Seasonal and interannual evolution of the mixed layer in the Antarctic Zone south of Tasmania

Abstract

Seasonal and interannual variations of the mixed layer properties in the Antarctic Zone (AZ) south of Tasmania are described using 7 WOCE/SR3 CTD sections and 8 years of summertime SURVOSTRAL XBT and thermosalinograph measurements between Tasmania and Antarctica. The AZ, which extends from the Polar Front (PF) to the Southern Antarctic Circumpolar Current Front (SACCF), is characterized by a 150 m deep layer of cold Winter Water (WW) overlayed in summer by warmer, fresher water mass known as Antarctic Surface Water (AASW). South of Tasmania, two branches of the PF divide the AZ into northern and southern zones with distinct water properties and variability. In the northern AZ (between the northern and southern branches of the PF), the mixed layer depth (MLD) is fairly constant in latitude, being 150 m deep in winter and around 40–60 m in summer. In the southern AZ, the winter MLD decreases from 150 m at the S-PF to 80 m at the SACCF and from 60 to 35 m in summer. Shallower mixed layers in the AZ-S are due to the decrease in the wind speed and stronger upwelling near the Antarctic Divergence. The WW MLD oscillates by ±15 m around its mean value and modest interannual changes are driven by winter wind stress anomalies.

The mixed layer is on annual average 1.7 °C warmer, 0.06 fresher and 0.2 kg m⁻³ lighter in the northern AZ than in the southern AZ. The Levitus (1998) climatology is in agreement with the observed mean summer mixed layer temperature and salinity along the SURVOSTRAL line but underestimates the MLD by 10–20 m. The winter MLD in the climatology is also closed to that observed, but is 0.15 saltier than the observations along the AZ-N of the SR3 line. MLD, temperature and density show a strong seasonal cycle through the AZ while the mixed layer salinity is nearly constant throughout the year. During winter, the AZ MLD is associated with a halocline while during summer it coincides with a thermocline.

Interannual variability of the AZ summer mixed layer is partly influenced by large scale processes such as the circumpolar wave which produces a warm anomaly during the summer 1996–1997, and partly by local mechanisms such as the retroflection of the S-PF which introduces cold water across the AZ-N.

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.