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

A Sirevaag,S De La Rosa,M G Mcphee,M Tjernström,M Nicolaus,I Fer

doi:10.5194/os-7-335-2011

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.

Highlights

The Arctic plays an important role in the global climate system; it is home to key processes for ocean-atmosphere exchange and water mass modification, and it is a strong indicator of the global climate change, as atmospheric temperature changes are amplified in the Arctic (Screen and Simmonds, 2010; Serreze et al, 2009)
As part of the multidisciplinary Arctic Summer Cloud Ocean Study (ASCOS) in 2008, a comprehensive upper ocean measurement program was performed during a 16-days long drifting ice station centred at 87.5◦ N and 5◦ E
The measurements show the development of the upper ocean stratification, heat content, and iceocean-atmosphere interaction in the transition period from the end of the melting season to the initialization of the freezing season

Summary

Introduction

The Arctic plays an important role in the global climate system; it is home to key processes for ocean-atmosphere exchange and water mass modification, and it is a strong indicator of the global climate change, as atmospheric temperature changes are amplified in the Arctic (Screen and Simmonds, 2010; Serreze et al, 2009). Due to an earlier onset of melting and later onset of freezing, the melting season in the central Arctic has increased by 10 days since 1979 (Markus et al, 2009), which reflects a shift towards a more seasonal ice cover (Kinnard et al, 2008; Maslanik et al, 2007; Nghiem et al, 2007). A shift towards a more seasonal Arctic sea ice cover have strong implications for maintaining the marine biological life, as biological productivity is maintained through the provision of nutrients by upwelling and eddies in the expanding open water areas (Niebauer and Alexander, 1985)

Results

Discussion

Conclusion