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Feature-Oriented Coastal Acoustic Tomography: Upwelling at Cabo

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Feature-Oriented Coastal Acoustic Tomography: Upwelling at Cabo Feature-oriented acoustic tomography: Upwelling at Cabo Frio (Brazil) Olivier Carriere` ∗, Jean-Pierre Hermand∗, Leandro Calado†, Ana ClaudiadePaula´ †, and Ilson Carlos Almeida da Silveira‡ ∗Environmental Hydroacoustics Lab., Universite´ libre de Bruxelles (U.L.B.) Avenue Franklin D. Roosevelt, 50 - B-1050 Brussels, Belgium Email: [email protected] †Marinha do Brasil - Instituto de Estudos do Mar Almirante Paulo Moreira Rua Kioto 253 - Arraial do Cabo - RJ 28930-000 Brazil ‡Instituto Oceanografico´ da Universidade de Sao˜ Paulo Prac¸a do Oceanografico´ 191 - Sao˜ Paulo - SP 05508-120 Brazil Abstract— The Cabo Frio region (Brazil) presents a unique coastal-oceanic system. Among several interesting oceanic phe- nomena that occur in this region, the coastal upwelling is the most important coastal feature and is mainly forced by persistent winds northeast. An oceanic feature model which specifically describes the Cabo Frio upwelling process is used as a parameterization scheme to track the time evolution of the sound-speed field of a vertical slice of the coastal waters. The tracker processes the repeated measurements of broad-band, multi-frequency (220–880 Hz), full-field acoustic field on a vertical receiver array. The acoustic data are assimilated in the feature model to continuously correct the prediction of the upwelling slope of the temperature field. To cope with the nonlinearity between the environmental parameters and the acoustic propa- gation data, advanced nonlinear extensions of Kalman filters are necessary. It is shown that an ensemble Kalman filter (EnKF) continuously tracks the upwelling conditions, outperforming the extended Kalman filter (EKF). Fig. 1. AVHRR image from 10th of January, 2001 exemplifying the coastal I. INTRODUCTION upwelling in Cabo Frio region. The blueish-yellow colors are associated with the cooler and fresher Coastal Water on the shelf, and the reddish colors mark The principles of ocean acoustic tomography were intro- the presence of the warmer and saltier Tropical Water. Image courtesy of W. Lins de Mello, Brazilian Navy. duced at the end of the 1970s by Munk and Wunsch [1]. Since, number of theoretical, numerical and experimental studies proved the efficiency of acoustic measurements to assess or monitor ocean environments. However, the coupling of ocean the surface winds rotate counter-clockwise and blow for a few modeling, acoustic inversion and data assimilation remains a days from the southern quadrant, inhibiting the upwelling [3]. complex problem, particularly in coastal environments. This The coastal upwelling in Cabo Frio occurs when the work investigates the use of an oceanic feature model for the South Atlantic Central Water, carried by Brazil Current (BC), ◦ acoustic monitoring of coastal upwelling in Cabo Frio (23 S) “climbs” the shelf break, and the isotherms (as well as in Brazil (Fig. 1). isopycnals) bend upwards in the vicinity of the continental The Brazilian coast at Cabo Frio region develops a unique slope. The main forcing of coastal upwelling in Cabo Frio is coastal-oceanic system when the coast orientation changes and the persistent winds northeast, that typically blow for several shelf break topography reinforces the interaction between the days, producing strong upwelling. On the other site, the surface oceanic and coastal systems [2]. The most important coastal layers of the shelf break region are normally occupied by feature in Cabo Frio region is the upwelling (Fig. 1). the Brazil Current (BC) waters, with high temperatures and During the summer months, when coastal upwelling is more salinities. The BC surface temperatures may range from 25◦C frequent and sustained due to favourable wind conditions over to 27◦C during the summer and from 22◦Cto24◦C during the the Cabo Frio area, the surface temperature difference between winter. Surface salinities normally vary from 36.5 to 37.0 [4], the Brazil Current front and upwelled waters near the coast [5]. As shown in [6], the interaction of these two systems (BC is most of the time greater than 10◦C. On the synoptic time and coastal upwelling) can create an environment that impacts scale of 6–11 days, as cold fronts pass through the region, the density field. Oceanic forecasts require the best possible 0-933957-38-1 ©2009 MTS specifications of density initial conditions. Therefore, precise models provide significant improvements on the prediction of knowledge or monitoring of the synopticity of these coastal transmission loss with respect to range-independent models. and oceanic systems are fundamental for the initialization of In this paper, the feature model imposes a strong apriori oceanic numerical forecasting model. constraint