ARTICLE IN PRESS Continental Shelf Research 29 (2009) 1070–1087 Contents lists available at ScienceDirect Continental Shelf Research journal homepage: www.elsevier.com/locate/csr Temperature variability in the Bay of Biscay during the past 40 years, from an in situ analysis and a 3D global simulation S. Michel a,Ã, A.-M. Treguier b, F. Vandermeirsch a a Dynamiques de l’Environnement Coˆtier/Physique Hydrodynamique et Se´dimentaire, IFREMER, BP 70, 29280 Plouzane´, France b Laboratoire de Physique des Oce´ans, CNRS-IFREMER-IRD-UBO, BP 70, 29280 Plouzane´, France article info abstract Article history: A global in situ analysis and a global ocean simulation are used jointly to study interannual to decadal Received 21 June 2008 variability of temperature in the Bay of Biscay, from 1965 to 2003. A strong cooling is obtained at all Received in revised form depths until the mid-1970’s, followed by a sustained warming over 30 years. Strong interannual 21 November 2008 fluctuations are superimposed on this slow evolution. The fluctuations are intensified at the surface and Accepted 27 November 2008 are weakest at 500 m. A good agreement is found between the observed and simulated temperatures, Available online 6 February 2009 in terms of mean values, interannual variability and time correlations. Only the decadal trend is Keywords: significantly underestimated in the simulation. A comparison to satellite sea surface temperature (SST) Bay of Biscay data over the last 20 years is also presented. The first mode of interannual variability exhibits a quasi- Interannual temperature variability uniform structure and is related to the inverse winter North Atlantic Oscillation (NAO) index. Regarding Climatic change the vertical structure, most cool and warm anomalies are generated at the surface, with the strongest Heat budget Air–sea flux ones penetrating down to 700 m and lasting up to 5 years. The complete heat budget from 1965 to 2004 Ocean circulation model is presented, including the contributions of vertical transport, freshwater flux and surface elevation. Interannual anomalies are mainly generated by the surface heat flux, while oceanic transports may become more important at longer time scales. & 2009 Elsevier Ltd. All rights reserved. 1. Introduction especially during Navidad years (Garcia-Soto et al., 2002). Slope water eddies (‘‘swoddies’’) are generated above various topo- During the last decades, the North Atlantic has been warming graphic accidents of the continental slope and carry warm more rapidly than any other ocean. This relatively small part of the anomalies inside the oceanic region (Pingree and Le Cann, 1991, world ocean would be responsible for one third of the global heat 1992). Considerable river discharges occur along the French coast, content increase from 1955 to 1998 (Levitus et al., 2005). This creating freshened water plumes which spread over several anomalous heating corresponds to a temperature increase by hundreds of kilometres on the wide French shelf (Puillat et al., 0.27 1C over the upper 700 m, from 1955 to 2003. The continuous 2004). Seasonally reversing winds induce upwelling- or down- trend is difficult to analyse, because it is superimposed on multi- welling-favorable conditions in particular areas of the narrow year oscillations. At regional scales, in situ data are often too Spanish shelf (Fonta´n et al., 2007). Moreover, this semi-enclosed sparse to get reliable estimates of long-term changes. None- area is connected to the deep ocean, mainly through the North theless, in the Bay of Biscay, many observations sources show Atlantic Drift, advecting Eastern North Atlantic Central Water strong temperature variations, both in terms of year-to-year (Gonza´lez-Pola et al., 2005). Air–sea heat fluxes also exhibit high oscillations and decade-long trends (Koutsikopoulos et al., 1998; interannual variability over this region, in relation with the North Garcia-Soto et al., 2002). However, this interannual variability is Atlantic Oscillation (Cayan, 1992). Because of this complexity, not well quantified, its geographic distribution is still largely three-dimensional (3D) general circulation models are very useful uncertain and its causes have yet to be identified. to improve the description of the circulation, as shown by The Bay of Biscay is affected by various physical processes, Friocourt et al. (2007). which act on a wide range of space scales. The Iberian Poleward The purpose of this paper is to document the variability of Current (IPC), flowing from the Spanish north-western corner temperature and heat content in the Bay of Biscay during the last along the Cantabrian coast, brings warm water during winter, 40 years. Developing a regional model requires a knowledge of the variability at the region boundaries: this information can be provided by basin-scale or global ocean model simulations. Ã Corresponding author. Global simulations are available from the DRAKKAR project (The E-mail address: [email protected] (S. Michel). DRAKKAR group, 2007), with a mesh size ranging from 28 