Partitioning of Kinetic Energy in the Arctic Ocean's Beaufort Gyre

Abstract

Kinetic energy (KE) in the Arctic Ocean's Beaufort Gyre is dominated by the mesoscale eddy field that plays a central role in the transport of freshwater, heat, and biogeochemical tracers. Understanding Beaufort Gyre KE variability sheds light on how this freshwater reservoir responds to wind forcing and sea ice and ocean changes. The evolution and fate of mesoscale eddies relate to energy pathways in the ocean (e.g., the exchange of energy between barotropic and baroclinic modes). Mooring measurements of horizontal velocities in the Beaufort Gyre are analyzed to partition KE into barotropic and baroclinic modes and explore their evolution. We find that a significant fraction of water column KE is in the barotropic and the first two baroclinic modes. We explain this energy partitioning by quantifying the energy transfer coefficients between the vertical modes using the quasi-geostrophic potential vorticity conservation equations with a specific background stratification observed in the Beaufort Gyre. We find that the quasi-geostrophic vertical mode interactions uphold the persistence of KE in the first two baroclinic modes, consistent with observations. Our results explain the specific role of halocline structure on KE evolution in the gyre and suggest depressed transfer to the barotropic mode. This limits the capacity for frictional dissipation at the sea floor and suggests that energy dissipation via sea ice-ocean drag may be prominent.

Circulation of Pacific Winter Water in the Western Arctic Ocean

“…A general anticyclonic PWW circulation during 2003–2016 is shown in Figure . The depth‐averaged velocity in 100–200 m shows similar circulation pattern (Zhao et al, ). Mooring B is dominated by a northeastward current, mooring C southeastward current, and mooring D southwestward current, while mooring A does not show a clear preference of current direction likely because it is close to the center of BG (which is revealed in the following discussion of freshwater content distribution).…”

Introduction to Special Collection on Arctic Ocean Modeling and Observational Synthesis (FAMOS) 2: Beaufort Gyre Phenomenon

“…Overall, the regional freshwater content increased by ~40% (i.e., 6,400 km 3 ) from 2003 to 2018, at a rate of approximately 4,420 ±1,300 km 3 per decade. These overall, seasonal, interannual, and longer term fluctuations depend on the complicated interplay of wind, ice, and ocean dynamics and thermodynamics regulating Beaufort Gyre spin‐up, dissipation, and stabilization (see for details Kelly, Proshutinsky, Popova et al, ; Liang & Losch, ; Manucharyan & Isachsen, ; Mensa et al, ; Regan et al, ; and Zhao et al, , etc., in this special collection).…”

“…It is concluded that without sea ice the major features of these dynamics resemble characteristics in the midlatitudes. The subsurface mesoscale eddy field is studied by Zhao et al () using horizontal water velocities measured by BGOS moorings. Partitioning of kinetic energy into barotropic and baroclinic modes suggests that the halocline structure plays a specific role in limiting the capacity for frictional dissipation at the sea floor.…”

Critical Role of Continental Slopes in Halocline and Eddy Dynamics of the Ekman‐Driven Beaufort Gyre

“…The lack of a planetary beta effect and weak dissipation result in a highly efficient inverse energy cascade that increases the equilibrium eddy size. These relatively large simulated eddies have the first baroclinic mode structure, being distinct from the O(10 km) higher baroclinic mode eddies detected by, for example, BG moorings (Zhao et al, ). Yet the salinity transport done by these large‐scale eddies is sufficiently strong to arrest the halocline deepening at a depth comparable to observations.…”

“…These eddies are large‐scale and deep. Because of the lack of surface buoyancy forcing, our idealized BG model does not generate a realistic mixed layer or any other isopycnal outcroppings, thus inhibiting the formation of higher baroclinic mode eddies—more energetic and localized eddies with peak velocities centered in the halocline that are commonly observed in BG moorings and Ice Tethered Profilers (Zhao et al, , ). By construction, the cumulative salt transport by the modeled large‐scale eddies must exactly counteract the Ekman transport and this balance leads to a realistic halocline depth of O(100 m).…”