on the oceanic feature that we want to invert. A The traditional acoustic tomography, which consists essen- previous work showed that the acoustic monitoring of the tially in the inversion of the range-integrated sound-speed central position of a seasonal thermal front was feasible by profile, is often not meaningful in coastal environments where coupling a basic feature model and appropriate inversion large inhomogeneities can occur rapidly and frequently at techniques involving broad-band and full-field acoustic propa- small- and meso-scales. For the same reason, common inver- gation modeling [17]. Here, the same technique is applied to a sion methods based on the linearization around a reference en- different feature model which describes the coastal upwelling vironment can fail because of the strong change of the environ- that occurs on the southeastern coast of Brazil and in particular mental conditions between successive time frames of acoustic the Cabo Frio area. probing. Therefore, range-resolving inversion schemes are be- In this work, the feature-oriented acoustic inversion is fo- ing developed to obtain accurate estimates of range-dependent cused on only one transect of the coastal environment. Further features of the environment, suitable for data assimilation in works will consider the inversion of several transects that cross oceanic model. Several works have shown that the inversion the upwelling front, thereby enabling the reconstruction of of range-dependent environments was possible to achieve the subsurface conditions over the whole zone by introducing using multiple pairs of source and receivers, with travel-time appropriate correlation functions. based methods [7], [8] or matched-field processing [9], [10]. The paper is organized as follow: Section II is dedicated However, the problem of recovering a range-dependent sound- to the basic principles of oceanic feature models and their speed field using a single pair of source-receiver is a more application to the particular case of the Cabo Frio upwelling. complicated problem, mainly because of the co-existence of After introducing some typical properties of the acoustic multiple solutions to the inverse problem. Different works propagation in the transect of interest, Section III develops proposed appropriate schemes to estimate the range-dependent the concept of acoustic monitoring of the upwelling using sound-speed field in a single vertical slice. To cope with an ad hoc feature model. Simulation results are presented in the existence of multiple solutions (sometimes non-physical), Section IV. Finally, the paper is concluded in Section V. different strategies are developed, basically to constrain the inverse problem by introducing aprioriknowledge of the II. FEATURE MODELS environment. Most of works use empirical orthogonal func- The philosophy of the feature modeling approach is to tions (EOF) to parameterize the temperature (or sound-speed) develop a first-order system for a very complex nonlinear profiles. In [11], the range-dependent sound-speed field of system such as a regional ocean where most processes strongly a deep water environment was inverted by introducing cor- interact and where processes cannot be studied separately. relation lengths in the inversion scheme. The tracking of Once the first-order structures are placed within a numeri- a cold filament was proposed in [12], for a shallow water cal models dynamical framework, the nonlinearity stimulates environment, by including the known position of the local further interaction among features and should create realistic perturbation. In [13], smoothness conditions on the solution four-dimensional complex fields. Thus, changing the parame- are imposed and the final estimate is given by averaging over ter values of the feature models, it could be possible to study the best solutions of the inversion algorithm. More recently, the interaction between features and the different dominant it was shown that the tracking of the fine hourly variations (and possibly candidate) processes in isolation [2], [18]. of a range-dependent sound-speed field in a vertical slice was In this framework, a single feature model is first developed possible by using an ensemble Kalman filter (EnKF) to invert to describe the upwelling feature near Cabo Frio region. full-field acoustic measurements on a vertical array [14]. In a The feature model approach enables the representation of shallow water environment, the acoustic propagation is highly the coastal feature in a low-dimensional scheme. Sensitivity dependent on the geoacoustic properties. Therefore, a precise studies can be done by varying the upwelling parameters to knowledge of the bottom and subbottom properties is essential generate different scenarios which can then be studied for to invert for the water column properties. All cited works made further
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