Jason H. Middleton

A comparison of observed upwelling mechanisms off the east coast of Australia

The East Australian Current (EAC) forms the western boundary current of the South Pacific sub-tropical gyre. Locally it plays an important role in the nutrient enrichment of the oligotrophic coastal waters of New South Wales (NSW), Australia. Observations from two detailed hydrographic surveys conducted during November 1998 and February 1999 are used to delineate the processes influencingnutrient enrichment across the continental shelf off the central east coast of Australia. Four nutrient enrichment mechanisms are identified: wind-driven upwelling, upwelling driven by the encroachment of the EAC onto the continental shelf, acceleration of the current resultingfrom the narrowingof the continental shelf at Smoky Cape, and the separation of the EAC from the coast. This study demonstrates that both the strength of the current and its proximity to the coast determine the nature of the upwellingresponse. An increase in nutrient concentrations occurs downstream as a result of each of the mechanisms identified. The highest nutrient concentration is attributable to the encroachment of the current onto the shelf, whilst separation induced upwellingis the most widespread. r 2002 Elsevier Science Ltd. All rights reserved.

On the East Australian Current: Variability, encroachment, and upwelling

[ 1] Observations from an intensive oceanographic field program which took place in 1998 - 1999 about the separation point of the East Australian Current (EAC) show significant spatial and temporal variability of the EAC. Upstream of the separation point, southward flowing currents are strong, with subinertial velocities of up to 130 cm s(-1) in the near-surface waters, whereas downstream currents are highly variable in both strength ( 1 - 70 cm s(-1)) and direction. Upwelling is observed to occur through both wind-driven and current-driven processes, with wind effects playing a lesser role. By contrast, the encroachment of the EAC upon the coast has a profound effect on the coastal waters, accelerating the southward (alongshore) currents and decreasing the temperature in the bottom boundary layer (BBL) by up to 5 degreesC. As the axis of the jet moves onshore, negative vorticity increases in association with an increase in nonlinear acceleration. During this time, bottom friction is increased, the Burger number is reduced, and the BBL shut-down time lengthens. The observed upwelling is attributed to enhanced onshore Ekman pumping through the BBL resulting from increased bottom stress as the southerly flow accelerates when the EAC encroaches across the continental shelf.

Topographically Induced Upwelling off Eastern Australia

Abstract A high-resolution, numerical study of an idealized western boundary current flow over variable topography is presented, with application to the East Australian Current (EAC). The results indicate that alongshelf topographic variations off Australia’s east coast cause the EAC to accelerate over the narrowing continental shelf near Cape Byron. This acceleration is sufficient to hinder the geostrophic adjustment in the bottom boundary layer (BBL), which would usually cause the EAC-driven BBL to shut down. Consequently, a region of persistent, high bottom stress was established off Cape Byron, which was responsible for driving an upwelling BBL. It is shown that the enhanced vertical mixing, associated with a low Richardson number flow beneath the EAC, reduced the local stratification. Consequently, the Burger number is decreased resulting in a long shutdown timescale of the BBL, which enables a nearshore thermal to be established and maintained. Such fronts are commonly observed in the region. As a p...

Mixing and bottom water formation in the shelf break region of the southern Weddell Sea

Abstract The shelf break region of the southern Weddell Sea was investigated during the period 1977–1980 with sections of closely spaced CTD stations and long-term moored current meters. An intrusion of warmer water onto the shelf from intermediate depths offshore was found in all cross-shelf sections. Evidence that this warmer water intrudes onto the shelf throughout the year was found in the current meter temperature records. Continental shelf waves appear to be the most likely mechanism for bringing this warmer water onto the shelf. Mixing of very cold water on the shelf with the warmer intruded water appears to be promoted by stirring induced by the semidiurnal tides. The mixing also may be directly caused by shear instabilities. Bottom water that forms near the shelf break and flows down the continental slope in the southern Weddell Sea appears to represent a mixture of the cold shelf water with the warmer intruded water and not cold, dense shelf water alone.

