Retention and Leakage of Water by Mesoscale Eddies in the East Australian Current System

Mesoscale eddies are ubiquitous in the ocean, transporting semi-isolated water masses as well as advecting tracers and biota. The extent to which eddies impact the environment depends on the time they retain water parcels. Here we quantify retention times of mesoscale eddies in a (1/10)° model of the East Australian Current and its extension along the southeast coast of Australia. We find that retention times vary widely, between 3 and 357 days, but peak around 24 and 27 days for anticyclones and cyclones, respectively. Changes in eddy shape, though not in eddy size, relate to water exchange between the eddy and the background flow. An increase in eccentricity (eddy elongation) often leads to water leakage, while a decrease is associated with water retention. Thus, the change in eddy eccentricity can be used as a diagnostic of the eddy's likelihood to exchange water with its surrounding. We find that water within a region of the eddy that is close to uniform rotation and rotating faster than uniform vorticity is more likely to be retained. Typical retention times are long enough for eddies to transport water across regions of contrasting hydrographic properties, develop a biogeochemical response, and influence connectivity patterns

Spurious sea ice formation caused by oscillatory ocean tracer advection schemes

Tracer advection schemes used by ocean models are susceptible to artificial oscillations: a form of numerical error whereby the advected field alternates between overshooting and undershooting the exact solution, producing false extrema. Here we show that these oscillations have undesirable interactions with a coupled sea ice model. When oscillations cause the near-surface ocean temperature to fall below the freezing point, sea ice forms for no reason other than numerical error. This spurious sea ice formation has significant and wide-ranging impacts on Southern Ocean simulations, including the disappearance of coastal polynyas, stratification of the water column, erosion of Winter Water, and upwelling of warm Circumpolar Deep Water. This significantly limits the model’s suitability for coupled ocean-ice and climate studies. Using the terrain-following-coordinate ocean model ROMS (Regional Ocean Modelling System) coupled to the sea ice model CICE (Community Ice CodE) on a circumpolar Antarctic domain, we compare the performance of three different tracer advection schemes, as well as two levels of parameterised diffusion and the addition of flux limiters to prevent numerical oscillations. The upwind third-order advection scheme performs better than the centered fourth-order and Akima fourth-order advection schemes, with far fewer incidents of spurious sea ice formation. The latter two schemes are less problematic with higher parameterised diffusion, although some supercooling artifacts persist. Spurious supercooling was eliminated by adding flux limiters to the upwind third-order scheme. We present this comparison as evidence of the problematic nature of oscillatory advection schemes in sea ice formation regions, and urge other ocean/sea-ice modellers to exercise caution when using such schemes

Giant kelp rafts wash ashore 450 km from the nearest populations and against the dominant ocean current

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The Strengthening East Australian Current, its eddies and biological effects - an introduction and overview

The poleward flowing East Australian Current (EAC) is characterised by its separation from the coast, 100–200 nautical miles north of Sydney,to form the eastward flowing Tasman Front and a southward flowing eddyfield. The separation zone greatly influences coastal ecosystems for the relatively narrow continental shelf(only15–50kmwide),particularly between 32–341S. In this region the continental shelf has a marked shift in the seasonal temperature-salinity relationship and elevated surface nitrate concentrations.This current parallels the portion of the coast where Australia’s population is concentrated and has a long history of scientific research. However,understanding of physical and biological processes driven by the EAC,particularly in linking circulation to ecosystems,is limited.In this special issue of 16 papers on the EAC,we examine the effects of climatic wind-stress forced ocean dynamics on EAC transport variability and coastal sealevel,from ENSO to multi-decadal timescales; eddy formation and structure;fine scale connectivity and larval retention.Comparisons with the poleward-flowing Leeuwin Current on Australia’s west coast show differences in ecosystem productivity that can be attributed to the under lying physics in each region. On average there is double the chlorophyll a concentration on the east coast than the west.In comparison to the Leeuwin, the EAC may have less local retention of larvae and act as a partial barrier to on shore transport,which may also be related to the local spawning and early life history of small pelagicfishoneach coast.Inter- annual variations in the EAC transport produce a detectable sea-level signal in Sydney Harbour,which could provide a useful fisheries index as does the Fremantle sea level and Leeuwin Current relationship. The EAC’s eddy structure and formation by the EAC are examined. A particular cold-coreeddy is shown to have a ‘tilt’ towards the coast,and that during a rotation the flow of particles may rise up to the euphotic zone and then down beneath.In a warm-coreeddy,surface floodingis shown to produce a new shallower surface mixed layer and promote algal growth.An assessment of plankton data from 1938–1942 showed that the local, synoptic conditions had to be incorporated before any comparison with the present. The reare useful relationships of water mass characteristics in the Tasman Sea and separation zone with larval fish diversity and abundance,as well as with long-line fisheries.These fisheries-pelagic habitat relationships are invaluable for fisheries management,as well as for climate change assessments. There is further need to examine the EAC influence on rainfall,storm activity, dust deposition, and on the movements by fish,sharks and whales. The Australian Integrated Marine Observing System (IMOS)has provided new infrastructure to determine the changing behaviour of the EAC and its bio-physical interaction with the coasts and estuaries. The forecasting and hindcasting capability developed under the Blue link project has provided a new tool for data synthesis and dynamical analysis. The impact of a strengthening EAC and how it influences the livelihoods of over half the Australian population, from Brisbane to Sydney, Hobart and Melbourne, is just being realised