Arctic Ocean under Melting ice

The Fram Strait is the only deep passage for the exchange of water masses between the Atlantic and Polahavet. On the eastern side of the strait, the West Spitsbergen Current transports warm Atlantic waters into the Arctic, on the west side of Framstredet, sea ice and ice water from the Arctic transported southward with the Est Greenland Current. The Alfred Wegner Institute and the Norwegian Polar Institute have operated an ocean observation system since 1997 to monitor this key area. This observation system has consisted of up to 17 rigs across the strait along 780 50? N, but still not been able to solve mesoscale variability and recirculation in the area. This results in significant errors in transport estimation. To support the ocean observations in the Fram strait, an acoustic conservation system has gradually been built up. In 2008, system consisting of one source and one receiver mooring was implemented in Fram Strait. This pilot was followed up with an extended system consisting of three low frequency sources in integration with vertical hydrophone cables. This system was operational from 2010-2012, and glider navigation was tested. These two first experiments were carried with funding through EU projects. In UNDER-ICE the experiences from these experiments were used to design and implement an upgraded acoustic thermometry experiment. The system in 2014 was designed to provide travel-time data along 8 sections with two source mooring and three stand alone receiver moorings. Each of the five moorings had 10 hydrophones placed on a 100 m long cable. The moorings were augmented with oceanographic point measurements. The system of five moorings was out for 2 years. After recovering the mooring system, the data was processed to provide time series of accurately measured times and oceanographic data. The acoustic travel-times are inverted to determine the average sound speed between the sound source and receiver. Sound speed is converted to the depth range average ocean temperature between the rigs, e.g. A change of 4.5 m / s corresponds to a change of 1 degree Celsius. The acoustic observation system for Framstredet is considered to be fully developed, but in order to make use of acoustic measurements in climate research it is important that time series be maintained. This requires resources, which is a general challenge for in situ observation systems in the ocean. Through UNDER-ICE, and earlier projects, it has been shown that tomographic data can help to evaluate ice-ocean models used for climate and operational services. However, the main objective of UNDER-ICE was to improve volume and heat transport into the Arctic Ocean by combining acoustic time data into a marine model. It proved difficult to use acoustic travel-time data directly into assimilation because the ocean model did not include small scale variability. In UNDER-ICE, we decided to use inversion results for assimilation, i.e. the depth and range averaged sound speed along the acoustic sections. By assimilating sound speed, it is achieved to update both average temperature and average salt. We now have the methodology ready to assimilate the data from the UNDER-ICE / ACOBAR experiments in a regional ice-ocean model. The test assimilations show that acoustic data improves state estimation. This will help to improve estimates for the exchange of water masses between the Atlantic and the Arctic Ocean. Based on our results we consider the combination of acoustic measurements, point measurements and ocean models through data assimilation as the best method for achieving the best estimate of the exchange through Fram Strait.

UNDER-ICE is an interdisciplinary proposal involving physical and biological oceanography, ocean acoustics and sea ice in the area of the Fram Strait and northwards to the Gakkel ridge. The proposal will focus on observational and modelling studies of wat er masses, fluxes and physical processes in ice-covered areas and the marginal ice zone. Innovative observing systems based on ocean acoustics, hovercraft and gliders will be applied to collect ocean and sea ice data. The new data will combined with exist ing data, including satellite data and state-of-the art numerical models to study processes of importance for the Arctic climate system. UNDER-ICE will contribute to improved understanding of ocean circulation, water mass distribution and mixing processes . Data sets provided by the project will be organised and disseminated by a data management system. The data analysis and model simulations in the project will be used to validate climate model simulations from CMIP5 and assess the future evolution of th e Arctic and sub-Arctic climate. UNDER-ICE is an international project proposal submitted by Nansen Environmental and Remote Sensing Center (NERSC) in collaboration with Scripps Institution of Oceanography (SIO), Institute of Oceanology Polish Academy o f Sciennces (IOPAS), University Center in Svalbard (UNIS) and Geophysical Institute (GFI), University of Bergen.