Sea salt aerosol (SSA) may influence regional climate directly through scattering of radiation or indirectly via its role as cloud-forming particles. While it is well known that SSA can be cloud condensation nuclei (CCN) forming cloud droplets, it has been shown only recently that SSA can also be a source of ice nucleating particles (INP) forming ice crystals, depending on its chemical composition and surface shape. Arctic clouds are poorly represented in climate models, which is partly due to a lack of understanding of source and nucleating capability of natural aerosol in the high Arctic. Aerosol models for example do currently not capture aerosol maxima in the Arctic winter/spring observed at high latitudes. Recent field campaigns provide first evidence of a hypothesized source of SSA from salty blowing snow (BSn) above sea ice. During storms salty snow gets lofted into the air and undergoes sublimation to generate SSA. Additional but minor SSA sea ice sources are frost flowers and open leads. The impact on radiation and clouds of SSA from this new source of SSA above sea ice is not known. However, a quantitative understanding of natural aerosol processes and climate interactions is needed to provide a baseline against which to assess anthropogenic pollution reaching the Arctic and evaluate the success of mitigation measures. We therefore propose to determine the SSA source, fate and potential impact on Arctic climate associated with blowing snow above sea ice and other sea ice sources. To do this we seek funding to participate in the year-long Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) to observe aerosol processes in the central Arctic ocean throughout all seasons. Proposed measurements on the sea ice and on-board "FS Polarstern" include particle size and concentration (sub-micron to snow particle size), INP concentrations, and a range of chemical properties using aerosol filters. Sampling of snow on sea ice, brine, frost flowers will constrain the local source of SSA. Tethered balloon launches will yield information on the fate of particles formed near the sea ice surface as they get lofted to heights where clouds may form.

The oldest, thickest sea ice in the 'last ice area' of the Arctic - a region thought to be most resilient to climate warming - unexpectedly broke up twice in the past year. Our current theories assume that the end-of-summer ice-covered area will steadily retreat into the Central Arctic Basin as global warming accelerates over coming decades. However, the dynamic break-up events witnessed in 2018 challenge this prevailing view. Here we hypothesise that a weaker, increasingly mobile Central Arctic ice pack is now susceptible to dynamic episodes of fragmentation which can precondition the ice for rapid summer melt. This mechanism of dynamic seasonal preconditioning is unaccounted for in global climate models, so our best current projections are overlooking the possibility for rapid disintegration of the Arctic's last ice area. Our team has demonstrated that seasonal preconditioning is already responsible for the neighbouring Beaufort Sea becoming ice-free twice in the past five years. Even ten years ago this region contained thick perennial sea ice, mirroring the Central Arctic Ocean, but it has now transitioned to a marginal Arctic sea. Could the processes responsible for the decline of the Beaufort Sea ice pack start to manifest themselves in the Central Arctic? Currently, a shortfall in satellite observations of the Arctic pack ice in summer prevents us from testing our hypothesis. We desperately require pan-Arctic observations of ice melting rates, but so far satellite observations of sea ice thickness are only available during winter months. Our project will therefore deliver the first measurements of Arctic sea ice thickness during summer months, from twin satellites: ESA's Cryosat-2 & NASA's ICESat-2. We have designed a new classification algorithm for separating ice and ocean radar altimeter echoes, regardless of surface melting state, providing the breakthrough required to fill the existing summer observation 'gap'. Exploiting the recent launch of multiple SAR missions for polar reconnaissance, our project will integrate information on ice-pack ablation, motion and deformation to generate a unique year-round sea ice volume budget in the High Arctic. This record will inform high-resolution ice dynamics simulations, performed with a suite of state-of-the-art sea ice models from stand alone (CICE), ocean-sea ice (NEMO/CICE), to fully coupled regional high resolution (RASM), and global coarser resolution (HadGEM) models, all now equipped with the anisotropic (EAP) sea ice rheology developed by our team. Using the regional and stand-alone models we will analyse the role of mechanics in this keystone region north of Greenland to scrutinise the coupling and preconditioning of winter breakup events - such as those witnessed in 2018 - to summer melting rates. Using the coupled models, we will quantify the likelihood of the Arctic's last ice area breaking up much sooner than expected due to oceanic and atmospheric feedbacks and how this will affect the flushing of ice and freshwater into the North Atlantic.

