CAATEX 2020 research cruise: Measuring the Arctic Ocean temperature below sea ice with sound

At this moment, the Norwegian Coast Guard icebreaker KV Svalbard is on a scientific expedition on the central Arctic Ocean with eleven scientists from different Norwegian and European institutions. Demanding operations are carried out to recover deep-water moorings deployed last summer. The moorings carry instruments to observe the ocean temperatures, salinity, ‘ocean sound’, carbon dioxide, and currents below the Arctic sea ice for one year. All this to calculate the ocean temperature below the sea ice. 

The Arctic is getting warmer, which is leading to a reduced sea ice cover in many regions of the Arctic Ocean and Seas. However, most of the central Arctic Ocean is still covered by year-round sea ice. During the CAATEX (Coordinated Arctic Acoustic Thermometry Experiment) cruise, sea ice observations are made using radars and drones as well as drilling holes in the ice to obtain information about the age of the sea ice. Several buoys are installed on the ice to monitor the ice drift during mooring recovery operations, and to calibrate ice drift models. Satellite radar images are extensively used during the cruise to support navigation and planning of complex operations in the ice.

Satellites are continuously monitoring the changes in sea ice, but little is known about the currents and ocean temperatures below the Arctic sea ice. To learn more about the ocean under the ice, instruments have to be put into the water column. Instruments can be lowered from ships, hung under buoys drifting with the ice, or they can be mounted into bottom anchored moorings. Last year, several moorings were deployed for the CAATEX and INTAROS projects. This year, the main focus is to recover moorings to obtain information about the ocean sound, the ocean temperatures, salinity, carbon dioxide, and currents below the Arctic sea ice for one year. 

How to measure ocean temperature below sea iceCAATEX 2020 research cruise: Researchers on sea ice in front of KV Svalbard in the Arctic Ocean. Credit: Andreas Kjøl, Norwegian Coastal Administration.CAATEX 2020 research cruise: Researchers on sea ice in front of KV Svalbard in the Arctic Ocean. Credit: Andreas Kjøl, Norwegian Coastal Administration.

The main purpose of CAATEX is to provide better estimates of the mean ocean temperature across the Arctic Ocean from north of Svalbard to the Beaufort Sea. To figure out how warm or cold the water below the ice layer is located, small instruments are mounted onto the moorings to measure temperature at selected depths. However, the really exciting bit is that we can use sound to get an accurate measure of the temperatures throughout the Arctic Ocean below the sea ice. When sound travels through water, the temperature of the water affects how fast the sound can move: Sound travels faster in warm water than in cold water. This makes it possible to make an acoustic thermometer.

The CAATEX setup and instruments

In acoustic thermometry, we use sources that emit a low frequency sound, and receivers, which – as their name indicates – receive the emitted sound at distances up to several thousand kilometers away. The CAATEX project makes use of low frequency sources and thousand-meter-long vertical receiver arrays on opposite sides of the central Arctic Ocean. The sources and receivers are attached to so-called fixed moorings. These fixed moorings are basically a long cable with a heavy anchor at the bottom resting on the seafloor, instruments and stabilizing floatation buoys along the cable, and a buoyant counterweight at the top floating just below the sea ice, to hold the cable upright. The instruments are clamped on to the mooring wire recording data year-round. However, to get the data the moorings must be recovered. The Norwegian part of CAATEX is responsible for the moorings in the Nansen Basin north of Svalbard, while the North American institutions are responsible for moorings in the Beaufort Sea north of Alaska. Therefore, the aim of this year’s cruises with KV Svalbard and USCG Healy is to recover six fixed moorings to collect the data on both sides of the Arctic Ocean.

One consecutive year of acoustic data and their implications

In the CAATEX project the sound is sent from the source on a regular basis of 3 days and received 30 minutes later on the respective other side of the Arctic Ocean. Based on the accurately measured arrival times the sound signal is analyzed with special methods to determine the mean ocean temperature at different depths. The analysis results will show how the large-scale ocean temperature in the entire water column below the Arctic sea ice is changing over the course of an entire year. The data will be compared to similar observations from 1994 and 1999. This comparison will reveal how much the mean ocean temperature has changed in the Arctic Ocean over the past two decades! The results will also give us a good idea of how accurately climate models represent the heat content in the Arctic Ocean.

The Nansen Environmental and Remote Sensing Center (NERSC) leads the KV Svalbard cruise. The Norwegian Coastal Administration (NCA) and the Ice Service at MET Norway are onboard to support the navigation and planning of operations with weather forecasts and interpretation of satellite remote sensing. Researchers from CNRS, UiB, and FFI are onboard to recover INTAROS moorings and ice buoys. NORCE is onboard to collect sea ice observations by drone, radar, and in-situ measurements as part of the Digital Arctic Shipping (DAS) project led by NERSC.

You can read more about the project objectives, the preparation and background of the 2019 CAATEX cruise, and the KV Svalbard reaching the North Pole during the cruise last year!


Projects and partners 

CAATEX is a research project funded by the Research Council of Norway and the US Office of Naval Research. The CAATEX is a joint project between NERSC, Scripps Institution of Oceanography (SIO), the Norwegian Polar Institute (NPI), Woods Hole Oceanographic Institution (WHOI), the University in Texas at Austin (UT Austin) and the Naval Postgraduate School. CAATEX collaborates with the Horizon2020 project INTAROS (INTegrated ARctic Observation System), and a CAATEX Post Doc participates in MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate).

INTAROS – Integrated Arctic Observation System is a H2020 project coordinated by NERSC. The University of Bergen (UiB) and the French National Centre for Scientific Research (CNRS) recover moorings for the INTAROS project. INTAROS has received funding from the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 727890.

Digital Arctic Shipping (DAS) is a project led by NERSC. DAS is funded by the Research Council of Norway. The Norwegian Research Centre (NORCE) collects sea ice information using radars, drones and in situ observations for DAS. Chinese partners will collect data in other regions of the Arctic Ocean from Chinese ice breakers.

KEPLER – Key Environmental monitoring for Polar Latitudes and European Readiness project is coordinated by the Norwegian Ice Service at the Norwegian Meteorological Institute (MET Norway), The project is evaluating end-user and stakeholder requirements to develop a roadmap for the next Copernicus phase (2021-2027). The overall aim for MET Norway during the CAATEX cruise is to give operational support by satellite images, weather forecasting and sea ice information, but also to perform Ice Watch (sea ice reporting), for collecting visual sea ice observations. These observations provide important information about the sea ice morphology that is not obtainable by remotely sensed data.

The NCA (Norwegian Coastal Administration) participates in the CAATEX expedition to gain more experience about operations in Norwegian areas with heavy ice and how to develop forecasting and monitoring information services of key ice/met/ocean conditions and climate indicators in the Arctic with respect to POLARIS (Polar Operational Limit Assessment Risk Indexing System). POLARIS is the most common decision support system in the mandatory Polar Code which regulates safety requirements for ships operating in Polar regions. POLARIS is used to determine which ice regime a ship can safely operate in. The future goal for NCA is to implement ice regime reporting and restrictions for ships operating in Norwegian Arctic waters.