We report observational existence of a large seasonal coastal upwelling system that establishes in austral summer (December–April) along Australian southern shelves. Wind‐driven upwelling events occur simultaneously in three upwelling centres spanning a distance of ∼800 km. During each summer period there are ∼2–3 major upwelling events, each lasting ∼1 week. The simultaneous, rapid response of SST to wind forcing in the upwelling centres, which display vastly different shelf widths, points to the existence of a larger‐scale process that carries cold water onto the shelf prior to the upwelling season. Exploration of a major upwelling event in March 1998 shows the evolution of peak surface chlorophyll‐a concentrations of >4 μg/L lagging the onset of upwelling by ∼1 week. The associated (exponential) growth rate can be estimated at 0.4 d−1. Another week later we found a distinct sub‐surface chlorophyll‐a maximum at a depth of 50 m centred along the upwelling front. Reasons for the formation of this maximum are not fully understood.

[1] Direct observations of microstructure near the Kuroshio Front were conducted in August 2008 and October 2009. These show negative potential vorticity (PV) in the mixed layer south of the front, where directly measured turbulent kinetic energy dissipation rates are an order magnitude larger than predicted by wind-scaling. These elevated dissipation rates scale better with an empirical scaling, which considers local wind and Ekman buoyancy flux driven by downfront wind. Near-zero PV in the thermocline under the Kuroshio mainstream is observed at 200-300 m depth, with dissipation exceeding open ocean thermocline values by factors of 10-100. Overall, the large turbulent dissipation rates measured in the Kuroshio can be categorized into two groups, one characterized by low Richardson number along the Kuroshio Front thermocline, and the other characterized by high stratification away from the Kuroshio mainstream. The former is attributed to mixing by unbalanced frontal ageostrophic flows, and the latter is attributed to internal wave breaking. On average, both groups appear in regions of large horizontal density gradients. Observed thermohaline structure shows low salinity tongues from the surface to over 300 m depth and deep cold tongues, extending upward from 500 to 100 m depth in a narrow (20 km) zone, suggesting down and upwelling driven by geostrophic straining, which is confirmed by Quasigeostrophic-Omega equation solutions. This implies that adiabatic along isopycnal subduction and diabatic diapycnal turbulent mixing acting in tandem at the Kuroshio Front likely contribute to NPIW formation.Citation: Nagai, T., A. Tandon, H. Yamazaki, M. J. Doubell, and S. Gallager (2012), Direct observations of microscale turbulence and thermohaline structure in the Kuroshio Front,

The microscale spatial distributions of viruses were investigated in three contrasting environments including oligotrophic open ocean, eutrophic coastal and estuarine habitats. The abundances of two discrete populations of both viruses and heterotrophic bacteria were measured at spatial resolutions of between 1 and 5 cm using purpose-designed microscale sampling equipment and £ow cytometric sample analysis. Within open water samples, virus distributions were characterized by non-normal distributions and by 'hotspots' in abundance where concentrations varied by up to 17-fold. In contrast to patterns generally observed at larger spatiotemporal scales, there was no correlation between bacterial and viral abundance or correspondence between bacteria and virus hotspots within these samples. Consequently, strong hotspots and gradients in the virus:bacteria ratio (VBR) were also apparent within samples. Within vertical pro¢les taken from above the sediment^water interface within a temperate mangrove estuary, distributions of planktonic viruses were characterized by gradients in abundance, with highest concentrations observed within the 1^2 cm immediately above the sediment surface, and virus distributions were correlated to bacterial abundance (P50.01). The patterns observed in these contrasting habitats indicate that microscale patchiness of virus abundance may be a common feature of the marine environment. This form of heterogeneity may have important implications for virus^host dynamics and subsequently in£uence microbial trophodynamics and nutrient cycling in the ocean.

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Enhanced turbulent dissipation, O(10−8–10−7) Wkg−1 in the thermocline on the cyclonic side of the Kuroshio Front was observed during a period of frontogenesis, using a microstructure profiler, XBT and ADCP along 143°E across the Kuroshio Front in August 2008. The eddy diffusivity corresponding to the mixing below the central jet is estimated to be O(10−4–10−3) m2 s−1. The strong turbulent mixing we observed in the Kuroshio is in sharp contrast to previous field measurements which found that small scale diapycnal mixing in western boundary currents remains at levels typically measured in the open ocean thermocline. The turbulence in the Kuroshio is attributed to frontogenesis arising out of an estimated confluence rate of O(10−5) s−1 based on satellite altimeters, suggesting the ubiquity of a forward energy cascade from mesoscale to microscale turbulence near ocean fronts as indicated by recent theoretical studies.