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Modeling the East Australian Current in the Western Tasman Sea

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Modeling the East Australian Current in the Western Tasman Sea 2956 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 30 Modeling the East Australian Current in the Western Tasman Sea PATRICK MARCHESIELLO* AND JASON H. MIDDLETON School of Mathematics, University of New South Wales, Sydney, Australia (Manuscript received 21 January 1998, in ®nal form 1 February 2000) ABSTRACT The East Australian Current (EAC) is a western boundary current ¯owing southward off the east coast of Australia. Its eddy variability has been shown to be vigorous, a typical feature being the formation of a large warm core eddy in the western Tasman Sea. The dynamics controlling the development of such an eddy are the subject of this paper. The Princeton Ocean Model was tuned for conditions that prevail in the western Tasman Sea, and initialized with features based on the Royal Australian Navy weekly temperature charts. A 70-day simulation initialized with summer conditions captures the formation of a large warm core eddy that matches fairly well the observations. Analyses of the results demonstrate that the formation of these eddies is associated with a wide range of dynamical aspects observed in the region, such as oscillation and propagation of the Tasman Front, EAC separation from the coast, formation of cold-core frontal eddies, and nutrient enrichment of coastal waters. 1. Introduction man Sea and the EAC as a basin edge system for climate dynamics studies. Modeling of western boundary currents such as the Our present knowledge of the EAC system is mainly East Australian Current (EAC), the Gulf Stream, the based on the compilation of many datasets. Little has Kuroshio, and the Brazil Current is a major challenge been produced from numerical models, and there is ob- of physical oceanography. The dynamics of these ¯ows viously a need for comprehensive studies of the pro- are essentially nonlinear and unstable, and they gener- cesses. In this paper, we describe a numerical investi- ally dominate the oceanographic conditions along their gation of the EAC dynamics using the Princeton Ocean paths. The EAC in particular is driven by the input of Model (POM) developed by Blumberg and Mellor the South Equatorial Current into the Coral Sea. As it (1987). Recent studies of the Gulf Stream (Mellor and ¯ows southward along the New South Wales (NSW) Ezer 1991) have shown the practical capability of the north coast, the EAC narrows to a strong shallow surface POM to simulate such complex ¯ows. A major problem current, impinging strongly but relatively steadily on area remains the initialization of such models. We pre- the continental shelf and slope. Farther south, at around sent here a ``feature model'' developed for the EAC 338S, the EAC often separates from the coast, deepening and ¯owing eastward into the Tasman Sea. At times, model, based on weekly temperature charts provided by the EAC convolutes itself to form large anticyclonic the Royal Australian Navy (RAN). Another challenge eddies. The primary objective of this paper is to further when con®guring a regional model is dealing with open elucidate the dynamics of the EAC and its interaction boundary conditions. We have applied successfully ra- with the continental shelf of the western Tasman Sea. diative boundary conditions developed in Barnier et al. The motivation is a need for an understanding of the (1998). Preliminary results of the con®guration of the physical processes that govern the cross-shelf exchanges POM for the EAC, and a comparison of model predic- of organic or inorganic materials, the circulation pat- tions with RAN charts of the same period show overall terns and thermodynamical balance of the western Tas- abilities for modeling and forecasting over a period of a few weeks (Gibbs et al. 1998). This paper presents longer simulations that capture the observed EAC var- iability in the western Tasman Sea, including the major * Current af®liation: IGPP, University of California, Los Angeles, feature that is the formation of a large anticyclonic Los Angeles, California. warm-core eddy. These simulations allow analyses of the physical processes involved. We present in section 2 an overview of the oceano- Corresponding author address: Dr. Patrick Marchesiello, IGPP, UCLA, 405 Hilgard Avenue, Los Angeles, CA 90095-1567. graphic conditions of the western Tasman Sea, and out- E-mail: [email protected] line the thermodynamical and dynamical characteristics q 2000 American Meteorological Society Unauthenticated | Downloaded 09/28/21 03:07 AM UTC NOVEMBER 2000 MARCHESIELLO AND MIDDLETON 2957 FIG. 1. Schematic diagram of currents around Australia (after Church and Craig 1998). to be studied. The East Australian Current model is ment and the Tropical Ocean±Global Atmosphere Ex- described in section 3. Section 4 presents a warm-core periment are focused on basin-scale