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A Comparative Study of Moored/Point and Acoustic Tomography/Integral Observations of Sound Speed in Fram Strait Using Objective Mapping Techniques

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A Comparative Study of Moored/Point and Acoustic Tomography/Integral Observations of Sound Speed in Fram Strait Using Objective Mapping Techniques OCTOBER 2016 D U S H A W A N D S A G E N 2079 A Comparative Study of Moored/Point and Acoustic Tomography/Integral Observations of Sound Speed in Fram Strait Using Objective Mapping Techniques BRIAN D. DUSHAW AND HANNE SAGEN Nansen Environmental and Remote Sensing Center, Bergen, Norway (Manuscript received 17 December 2015, in final form 5 July 2016) ABSTRACT Estimation of the exchange of seawater of various properties between the Arctic and North Atlantic Oceans presents a challenging observational problem. The strong current systems within Fram Strait induce recirculations and a turbulent ocean environment dominated by mesoscale variations of 4–10-km scale. By employing a simple parameterized model for mesoscale variability within Fram Strait, the authors examine the ability of a line array of closely spaced moorings and an acoustic tomography line to measure the average sound speed, a proxy variable for ocean temperature or heat content. Objective maps are employed to quantify the uncertainties resulting from the different measurement ap- proaches. While measurements by a mooring line and tomography result in similar uncertainties in estimations of range- and depth-averaged sound speed, the combination of the two approaches gives uncertainties 3 times smaller. The two measurements are sufficiently different as to be complementary; one measurement provides resolution for the aspects of the temperature section that the other misses. The parameterized model and its assumptions as to the magnitudes and scales of variability were tested by application to a hydrographic section across Fram Strait measured in 2011. This study supports the deployment of the 2013–16 Arctic Ocean under Melting Ice (UNDER-ICE) network of tomographic transceivers spanning the ongoing moored array line across Fram Strait. Optimal estimation for this ocean environment may require combining disparate data types as constraints on a numerical ocean model using data assimilation. 1. Introduction important aspects of ocean circulation, with profound impacts on the ocean’s climate. Fram Strait, the passage between Spitsbergen and The details of these current systems are, however, Greenland, is a significant ‘‘choke point’’ for the general difficult to observe. Not only are the natural scales of circulation of the world’s oceans (Fieg et al. 2010; variability small at these high latitudes but the powerful Schauer et al. 2008). This strait is the only deep con- current systems have turbulent and recirculating fea- nection between the Arctic and the world’s oceans. tures. These features influence and obscure transports of Through this strait, warm, salty North Atlantic water mass or heat. Eddy variability may be an important flows northward in the West Spitzbergen Current, while contributor to these transports. The Fram Strait moored cold, fresher water, together with considerable quanti- array (Fig. 1) has been deployed across the strait since ties of ice (Smedsrud et al. 2011), flows southward in the 1997 to measure the properties of these current systems East Greenland Current. The transports of heat and salt (Fieg et al. 2010; Schauer et al. 2008). Temperature, between the Atlantic basin and the Arctic Basin by the salinity, and current data from this array have been deep and shallow current systems in Fram Strait are noted for their great variability (von Appen et al. 2015). Even with 16 moorings deployed along a 325-km line across the strait, however, the separation of the moor- Denotes Open Access content. ings (20–28 km) is a few times larger than the natural scales of variability, 4–10 km (Fieg et al. 2010; Nurser and Bacon 2014). The moored array has therefore un- Corresponding author address: Brian D. Dushaw, Nansen En- vironmental and Remote Sensing Center, Thormøhlens Gate 47, dersampled the ocean variability; the Fram Strait N-5006 Bergen, Norway. moored array forms an incoherent observing array. This E-mail: [email protected] situation has made it challenging to employ the moored DOI: 10.1175/JTECH-D-15-0251.1 Ó 2016 American Meteorological Society 2080 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 33 modelers and introduces the effects of aliasing. Other observing approaches, such as glider or conductivity– temperature–depth (CTD) sections, have obvious, different sets of complications or deficiencies. It is clear that no one type of measurement offers a com- prehensive solution to the observation problem in Fram Strait. Ocean acoustic tomography offers a natural in- tegrating measurement type for sound speed that is complementary to the point (moored or CTD) data type (Munk et al. 1995). The technique has been previously explored as a way to measure temperature or currents in Fram Strait using realistic simulations (Chiu et al. 1987; Naugolnykh et al. 1998). Tomography offers a natural integral measurement of large-scale temperature. If