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

Abstract

[1] Physical processes forced by alongshore winds 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 upwelling/downwelling and current-driven upwelling events off southeast Australia. Remote-sensed indices are developed and calibrated with multiannual 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 S) occurs twice as often as downstream. Using independent in situ data sets, 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 destratified 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 favored at specific latitudes and seasons, potentially impacting coastal ecosystems.Citation: Rossi, V., A. Schaeffer, J. Wood, G. Galibert, B. Morris, J. Sudre, M. Roughan, and A. M. Waite (2014), Seasonality of sporadic physical processes driving temperature and nutrient high-frequency variability in the coastal ocean off southeast Australia,

Is the East Australian Current causing a marine ecological hot-spot and an important fisheries near Fraser Island, Australia?

“…Gräwe et al (2010) found evidence of a sporadic upwelling event just to the north of Sandy Cape at 24.5 o S in hydrography data from a Hervey Bay survey during December 2007. Rossi et al (2014) focused their analysis of eastern coastal upwelling on a region south of 28 o S using 10 years of remotely sensed wind data. It was found that upwelling favourable winds may occur 3-6 days per month on average during the August to December period, at the same time when current-driven upwelling is at a maximum, and less than 2 days per month during other months that appear to be dominated by wind-driven downwelling conditions.…”

“…Along the EAC-influenced eastern coast of Australia, studies by Andrews and Gentien (1982), Griffin et al (1987), Hallegraef and Jeffrey (1993), Middleton et al (1994), Oke andMiddleton (2000, 2001), Middleton (2002, 2004), Weeks et al (2010), Thompson et al (2011), Wood et al (2012), Schaeffer et al (2013), Everett et al (2014), Armbrecht et al (2014), and Rossi et al (2014) provided significant insight into the location, frequency and the physical mechanisms of Australian east coast upwelling.…”

“…Hallegraef and Jeffrey (1993) and Oke andMiddleton (2000, 2001) stressed the role of this current-driven upwelling associated with an encroachment of the EAC on the coast that is modified by local bathymetry (Roughan and Middleton, 2004). Most recently, Armbrecht et al (2014), Everett et al (2014), Schaeffer et al (2014), and Rossi et al (2014) quantify upwelling favourable conditions and the relative contribution made by windand current-driven upwelling for the east coast south of about 28 o S. linked field data from about 30.3 o S (near Coffs Harbour, NSW) and historical observations from about 33.9 o S (near Sydney, NSW) with climatological data to quantify wind-and current-driven upwelling processes and their relative contributions to surface layer nutrient enrichment in those locations. For both locations, Schaeffer et al (2014) documented changes in shelf water characteristics that appear to be linked to EAC fluctuations on time scale of 90-100 days.…”

Baseline biogeochemical data from Australia's continental margin links seabed sediments to water column characteristics

“…The more productive regions of Australia's Exclusive Economic Zone (EEZ) include the northern continental shelves along the Gulf of Carpentaria and around to the Kimberley (Condie and Dunn 2006;Furnas and Carpenter 2016). The Leeuwin Current and the EAC have generally lower levels of productivity (Hanson et al 2005;Thompson et al 2011a;Everett and Doblin 2015), despite the existence of hot spots (Rossi et al 2014;Brieva et al 2015). The more productive regions of Australia's EEZ include the northern continental shelves along the Gulf of Carpentaria and around to the Kimberley (Rothlisberg et al 1994).…”

“…Likewise, intermittent high productivity events caused by physical forcing are known to occur in the region (e.g. McKinnon et al 2011;Rossi et al 2013aRossi et al , 2014. The Sahul Shoals, located to the east of the CSB stations at water depths of 30-40 m, are also a potential source of highly labile OM to the CSB sediments that is derived from benthic production.…”