Mixing, heat fluxes and heat content evolution of the Arctic Ocean mixed layer

Abstract

Abstract.A comprehensive measurement program was conducted during 16 days of a 3 week long ice pack drift, from 15 August to 1 September 2008 in the central Amundsen Basin, Arctic Ocean. The data, sampled as part of the Arctic Summer Cloud Ocean Study (ASCOS), included upper ocean stratification, mixing and heat transfer as well as transmittance solar radiation through the ice. The observations give insight into the evolution of the upper layers of the Arctic Ocean in the transition period from melting to freezing. The ocean mixed layer was found to be heated from above and, for summer conditions, the net heat flux through the ice accounted for 22 % of the observed change in mixed layer heat content. Heat was mixed downward within the mixed layer and a small, downward heat flux across the base of the mixed layer accounted for the accumulated heat in the upper cold halocline during the melting season. On average, the ocean mixed layer was cooled by an ocean heat flux at the ice/ocean interface (1.2 W m −2 ) and heated by solar radiation through the ice (−2.6 W m −2 ). An abrupt change in surface conditions halfway into the drift due to freezing and snowfall showed distinct signatures in the data set and allowed for inferences and comparisons to be made for cases of contrasting forcing conditions. Transmittance of solar radiation was reduced by 59 % in the latter period. From hydrographic observations obtained earlier in the melting season, in the same region, we infer a total fresh water equivalent of 3.3 m accumulated in the upper ocean, which together with the observed saltier winter mixed layer indicates a transition towards a more seasonal ice cover in the Arctic.

neXtSIM: a new Lagrangian sea ice model

“…The heat flux resulting from the nudging is usually slightly below 0.5 W m −2 in the central Arctic in midwinter, which is a reasonable value (see e.g. Sirevaag et al, 2011).…”

Advances in understanding and parameterization of small-scale physical processes in the marine Arctic climate system: a review

“…Net surface energy residuals, however, were significantly reduced by redistribution of heat and moisture via near-surface turbulence and heat conduction in snow/ice. The increase of the surface albedo, that eventually put the energy balance beyond recovery, was not gradual but a result of heavy frost formation and melt pond freezing during a short colder period with new snowfall Sirevaag et al, 2011;Tjernström et al, 2012). The onset of freeze-up was not realized until the low-level Arctic MPS became tenuous and cloud LWP decreased below 20 g m −2 -essentially diminishing the cloud greenhouse effect.…”

Meteorological conditions in the central Arctic summer during the Arctic Summer Cloud Ocean Study (ASCOS)

“…A weather system on 23 August (DoY 236) additionally covered the surface with a layer of new snow. The transmission of solar radiation through the ice also went through an abrupt change on 24 August Sirevaag et al, 2011); this also ended the 3rd period. The surface albedo increased from ∼ 70 % to ∼ 85 % from before to after the 3rd period (not shown) and the surface energy balance did not recover to sustained positive values again.…”