Description: The Arctic Ocean is currently undergoing a dramatic change. Decreasing sea-ice extent, thickness and age are changing important processes in the climate system. An increasing coverage of the sea ice by melt ponds and an increased amount of light transmitted to the upper ocean are also affecting the ice associated ecosystem. To document these changes, we operated different remotely operated vehicles (ROV) underneath the drifting sea ice of the Central Arctic Ocean. The newest underwater technology combined with a highly interdisciplinary sensor suite was successfully used for scientific investigations directly under the sea ice. The unique dataset of novel observations provided insights into the partitioning of solar shortwave radiation in and under sea ice, the deformation and topography of the ice cover, the distribution of sea-ice algae and ice algal aggregates and the ice associated primary production. The large range covered by the ROV surveys enabled us to quantify the spatial variability of physical as well as habitat properties. Despite the harsh climatic conditions and logistical difficulties in the high Arctic, the latest ROV technology proved to be a valuable tool for interdisciplinary sea-ice research.

Description: In the paper “Changes in Arctic sea ice result in increasing light transmittance and absorption” by Nicolaus et al. (Geophysical Research Letters, 39(24), L24501, doi:10.1029/2012GL053738, 2012), the presented data on solar surface irradiance are erroneous. In order to generate monthly means of the solar heat input into the Arctic Ocean, among others, the ERA interim (European Centre for Medium-Range Weather Forecasts) data set of solar surface irradiance was used. The original data set consists of eight time slices with integrated fluxes over 3 h each. But in the presented results, only the mean (not sum) of two slices (from 00:00 to 03:00 and from 12:00 to 15:00) was considered, resulting in too low fluxes, approx. by a factor of eight. As a consequence, two text passages (in paragraph 3.1 and 3.3) and three figures (Figure 4 of the main article and Figures S3 and S6 of the auxiliary material) contain too low fluxes. However, the main conclusions of the manuscript remain completely valid and unchanged, since those are only based on relative fluxes, which are not affected by this mistake.

Description: Changes in the Arctic sea ice result in a thinner and younger ice cover with changing physical properties and strong impacts on the energy budget. In addition to long-term trends, the physical properties of sea ice and its snow cover underlay a strong seasonal and spatial variability that impact observations and conclusions. Here, I present methods and results of recent studies to observe the sea ice energy budget on different time and length scales. Radiation measurements over and under sea ice reveal changes in the optical properties of sea ice and their implications for the energy and mass budget. The combination of different measurement platforms, incl. remotely operated vehicles (ROV) and autonomous stations, with large-scale observations and parameterizations allows discussing key processes over large regions and during different seasons.

Description: Arctic sea ice has not only decreased in volume during the last decades, but has also changed in its physical properties towards a thinner and more seasonal ice cover. These changes strongly impact the energy budget, and might affect the ice-associated ecosystems. In this study, we quantify solar shortwave fluxes through sea ice for the entire Arctic during all seasons. To focus on sea-ice-related processes, we exclude fluxes through open water, scaling linearly with sea ice concentration. We present a new parameterization of light transmittance through sea ice for all seasons as a function of variable sea ice properties. The maximum monthly mean solar heat flux under the ice of 30 × 105 Jm−2 occurs in June, enough heat to melt 0.3 m of sea ice. Furthermore, our results suggest that 96% of the annual solar heat input through sea ice occurs during only a 4-month period from May to August. Applying the new parameterization to remote sensing and reanalysis data from 1979 to 2011, we find an increase in transmitted light of 1.5% yr−1 for all regions. This corresponds to an increase in potential sea ice bottom melt of 63% over the 33-year study period. Sensitivity studies reveal that the results depend strongly on the timing of melt onset and the correct classification of ice types. Assuming 2 weeks earlier melt onset, the annual transmitted solar radiation to the upper ocean increases by 20%. Continuing the observed transition from a mixed multi-year/first-year sea ice cover to a seasonal ice cover results in an increase in light transmittance by an additional 18%.

Description: The observed changes of Arctic sea ice during the last decades have a strong impact on interactions with the atmosphere and ocean. Due to a more seasonal ice cover the transmitted and absorbed solar radiation (light) of Arctic sea ice increases significantly. This, in turn, affects sea ice melt as well as biological and geochemical processes in and under Arctic sea ice. Up to now, it is not possible to observe light transmission sufficiently well over large regions and during different seasons. Hence, to obtain Arctic-wide estimates of light under sea ice, it is necessary to develop new methods. Here we present an upscaling method based on parameterization of light transmittance and remote sensing and reanalysis data.

Description: Arctic sea ice has declined and become thinner and more seasonal during the last decade. One consequence of this is that the surface energy budget of the Arctic Ocean is changing. Solar light transmitting into and through sea ice is of critical importance for the state of sea-ice and the timing and amount of primary production. The light field in and under sea ice is highly variable: horizontally, vertically, and over seasons. At the same time, observations of light transmittance through sea ice are still sparse, because the under-ice environment is difficult to access and high quality measurements are challenging. Furthermore, it is necessary to generalize measurements in order to obtain Arctic-wide estimates of light conditions and energy budgets.

Description: The mass and energy balance of sea ice are strongly connected through the transfer of solar radiation from the atmosphere through snow and sea ice into the ocean. Recent studies show that a major uncertainty in quantification of the sea ice mass balance is related to the timing and duration of the melt season as well as the very limited knowledge of the characteristics of the snow layer on top. Therefore, we are working on (1) improving our understanding of radiative transfer into and through Arctic and Antarctic sea ice and its impacts on sea-ice melt, and (2) improving existing and developing new remote sensing tools and data products. This allows for estimates of sea-ice melt and freeze rates, and large-scale estimates of heat fluxes in and under sea ice. Here we show established methods for melt onset detection on sea ice based on passive microwave data, and we present first new ideas for future improvements for onset detection methods.

Description: Arctic sea ice extent decreased considerably along with the ice cover becoming thinner and more seasonal during the last decades. These observed changes have a strong impact on interactions between atmosphere and ocean and thus play a major role in Earth’s climate system. Until now, it is not possible to quantify shortwave energy fluxes through sea ice sufficiently well over large regions and during different seasons. In order to obtain Arctic-wide estimates of solar radiation under sea ice, new methods are necessary. In this thesis, an upscaling method combining a newly developed parameterization of light transmittance and remote sensing and reanalysis data is presented. The main result suggests that 96% of the total annual solar heat input under Arctic sea ice occurs in the time from May to August, hence in the course of only four months of the year. Sensitivity studies indicate that once the melt season begins two weeks earlier, an increase by 20% of the total annual solar heat input through sea ice is shown. Therefore, the transition period from spring to summer, particularly the timing of the melt season, substantially affects the light availability under ice. Furthermore, a more seasonal ice cover and a higher melt pond coverage lead to higher fraction of solar radiation being transmitted through the sea ice in summer. This positive correlation between enhanced melting and increasing transmittance can be described as ’transmittance-melt feedback’. Assuming an ongoing ice thinning, the transmittance-melt feedback results in a further increase in transmitted and absorbed heat fluxes. Changes in timing and amount of light penetrating through Arctic sea ice might also influence biological and geochemical processes as well as basal and internal melt and freeze rates. These positive feedbacks affect the mass and energy budget of sea ice and alter crucially the interaction of atmosphere and the upper ocean.