The decline of Arctic Ocean seasonal sea ice cover over the past two decades is a major indicator of polar climate change. Over the same period satellite observations have implied that net primary productivity (NPP) has increased by at least 30%. However, the observed increase in net primary productivity is greater than the predicted response to the declining sea ice, and the consequent lengthening of the ice-free season. This implies that the nitrate-limited Arctic marine ecosystems may also be experiencing increasing nutrient availability. Whilst the impact of riverine nutrients is limited to coastal areas the greatest net primary production increases are observed over the shelf break regions. In these regions the primary source of nutrients is intruding Pacific and Atlantic Water. However this water can reside at depths of 100 or more metres and so physical mixing processes are required to transport nutrients up to the nutrient replete euphotic zone. This leads us to hypothesize that the observed increases in net primary production in the shelf break regions are driven by escalating nutrient fluxes from the deep waters (Atlantic, Pacific) into the euphotic zone as a result of enhanced vertical mixing rates. However, there is a very strong seasonality in the availability of light in the Arctic, due to both the formation of sea ice and also changing day length - from the perpetual darkness of winter to the mid-night sun - enabling accumulation of nutrients close to the surface in winter and so implying a strong seasonal cycle in nutrient fluxes to the surface layer. Furthermore our own turbulence measurements have shown mixing in the Arctic to be highly intermittant (and in consequence fluxes varying by up to 3 orders of magnitude) on timescales as short as an hour. These facts imply that in order to quantify the flux of nutrients from intermediate depths towards the sea surface measurements are required which resolve timescales, from hourly to seasonally. The aim of this project is to test the hypothesis that increased primary production is promoted by increased availability of nutrients resulting from increased nutrient fluxes. Data will be collected to test this hypothesis, including employing novel acoustic Doppler techniques developed at Bangor University, to make turbulent mixing rate and nutrient flux estimates using profilers and from moorings at contrasting locations around the Arctic shelf break and interior, on timescales from hourly to the full seasonal cycle. These will then be compared to baseline measurements made at these locations by ourselves and others during the recent 2007/8 International Polar Year. The new measurements will be integrated with coincident fluorescence timeseries measurement, within the framework of a biogeochemical model, to quantify the impact of the observed changes in the nutrient environment, on net primary productivity, and to deduce intra-seasonal ecosystem responses to specific flux events.

At the base of the Arctic food web, there are three major primary producers: small flagellates, diatoms living in open water (pelagic) and diatoms growing in sea ice (sympagic). The role of the sea ice diatoms is perceived differently across the research community. For ecologists they are central to the polar ecosystem, while those looking at global ocean scales consider them less important and have not incorporated them into their models projecting climate change feedbacks. This may reflect their minor (<10%) contribution to the total primary production in Arctic waters. However, two newly developed trophic marker approaches that can trace diatoms from sea ice and open water within the food web, consistently find a strong ice algae 'signal' in polar consumers. Even in whales, seals and polar bears, as much as 80% of their body fat reserves are from carbon originally fixed by ice algae. How is this possible? How will this change in a warming Arctic? Our project aims to answer this puzzle and to bridge the gap between the contrasting perceptions of ice algae. We propose to quantify the relative importance of ice algae vs. open water diatoms for consumers living in the high Arctic - considering different species, regions and times of the year. We will also look at material that sinks to the seabed, and is collected in sediment traps. Our first hypothesis is that the input of ice algae to Arctic food webs and to export fluxes is disproportionately higher than their contribution to total primary production. Our second hypothesis examines the mechanisms behind these energy transfers, focussing on the more subtle concept of food benefit. It is not just the total annual amount of food that matters; it also has to arrive at the right time, be accessible and be nutritious. To test these hypotheses, we have developed a method based on "Highly Branched Isoprenoids" (HBIs). These lipid molecules are specific to a series of diatom species specific either to sea ice or open water. Using the ratio of ice-versus water column-derived HBIs, we can now trace the relative roles of these energy inputs to the food web. The chemical stability of these molecules as they pass through the food web is a key advantage of this tracer method, as previously it has been very difficult to follow the fate of ice- or water column derived algae. We propose to take part in an ice drift across the Central Arctic Ocean (MOSAiC) that will give the opportunity to sample the foodweb and material from sediment traps for subsequent HBI analysis in our lab in Plymouth. We will also determine the body condition of various consumers as an integrator of net benefit derived from each food type over the season. The cruise data set will be complemented with data from other Arctic expeditions and those estimated with a second, independent diet method by our Project partners. This will give a pan-Arctic overview of the importance of ice algae to the lipid stores of key consumers. Then, simulation model outputs of future climate projection will allow scaling up to the whole Arctic Basin. First, we will work with Project partners modelling life cycles of key zooplankton species, to estimate their potential to colonise a future, more ice-free central Arctic Ocean. Second, we will use NEMO-MEDUSA - the oceanic component of the UK's Earth system model (UKESM1) - to determine whether projected increases in pelagic primary production could compensate for loss of ice algae as a food source for zooplankton. Our findings, and those of other participants in MOSAiC, will be used to initiate a "roadmap" for the incorporation of ice algae into NEMO-MEDUSA. By helping to bridge between the physical, biogeochemical and ecological functions of sea ice and requirements of large-scale modelling, we aim to improve our understanding of the changing Arctic and its provision of services to mankind.