dynamics relevant eddy formation as simulated by the model. We analyze to climate variability. Synoptic mappings of the tridi- the mechanisms and compare with other studies and mensional hydrographic and dynamic structure of the observations in section 5. Section 6 is devoted to dis- western Tasman Sea are still needed, and we can expect cussion of the results. large improvements in this respect from numerical mod- els. Figure 1 depicts the location of the EAC in relation 2. Oceanographic conditions to the large-scale circulation. The current extends south- The EAC is a major oceanographic feature of the ward from about 188S at its northern extreme sometimes Tasman Sea. Since the eastern Australian shelf is nar- as far as 428S near Bass Strait at its southern extreme. row, the current can be a dominant factor determining South of the Great Barrier Reef (258S) the shelf is much the shelf and slope circulations. Previously, the EAC narrower and the EAC is stronger (having its maximum has been observed to display signi®cant temporal and between 258 and 308S). As a result, the current has a spatial variability. Nilsson and Cresswell (1981) have signi®cant impact on the conditions on the shelf. It fre- presented a synoptic description of its variability, while quently crosses onto the continental shelf and moves Bennett (1983) has compiled a summary of EAC dy- close inshore, causing noticeable wakes behind head- namics, including a discussion of separation, instabili- lands (Cresswell et al. 1983). As the current moves ties, and eddy formation. More recently an overview southward, following along the shelf topography it may has been given by Church and Craig (1998). However, diverge or separate and move out to sea (Fig. 2). Cape apart from the Australia Coastal Experiment (Huyer Byron (28.438S, 153.348E), Smoky Cape (30.558S, 1988; Louis 1989), there have been few comprehensive 153.058E), and Sugarloaf Point (32.258S, 152.308E) are studies of the EAC dynamics. Our knowledge is mainly places generally noted for this occurrence. Sugarloaf derived from the compilation of many individual da- Point appears as a major separation point (Godfrey et tasets. It is also derived from satellite observation: sea al. 1980). Some of the separating ¯ow returns to the surface temperature (Cresswell et al. 1983) or altimetry north and some can be tracked as the Tasman Front (Wilkin and Morrow 1994), but this concerns only the across the Tasman Sea and around the northern tip of surface and is limited by cloud cover. Recent obser- New Zealand. The zonal current associated with the vations linked to the World Ocean Circulation Experi- Tasman Front is seen as the ``bridge'' linking the west- Unauthenticated | Downloaded 09/28/21 03:07 AM UTC 2958 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 30 the Tasman Sea. Characteristic values for the salinity of Coral Sea water are found to be 35.4±35.6 psu in the 208±268C temperature band. Over the central Tasman Sea a core of high salinity water (.35.7) is found to extend from 1608E, but along the coast of Autralia, Tas- man Sea salinity drops due to advection of tropical wa- ters. In the western Tasman Sea, the vertical structure is composed of three main water masses. The water in the upper 1000 m consists of various central, equatorial, or subtropical water masses. Antarctic Intermediate Wa- ter is below, characterized by a salinity minimum of 34.4 psu and 5.58C. Deep water has a salinity maximum of 34.7 psu and 28C. The Tasman Sea is bound at its southern limit by the subtropical convergence zone lo- cated near 428S. The EAC is present at all times of the year, but there is a marked seasonal cycle, with strongest ¯ow between December and April, the austral summer (Ridgway and Godfrey 1997). In the western Tasman Sea, the EAC is a narrow (40±50 km), shallow (300±500 m), and strong (1±2 m s21) surface current, decreasing to approximately half this magnitude at 250 m. Below this depth, the EAC decreases more gradually and there is evidence from density structure that the EAC extends down to at least 2000-m depth. Along the sea¯oor beneath the EAC a compensating northward ¯owing current has been ob- served. The mean EAC volume transport has been es- timated at 27 Sv (Sv [ 106 m3 s21) by Ridgway and Godfrey (1994). FIG. 2. Grayscale image of sea surface temperature in the western Tasman Sea. The EAC clearly separates around Sugarloaf Point and Meanders and instabilities may develop along the presents energetic turbulence at different scales. coast of New South Wales. A U-shaped meander is formed between 1528 and 1588E and may extend as far south as 358S. At its southernmost tip the meander may ern boundary currents of Australia and New Zealand, become unstable. As instabilities in the current develop, which would explain its location. The process actually the meander pinches off to form eddies at a rate of once involves westward propagation of Rossby waves (God- or twice per year.
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