On Thin Ice: Role of Ocean Heat Flux in Sea Ice Melt

The present area, thickness and mobility of the Arctic ice cover are a manifestation of climate change with substantial implications for ocean circulation, global weather, economics and governance. The difference between sea ice extent maximum in winter and minimum in summer is increasing. Consequently, the chemical, biological and physical cycles associated with freeze up and melt back are also potentially amplified. The role of ocean heat content in driving variations in the ice cover, either directly or through feedback mechanisms, is not yet fully understood. NICE aims to fill this gap through observational-based knowledge and process understanding. April 2014 to summer 2015 has been a NICE data collection period. Synergy is gained by collaborating with other national and international groups. In April 2014, we have sent a microstructure sensor package to the North Pole Environmental Observatory (NPEO) to be installed on the profiling frame that is deployed from drifting pack ice close to the North Pole. Below the drifting ice, the Arctic Ocean is quiet, not turbulent, and has a unique vertical temperature (T) and salinity (S) distribution above the warm Atlantic layer. The T and S profiles resemble a staircase with thin (approximately 10 cm) steps where T/S change rapidly and several meters thick layers where T/S is uniform. The measurements resolve the details of these structures and are used to quantify the upward transfer of heat from the warm deep layer. In summer 2014, we conducted the NICE cruise aboard the research vessel Håkon Mosby, north of Svalbard on the southern flanks of the Yermak Plateau. In addition to measurements from the ship, NICE utilizes bottom-anchored moored instruments to measure temperature, salinity and ocean currents in the water column for one year. The moorings were deployed in ice covered waters using the icebreaker coast guard vessel Svalbard, in collaboration with the Nansen Center, whereas our sampling from the Håkon Mosby concentrated on the ice-free waters near the ice edge. In summer 2015 we recovered all moorings and conducted process studies north of Svalbard. From one year of mooring measurements, we observe several events of enhanced near-inertial internal wave energy. The events relate to wind forcing at the surface, or to tidal currents interacting with rough topography at the bottom. Wind-generated internal waves reached depths of several hundred meters. Measurements from the ship also include data on turbulence in the water column which allow us to relate forcing to turbulence and mixing in the interior. From early 2015, we have participated in the drift of R/V Lance. Unique measurements of under-ice turbulence were made from a drifting ice camp north of Svalbard in the winter and spring of 2015. Spanning nearly half a year, the data set comprises observations of cold winter freezing conditions, winter storms and rapid melt events in spring. The camp drifted over the quiescent deep Nansen basin, across the shelf break onto the energetic Yermak plateau and into the marginal ice zone. The measurements have showed that oceanic heat reaches sea ice, even in winter, far from the ice edge. Passing storms increased the heat exchange significantly. Using winter-time drift data as input to a one-dimensional model, we show that, away from topography, observed salinity increase in the upper ocean was mostly mixed up from below, rather than salt released from ice growth. Closer to open water, and in spring, the vertical heat exchange was typically 10 to 100 times greater than in winter. During melting conditions, we observed sinking plumes of very salty water, previously not observed from drifting sea ice in the Arctic Ocean. The data provides new insight into processes of vertical mixing in the ice-ocean boundary layer in a region where warm Atlantic-originating water threatens the shrinking ice cover.

NICE is a small research project proposal to primarily support one PhD study and the associated field work expenses. The study aims to identify principal mechanisms for diapycnal mixing and ocean heat transport processes and quantifying their contribution for the Arctic heat and biogeochemical budgets. NICE builds on recently completed US-Norway collaboration projects and the synergy gained from recently funded or pending multidisciplinary, national and international initiatives. The main approach is in novative observation techniques including microstructure measurements up to the seawater-ice interface and eddy-covariance flux measurements in the under-ice boundary layer. Observations will be made in the MIZ, over topographic features north of Svalbard , in the central Arctic, as well as along the continental slope in the eastern Eurasian Basin and Makarov Basin, in conditions of retreating and advancing sea ice, allowing us to address various vertical mixing processes and evaluate their role in the ove rall heat and biogeochemical budgets of the Arctic Ocean. Overall, by sampling at various times under differing conditions of ice concentration, roughness, surface stress, sun angle, etc., we will be better able to characterize and model the exchange of s calar contaminants in the turbulent ice-ocean boundary layer.