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Hydrodynamics Along Meridional Transects in the Southwest Indian Ocean Sector of the Southern Ocean During Austral Summer 2007

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Hydrodynamics Along Meridional Transects in the Southwest Indian Ocean Sector of the Southern Ocean During Austral Summer 2007 73 Twenty Sixth Indian Antarctic Expedition 2006-2008 Ministry of Earth Sciences, Technical Publication No. 24, pp 73-99 Hydrodynamics along Meridional Transects in the Southwest Indian Ocean Sector of the Southern Ocean during Austral Summer 2007 Alvarinho J. Luis National Centre for Antarctic and Ocean Research, Earth System Science Organization, Ministry of Earth Sciences, Headland Sada, Goa ABSTRACT Preliminary analysis of expendable CTD profiles collected during the 26th Indian Antarctic Expedition along the ship route from Durban to India Bay, Antarctica (Track-1) and Prydz Bay to Mauritius (Track-2) during February– March 2007 are discussed. Vertical thermohaline structure reveal that the northern and southern branches of Subantarctic Front on both Tracks merge; likewise, the Agulhas Retroflection Front (AF) and South Subtropical Front (SSTF) merge between 43° and 44°S on Track-2. The southern branch of the Polar Front (PF2) meanders by 550 km southward towards east. The Subtropical Surface Water, Central Water and Mode Water are located north of 43.5°S, while the Subantarctic Surface Water, Antarctic Surface Water, Antarctic Intermediate Water and Circumpolar Deep Water are encountered in the Antarctic Circumpolar Current (ACC) region. Baroclinic transport relative to 1000 db reveals that the ACC volume is enhanced by 10 × 106 m3s–1 eastward, and a four- fold increase in transport occurs south of ACC. Nearly half of the ACC transport occurs in the 100–500 m slab. INTRODUCTION The wind driven circulation in the Indian sector of the Southern Ocean (SO) consists of the anticyclonic subtropical gyre (STG), the eastward flowing Antarctic Circumpolar Current (ACC) and the westward flowing Antarctic Coastal Current. The STG comprises of the Agulhas Current (AC) to the west, the Agulhas Return Current (ARC) to the southwest, the South Indian Ocean Current to the south, the equatorward flowing West Australian Current to the east and the South Equatorial Current to the north. This gyre differs from its counterparts elsewhere in that most of the water re-circulates in the western and central parts of the 74 Alvarinho J. Luis ocean basin (Stramma and Lutjeharms, 1997). The AC, which is the strongest western boundary current in the southern hemisphere, transports southwards warm and saline water of tropical and subtropical IO origin amounting to 65 × 106 m3 s-1 (Stramma and Lutjeharms, 1997). These characteristics promote strongest eddy activity in the southwestern IO and generate quasi-permanent meanders and eddies (Weeks and Shillington, 1994). The Antarctic Circumpolar Current (ACC) is organized into a small number of relatively narrow and deep-reaching jets associated with the baroclinic shear (Nowlin and Klinck, 1986). Spanning 45° to 55°S, the ACC flows uninterrupted from west to east (Orsi et al., 1995), linking the different ocean basins, thereby allowing the exchange of climate relevant properties, which constitutes an important part of the global overturning circulation (Schmitz, 1996). The IO sector of the SO has an intricate frontal system of quasi-zonal fronts that merge, split and steer over the uneven bottom topography in the Crozet region (Park et al., 1993) and Kerguelen region (Belkin and Gordon, 1996 (hereafter BG96); Holliday and Reed, 1998). The identification of fronts and their meandering properties are essential elements in tracing the upper-level ocean circulation. At the choke points, such as the Drake Passage, south of Africa, south of Tasmania, the meridional spread of the SO dynamics is constrained, meaning that the ACC transport, water mass and frontal characteristics can be accurately monitored. Hydrodynamic characterization of the southwestern IO is warranted on the basis of a number of meteorological and oceanographic features. First, the positive wind stress curl promotes downwelling throughout the year at the rate of 0–20 cm-1 day (Hellerman and Rosenstein, 1993). Second, the coastally trapped waves are formed in the atmosphere around the south African coast, which propagate eastward as coastal low pressure system during the passage of synoptic pressure systems to the south (Gill, 1977); therefore the winds blow parallel to the east African coast. Third, the general wind patterns over the southern IO drive major currents; consequently, the wind-driven circulation is dominant over thermohaline circulation (Hellerman and Rosenstein, 1993). Fourth, the region exchanges a large heat with the atmosphere, which is received largely via the warm (16–26°C) and saline (35.5, salinity is unitless because it is measured from the ratio of conductivities) AC, which Hydrodynamics along Meridional Transects in the Southwest ... 