fig. 1). Conductivity–temperature–depth (CTD) sensors (SeaBird Electronics 911+) were used to measure the background hydrographic structures (i.e., potential temperature and salinity), and the vertical profiles of current data were observed using a 300-kHz lowered acoustic Doppler current profiler (LADCP; Teledyne RD Instruments).
The long-term data were collated by the moored stations from February 2012 to January
Westward coastal current and hydrographic condition
Fig. 2a shows the horizontal baroclinic current distribution based on LADCP data, which reflects the vertically averaged ocean current of the upper 200 m in the ASP in 2012. A strong westward flow can be seen along the coastline near the Dotson Ice Shelf (southern part of the Amundsen Sea Polynya) and the maximum current velocity appears in front of the ice shelf where the westward flow is dominant. However, a dominant northwestward current flows along the isobaths to the north of the Martin
Dynamics
Fig. 7 is a schematic explaining the relationship between the baroclinic pressure gradient and the westward AACC in the coastal area. The easterly wind drives the southward Ekman transport toward the ice shelf, leading to an increase in the geopotential anomaly at the coast, which induces downwelling of the isopycnals (Heywood et al., 1998). This causes a mounding of the sea surface and a thickening of the pycnocline adjacent to the continent (Talley et al., 2011). The force associated with the
Conclusions and summary
The strong westward flow of the AACC, generated by prevailing easterly winds, has been observed in the Dotson Ice Shelf of the Amundsen Sea. This study examined the variability of the baroclinic component of AACC induced by four driving mechanisms: local zonal wind, SIC, thermohaline forcing, and Ekman vertical velocity. The effects of these mechanisms on the seasonal and non-seasonal variabilities of the coastal current around the Dotson Ice Shelf were estimated. The high-frequency variation
Acknowledgments
We would like to thank Chang-Su Hong for the acquisition and analysis of the moored station data. We also thank Anna K. Wåhlin for discussion on the mechanisms of the ocean circulation in front of ice shelves. This work was supported by grants (PP15020 and PE15040) from the Korea Polar Research Institute.
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