One‐dimensional evolution of the upper water column in the Atlantic sector of the Arctic Ocean in winter

Abstract

A one‐dimensional model is employed to reproduce the observed time evolution of hydrographic properties in the upper water column during winter, between 26 January and 11 March 2015, in a region north of Svalbard in the Nansen Basin of the Arctic Ocean. From an observed initial state, vertical diffusion equations for temperature and salinity give the hydrographic conditions at a later stage. Observations of microstructure are used to synthesize profiles of vertical diffusivity, K, representative of varying wind forcing conditions. The ice‐ocean heat and salt fluxes at the ice‐ocean interface are implemented as external source terms, estimated from the salt and enthalpy budgets, using friction velocity from the Rossby similarity drag relation, and the ice core temperature profiles. We are able to reproduce the temporal evolution of hydrography satisfactorily for two pairs of measured profiles, suggesting that the vertical processes dominated the observed changes. Sensitivity tests reveal a significant dependence on K. Variation in other variables, such as the temperature gradient of the sea ice, the fraction of heat going to ice melt, and the turbulent exchange coefficient for heat, are relatively less important. The increase in salinity as a result of freezing and brine release is approximately 10%, significantly less than that due to entrainment (90%) from beneath the mixed layer. Entrainment was elevated during episodic storm events, leading to melting. The results highlight the contribution of storms to mixing in the upper Arctic Ocean and its impact on ice melt and mixed‐layer salt and nutrient budgets.

Turbulent heat and momentum fluxes in the upper ocean under Arctic sea ice

“…The similar TS properties (Figure c) indicate that the water mass is essentially the same, and the change in the profiles is likely due to local processes, rather than, e.g., a shift in the AW layer. This is supported by the findings of Fer et al []. The divergence of the ice field in response to northerly winds could also drive upwelling of the pycnocline, as was previously observed by McPhee et al .…”

“…Using idealized one‐dimensional modeling, Fer et al . [] attribute this mainly to entrainment of warm water from below. On 14 March, a storm accelerated the ice drift, but the ice broke up, and the instruments lost power before the storm peaked.…”

“…The occurrence distribution of the observed heat fluxes is shown in Figure , and it is clear that storm events in winter shift the distribution toward higher, although still modest, heat fluxes. Using a simple one‐dimensional model, Fer et al [] show that the hydrographic evolution of the upper ocean during the N‐ICE2015 winter drift can be fairly well reproduced from realistic forcing from variable profiles of eddy diffusivity in the water column. The increase in heat flux during winter storms can be attributed to entrainment of relatively warm waters from deeper in the water column.…”

Effects of sea‐ice and biogeochemical processes and storms on under‐ice water fCO2 during the winter‐spring transition in the high Arctic Ocean: Implications for sea‐air CO2 fluxes

“…Fer et al . [] investigated the proportions of vertical mixing and brine contribution to salinity in winter to be 90 and 10%, respectively.…”

Atmosphere‐Ice‐Ocean‐Ecosystem Processes in a Thinner Arctic Sea Ice Regime: The Norwegian Young Sea ICE (N‐ICE2015) Expedition

“…The spring/summer storms were less intense and traveled north through the Barents and Kara Seas (Cohen et al, ). The passing storms made a significant impact, for example, on the surface energy budget (Walden et al, ), the entire tropospheric column and boundary layer characteristics (Kayser et al, ), sea ice deformation and drift (Itkin et al, ), and upper ocean mixing and thus turbulent ocean heat fluxes (Fer et al, ; Meyer et al, ; Peterson et al, ). Winter observations and direct observations from Arctic storms are rare, and the N‐ICE2015 campaign provided the first direct integrated observations of the effect of such storms in the Arctic Basin.…”