75 becomes trapped in the Agulhas Retroflection. Fifth, among the world oceans, about 67% of total water volume with temperature between –2° and 2°C is present in the southwest IO, as this region lies immediately downstream of the Weddell Sea, where most of this water is formed (Emery and Meincke, 1986). Despite the above features, the study of hydrodynamics in the southwest IO is hampered by a lack of good quality and spatially- resolved hydrographic data. Although numerical models have been used to simulate specific hydrodynamic features (Wang and Matear, 2001), verification of the numerical solutions requires in-situ measurements. The criteria required to identify large-scale fronts, characterize water masses, and calculate volume transport within the ACC, as well as to determine the spatial and temporal variability of fronts in the Indian ocean sector of the SO, have been outlined in previous studies based on the historical hydrographic data (BG96; Holliday and Read, 1998; Lutjeharms and Valentine, 1984; Orsi et al., 1995; Park et al., 1993, 2001 (hereafter PEP01)Belkin and Gordon (1996). I.M. Belkin and A. Gordon, Southern Ocean fronts from the Greenwich meridian to Tasmania. Journal of Geophysical Research 101 (1996), pp. 3675–3696. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (235); Sparrow et al., 1996), sea surface temperature (Ts) images obtained by passive radiometer (Kostianoy et al., 2004), and by a combination of data from expendable bathythermograph (XBT) and satellite altimetry (Swart et al., 2008). Anilkumar et al. (2006) (hereinafter AN06) surveyed the meridional sections along 45°E and 57.5°E during the austral summer of 2004. They reported that the merged font, consisting of the Agulhas Retroflection Front (ARF) and Subtropical Front (STF), exhibits meandering by more than 220 km eastwards, and the Polar Front (PF) and Subantarctic Front (SAF) split into northern branch (PF1 and SAF1, respectively) and southern branch (PF2 and SAF2, respectively). Literature survey indicates that the choke point south of Africa has yet to be systematically and repeatedly sampled, except for several synoptic cruises such as KERFIX, SUZIL, WOCE, Antaras, Civa1 and Civa2 (Park et al., 1991; 1993; 1998a; 1998b; PEP01) and the Japanese Antarctic Research Expeditions (Aoki, 1997; Aoki et al., 2003 and references therein). These extensive studies provide a detailed background to spatial trends in the ocean-based synoptic surveys. 76 Alvarinho J. Luis OBJECTIVES Repeated hydrographic observations are required to compare and quantify changes in the hydrodynamics over a period of many years; however, such observations are impractical because of the high costs of chartering research vessel. To overcome this problem, I have initiated a project to collect hydrographic data in the southwestern IO sector of the SO by using expendable CTD (XCTD) probes which can be launched from a moving ship. The hydrographic surveys were undertaken under the International Polar Year (IPY)-endorsed project (IPY#70) entitled “Monitoring of the Upper Ocean Circulation, Transport and Water Masses between Africa and Antarctica” (http://classic.ipy.org/ development/eoi/index.htm). The data were collected aboard M. V. Emerald Sea chartered for the 26th Indian Antarctic Expedition (IAE). Using the hydrographic data recorded along the ship route from Durban to India Bay and Prydz Bay to Mauritius, the aims of the study are to (1) characterize vertical structures of temperature, salinity and density; (2) identify and compare the location of the hydrological fronts with previous studies; (3) identify various water masses; and (4) compare the baroclinic volume transport between the two sections. DATA AND METHODS The vertical profiles of temperature and salinity (% in Fig. 1) were recorded using XCTD manufactured by Tsurumi Seiki Company Limited (model: XCTD-3; terminal depth: 1000 m; temperature accuracy: ±0.02°C and salinity accuracy: ±0.03 mS cm–1). In shallow regions, only temperature profiles were recorded using Sippican-make T–7 XBT probes (accuracy: ±0.15°C depth resolution: 0.65 m) (+ in Fig. 1). The section from Durban, South Africa (29.87°S, 31.03°E) to New India Bay, Antarctica (69.9°S, 12.6°E) was occupied during 9–14 February 2007 (hereafter Track-1), while the section from Prydz Bay (69.3°S, 76.35°E) to Port Louis, Mauritius (20.7°S, 57.35°E) (hereafter Track-2) was surveyed during 19–26 March 2007. A comparison of XCTD-3 and Sea Bird CTD profiles reveals that the former is consistent with the manufacturer’s specified accuracy for temperature and salinity (Mizuno and Watanabe, 1998), and the fall rate for XCTD shows no systematic bias in the fall equation provided by the manufacturer (Kizu et al., 2008). The quality control for the profiles was carried out by adopting the following procedure (CSIRO Cookbook for Quality Control of Expendable Hydrodynamics along Meridional Transects in the Southwest ... 77 Fig. 1: Bathymetry from TOPEX/Poseidon (light gray contours, km; Smith and Sandwell, 1997) overlaid with XCTD (•) and XBT (+) stations covered along Track-1 during 9–14 February and along Track-2 during 19–26 March 2007. Also shown are Civa 2 track occupied during February–March 1996. Schematics in thick gray lines indicate the Agulhas Current system Bathythermography (XBT) Data (1993). CSIRO Cookbook for Quality Control of Expendable Bathythermography (XBT) Data, 1993.
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