Coordinated Arctic Acoustic Thermometry Experiment

Almost 30 years ago (1994) a 19.6 Hz signal was for the first time successfully transmitted across the Arctic Basin in the Transarctic Acoustic Propagation (TAP) experiment. In CAATEX we wanted to set up an acoustic thermometry experiment to again measure the mean ocean temperature across the Arctic Basin and compare with the measurements from the TAP experiment. In acoustic thermometry we measure the time it takes from a sound signal is transmitted till it is received at a location away from the sound source. The travel time measurements are used to find the sound speed. Accurate sound speed requires high accuracy in time keeping and distance between source and receiver. The sound speed is related to the temperature, salinity, and pressure through empirical formulas. Because of the declining sea ice extent and thickness, it was assumed that slightly higher frequency sources could be used. Two sources with center frequency at 35 Hz were selected, prepared, and programmed to transmit every third day for one-year. Two source moorings and four other receiving moorings were equipped with 25-40 hydrophones distributed over a 1000 m vertical array. All six moorings were equipped with oceanographic instruments for salinity and temperature measurements, an upward-looking sonar to measure the ice thickness, an acoustic current profiler, and pressure gauges to measure the ocean bottom pressure. The moorings were deployed along the path crossing the Arctic Ocean. One source transmitted from a mooring in the Nansen Basin in the Eastern Arctic, and a second source transmitted from the Beaufort Sea in the Western Arctic. The use of several moorings along the path makes it possible to measure travel times over different segments, and thereby provide more regional information about the travel times. The receiver arrays successfully recorded transmissions from each source during the yearlong deployment from fall 2019 to fall 2020. Due to the strong vertical stratification of the ocean under the sea ice, the acoustic propagation is best described by splitting up the acoustic field into depth-dependent modes. Mode 1 is trapped and propagates in the around 200 m thick layer with polar water, mode 2 propagates partly in the cold polar water and Atlantic water, while mode 3 and higher modes propagates in warmer Atlantic water and deeper water masses. The different modes will therefore have different group speeds reflecting temperatures in the different water layers. Due to earlier reports about a significant change in acoustic propagation, it was expected that there would be a significant change in acoustic travel times/group speeds of the modes due to warming of the Atlantic water. However, the analysis of the acoustic measurements does not indicate any significant warming of the ocean under the Arctic Sea ice in the central Arctic. A closer look is needed to investigate if we can observe other changes in the acoustic measurements, and if they can be explained by changes in sea ice and ocean stratification. The oceanographic data from various cruises show a weak increase of about 0.3°C from 1995 to 2021 in the Atlantic Layer (300-700 m) when considering the Eurasian part of the CAATEX section. Considering the full section from North of Svalbard to the Beaufort Sea, all CMIP6 models keep well below 0.5°C, up to 2020 in both the Atlantic Layer and Halocline Layer (100-300 m). At this time, the model spread is low, but afterwards there is a considerable increase in the model spread. Here we have used the weak future scenario (ssp126). In 2040, most CMIP6 models show a warming below 1°C in the Atlantic Layer and Halocline Layer, whereas 1-2 models show a larger temperature change. Another recent study published in science used the more aggressive ssp585 scenario and showed a similar warming in 2040; the multi-model mean shows a warming below 1°C in 0-700 m. Arctic Amplification in the atmosphere is already taking place, where the Arctic is warming nearly 4 times faster than globally. It seems that it will take longer until the ocean follows. The recent study from Science showed that an Arctic Ocean Amplification will occur in this century and projected that the Arctic Ocean will warm twice as fast than globally, averaged over this century. In our study we see a similar tendency with accelerated model spread and warming. Thus, it will be important to continue year-round monitoring of the Arctic Ocean, to follow-up when large changes are expected to occur in the coming years. A follow-up 5 year long project HiAOOS has been funded by HORIZON EUROPE. This project started up 1 January 2023.

CAATEX have shown by acoustic and oceanographic observations that the ocean temperature under the sea ice in the central Arctic have not yet changed significantly.This is also supported by reanalysis and climate models. However, the CMIP models indicates that we are now moving into decades where the ocean under the ice will change significantly, and therefore year round autonomous monitoring should be continued and intensified. CAATEX has established baseline data for future detection of changes in the ocean under the ice. It is therefore, important to continue and extend the monitoring of the ocean under the ice with moorings equipped with sensors ands instruments for several ocean parameters at fixed locations over several years. Furthermore by enabling the network for acoustic tomography and underwater geopositiong of underwater floats measurments from a larger region will be obtained. INTAROS and CAATEX has addressed the data delivery chain from advanced mooring systems. An INTAROS roadmap has been developed by more than 300 scientists with significant input from CAATEX. Roadmap will impact how the future Arctic Ocean Observation System is designed and implemented. CAATEX an INTAROS is followed up in the recently EU funded High Arctic Ocean Observation System (HiAOOS). The new project is coordinated by NERSC, and information will be available at https://hiaoos.eu. CAATEX has provided results which will be important for the development of methods and digital tools in HiAOOS and other projects. The CAATEX experiments in 2019 and 2020 required coordination and collaboration between several actors from research organizations, ice services and the coast guard. The operations included required navigation combining ship-radar and satellite remote sensing data to reach the planned mooring sites deep into the Nansen and Amundsen Basin and in the Beaufort sea. Deploying and recovering 4-kilometre-long moorings in harsh ice and weather conditions are complex operations that require clever mooring design, safety procedures, sea ice management, monitoring of the sea ice drift, bathymetric mapping, and accurate positioning the moorings. The CAATEX operations have given the coast guard and the involved scientists expertise in the planning and execution of complex operations in ice covered regions under harsh conditions. Good practices for deep water mooring design and procedures for deployment and recovery in ice covered regions has been developed. The expertise the ice service at Met.no, and the coast guard and Kv Svalbard have built up is important for Norway’s capability to take SAR responsibility in the sector from North of Svalbard and up to the North Pole. It is important to maintain this competence and to share it with other operators in the Arctic region.

The Arctic region experiences strong climate change, but yet the central Arctic Ocean under the sea ice is poorly observed and remains largely unknown. A particular focus in CAATEX will be to obtain regional to basin scale information about ocean temperature from acoustic thermometry and standard oceanographic instrumentation. A major effort will be to design and implement the basin wide acoustic thermometry experiment. Six fixed mooring thermometry will provide yearlong times series of mean ocean temperature between each of the moorings and across the basin. Canadian scientists will deploy drifting buoys to receive signals from two sources in fixed moorings providing mean ocean temperature along each section connecting the buoys and moorings.This scanning concept will systematically map a very large portion of the upper and lower part of the central Arctic Ocean, manifesting spatial variability. The yearlong time series of mean ocean temperature and point measurements between the fixed moorings will provide information about the temporal variability on local to basin-wide scale. CAATEX will contribute to MOSAIC with measurements from sea ice buoys. The prime result from CAATEX field experiment is baseline data on mean ocean temperature and heat content of the central Arctic Ocean, which will be used for estimation of ocean climate change, as well as evaluation of global climate models. The new observations from the fixed moorings will be compared to similar acoustic observations made in 1994 and 1999 and thereby quantify how much the mean ocean temperature along the mooring array has changed over two decades. To obtain an improved estimate of the heat content of the central Arctic Ocean, the new data obtained in CAATEX will be used in combination with a high-resolution ice-ocean model. This estimate will be highly valuable for benchmarking the skill of climate models to represent Arctic Ocean heat content.