one of the aims of the Fram Strait observations is an accurate estimate of the transport of net heat through the strait, then large-scale temperature may be one relevant pa- rameter. For these reasons, the Nansen Environmental and Remote Sensing Center (NERSC) in Bergen, Nor- way, developed the Developing Arctic Modeling and Observing Capabilities for Long-Term Environmental Studies (DAMOCLES) tomography experiment as a pilot study. The study included the deployment of a single tomographic path of about 130-km range during 2008–09 (Skarsoulis et al. 2010; Mikhalevsky et al. 2015) (Fig. 1). Tomographic observations were subsequently expanded with the 2009–11 Acoustic Technology for Observing the Interior of the Arctic Ocean (ACOBAR) deployments (Fig. 1), and they were sustained with the Arctic Ocean under Melting Ice (UNDER-ICE) program with observations spanning 2013–16. Not surprisingly, the nature of the data from the actual measurements is con- FIG. 1. (top) Observational deployments within Fram Strait. The 2008–09 single DAMOCLES tomography path is at lower right, the siderably different than those employed in the previous 2009–11 ACOBAR tomography triangle spans the deep regions of simulation studies (Dushaw et al. 2016b). Fram Strait, and the 16 moored array moorings are along the In addressing the measurement problem by employing 78.8338N line of constant latitude. (bottom) The moored array basic objective mapping techniques, this paper has three moorings (black dots) and a fictive acoustic tomography path across main aims. The first aim is to describe the different Fram Strait coinciding with the moored array. The panel illustrates several approximations employed for this study. As deployed, the measurement properties of the moored array and to- moored array positions do not lie along a geodesic, but for the mography. The difference in these properties illustrates purposes of this study, the positions and tomographic line were the mutual advantage of employing both data types in an assumed to be coincident. The tomographic path, between the observation system. The second aim is to document the magenta dots, does not span the entire the moored array, because inverse of the tomography data to obtain an estimate it is limited by the shoaling topography at either end of the moored array. The moored array length is 325.688 km. The to- for ocean temperature. This approach was used by mographic range is 249.444 km, with the source mooring located Sagen et al. (2016) to estimate temperature from the 45.377 km from the westernmost mooring. Azimuthal equal area DAMOCLES tomography data. The third aim is to use projection. the analysis as a first step toward assimilation of the to- mography data type to constrain numerical ocean array data directly to estimate heat flow (Schauer et al. models. The results of this analysis are intended to build 2008; Schauer and Beszczynska-Möller 2009), or as intuition as to the nature of the information content of the constraints on numerical ocean models through data tomography data; these results suggest how a numerical assimilation. The high noise of the observations ocean model might respond to the constraints of this in- overwhelms the signals of interest to the ocean formation. In this paper it is important to distinguish OCTOBER 2016 D U S H A W A N D S A G E N 2081 between a simple ocean model of an objective map, matched the observations. The objective map approach consisting of a parameterization employing small sets of discussed here is a more versatile and less computa- vertical and horizontal basis functions, and a complex tionally intensive approach to an inverse. Objective numerical ocean model, consisting of atmospheric forc- maps require the design of a parameterized ocean ing, ocean dynamics, etc., and a gridded scheme for model for an infinite number of possible ocean states computing the temporal evolution of the ocean state. This are consistent with known ocean properties. The aim paper is an application of the former but with an eye on of the objective map is to determine the solution the latter. from this infinite pool of possible solutions that best As shown by Dushaw et al. (2016b)andSagen et al. fits the data, within the assumed constraints of as- (2016), the non-bottom-interacting acoustic propaga- sociated model variances and data noise. With any tion associated with the DAMOCLES measurements objective mapping approach, the results are de- was mainly confined to the upper half of the water pendent on the assumed model employed for the column. In addition, as shown by Dushaw et al. study. This paper therefore begins with a brief dis- (2016a), sound speed is an accurate proxy variable to cussion of possible ocean models that might capture temperature to good approximation, except, perhaps, the Fram Strait oceanography. Within the limitations in near-surface conditions. Although salinity is obvi- of a parameterization of the ocean by a simplistic ously an important quantity governing and marking ocean model (a ‘‘toy’’ model), the study will quan- oceanographic variability, its influences on the acous- titatively illustrate the value of the integrating, to- tic propagation and analysis here can be neglected.
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