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The Under-Ice Experiment – Overview and Preliminary Results

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The Under-Ice Experiment – Overview and Preliminary Results UACE2017 - 4th Underwater Acoustics Conference and Exhibition THE UNDER-ICE EXPERIMENT – OVERVIEW AND PRELIMINARY RESULTS Espen Storheima, Hanne Sagena, Peter F. Worcesterb, and Matthew A. Dzieciuchb aNansen Environmental and Remote Sensing Center, Thormøhlens gate 47, N-5006 Bergen, Norway bScripps Institution of Oceanography, University of California at San Diego, La Jolla California, 92093-0225, USA Espen Storheim, Thormøhlens gate 47, N-5006 Bergen, Norway. Fax: 55205801. [email protected] Abstract: Fram Strait is the main deep passage through which the major ocean mass and heat exchange between the Atlantic and Arctic Ocean takes place. To understand the on going climate change it is important to estimate the transports through the strait. The ultimate approach is to combine data from satellites, acoustic systems, moorings, and high- resolution ice-ocean models through data assimilation. An acoustic system for acoustic thermometry was developed under the ACOBAR project. Results from this experiment showed that it was important to monitor the heat content in the north going current and the south going current separately. Furthermore, the acoustic propagation conditions in this region make it difficult to separate the arrivals in time domain. In September 2014 a continuation and extension of the acoustic system was implemented within the UNDER-ICE project funded by the Research council of Norway. The acoustic system was designed to provide accurately measured travel times along 7 sections in the deep part of the Fram Strait every three hours for two years. In this upgraded system the goal was to separate the north going current with warm water, from the south going current with cold water. To improve resolution of arrivals the receiver arrays were extended with additional receivers. To support the oceanographic interpretation of the acoustic results the moorings were augmented with a significant number of oceanographic instruments, e.g. thermistors and Acoustic Doppler Current Profilers. This system was recovered in July 2016. In this paper, preliminary results are shown from the processing and analysis of the acoustic data, oceanographic measurements, and comparison between acoustic observations and modeling results. A technical assessment of the experiment will also be given. Keywords: Tomography, seismic, airgun, temperature Page 273 UACE2017 - 4th Underwater Acoustics Conference and Exhibition 1 INTRODUCTION The only deep-water passage between the Arctic Ocean (AO) and the North Atlantic Ocean is through the Fram Strait, between Svalbard and Greenland. Warm saline Atlantic water is transported into the AO by the West Spitsbergen Current (WSC), while cold fresh water is transported out from the AO by the East Greenland Current (EGC) [1]. Ocean acoustic tomography [2] is an efficient method to obtain the range-averaged temperature along a source-receiver transect, that can span several hundred kilometers. Although the Fram Strait is a challenging acoustic environment due to strong recirculation of water masses, and high mesoscale variability [3], use of tomography in this region has been successfully demonstrated in the ACOBAR and DAMOCLES projects [3-7]. In the UNDER-ICE experiment [8], the sources and receivers are set up in a configuration that allows for monitoring of both the northbound WSC, and the southbound EGC, in addition to propagation through the marginal ice zone (MIZ), and under the sea ice. The number of moorings has been increased compared to the previous experiments, and also the number of receiving hydrophones to improve the vertical resolution. The objective of the present work is to give a brief overview of the main experiment carried out under the UNDER-ICE project, and to illustrate some of the differences compared to ACOBAR. Some preliminary results from the processing of the recovered data will also be presented. The focus in this paper is on some of the oceanographic data that are available, not the acoustic tomographic data. The experimental configuration and details are presented in Section 2. Some preliminary results are presented in Section 3, before a brief summary of the processing so far is given in Section 4. 