Deep Sea Research Part I: Oceanographic Research Papers
Seasonal and interannual evolution of the mixed layer in the Antarctic Zone south of Tasmania
Introduction
The Antarctic Zone (AZ) of the Southern Ocean lies between the Polar Front (PF) and Southern Antarctic Circumpolar Current Front (SACCF), and is characterized by cold Winter Water (WW) (Mosby, 1934; Orsi et al., 1995; Toole, 1981) which extends in a well-mixed layer down to around 150 m depth in winter, and which is overlain by fresher and warmer Antarctic Surface Water (AASW) in summer. The AZ spans the Antarctic Divergence where the zonal wind regime reverses, resulting in the upwelling of Circumpolar Deep Water (CDW) (Speer et al., 2000), bringing warmer and saltier water into the surface layer. The deep water heat is lost to the atmosphere, and on the annual mean the deep water salt mixes with freshwater supplied by excess precipitation and by ice melt to form the relatively fresh AASW. Upwelling rates suggest that the AASW has a residence time of only 1.5 years (Gordon and Huber, 1990). Studies of the upwelling of relatively warm CDW (Gordon and Huber, 1990; Martinson, 1990; McPhee et al., 1996) reveal the very delicate nature of static stability within the AZ. Slight changes to the pycnocline stability would alter the exchange of the CDW heat and salt with the surface layer, and in turn influence the atmospheric and sea–ice dynamics. For example, the immense heat supply of the CDW could remove the winter ice cover, should the winter pycnocline weaken sufficiently. Therefore, it is important to determine what sets and maintains the mixed layer properties in the AZ.
The upwelling balances the divergence of surface water; the southern branch is cooled and freshened as it mixes with Antarctic shelf waters. Brine rejection from the winter sea–ice formation can increase the surface salinity of this cold water creating the densest waters known as Antarctic Bottom Water. North of the divergence, the upwelled CDW will be modified by surface fluxes to form WW or AASW. AASW acquires its low salinity during its flow around Antarctica (Gordon and Molinelli, 1982; Sievers and Nowlin, 1984) as freshwater is supplied by melting of glacial ice and sea ice, and from the excess of precipitation over evaporation. This continuously modified water mass is advected northward by Ekman transport and eastward by the geostrophic flow. Crossing the PFs to the north, it can modify the properties of the mode and intermediate water masses which are formed and subduct north of these fronts (Rintoul and England, 2002).
The mechanisms by which the AASW is transformed into Subantarctic Mode Water (SAMW) or Antarctic Intermediate Water (AAIW) are subject to debate. Proposed mechanisms include cross-frontal isopycnal exchanges (Molinelli, 1981), Ekman drift (Sloyan and Rintoul, 2001a; England et al., 1993) or eddy fluxes (Morrow et al., 2004). Ribbe and Tomczak (1997) concluded that both circumpolar cross-frontal mixing and local deep mixing play a role, an idea further supported by recent observational evidence (Rintoul and Bullister, 1999). Although these mechanisms are varied, there is a clear link between the AASW and the regions of mode and intermediate water formation.
Very few studies have investigated the characteristics of AASW and WW despite their importance in “feeding” the mode and intermediate water masses and their marginal stability which can eventually influence heat transfer in the AZ. Orsi et al. (1995) have described the mean property distributions in the AZ at various Southern Ocean sites. Park et al. (1998) examined the AASW structure close to the Antarctic continent in the Indian sector based on 2 conductivity–temperature–depth (CTD) sections. The temporal evolution of the AASW characteristics has not been addressed, due to the limited number of repeat hydrographic sections in the AZ.
In the region south of Tasmania, recent long transects between Tasmania and Antarctica (WOCE SR3 line and SURVOSTRAL project) have helped to define the Antarctic Circumpolar Current (ACC) circulation and its variability (Rintoul and Bullister, 1999; Rintoul and Sokolov, 2001; Yaremchuk et al., 2001; Rintoul et al., 2002). A recent multidisciplinary study has addressed the dynamics of the Subantarctic Zone (SAZ) (Trull et al., 2001; Rintoul and Trull, 2001), but no study has dealt with the mixed layer characteristics of the AZ. The main goal of this paper is thus to use the 7 WOCE SR3 repeat CTD sections and 50 SURVOSTRAL expendable bathythermograph (XBT) profiles to describe the temporal evolution of the AASW and WW in the AZ south of Tasmania during the 1990s. With nearly 10 years of measurements, we will examine the seasonal and interannual variations of the AASW and WW mixed layer properties.