km at 0278-4343/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2008.11.019 ARTICLE IN PRESS S. Michel et al. / Continental Shelf Research 29 (2009) 1070–1087 1071 the equator to 12 km in the Arctic ocean (1/41 resolution). Constructing surface forcings for multi-decadal simulations of Regarding in situ observations, a new regional analysis is being the ocean is a challenge. Atmospheric reanalyses, such as ERA40 developed for the Bay of Biscay (Vandermeirsh, personal commu- from the European Centre for Medium-range Weather Forecast nication), but a global analysis at coarse resolution (11) is already (ECMWF), are burdened by well-known weaknesses regarding available (World Ocean Database, hereafter ‘‘WOD’’, Levitus et al., radiation and precipitations (Brodeau et al., 2007). We use a 2005). In the present paper, we describe the variability of strategy proposed by Large (2007), implemented in global model temperature in the Bay of Biscay, based on both the global model intercomparisons by Griffies et al. (2008) and adapted to ERA40 and the global data analysis. Our results provide a basis for future by Brodeau et al. (2007). Radiation is obtained from satellite data, regional analysis and modelling at higher resolution. precipitation from a composite product built from in situ and After introducing the observational and modelled datasets satellite data, and turbulent fluxes are computed using the bulk (Section 2), we describe the observed temperature variability and formulae of Large and Yeager (2004), combined with meteorolo- assess the simulation reliability (Section 3). The in situ and gical variables from ECMWF (air temperature at 2 m, air humidity simulated temperatures are compared with satellite sea surface at 2 m and wind speed at 10 m). One aim of improving atmo- temperature (SST), over the last 20 years. The interannual spheric forcing is to minimise unrealistic relaxation of sea surface variability is analysed using empirical orthogonal functions (EOFs) salinity towards climatology in models (Griffies et al., 2008); and related to the North Atlantic Oscillation (NAO) index (Section however, in ORCA, it has been found necessary to keep a 3.4). The in situ analysis and the simulation are used to describe relaxation to climatological salinity, with a time-scale of 36 days the three-dimensional structure of heat content anomalies and in the top model layer (6 m), to avoid unrealistic drift in polar their propagation (Section 3.5). Then, we compute a complete regions. River run-offs are provided by a monthly climatology, heat budget from the simulation, in order to estimate how including 99 major rivers as well as coastal run-off (Dai and advective processes, surface heat fluxes and turbulent mixing Trenberth, 2002). Note that over the Bay of Biscay area, only the contribute to the heat content variability (Section 4). Gironde and Loire rivers are explicitly represented in the run-off forcing. The simulation covers the full duration of the ERA40 reanalysis, 2. Datasets from 1958 to July 2002, plus 2.5 additional years using the ECMWF analysis until the end of year 2004. The model outputs are averaged 2.1. Global in situ analysis and saved every five days. The simulation is initialised in 1958 with the climatology of Levitus et al., 1998 and undergoes substantial As a reference, we use the global temperature analysis by adjustment during the first years. For this reason, only the period Levitus et al. (2005), which was designed to estimate the World after 1965 is considered in this paper. Although the simulation is Ocean heat content and its decadal evolution. This dataset is not fully equilibrated (reaching equilibrium on the global scale based on in situ data of the World Ocean Database 2001, made requires centuries), the model drift in the upper layers of our region available by the National Oceanographic Data Center. Approxi- of interest seems moderate enough (at least, it does not override mately seven millions temperature profiles are included in the interannual variability). The model resolution does not allow a WOD01 database and more than 300,000 profiles were added by realistic representation of the Mediterranean Water outflow at Levitus et al. Over the global ocean, these measurements were Gibraltar. Therefore, a relaxation to climatology has been added in mostly acquired by expandable bathythermographs (XBTs), me- the Gulf of Cadiz, in order to prevent temperature and salinity drifts chanical bathythermographs (MBTs) and ocean stations (OSD). in the Mediterranean Water layer (800–1200 m). The remaining data was obtained from moored buoys (MRBs), This simulation has been designed to study basin-scale conductivity-temperature-depth sensors (CTDs), profiling floats variability and global changes, thus it does not resolve all the (PFLs), bottle samples, etc. processes occurring over the continental shelf and slope. For The horizontal resolution of the gridded dataset is 11 and its time instance, tides are not included in the model physics, so tidal period ranges from 1955 to 2003.