Proceedings of the Institute of Acoustics

Acoustic tomography and ocean models in Fram Strait

Brian Dushaw, Nansen Environmental and Remote Sensing Center, Thormøhlens Gate 47, Bergen, Norway. brian.dushaw@nersc.no
Hanne Sagen, Nansen Environmental and Remote Sensing Center, Thormøhlens Gate 47, Bergen, Norway. hanne.sagen@nersc.no

1 INTRODUCTION

A pilot acoustic program in Fram Strait during 2008 ‐ 2009 called DAMOCLES measured a year-long record of acoustic travel times along a 130 ‐ km range acoustic path crossing the West Spitsbergen Current (Figure 1). One aim of this program was to exploit long-range acoustic techniques to measure temperature in this extraordinary ocean region by acoustic tomography. The salient features of the region are the warm, salty West Spitsbergen Current flowing north along the Spitsbergen coast and the cold, fresh East Greenland current flowing south along the Greenland coast. Both current systems influence the climatic states of the North Atlantic and Arctic Oceans. These current systems interact within Fram Strait through complicated mesoscale, recirculation, and entraining processes (Figure 1). These processes are difficult to observe.

Unlike the nature of long-range acoustic propagation in mid-latitude regions, individual ray arrivals were not observed in long-range acoustic signals recorded in Fram Strait. Rather, the arrival patterns consisted of a single, stable, broad arrival pulse of about 100 ms duration. The nature of these arrival patterns was mostly a result of acoustic scattering and thermal variations caused by the extraordinary small-scale variability within Fram Strait (Dushaw et al. 2016b; Sagen et al. 2016b). Travel time variations of ±0.15 s recorded the vigorous mesoscale environment of the region and the seasonal cycle.

To estimate ocean temperature from the tomography data, an inverse scheme employed a high-resolution ocean model (HYCOM) for Fram Strait as the reference ocean. The variations in sound speed caused by variations in salinity in Fram Strait contribute only a negligible error to this inverse (Dushaw et al. 2016a). The information from the tomographic measurements is primarily average temperature, and this average is a natural complement to existing moored array observations (Dushaw and Sagen 2016). Estimated temperatures, averaged over 0-1000 m depth and over range, had a mean of 1.11ºC and variations of ±0.33ºC. The formal uncertainty of the tomography estimates was about 60 mºC (Sagen et al. 2016a) (Figure 2). Agreement with an alternate inverse approach based on Empirical Orthogonal Functions (EOF)s and a Markov Chain Monte Carlo inversion scheme relying on a matched- peak approach (Skarsoulis et al. 2010) was excellent, indicating a robust estimate for ocean temperature. The inverse estimates for average temperature agreed with the equivalent estimates from hydrographic sections obtained along the acoustic path at the start and end of the program.

Among other deficiencies, the ocean model greatly underestimated the intensity of the mesoscale fluctuations and exhibited a warm bias of about 0.38ºC in section-averaged temperature. Additional tomography data were obtained in 2011 ‐ 2012 on three acoustic paths (ACOBAR, Figure 1) and in 2014- 2016 on seven acoustic paths (UNDER-ICE) during follow on experiments. Tomographic measurements in Fram Strait offer unique large-scale temperature constraints for ocean models through data assimilation. It is anticipated that these constraints, together with the on-going evolution in high-resolution numerical ocean models, will lead to more accurate estimates of the circulation and transports in Fram Strait.

Figure 1: A snapshot of (left) temperature and (right) salinity at 300-m depth derived from the Fram Strait Model (HYCOM) for 26 March 2008. Depths shallower than 300 m are white. The edge of the ice is denoted by the magenta line. The 2008-2009 DAMOCLES (red line) and 2010-2012 ACOBAR (black lines) tomography paths are indicated. The ocean variablity of Fram Strait is characterized by 4-10 km scales. From Sagen et al. 2016a.

Figure 2: The time series of estimated sound speed was used to obtain estimates of temperature (blue line), with an uncertainty of about 60 mºC. The temperature time series was low-pass filtered using a 2-day running mean. The equivalent time series of temperature derived from the Fram Strait Model (red line), and the temperature estimates of Skarsoulis et al (2010) (green line) are shown. Some of the fluctuations of the acoustic estimates are a product of the randomness of picked-peak travel times. Single points with error bars indicate equivalent average and RMS temperature estimated from CTD (17/8/2008, 6/8/2009) or XBT (20/9/2008) sections, roughly along the DAMOCLES path. From Sagen et al. 2016a.

References

Brian D. Dushaw, Hanne Sagen and Agnieszka Beszczynska-Möller, (2016a). Sound speed as a proxy variable to temperature in Fram Strait, J. Acoust. Soc. Am. , 140 , 622-630. http://dx.doi.org/10.1121/1.4959000

Brian D. Dushaw, Hanne Sagen and Agnieszka Beszczynska-Möller, (2016b). On the effects of small- scale variability on acoustic propagation in Fram Strait: The tomography forward problem, J. Acoust. Soc. Am. , 140 , 1286-1299. http://dx.doi.org/10.1121/1.4961207

Brian D. Dushaw and Hanne Sagen, (2016). A Comparative Study of Moored/Point and Acoustic Tomography/Integral Observations of Sound Speed in Fram Strait Using Objective Mapping Techniques, J. Atmos. Oceanic Tech. , 33 , 2079-2093. http://dx.doi.org/10.1175/JTECH-D-15-0251.1

Hanne Sagen, Brian D. Dushaw, Emmanuel K. Skarsoulis, Dany Dumont, Matthew A. Dzieciuch, and Agnieszka Beszczynska-Möller, (2016a). Time series of temperature in Fram Strait determined from the 2008–2009 DAMOCLES acoustic tomography measurements and an ocean model, J. Geophys. Res. , 121 , doi:10.1002/2015JC011591. http://dx.doi.org/10.1002/2015JC011591

Hanne Sagen, Florian Geyer, Stein Sandven, Mohamed Babiker, Brian D. Dushaw, Peter F. Worcester, Matthew A. Dzieciuch, Bruce Cornuelle, and Agnieszka Beszczynska-Möller, (2016b). Resolution, identification, and stability of broadband acoustic arrivals in Fram Strait, J. Acoust. Soc. Am., s ubmitted May 2016.

Emmanuel Skarsoulis, G. Piperakis, M. Kalogerakis, H. Sagen, S. Haugen, A. Beszczynska-Möller, and P. Worcester (2010). Tomographic inversions from the Fram Strait 2008-9 experiment, in Proceedings of the European Conference on Underwater Acoustics, Istanbul, Turkey, pp. 265-271.