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Seasonality of sporadic physical processes driving temperature and nutrient high-frequency variability in the coastal ocean off southeast Australia

Rossi, Vincent; Schaeffer, A.; Wood, J.; Galibert, G.; Morris, B.; Sudre, J.; Roughan, M.; Waite, A.
Journal of Geophysical Research-Oceans 119, 445–460 (2014)

Physical processes forced by alongshore wind and currents are known to strongly influence the biogeochemistry of coastal waters. Combining in-situ observations (moored platforms, hydrographic surveys) and satellite data (Sea Surface Wind and Sea Surface Height), we investigate the transient occurrence of wind-driven up/downwelling and current-driven upwelling events off southeast Australia. Remote-sensed indices are developed and calibrated with multi-annual time-series of in-situ temperature and current measurements at two shelf locations. Based on archives up to 10 years long, climatological analyses of these indices reveal various latitudinal regimes with respect to seasonality, magnitude, duration of events and their driving mechanisms (wind or current). Generally downwelling-favorable winds prevail in this region, however we demonstrate that up to 10 wind-driven upwelling days per month occur during spring/summer at 28-33.5°S and up to 5 days in summer further south. Current-driven upwelling upstream of the East Australian Current separation zone (~32.5°S) occurs twice as often as downstream. Using independent in-situ datasets, we show that the response of the coastal ocean is consistent with our climatology of shelf processes: upwelling leads to a large range of temperatures and elevated nutrient concentrations on the shelf, maximized in the wind-driven case, while downwelling results in de-stratified nutrient-poor waters. The combination of these sporadic wind- and current-driven processes may drive an important part of the high-frequency variability of coastal temperature and nutrient content. Our results suggest that localized nutrient enrichment events of variable magnitude are favoured at specific latitudes and seasons, potentially impacting coastal ecosystems.