Abstract
Abstract. Different sea ice models apply unique approaches in the computation of nutrient diffusion between the ocean and the ice bottom, which are generally decoupled from the calculation of turbulent heat flux. A simple molecular diffusion formulation is often used. We argue that nutrient transfer from the ocean to sea ice should be as consistent as possible with heat transfer, since all of these fluxes respond to varying forcing in a similar fashion. We hypothesize that biogeochemical models that do not consider such turbulent nutrient exchanges between the ocean and the sea ice, despite considering brine drainage and bulk exchanges through ice freezing and melting, may underestimate bottom-ice algal production. The Los Alamos Sea Ice Model (CICE + Icepack) was used to test this hypothesis by comparing simulations without and with diffusion of nutrients across the sea ice bottom that are dependent on velocity shear, implemented in a way that is consistent with turbulent heat exchanges. Simulation results support the hypothesis, showing a significant enhancement of ice algal production and biomass when nutrient limitation was relieved by bottom-ice turbulent exchange. Our results emphasize the potentially critical role of turbulent exchanges to sea ice algal blooms and thus the importance of properly representing them in biogeochemical models. The relevance of this becomes even more apparent considering ongoing trends in the Arctic Ocean, with a predictable shift from light-limited to nutrient-limited growth of ice algae earlier in the spring, as the sea ice becomes more fractured and thinner with a larger fraction of young ice with thin snow cover.
Highlights
Momentum, heat, and mass fluxes between the ocean and sea ice are of utmost importance to predict sea ice motion, thermodynamics, and biogeochemistry
In this work we will focus on the differences related to the vertical diffusion of tracers between the water column and the bottom ice and attempt to explore their consequences on nutrient limitation for sea ice algal growth
All simulations with turbulent diffusion (RL_Sim2– RL_Sim5, Table 1) predict higher bottom chlorophyll a (Chl a) concentration than with the standard molecular diffusion formulation (RL_Sim1) (Fig. 1a)
Summary
Heat, and mass fluxes between the ocean and sea ice are of utmost importance to predict sea ice motion, thermodynamics, and biogeochemistry. We may divide the ocean–ice exchange processes into those related to (i) entrapment during freezing; (ii) flushing and release during melting; (iii) brine gravity drainage, driven by density instability, parameterized as either a diffusive or a convective process; (iv) molecular diffusion; and (v) turbulent diffusion at the interface between the ocean and the ice induced by velocity shear – the latter process being the focus of this study (e.g., Arrigo et al, 1993, and references therein; Jin et al, 2006; McPhee, 2008; Notz and Worster, 2009; Turner et al, 2013; Tedesco and Vichi, 2010, 2019; Jeffery et al, 2011; Vancoppenolle et al, 2013) These processes are considered in several sea ice models.
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