Modification by lateral mixing of the Warm Deep Water entering the Weddell Sea in the Maud Rise region

Abstract

Deep water originating in the North Atlantic is transported across the Antarctic Circumpolar Current by eddies and, after circumnavigating of the Antarctic, enters the Weddell Gyre south of Africa. As it does so, it rises up from mid-depth towards the surface. The separate temperature and salinity maxima, the Upper and Lower Circumpolar Deep Waters, converge to form the Warm Deep Water. Cores of this water mass on the southern flank of the eastern Weddell Gyre show a change in characteristic as they flow westward in the Lazarev Sea. Observations have been made along four meridional sections at 3°E, 0°, 3°W and 6°W between 60 and 70°S during the Polarstern Cruise ANTXXIII/2 in 2005/2006. These show that a heterogeneous series of warm and salty cores entering the region from the east both north and south of Maud Rise (65°S, 3°W) gradually merge and become more homogeneous towards the west. The gradual reduction in the variance of potential temperature on isopycnals is indicative of isopycnic mixing processes. A multiple regression technique allows diagnosis of the eddy diffusivities and, thus, the relative importance of isopycnic and diapycnic mixing. The method shows that the isopycnic diffusivity lies in the range 70-140 m 2 s −1 and the diapycnic diffusivity reaches about 3×10 −6 m 2 s −1 . Scale analysis suggests that isopycnic diffusion dominates over diapycnic diffusion in the erosion of the Warm Deep Water cores.

A gridded data set of upper-ocean hydrographic properties in the Weddell Gyre obtained by objective mapping of Argo float measurements

“…There is a considerable deepening of the sub-surface temperature maximum at about 65 • S, just east of the Prime Meridian, from about 200 m in the surrounding region to roughly 400 m, which occurs directly over Maud Rise (note that the mapping error is relatively small in this region due to the large availability of profiles despite the large change in bathymetry). This is in agreement with the literature, which shows the presence of trapped water in a Taylor column over Maud Rise, identifiable by a localized cooler sub-surface temperature maximum in comparison to surrounding regions (Bersch et al, 1992;Muench et al, 2001, andLeach et al, 2011). The sub- surface temperature maximum is shallowest within the gyre centre and deepest towards the gyre peripheries, demonstrating the domed structure associated with the cyclonicity of the gyre.…”

Acoustic insights into the zooplankton dynamics of the eastern Weddell Sea

“…Three expedition cruises were conducted with R. V. (Bersch et al, 1992;Gordon and Huber, 1995;Muench et al, 2001;de Steur et al, 2007;Leach et al, 2010;Cisewski et al, 2011).…”

“…Since ensembles, respectively, which corresponds to the theoretical precision of w averaged over a month. According to the method first introduced by Plueddemann and Pinkel (1989) and modified by Luo et al (2000) and Cisewski et al (2010), we also infer the migration velocity for the scattering layers from e "slope" velo i y Δz/Δt of individual MVBS contours.…”

“…Since the pioneering work of Flagg and Smith (1989), many investigators have utilized the echo intensity measured by acoustic Doppler current profilers (ADCPs) to examine the temporal and spatial variability in the distribution of zooplankton biomass and diel vertical migration for over two decades, presuming that zooplankton are the main scatterers of sound in the typically used frequency range of 10s to 100s kHz (e.g., Heywood et al, 1991;Fischer and Visbeck, 1993;Flagg et al, 1994;Heywood, 1996;Ashjian et al, 1998;Luo et al, 2000;Pinot and Jansá, 2001;le -el e al., 2002;Jiang et al, 2007;Cisewski et al, 2010;Radenac et al, 2010;Wallace et al, 2010). ADCPs can complement conventional net sampling in an excellent way because they can be deployed in extreme environments and can operate autonomously year-round, and thus provide sufficient spatial and temporal resolution to yield precise estimates of timing, velocity and extent of DVM.…”

“…The presented time series data enable to track the changes that occur over three years at hourly resolution. The particular purposes of this study are (1) to analyse and interpret the derived DVM patterns and abundance variations for their seasonal modulation and interannual variability, (2) to scrutinize the annual asymmetry of the diel vertical migration first described by Cisewski et al (2010) for its recurrence, and (3) to investigate how the environmental conditions control the distribution of zooplankton spatially and temporally.…”