Arctic sea ice area has been mapped for nearly four decades using the long-term data record provided by successive passive microwave satellite missions; showing an accelerated pace of ice loss since 1979. Less is known about how much the ice has also thinned, in part because of the lack of a similarly long-term and consistent data record on sea ice thickness. Radar altimeters, such as the one flown on the European Space Agency (ESA)'s CryoSat-2 (CS2) since April 2010, and the SARAL/AltiKa satellite, launched in February 2013 as part of a joint mission by the Centre National d'Etudes Spatiales (CNES) and the Indian Space Research Organization (ISRO), are now providing pan-Arctic (or up to 81.5N for AltiKa) thickness observations. However, one key uncertainty in using these data is how far the radar actually penetrates into the overlying snow cover. The general assumption has been that the radar return is from the snow-ice interface at Ku-band (CS2) frequencies, and from the snow-air interface at Ka-band (AltiKa) frequencies. Using this information together with assumptions on the depth of the overlying snow pack and its density, scientists can then convert the radar returns into total ice thickness assuming hydrostatic equilibrium. However, field evidence has put this general assumption into question, even for a homogeneous snowpack. A further complication is the lack of knowledge on how deep the snow pack is and its density. Typically, snow depth and density information based on a climatology constructed over thick multiyear ice in the 1980s have been used. However, as the total area in the sea ice cover has declined, there is now a larger proportion of first-year sea ice in the Arctic Basin. Snow over first-year ice tends to be more saline than over multiyear ice, and as such it has the potential for a significant impact on the radar returns. In addition, autumn and winter freeze-up has been delayed by several weeks to months in certain regions of the Arctic, shortening the duration for accumulation of snow. Given these current uncertainties, it is difficult to accurately assess how sea ice thickness is changing from year to year and over the long-term. Because sea ice is an important indicator of climate change, plays a fundamental role in the Arctic energy and freshwater balance, and is a key component of the marine ecosystem, it is essential that we improve the accuracy of thickness retrievals from radar altimetry. This project aims to do just that by making ground-based observations of the radar penetration depth over a full annual cycle at both Ku- and Ka-band frequencies, from autumn freeze-up, through winter snow metamorphism and summer melt. This information, together with detailed snow pack characteristics, will allow us to assess how changes in snow accumulation, snow morphology and snow salinity impact Ku- and Ka-band penetration factors. The MOSAiC drifting station provides a unique opportunity, possibly the only opportunity, to obtain a benchmark dataset that involves coherent field, airborne and satellite data. Analysis of this information will enable scientists to better characterize how the physical properties of the snow pack (above different ice types) influence the penetration of Ka and Ku band radar. Importantly, we will be able to evaluate the seasonal evolution of the snow pack over first-year (sea ice greater than a few cm) and multiyear sea ice. MOSAiC additionally provides the opportunity for year-round observations of snow depth and density that will allow for assessment of the validity of climatological assumptions typically employed in thickness retrievals from radar altimetry and provide data for validation of snow depth products. These activities are essential in order to improve sea ice thickness retrievals from radar altimetry over the many ice and snow conditions found in the Arctic.