2 OVERVIEW OF THE EXPERIMENT The UNDER-ICE experiment consists of five moorings, denoted UI1 to UI5, deployed in September 2014 in the Fram Strait. Fig. 1 shows the placement of the UNDER-ICE moorings, compared to the locations of the ACOBAR moorings. The contour plot illustrates the bathymetry obtained from the International Bathymetric Chart of the Arctic Ocean (IBCAO) [9]. Two of the moorings, UI2 and UI5, are equipped with Teledyne Webb Research sweeper sources that transmit 90 s linear FM sweeps from approximately 200-300 Hz. UI2 transmits every 3rd hour (0000, 0300, …, UTC) on odd year days, while UI5 transmits every 3rd hour of every day, six minutes after UI2. Mooring UI4 is located in a region that is covered by ice in the winter, for transmissions through the marginal ice zone (MIZ). Each mooring is equipped with 10 hydrophone modules (HM), recording a total of 130 seconds at a sampling frequency of 1953.125 kHz. The HM’s are equidistantly spaced by 9 m, creating an array with an aperture of 81 m. The sources and receivers are synchronized by the DSTAR controllers. Four transponders are placed on the ocean floor in a square shape around the moorings. These are used to position the DSTAR’s, sources, and HM’s before and after each transmission, in three dimensions. With this mooring configuration, a total of 7 different source-receiver paths are available, ranging from 130 km (UI2-UI1) to 278 km (UI2-UI4). The HM’s are equipped with thermistors that measure the temperature at regular intervals, and the DSTAR’s are equipped with calibrated pressure sensors. Moorings UI4 and UI5 are also augmented with oceanographic instruments (Sea-Bird SBE37 and SBE39) that measure the temperature, salinity, and pressure at 5 or 10 min intervals, and in total three Acoustic Doppler Current Meters (ADCP). This additional information supports the oceanographic interpretation of the acoustic results. Page 274 UACE2017 - 4th Underwater Acoustics Conference and Exhibition Fig. 1. Geometry of the moorings in the UNDER-ICE project (red circles, denoted UI1- UI5), compared to the ACOBAR project (yellow squares, denoted A-D). The contour plot indicates the bathymetry obtained from IBCAO. 3 RESULTS 3.A MOORING MOTION Fig. 2 shows the nominal pressure obtained from the DSTAR pressure sensors over the course of the experiment. The same vertical scale is used to illustrate the differences between the moorings. Although the data shown is not calibrated, it still provides a good indication of the relative variation in the depth of the DSTAR’s. Calibration data for the sensors is however available and will be included in the further work. UI1 and UI4 have relatively little movement compared to the other three moorings, with a total span (i.e. the difference between max/min depths) of 28 m and 8 m, respectively. A similar stable behavior is also seen at UI3, but there are some pulldowns in early October 2014, May 2015, etc. The span for this mooring is 115 m, with a mean depth of 326 m. The most significant mooring movement is at UI2, located in the central part of the Fram Strait, from January 2015 to September 2015, and the span is 411 m, while the mean depth is 461 m. The mooring depth is fairly stable until January 2015, and after September 2015, except for a dip in October 2014 and in November 2015. At UI5 there is also some significant movement, but the depth is seen to be somewhat more constant during periods, e.g. in January or June to July in 2015, compared to UI2. The span of this mooring is 253 m, and the mean depth is 653 m. The pressure sensor onboard the DSTAR units only provide information about the depth of the unit, not the orientation of the mooring. For example at UI1, where the depth is fairly constant, the results does not distinguish if the mooring is moving in a circular pattern along a constant depth, or if it’s stationary in a certain direction. However, there is also a compass and a tilt sensor onboard the DSTAR’s, which in addition to the 3D positioning by acoustic methods, can be used to qualitatively analyze the oceanographic conditions at the mooring location, e.g. currents, eddies, etc. Page 275 UACE2017 - 4th Underwater Acoustics Conference and Exhibition Fig. 2. Plot of the nominal pressure recorded by the DSTAR controllers on the five different moorings. Note that the same vertical range is used on all plots to illustrate the difference. The horizontal red dashed line indicates the design depth of the given DSTAR. 3.B TEMPERATURE Fig. 3 shows the temperature recorded by the HM’s and the oceanographic instruments on UI4, as a function of time and depth (a), and the average temperature from the HM measurements. From September 2014 to January 2015 there are alternating periods of warm and cold water. Between January and July the water is fairly cold, between 0.5ºC and 1.0ºC. The warmest water is present from July to October in 2015, where the temperature is approximately 3ºC. The water in September 2015 is warmer than 2014. The average temperature for the ten HM’s as a function of depth, is calculated to better illustrate the variability in this period. The average value is seen to vary between 0.5ºC and 3ºC. The mean value of the average HM temperature is approximately 1.1ºC. There are five distinct heat spikes between July and October 2015, where the temperature exceeds 2.0ºC, and cooler water comes in between these spikes. Although the distribution of the oceanographic instruments is sparse compared to the length of the array, these measurements still provide very important information about the water masses at these locations.
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