Solar radiation over and under sea ice at ROV station PS86/081-1 during POLARSTERN cruise PS86 (AURORA) in July 2014

The observed changes in physical properties of sea ice such as decreased thickness and increased melt pond cover severely impact the energy budget of Arctic sea ice. Increased light transmission leads to increased deposition of solar energy in the upper ocean and thus plays a crucial role for amount and timing of sea-ice-melt and under-ice primary production. Recent developments in underwater technology provide new opportunities to study light transmission below the largely inaccessible underside of sea ice. We measured spectral under-ice radiance and irradiance using the new Nereid Under-Ice (NUI) underwater robotic vehicle, during a cruise of the R/V Polarstern to 83°N 6°W in the Arctic Ocean in July 2014. NUI is a next generation hybrid remotely operated vehicle (H-ROV) designed for both remotely piloted and autonomous surveys underneath land-fast and moving sea ice. Here we present results from one of the first comprehensive scientific dives of NUI employing its interdisciplinary sensor suite. We combine under-ice optical measurements with three dimensional under-ice topography (multibeam sonar) and aerial images of the surface conditions. We investigate the influence of spatially varying ice-thickness and surface properties on the spatial variability of light transmittance during summer. Our results show that surface properties such as melt ponds dominate the spatial distribution of the under-ice light field on small scales (<1000 m**2), while sea ice-thickness is the most important predictor for light transmission on larger scales. In addition, we propose the use of an algorithm to obtain histograms of light transmission from distributions of sea ice thickness and surface albedo.

Discarge compilation from The Global River Discharge (RivDIS) Project

The Global River Discharge (RivDIS) data set contains monthly discharge measurements for 1018 stations located throughout the world. The period of record varies widely from station to station, with a mean of 21.5 years. These data were digitized from published UNESCO archives by Charles Voromarty, Balaze Fekete, and B.A. Tucker of the Complex Systems Research Center (CSRC) at the University of New Hampshire. River discharge is typically measured through the use of a rating curve that relates local water level height to discharge. This rating curve is used to estimate discharge from the observed water level. The rating curves are periodically rechecked and recalibrated through on-site measurement of discharge and river stage.

Data compilation of ESOP

The work in this sub-project of ESOP focuses on the advective and convective transforma-tion of water masses in the Greenland Sea and its neighbouring areas. It includes observational work on the sub-mesoscale and analysis of hydrographic data up to the gyre-scale. Observations of active convective plumes were made with a towed chain equipped with up to 80 CTD sensors, giving a horizontal and vertical resolution of the hydrographic fields of a few metres. The observed scales of the penetrative convective plumes compare well with those given by theory. On the mesoscale the structure of homogeneous eddies formed as a result of deep convection was observed and the associated mixing and renewal of the intermediate layers quantified. The relative importance and efficiency of thermal and haline penetrative convection in relation to the surface boundary conditions (heat and salt fluxes and ice cover) and the ambient stratification are studied using the multi year time series of hydro-graphic data in the central Greenland Sea. The modification of the water column of the Greenland Sea gyre through advection from and mixing with water at its rim is assessed on longer time scales. The relative contributions are quantified using modern water mass analysis methods based on inverse techniques. Likewise the convective renewal and the spreading of the Arctic Intermediate Water from its formation area is quantified. The aim is to budget the heat and salt content of the water column, in particular of the low salinity surface layer, and to relate its seasonal and interannual variability to the lateral fluxes and the fluxes at the air-sea-ice interface. This will allow to estimate residence times for the different layers of the Greenland Sea gyre, a quantity important for the description of the Polar Ocean carbon cycle.