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

Abstract

Western boundary currents strongly influence the dynamics on the adjacent continental shelf and in particular the cross-shelf transport and uplift through the bottom boundary layer. Four years of moored in situ observations on the narrow southeastern Australian shelf (in water depths of between 65 and 140 m) were used to investigate bottom cross-shelf transport, both upstream (30 S) and downstream (34 S) of the separation zone of the East Australian Current (EAC). Bottom transport was estimated and assessed against Ekman theory, showing consistent results for a number of different formulations of the boundary layer thickness. Net bottom cross-shelf transport was onshore at all locations. Ekman theory indicates that up to 64% of the transport variability is driven by the along-shelf bottom stress. Onshore transport in the bottom boundary layer was more intense and frequent upstream than downstream, occurring 64% of the time at 30 S. Wind-driven surface Ekman transport estimates did not balance the bottom crossshelf flow. At both locations, strong variability was found in bottom water transport at periods of approximately 90-100 days. This corresponds with periodicity in EAC fluctuations and eddy shedding as evidenced from altimeter observations, highlighting the EAC as a driver of variability in the continental shelf waters. Ocean glider and HF radar observations were used to identify the bio-physical response to an EAC encroachment event, resulting in a strong onshore bottom flow, the uplift of cold slope water, and elevated coastal chlorophyll concentrations.

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

“…Schaeffer and Roughan (2015) and both evidenced greater temperature gradients across than along the shelf, based on satellite, model and glider data sets. This directional anisotropy is also evident in density (not shown), which has been shown to be mostly temperature driven (Schaeffer et al, 2014b), and even more intensified for DO. While DO is characterized by dominant cross-shelf scales similar to salinity and CDOM (8-15 km), the along-shelf spatial variability seems to be linked to the shallow EAC water mass, resulting in decorrelation scales of 27 to 35 km (surface and MLD) similar to temperature.…”

“…The anisotropic shape of the temperature variance is consistent with a highly dynamic circulation (Tandeo et al, 2014), here driven by the EAC, characterized by a greater signature in temperature than in salinity. Spatial variability in salinity is predominantly isotropic and similar to CDOM with decorrelation length scales of 9-15 km, corresponding to the first Rossby baroclinic radius of deformation (12-15 km based on local moored observation, Schaeffer et al, 2014b), and high surface variance, suggesting a predominant influence of coastal processes and river input.…”

“…It is a warm and dynamic poleward flowing current, encroaching on the continental shelf of southeastern Australia between around 18 • S (Ridgeway and Godfrey, 1994) and usually 30.7-32.4 • S (Cetina-Heredia et al, 2014) where it bifurcates eastward, forming the Tasman Front. Further south, eddies are shed (Everett et al, 2012), leading to high variability in the velocity field and water masses on the shelf (Schaeffer et al, 2014b;Schaeffer and Roughan, 2015).…”

Mechanisms of bottom boundary fluxes in a numerical model of the Shetland shelf

“…We investigate both linear and quadratic τ parameterisations for our model of BBL fluxes (Section 3.4.1) to see if there are any differences in approach. An additional flux considered by Holt et al (2009) andSchaeffer et al (2014) is a wind-driven two-dimensional downwelling circulation. Here, a local wind-driven surface Ekman transport (that is perpendicular to the 200 m isobath), Q ⊥ W , may lead to an additional BBL flux below.…”

“…Furthermore, Holt et al (2009) propose that the contribution of the wind to BBL fluxes should be valid across many shelf seas. However in the south-eastern Australian shelf Schaeffer et al (2014) have rejected the wind as a contributor. Can a wind-driven Ekman term, potentially driving a classical 2-D coastal downwelling, help to understand BBL fluxes on the Shetland shelf-edge which is part of the north-western European continental shelf?…”