Solar heating of the Arctic mixed layer

Abstract

Data from the 1975 Arctic Ice Dynamics Joint Experiment (AIDJEX) are used to examine energy exchange between the Arctic mixed layer and the ice pack. Conductivity‐temperature‐depth profiles from four drifting stations reveal significant heat storage in the upper 50 m of the water column during summer, with mixed layer temperature elevation above freezing δT reaching as high as 0.4°C. Combining δT with turbulent friction velocity obtained from local ice motion provides an estimate of heat flux from the ocean to the ice Fw which was found to be strongly seasonal, with maximum values reaching 40–60 W m−2 in August. The annual average value of Fw was 5.1 W m−2, about half again as large as oceanic heat flux inferred from bottom ablation measurements in undeformed ice at the central station. Solar heat input to the upper ocean through open leads and thin ice, estimated using an ice thickness distribution model, totaled about 150 MJ m−2, in general agreement with integrated values of Fw. Results indicate that oceanic heat flux to the ice in the central Arctic is derived mainly from shortwave radiation entering the ocean through the ice pack, rather than from diffusion of warm water from below. Indeed, during the AIDJEX project the mixed layer appears to have contributed 15–20 MJ m−2 of heat to the upper pycnocline. During the summer, Fw was found to vary by as much as 10–30 W m−2 over separations of 100 to 200 km and thus represents an important term in the surface heat budget not controlled by purely local deformation and thermodynamics.

Observations of the annual cycle of sea ice temperature and mass balance

“…Surface ablation in ponded ice was notably greater, totaling 85 cm. During AIDJEX the average ablation for unponded ice was 26 cm on the surface and 34 cm on the bottom (Maykut and McPhee, 1995). Total ablation during SIMI was 50% greater than observed during AIDJEX and nearly double that observed during Alpha II on unponded ice.…”

“…This is due, in large part, to the expense involved in operating a long-term field camp, which typically serves as the base for these studies. The combined results of four sea ice mass balance studies (Untersteiner, 1961;Hanson, 1965; Koerner, 1973;Maykut and McPhee, 1995) suggest that there is significant interannual variability in the ablation/accretion rate This paper is not subject to U.S. copyright. Published in !997 by the American Geophysical Union.…”

Field observations and results of a 1-D boundary layer model for developing near-surface temperature maxima in the Western Arctic

“…Early studies of the NSTM during AIDJEX (Maykut and McPhee, 1995) and SHEBA (McPhee et al, 1998) found that the layer was present directly below the summer surface mixed layer, at depths between 25 and 35 m. However, the Canada Basin upper ocean is freshening (McPhee et al, 2009) Summer sea ice extent in the Western Arctic has decreased significantly in recent years resulting in increased solar input into the upper ocean. Here, a comprehensive set of in situ shipboard, on-ice, and autonomous ice-ocean measurements were made of the early stages of formation of the near-surface temperature maximum (NSTM) in the Canada Basin.…”

“…Early studies of the NSTM during AIDJEX (Maykut and McPhee, 1995) and SHEBA (McPhee et al, 1998) found that the layer was present directly below the summer surface mixed layer, at depths between 25 and 35 m. However, the Canada Basin upper ocean is freshening (McPhee et al, 2009) through a combination of sea ice melt, river runoff, and convergence of Ekman boundary layer transports under the Beaufort Gyre (MacDonald et al, 1999;…”

“…Although NSTM heat is gained in the summer, the release of this heat often occurs in later seasons. Observations in the Canada Basin show that the NSTM often survives into fall, and that heat from this layer can be mixed into the surface mixed layer to delay or slow freeze up (Steele et al, 2008;Jackson et al, 2010;Steele et al, 2011;Timmermans, 2015).Early studies of the NSTM during AIDJEX (Maykut and McPhee, 1995) and SHEBA (McPhee et al, 1998) found that the layer was present directly below the summer surface mixed layer, at depths between 25 and 35 m. However, the Canada Basin upper ocean is freshening (McPhee et al, 2009) Summer sea ice extent in the Western Arctic has decreased significantly in recent years resulting in increased solar input into the upper ocean. Here, a comprehensive set of in situ shipboard, on-ice, and autonomous ice-ocean measurements were made of the early stages of formation of the near-surface temperature maximum (NSTM) in the Canada Basin.…”