Dr. Sara de la Rosá defended her doctoral dissertation titled Temporal changes of sea ice affected by waves and thermal forcing at the Nansen Center on September 30.

The motivation of the study is based on the fact that the summer Arctic sea ice conditions have changed during the last decades. The area cover has steadily decreased and much of the thick and old multi-year sea ice also been fractionated and lost, leaving thousands of square kilometers of open and wave-exposed ocean areas where one would have found thick pack ice twenty years ago. Whether this tendency can be reversed or not in the future will depend on how much ocean water can re-freeze during each winter season and survive successive summers.

Ice formation in open water initially starts as small crystals known as ‘frazil’, which gradually pile-up through the mechanical action of waves, compact and freeze together into circular patches known as ‘pancake ice’. As ocean water freezes, the salt is ejected from the crystals. This increases the ocean density below the ice, enough for it to sink to great depths. The dense, cold and salty waters formed play a crucial role in the thermohaline circulation between the Arctic and Atlantic Oceans. As new ice is becoming more common in the Arctic, along the seasonal sea-ice edge as well as in coastal regions, a better description of the thermodynamic and mechanical ice formation characteristics is necessary to numerically model the total ice-production in the Arctic.  

The Thesis consists of 4 scientific papers. Two are published in international journals, while the other two are currently under review:

Paper I: Thermodynamic investigation of a grease to pancake ice cover. De la Rosa, S., Maus, S. and Kern, S. (2011). Annals of Glaciology, 52 (57), 206–214.

Paper II: Laboratory study of frazil ice accumulation under wave conditions. De la Rosa, S. and Maus, S. (2011). The Cryosphere, (under review)

Paper III: Salinity and solid fraction of frazil and grease ice. Maus, S. and De la Rosa, S. (2011). Journal of Glaciology, (under review)

Paper IV: Mixing, heat fluxes and heat content evolution of the Arctic Ocean mixed layer. Sirevaag, A., de la Rosa, S., Fer, I., Nicolaus, M., Tjernstrom, M. and McPhee, M. (2011). Ocean Science, 7, 335–349.

A central focus of this work was to study the properties of processes required to model ice-growth under wavy conditions. Emphasis is given on the volume of compaction, which determines the transition between the loose frazil crystals and pancake ice. Data was obtained from a set of complex laboratory experiments, which allowed to study the combined effects of surface cooling, new sea ice formation and waves in turbulent waters. A methodological review of the various approaches that may be used to determine the bulk salinity and the solid ice volume fraction of young grease ice is given.

The fourth paper of the thesis focuses on the heat content evolution of the uppermost 30 m of water underlying the thick multi-year sea ice in the central Arctic Ocean. Ice-melt during the summer months greatly affects the transfer of solar radiation through the ice cover and open ocean.  The accumulation of heat in the ocean underlying the ice is of particular interest as it may change the under-ice dynamics and melting rates during successive summers. Results of this study illustrate the variability caused by changing surface conditions over relatively short-time scales, and also suggest that there is a transition towards a more seasonal ice cover in the Arctic.  These are all processes that need to be investigated further in future studies to assess seasonal and interannual changes.