The paper is organized as follows: in Section 2 we describe the data sets used in this paper. After a brief discussion of the circulation and water masses south of Tasmania in Section 3, we examine the seasonal and interannual variations of the AASW properties in the AZ along the WOCE/SR3 and SURVOSTRAL lines, and the role of surface forcing on these variations (4 Typical mixed layer properties in the AZ, 5 Seasonal cycle of mixed layer properties in the AZ south of Tasmania, 6 Role of surface forcing on the MLD, 7 Interannual variations of AASW properties). The WW properties and their variability along each line of measurements are analysed in Section 8. Finally, the discussion in Section 9 compares our main seasonal results with the Levitus (1998) climatology, and estimates how AASW/WW anomalies would potentially influence the mode and intermediate waters characteristics north of the main ACC fronts.
Section snippets
Data
We have available seven full depth repeat CTD sections from the World Ocean Circulation Experiment (WOCE) SR3 line which were collected between Tasmania and Adelie Land, Antarctica during voyages of the research vessel R. V. Aurora Australis between October 1991 and November 2001, roughly covering each season of the year (Rintoul and Sokolov, 2001; Yaremchuck et al., 2001; Rintoul and Trull, 2001; Rintoul et al., 2002). The station spacing for these CTD sections is around 55 km with more tightly
Fronts, circulation and water masses south of Tasmania
The ACC is composed of a series of sharp temperature and salinity fronts, which separate the subtropical and subpolar water masses. In the region south of Tasmania, these fronts are fairly tightly grouped, due to the presence of the bathymetric ridges to the north and south. Readers interested in more details on the front locations in this sector of the Southern Ocean are referred to Sokolov and Rintoul (2002) and Chaigneau and Morrow (2002). Between the fronts lie zones of weaker flow and
Typical mixed layer properties in the AZ
In order to show the seasonal variability of the AZ mixed layer and its properties and lines we will start with a detailed description of two “typical” winter and summer sections along the SR3 section, then consider a “typical” seasonal heating cycle over one summer using SURVOSTRAL sections.
Seasonal cycle of mixed layer properties in the AZ south of Tasmania
In this section we use the 7 SR3 sections and the seasonal means from the available SURVOSTRAL data from 1993 to 2000 to illustrate that the differences between AZ-S and AZ-N persist throughout the year. The SURVOSTRAL XBTs allow us to determine the monthly mean mixed layer temperature (MLT) and MLD from the end of October to the beginning of March while the TSG give us SSS. As the mixed layer salinity (MLS) and SSS are very similar in the AZ along SR3 (Figs. 2b and e), we expect that SSS along
Role of surface forcing on the MLD
To illustrate the role of surface forcing on the variability of the MLD in the AZ, we plot the MLD as a function of latitude for the 7 WOCE SR3 repeat sections (Fig. 6). The mean properties described in Section 5 are clearly evident in the individual sections: in the AZ-N the winter mixed layers extend to 100–150 m depth and the summer temperature stratification induces a shallower mixed layer of 20–70 m depth. Except for January 1994, all sections have a MLD which shoals south of the S-PF.
These
Interannual variations of AASW properties
Although we have six regular XBT sections per year, bad weather and faulty probes can lead to substantial data gaps. This gives us irregular space–time data sampling even during the summer months. In addition, in October and November, the AZ-S MLD cannot always be deduced with temperature profiles only because the thermocline does not always coincide with the pycnocline (see Appendix). For these reasons we have decided to average the MLD, temperature, density and SSS over each summer season
WOCE/SR3 line
With the 7 WOCE/SR3 sections we can investigate the properties of the WW layer. Just below this layer the vertical temperature gradient is maximum and we use this to define the base of WW layer during all seasons. Fig. 9 shows the WW properties measured at this level for each SR3 transect. The base of the WW (Fig. 9a) is at about 200 m depth between the two branches of the PF, but shoals gradually south of the S-PF to a minimum depth of around 80 m at the Antarctic Divergence. It then deepens
Discussion
Our analysis of the 7 SR3 and 50 SURVOSTRAL transects south of Tasmania has documented the seasonality and interannual variability of the mixed layer characteristics in the AZ, including an analysis of the AASW and WW characteristics. These results are summarised in Table 2, Table 3, and together with the mixed layer criteria (Appendix) provide a useful metric to validate numerical models in the AZ south of Tasmania.
In general, the mean AASW mixed layer characteristics in the AZ of the
Acknowledgements
We would like to thank the captain and crew onboard the Astrolabe and Aurora Australis, as well as our numerous volunteer observers, for helping us collect these measurements in the frequently inhospitable weather conditions. Special thanks to Ann Gronell and Peter Jackson of CSIRO, Australia for their work in handling the logistics of the SURVOSTRAL program, and the data preparation and quality control. Our thanks to Guy Caniaux for providing the ECMWF data and to Gilles Reverdin and our 3
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