Vertical scales and dynamics of eddies in the Arctic Ocean's Canada Basin

Abstract

A decade of moored measurements from the Arctic Ocean's northwestern Beaufort Gyre (collected as a component of the Beaufort Gyre Exploration Project) are analyzed to examine the range of mesoscale eddies over the water column and the dynamical processes that set eddy vertical scales. A total of 58 eddies were identified in the moored record, all anticyclones with azimuthal velocities ranging from 10 to 43 cm/s. These are divided into three classes based on core depths. Shallow eddies (core depths around 120 m) are shown to be vertically confined by the strong stratification of the halocline; typical thicknesses are around 100 m. Deep eddies (core depths around 1200 m) are much taller (thicknesses around 1300 m) owing to the weaker stratification at depth, consistent with a previous study. Eddies centered around mid‐depths all have two cores (vertically aligned and separated in depth) characterized by velocity maxima and anomalous temperature and salinity properties. One core is located at the base of the halocline (around 200 m depth) and the other at the depth of the Atlantic Water layer (around 400 m depth). These double‐core eddies have vertical scales between those of the shallow and deep eddies. The strongly decreasing stratification in their depth range motivates a derivation for the quasi‐geostrophic adjustment of a nonuniformly stratified water column to a potential vorticity anomaly. The result aids in interpreting the dynamics and origins of the double‐core eddies, providing insight into transport across a major water mass front separating Canadian and Eurasian Water.

Circulation of Pacific Winter Water in the Western Arctic Ocean

“…In this case, the PWW layer is compressed, so that relative vorticity is generated to satisfy PV conservation. This is in contrast to the cross‐front flow that compresses water from the Eurasian Water to the Canadian Water, which is described in Zhao and Timmermans (). For a water column of the PWW layer initially with zero relative vorticity, the generated relative vorticity is ζ F = f ( h F − h I )/ h I , where h I and h F are the initial PWW thickness and final thickness, respectively.…”

“…The Rossby number of the resulting mesoscale eddy by this process would be: R 0 = V /( fR ) = ζ /(2 f ) = ( h F − h I )/2 h I ≈ 0.18. For the lower halocline eddies at mooring B (78°N) with a typical maximum rotational velocity of 30 cm/s and a Rossby deformation radius of ~12 km (Zhao et al, ; Zhao & Timmermans, ), the Rossby number of these mesoscale eddies is R 0 = V /( fR ) = 0.3m/s/(12km × 1.42 × 10 −4 /s) ≈ 0.18. This is consistent with the Rossby number of mesoscale eddies that could be generated by the PWW layer compressed through its advection.…”

“…Both regions may exhibit baroclinic instability: in the north associated with the front between Pacific and Atlantic water mass assemblies (hereafter referred to as the P/A front; McLaughlin et al, 1996) and in the south associated with a strong boundary current (Pickart et al, 2005;Spall et al, 2008;Timmermans et al, 2008). The P/A front is thought to be a source region for double-core eddies, with their shallow cores at the lower halocline (Zhao & Timmermans, 2015). Relatively high EAPE shows a regional shift from the southern basin in 2007-2010 to the western basin and the Chukchi Borderland in 2011-2016, a shift that is coherent with the changing pathway of PWW (Figures 8 and S6).…”

Observations of an Extra-Large Subsurface Anticyclonic Eddy in the Northwestern Pacific Subtropical Gyre

“…The misalignment between ADCP transducers and the ship's GPS gyrocompass has been calibrated carefully since small misalignments can generate significant velocity errors [35]. Vertical scale of the mesoscale eddy can be detected by the presence of large horizontal speeds accompanied by T-S anomalies Zhao et al [34]. According to previous works, this speed criterion is set to be 0.1 m s -1 , which is faster than the tidal flow speeds in the open ocean.…”

Inorganic Carbon and pCO2 Variability During Ice Formation in the Beaufort Gyre of the Canada Basin

“…The eddy event at the end of the record (late September) was more intense, pulling the sensors down hundreds of meters. In this case, the cooler, saltier, and higher p CO 2 water was likely a middepth eddy with its core at the base of the halocline formed at the front to the west of mooring B separating Eurasian and Canadian basin water types (Carpenter & Timmermans, ; Zhao & Timmermans, ). Notably, after the eddies departed, the time series resumed their pre‐eddy trend.…”