The Dynamics of the East Australian Current System: The Tasman Front, the East Auckland Current, and the East Cape Current

Abstract

The dynamics of the flow field surrounding New Zealand are investigated using a series of global ocean models. The physical mechanisms governing the direction, magnitude, and location of the East Australian Current (EAC), the Tasman Front, the East Auckland Current (EAUC), and the East Cape Current (ECC) are studied using numerical simulations whose complexity is systematically increased. As new dynamics are added to each successive simulation, their direct and indirect effects on the flow field are examined. The simulations have horizontal resolutions of 1/8Њ, 1/16Њ, or 1/32Њ for each variable, and vertical resolutions ranging from 1.5-layer reduced gravity to 6-layer finite depth with realistic bottom topography. All simulations are forced by the Hellerman and Rosenstein monthly wind stress climatology. Analysis of these simulations shows that several factors play a critical role in governing the behavior of the examined currents. These factors include 1) mass balance of water pathways through the region, 2) gradients in the wind stress curl, 3) nonlinear flow instabilities, and 4) upper-ocean-topographic coupling due to mixed baroclinic and barotropic instabilities. Transport streamfunctions of a linear reduced gravity model reproduce the large-scale features well but produce an EAUC that flows counter to the observed direction. The residual of the mass balance of the transport through the Tasman Sea, the basinwide transport at 32ЊS, and the transport of the South Pacific subtropical gyre east of New Zealand determines the direction of the EAUC. The 6-layer nonlinear model allows isopycnal outcropping, which changes the transport through the Tasman Sea and produces an EAUC flowing in the observed direction. Gradients in the zonally integrated wind stress curl field determine the coastal separation points of the EAC, the EAUC, and the ECC, while a combination of nonlinear flow instabilities and upper-ocean-topographic coupling contribute to the formation of meanders in the Tasman Front. Increased resolution results in greater mixed baroclinicbarotropic instabilities and thus more upper-ocean-topographic coupling and surface variability, giving a more accurate simulation of topographically controlled mean meanders in the Tasman Front.

Coupled productivity and carbon isotope records in the southwest Pacific Ocean during the late Miocene–early Pliocene biogenic bloom

“…1) manifests as an eastward 'zonal jet' between 152 ‡ and 173 ‡E, connecting the East Australian Current (EAC) with the East Cape Current (ECC) northeast of New Zealand (Warren, 1970). Numerous eddies are associated with this frontal zone (Tilburg et al, 2001) and surface waters are characterized by an abrupt change in temperature and salinity (Heath, 1985). Scott (1981) observed seasonally elevated phytoplankton £uorescence within some eddies along the TF, and Comiso et al (1993) found a positive correlation between seasonally elevated chlorophyll concentrations and the strength of Ekman upwelling in the Tasman Sea.…”

Large-scale movements and high-use areas of western Pacific leatherback turtles,Dermochelys coriacea

“…Further, water temperatures within the high latitude foraging areas of the EAC are warmer than temperate waters frequented by leatherbacks in the northern hemisphere, thus potentially providing an energetic advantage. Tasman Front (TAS).-The TAS is a semipermanent frontal feature created by the East Australia Current as it moves eastward and offshore, and is characterized by marked meanders and eddies and a zone of enhanced CHL (Andrews et al 1980, Tilburg et al 2001, Belkin and Cornillon 2007, Baird et al 2008. Two leatherbacks from SI nesting beaches arrived in the TAS region during April-June and engaged in ARS behavior in an area of low currentvelocity and potential zooplankton retention just south of the TAS prior to passing New Zealand's North Cape (Fig.…”

Patterns of demography for rocky-shore, intertidal invertebrates approaching their geographical range limits: tests of the abundant-centre hypothesis in south-eastern Australia

“…The EAC flows dominantly southwards from the Coral Sea to the north of Sydney. Here, a large proportion of it deflects eastwards or recirculates (Ridgway and Godfrey 1997), whereas about a third continues southwards to the Tasman Sea, as a weaker, less reliable current (Tilburg et al 2001). This most southerly flow is twice as strong in the austral summer than in winter, reflecting seasonal changes in the intensity of anti-cyclonic eddies formed along its path (Ridgway and Godfrey 1997).…”