Cross-Shelf Dynamics in a Western Boundary Current Regime: Implications for Upwelling

Abstract

The cross-shelf dynamics up- and downstream of the separation of the South Pacific Ocean’s Western Boundary Current (WBC) are studied using two years of high-resolution velocity and temperature measurements from mooring arrays. The shelf circulation is dominated by the East Australian Current (EAC) and its eddy field, with mean poleward depth-integrated magnitudes on the shelf break of 0.35 and 0.15 m s−1 up- and downstream of the separation point, respectively. The high cross-shelf variability is analyzed though a momentum budget, showing a dominant geostrophic balance at both locations. Among the secondary midshelf terms, the bottom stress influence is higher upstream of the separation point while the wind stress is dominant downstream. This study investigates the response of the velocity and temperature cross-shelf structure to both wind and EAC intrusions. Despite the deep water (up to 140 m), the response to a dominant along-shelf wind stress forcing is a classic two-layer Ekman structure. During weak winds, the shelf encroachment of the southward current drives an onshore Ekman flow in the bottom boundary layer. Both the bottom velocity and the resultant bottom cross-shelf temperature gradient are proportional to the magnitude of the encroaching current, with similar linear regressions up- and downstream of the WBC separation. The upwelled water is then subducted below the EAC upstream of the separation point. Such current-driven upwelling is shown to be the dominant driver of cold water uplift in the EAC-dominated region, with significant impacts expected on nutrient enrichment and thus on biological productivity.

Observed bottom boundary layer transport and uplift on the continental shelf adjacent to a western boundary current

“…Here we focus on the subinertial and subtidal variability, hence the PL-64 low-pass filter (half amplitude 33 h, half power 38 h) [Rosenfeld, 1983] has been applied to hourly averaged time series (as per Schaeffer et al [2013]). Missing velocity data at the vertical extremities are obtained by extrapolation of the shallowest (deepest) measurement available for the surface (bottom).…”

“…A total of 4 years of concomitant observations are used, from 1 January 2010 to 31 December 2013. The complete description of the moorings and their temporal coverage are detailed in Roughan et al [2013] and Schaeffer et al [2013Schaeffer et al [ , 2014 (in particular Table 1). We consider a cross-shelf and along-shelf coordinate system (x,y) based on the local isobath orientation as shown on Figure 1, respectively, 22 and 24 upstream and 20 to 32 downstream, which agrees well with the depth-averaged major axis orientation.…”

“…However, their results only apply to large-scale upwelling, as they considered events longer than 2 days on a 0.25 latitudinal grid. Schaeffer et al [2013] focused on the specific response to individual upwelling-favorable forcings (current or wind or both), by identifying patterns of cross-shelf velocity and temperature gradients from moored observations. They revealed a similar response to current-driven uplift upstream and downstream with predictable mean behavior when EAC encroachment is the major driver and wind stress is weak.…”

Physical and biogeochemical spatial scales of variability in the East Australian Current separation from shelf glider measurements

“…The dynamics on the shelf are influenced both by local coastal processes and the episodic intrusion of the large-scale East Australian Current (EAC) and its eddies (Fig. 1, Schaeffer et al, 2013, 2014a. The EAC is the western branch of the subtropical gyre in the South Pacific.…”

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

“…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.…”