Buoyancy-Driven Coastal Currents off Oregon during Fall and Winter

Abstract

During fall/winter off the Oregon coast, oceanographic surveys are relatively scarce because of rough weather conditions. This challenge has been overcome by the use of autonomous underwater gliders deployed along the Newport hydrographic line (NH-Line) nearly continuously since 2006. The discharge from the coastal rivers between northern California and the NH-Line reach several thousands of cubic meters per second, and the peaks are comparable to the discharge from the Columbia River. This freshwater input creates cross-shelf density gradients that together with the wind forcing and the large-scale Davidson Current results in strong northward velocities over the shelf. A persistent coastal current during fall/winter, which the authors call the Oregon Coastal Current (OCC), has been revealed by the glider dataset. Based on a two-layer model, the dominant forcing mechanism of the OCC is buoyancy, followed by the Davidson Current and then the wind stress, accounting for 61% (±22.6%), 26% (±18.6%), and 13% (±11.7%) of the alongshore transports, respectively. The OCC average velocities vary from 0.1 to over 0.5 m s−1, and transports are on average 0.08 (±0.07) Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1), with the maximum observed value of 0.49 Sv, comparable to the summertime upwelling jet off the Oregon coast. The OCC is a surface-trapped coastal current, and its geometry is highly affected by the wind stress, consistent with Ekman dynamics. The wind stress has an overall small direct contribution to the alongshore transport; however, it plays a primary role in modifying the OCC structure. The OCC is a persistent, key component of the fall/winter shelf dynamics and influences the ocean biogeochemistry off the Oregon coast.

Optics of the offshore Columbia River plume from glider observations and satellite imagery

“…Cross‐shore glider transects recorded many appearances of the CRP through the 7 years of study, however, here we only present results from the glider trajectories that are concordant with clear MODIS nLw(555) fields identifying both the presence and absence of the CR plume. The MODIS imagery also revealed that some events with low‐salinity water were consistent with the signal from the small local rivers of central Oregon [ Mazzini et al ., ] under upwelling conditions and mostly during early spring. Thus, glider data suggesting the presence of the CR plume but not supported by a clear MODIS image showing the presence of the CR plume were discarded in order to avoid the inclusion of glider data under the influence of these small rivers.…”

Ocean Research Enabled by Underwater Gliders

“…Deploying gear in the ocean involves some risk, as described for gliders by Brito et al (2014), whose view is based on many users new to gliders and is therefore rather bleak. My opinion is that more experienced glider operators have better performance, as must be the case for the glider programs that have been gathering data continuously for several years , Perry et al 2008, Todd et al 2011a, Pierce et al 2012, Pelland et al 2013, Mazzini et al 2014, Johnston & Rudnick 2015. The optimistic view is 10 August 2015 14:1 that ocean scientists willing to make an investment can approach the performance of experienced glider operators.…”

“…Davis (2010) used gliders equipped with ADCPs to show that the return flow is just beneath the surface wind-forced Ekman layer, implying that the heat flux is less than what would be expected if the return flow extended to the bottom. Sustained observations off Oregon have elucidated the buoyancy-driven currents associated with river discharge (Mazzini et al 2014). Timmermans & Winsor (2013) used two glider missions in the Chukchi Sea to examine the submesoscale horizontal structure in the surface layer.…”

The Impact of Wind Forcing on the Thermal Wind Shear of A River Plume

“…The assumption of geostrophic balance has been widely used in the literature to estimate along‐shelf baroclinic transport in river plumes, both from numerical model results (e.g., Fong & Geyer, ; Moffat & Lentz, ; Pimenta & Kirwan, ; Pimenta et al, ) and observations (e.g., Chant et al, ; Chant et al, ; Geyer et al, ; Mazzini et al, ). Pimenta et al () provided a thorough discussion of scalings to estimate plume baroclinic transports under the assumption of geostrophic balance.…”