Nutrient enrichment off Port Stephens : the role of the East Australian Current

In January 1997, a bloom of the dinoflagellate Noctiluca scintillans occurred off Port Stephens, on the New South Wales (NSW) central coast. The nutrient enrichment that presumably caused the bloom was apparently unrelated to local winds. The purpose of this study is to investigate the role of the East Australian Current (EAC) in promoting nutrient-rich slope water into the euphotic zone off Port Stephens. To this end, a regional model of the NSW coast is presented and a new mechanism for upwelling is proposed. The simulations indicate that uplifting of slope water results from the interaction of the EAC with the continental shelf topography near Laurieton, located to the north of Port Stephens. The uplifted water is consequently advected along the shelf to Port Stephens, where the EAC separates from the coast. As a result of this divergence, the uplifted slope water is upwelled to the surface, and outcrops over the shelf. In situ velocity and temperature measurements are presented as evidence for the upwelling mechanism and the proposed path of the upwelled slope water is inferred by modelling the dispersal of a passive concentration tracer injected to the north of the upwelling region. The proposed upwelling mechanism may provide insight into topographically induced upwelling in other western boundary current regions. # 2001 Elsevier Science Ltd. All rights reserved.

Modeling the East Australian Current in the Western Tasman Sea

The East Australian Current (EAC) is a western boundary current flowing 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.

On the Dynamics of the Leeuwin Current

Abstract Two models, one numerical and one analytical, are used to investigate mechanisms for the generation and flow of the Leeuwin Current observed off the west coast of Australia. Three numerical experiments are conducted using the Bryan–Cox Ocean General Circulation Model. In the first experiment an alongshore density gradient is imposed, while in the latter two experiments the additional effects of warm, fresh North West Shelf waters are considered. The alongshore density gradient in the Indian Ocean produces an onshore geostrophic flow which turn southward and intensifies as it flows along the coast of Western Australia and into the Great Australian Bight. Maximum alongshore surface velocities occur just off the shelf break and below this poleward current is a weak equatorward flow. The magnitudes of the velocity vectors and associated advection of temperature and salinity, and the width, structure and geographical location of the current all agree well with field observations. The effects of the wa...

The oceanographic structure of the eastern Scotia Sea. I: Physical oceanography

Abstract Lines of closely spaced hydrographic stations across the boundary region in the eastern Scotia Sea between waters flowing out of the Weddell Sea and those flowing through the Darke Passage were occupied in the austral summer of 1981. A clearly defined front between the two waters was not observed except in the section just north of the South Orkneys near where the two currents evidently first come together. Downstream of this front is a region of eddy-like structures that increases in width almost linearly with distance. Energy spectra of the geostrophic velocities show that the strengths of the eddies also increase downstream. The boundary region thus has some of the appearances of a turbulent shear layer induced by the difference in velocity between the two currents, but as the size of the eddies remains nearly constant, baroclinic instability may also contribute to their formation.

Low-Frequency Currents and Continental Shelf Waves in the Southern Weddell Sea

Abstract The salient features of low-frequency current fluctuations, obtained from an analysis of eight current meter records from the continental shelf and slope of the southern Weddell Sea, are compared to baroclinic and barotropic theories. A simple baroclinic theory of internal waves is used successfully to predict high-frequency spectral cutoff values from low-frequency velocity ellipse calculations made from the continental slope mooring data. The success of this theory indicates that the higher spectral energy levels observed over the slope compared to the shelf are probably due to baroclinic motions. A barotropic model of free continental shelf waves probably by Saint-Guily (1976) is adapted for the local topography and the predictions of the model compared to observations. Coherences and phases between moorings separated by 10 and 160 km in the along-shelf direction provide substantial evidence of the existence of shelf waves as predicted by the theory for periods of 3–60 days and for the lowest ...

Baroclinic Response of Sydney Shelf Waters to Local Wind and Deep Ocean Forcing

A study of the forcing processes responsible for upwelling events in the coastal ocean of Sydney, Australia, has been performed using data collected over the summer of 1994 from a shore-normal-aligned mooring array and a numerical model. Analyses of the data show that vertical displacements of fluid in the nearshore zone responded principally to the local wind stress during the experimental period. However, intrusions of mesoscale East Australian Current features are shown to significantly influence the vertical structure of the water column over the middle and outer shelf regions. Numerical simulations are performed to investigate the internal processes occurring during strong steady intrusion events of the East Australian Current onto the Sydney shelf. These simulations suggest that Ekman transport in the bottom boundary layer underlying an intrusion event is not an efficient mechanism for advecting colder water into the nearshore zone on the Sydney shelf. Preconditioning of shelf waters by the East Australian Current and concurrent forcing by local winds is suggested as an efficient process by which upwelling states may be achieved in the Sydney coastal ocean.