The Antarctic Coastal Current and its influence on the formation of deep and bottom water in the Weddell Sea
Ismael Núñez-Riboni and Eberhard Fahrbach
Introduction
Antarctic Bottom Water (AABW) is supposed to have an important impact on the meridional overturning circulation of the global ocean. This renders AABW to an important element of the thermohaline circulation. About 50 to 70% of the AABW originates in the Weddell Sea.
Fig. 1: Formation of Antarctic Bottom Water in the Weddell See. The mixture of North Atlantic Deep Water (NADW) with recently formed AABW in the Antarctic Circumpolar Current (ACC; olive arrows) yields Circumpolar Deep Water (CDW), which enters into the Weddell gyre. There, it is then called Warm Deep Water (WDW; red arrows). WDW is transported in the southern limb of the gyre and contributes to the formation of Western Shelf Water (WSW) which can sink along the Antarctic continental slope to form Weddell Sea Bottom Water (WSBW) by mixing with WDW. WSBW flows to the northwest (blue arrows) and mixes again to form Weddell Sea Deep Water (WSDW) which then can leave the Weddell Sea. The Antarctic Coastal Current (ACoC; magenta arrows) surrounds Antarctica flowing westwards, counter to the ACC. In the Atlantic sector of the Southern Ocean it is the major transport way to carry the WDW in the deep and bottom water formation area in the south-western Weddell Sea.
Goal
The present study attempts to clarify the role that the Antarctic Coastal Current (ACoC) might play in the formation of AABW in the Weddell Sea, by relating the low frequency variability of the current to variations of the water mass properties of the deep and bottom waters. The investigations are a contribution the the IPY project SASSI initiated by iAnZone.
Data
The forces driving the ACoC are studied with eight-years-long time series of wind (surface analysis of the European Centre for Medium-Range Weather Forecast; ECMWF) and sea ice concentrations from the final data set of passive microwave data of the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) and the Special Sensor Microwave/Imager (SSM/I) of the Defence Meteorological Satellite Program (USA). The ACoC is analysed with time series of instruments moored at the prime meridian, as well as nine CTD sections of RV-Polarstern at the prime meridian (spanning from 1992 to 2005).
Results
A strong horizontal density gradient between Shelf Water and Surface/Winter Water with a significant annual variation induced by heat gain/loss and ice melting/formation, gives rise to the baroclinic core of the ACoC (Figure 2). This geostrophic analysis together with composites of monthly averages of baroclinic and barotropic component of the current show that barotropic and baroclinic components have a clear annual cycle and are roughly in phase, with maxima and minima occurring around austral autumn and spring, respectively (Figure 3). This synchronization is related to ice covering hindering the effect of the wind, while the baroclinic component of the current could be affected by the wind through downwelling.
Fig. 2: Geostrophic velocities (cm s-1) from hydrographic data of RV-Polarstern expeditions ANT-X/4 (winter, upper left panel), ANT-XVIII/3 (late spring, upper panel), ANT-XXII/3 (summer, lower left panel) and ANT-XV/4 (fall, lower right panel). The mean depths of the moored instruments are shown with dots joined with a thick line.
Fig. 3: Composite of (from upper to lower panel): minimum atmospheric
pressure, wind, barotropic component of AWI-233, barotropic component
of AWI-232 and surface baroclinic component of AWI-233. The shaded
areas are the unbiased standard deviations.
Squared coherencies involving the wind and the ice concentration, on the one hand, and the barotropic and baroclinic components, on the other, suggest that both the wind and the ice contribute to the ACoC’s variability mostly through the semi-annual and the annual components, which together span ca. 42 % of the current's energy. Ca. 53 % of this shared energy is due to the wind, while 47 % is due to ice processes. While the great bulk of high-frequency signals (tides and inertial waves) are not coherent with the wind or the ice signal, their contribution to the variability of the current is comparable to the one of ice and wind together. Most of the time series are at least 95 % barotropic, while only at the upper layer and near the ice shelf, 38 % of the current is baroclinic. With one word, the spectral analysis suggests that the ACoC is mainly a barotropic current with a strong annual cycle driven by the wind and ice melting/formation.
The linear trends of the winds and the current’s speed suggest deceleration but are not significant at the present state of observation. To determine if the trends could become significant, it is necessary to extend the observation period for 6 more years. Since it is planned to continue the fieldwork, we expect to approach to that aim with our work during IPY where this project is part of the Synoptic Antarctic Shelf-Slope Interactions Study (SASSI) lead project.
Outlook
Since the measured time series in the ACoC are still too short, it is planned to relate the ACoC variability to its atmospheric driving forces and to find correlations in the longer time series of the atmospheric variables and the ones of the ACoC
Last actualisation: 16th of March, 2007.
Responsible: Ismael Núñez-Riboni