Vertical Heat Fluxes beneath Idealized Sea Ice Leads in Large-Eddy Simulations: Comparison with Observations from the SHEBA Experiment

Abstract

Large-eddy simulations (Δx = Δz = 1 m) are used to examine vertical ocean heat fluxes driven by mechanical and buoyancy forcing across idealized sea ice leads. Forcing parameters approximate conditions from a shear event during the Surface Heat Budget of the Arctic (SHEBA) experiment in March 1998. In situ measurements near the lead showed isopycnal displacements of 14 m and turbulent vertical heat fluxes up to 400 W m−2, both of which were attributed to a strong cyclonic stress curl localized along the lead axis. By contrast, the large-eddy simulations show cyclonic shear across the lead to produce no turbulence, with vertical heat transport instead related to an overturning cell that connects a broad upwelling near the lead to downwelling farther away. Anticyclonic forcing produces an opposite-signed overturning cell, but with an intense, narrow downwelling jet and strong turbulent heat fluxes (~100 W m−2) near the lead. For both signs of curl, domain-integrated heat transport due to the overturning cells is somewhat larger than the turbulent heat flux, the latter being confined to the vicinity of the lead. Buoyancy forcing related to sea ice formation in the lead was found to increase both the turbulent and the cell-related heat fluxes (by up to 50% and 10%, respectively). Vertical isopycnal displacements for the upwelling case were found to increase linearly with the strength of the forcing. Possible reasons for the discrepancies with the observations include finer scale variation in the surface ocean stress and turbulence associated with the formation of a ridge during the shear event.

Numerical Simulations of Internal Solitary Wave Evolution Beneath an Ice Keel

“…The heat fluxes induced by both vertical mixing and advection support vertical heat transport (Bourgault et al., 2020; Peterson et al., 2017; Rippeth et al., 2015). The heat flux induced by vertical mixing and vertical advection are denoted by $${F}_{vm}$$ and $${F}_{adv}$$, respectively (Bourgault et al., 2020; Lenn et al., 2011): $$${F}_{vm}=-{\rho }_{0}{c}_{p}{K}_{z}{T}_{z}$$$ $$${F}_{adv}={\rho }_{0}{c}_{p}{w}_{-h}{T}_{-h}$$$ where $${\rho }_{0}$$ is the reference density, $${c}_{P}$$ is the specific heat capacity of sea water. $${T}_{z}$$ is the temperature gradient and $${K}_{z}$$ is the eddy diffusivity.…”

“…The heat fluxes induced by both vertical mixing and advection support vertical heat transport (Bourgault et al, 2020;Peterson et al, 2017;Rippeth et al, 2015). The heat flux induced by vertical mixing and vertical advection are denoted by 𝐴𝐴 𝐴𝐴𝑣𝑣𝑣𝑣 and 𝐴𝐴 𝐴𝐴𝑎𝑎𝑎𝑎𝑎𝑎 , respectively (Bourgault et al, 2020;Lenn et al, 2011):…”

Intense Downwelling and Diffuse Upwelling in a Nonlinear Ekman Layer

“…This produced an intense vorticity forcing over the lead balanced by a broad vorticity forcing on the opposite sign elsewhere. Their results show strong asymmetry between anticyclonic and cyclonic forcing, with a focused downwelling associated with strong turbulence, and diffuse upwelling with weak turbulence [Bourgault et al, 2020]. This asymmetry contributes to the additional heat flux and turbulence heat fluxes underneath the lead.…”

“…In Arctic sea ice leads, the forcing varies sharply across the width of a narrow lead as shown in Figure 1.7, meaning even weak friction layer currents can produce large Rossby numbers [Bourgault et al, 2020]. This sharp gradient in stress is responsible for increased small-scale horizontal activity which in turn leads to strong surface layer nonlinearity [Bourgault et al, 2020].…”

Pacific Waters Pathways and Vertical Mixing in the CESM1‐LE: Implication for Mixed Layer Depth Evolution and Sea Ice Mass Balance in the Canada Basin

“…The NSTM is remnant solar heat trapped beneath the mixed layer at the onset of ice formation (Jackson et al., 2010; Maykut & McPhee, 1995; Perovich et al., 2008; Steele et al., 2011). Heat from the NSTM is ventilated during the fall and winter (Jackson et al., 2012) via convection associated with ice formation and brine rejection (Rudels et al., 1996; Timmermans, 2015), enhanced turbulent mixing associated with inertial oscillations (Rainville et al., 2011), and Ekman pumping along active leads where large ice‐ocean surface stress curl is present (Bourgault et al., 2020; McPhee, 2002). Warm Pacific Waters enter through the Bering Strait and are divided into three branches (Figure 1).…”