Central Water Masses Variability in the Southern Bay of Biscay from Early 90'S. the Effect of the Severe Winter 2005. ICES C
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ICES CM 2006/C:26 ICES Annual Science Conference. Maastricht. September 2006 NOT TO BE CITED WITHOUT PREVIOUS NOTICE TO AUTHORS Central water masses variability in the southern Bay of Biscay from early 90's. The e®ect of the severe winter 2005 C¶esarGonz¶alez-Pola*y, Alicia Lav¶³nz, Raquel Somavillaz and Manuel Vargas-Y¶a~nezx Instituto Espa~nolde Oceanograf¶³a y C.O. de Gij¶on, z C.O. de Santander, x C.O. de M¶alaga * Avda Pr¶³ncipe de Asturias 70, 33212 Gij¶on,Spain, [email protected] Abstract A monthly hydrographical time series carried out by the Spanish Institute of Oceanography in the southern Bay of Biscay (eastern North Atlantic), covering the upper 1000 m, have shown local warming rates for the last 10-15 years that are much higher than the long-term ocean trend in the 20th century. At Mediterranean Water (MW) level this warming is linked to an e®ective modi¯cation of the termohaline properties but at the East North Atlantic Central Water (ENACW) levels the warming was mainly related to isopycnal sinking (heave) until winter 2005. The overall picture is consistent with the fact that climatic warming has accelerated over the last few decades. The anomalous winter of 2005 in south-western Europe (extremely cold and dry) caused the lowest temperature record of the time-series 1993-2005 for the surface waters in the southern Bay of Biscay, and the mixed layer reached unprecedented depths greater than 300 m. The isopycnal level σθ = 27:1 classically used to analyze ENACW variability disappeared (outcrops further south) and as a result the hydrographical structure of the upper layers of the ENACW was strongly modi¯ed remaining in summer 2006 completely di®erent than what it was in the previous decade. However, the local warming trend was only disrupted down to 300-400 dbar but there is a noticeable increase in salinity much deeper. Keywords: Bay of Biscay, warming trend, air-sea fluxes, climatic variability, North Atlantic, water masses 1 Introduction tions during the past decade to the present. Station 6 is located at the slope very close to the shelf- From late 80's the Spanish Institute of Oceanog- break and stations 7 and 8 are located over depths raphy (IEO) carries out the ambitious monitor- greater than 2000 m and can be considered oceanic ing program \RADIALES" which regularly occu- stations. pies some oceanographic sections along the Spanish The main result of the Santander Standard Sec- coasts sampling hydrographical and biological pa- tion regarding water masses variability is the de- rameters (Vald¶eset al., 2002). All sections are cov- tection of a progressive warming trend at all levels ered with small ships in one-day journeys so they during the 90's and early 00's, which seems to be are coastal (mainly sample the continental shelf). related to the mild atmospheric conditions at the However, due to the proximity of the shelf-break formations areas of the water masses reaching our near Santander (south-eastern corner of the Bay sampling location (Gonz¶alez-Polaet al., 2005). In of Biscay, see ¯gure 1), the external stations of the the present work we extend the analysis of water standard section located there are situated over the masses variability to the present giving special at- slope making possible for us to dispose of a monthly tention to the recent strong changes in the thermo- timeseries of pro¯les covering the intermediate wa- haline structure of shallower levels probably linked ters (down to 1000 m depth) at the same ¯xed sta- to the anomalous winter 2005 in the area. Through 1 ICES CM 2006/C:26 Gonz¶alez-Polaet. al. 2 52oN NAC France 48oN o Latitude 44 N Spain 8 AC 7 6 40oN Santander 36oN 30oW 24oW 18oW 12oW 6oW Longitude Figure 1: Composition of ENACW circulation in the Bay of Biscay taken from several sources, mod- i¯ed from Gonz¶alez-Polaet al. (2005). The Santander Standard Section (43±30:000N{43±54:000N, 003±47:000W) is magni¯ed over the Iberian Peninsula. the manuscript we will mainly use data from sta- ure 1 summarizes the main circulation in the Bay tion 7, which cover the whole water column down of Biscay and the nearest part of the Atlantic at to 1000 dbar and are not influenced by shelf-break the level of Eastern North Atlantic Central Water e®ects, but we will sometimes take timeseries from (ENACW). This water mass is found just below the station 6 which have a better coverage regarding mixed layer and it is formed by winter mixing on ENACW variability. a wide region from northeast Azores to the Euro- pean margin in the area bounded by the two main currents of the sub-basin, the North Atlantic Cur- 2 Waters Masses Variability rent (NAC) and the Azores Current (AC) (Pollard and Pu, 1985; Pollard et al., 1996). In the Bay of The Bay of Biscay is a marginal basin of the East- Biscay ENACW gets quite stagnated and the circu- ern North Atlantic with weak circulation patterns. lation is somehow residual of the main current core Water masses below the mixed layer are described west from the Iberian Peninsula, following an an- in several climatologic studies and one of the most ticyclonic loop with southward velocities around 1 recent descriptions of the region compiling a high cm per second (Paillet and Arhan, 1996; van Aken, quantity of recent and historical data can be found 2001, 2002; Colas, 2003; Pingree, 1993). The inter- in the works of van Aken (2000a,b, 2001). Fig- annual variability of ENACW is linked to winter ICES CM 2006/C:26 Gonz¶alez-Polaet. al. 3 Figure 2: θS diagram of external stations of the Santander Standard Section (see ¯gure 1). Colour indicates time periods. air-sea interaction in its wide formation area and it the limit of our sampling|. It is evident from the is also related to advection from further west. P¶erez data set that MW have increased its temperature et al. (1995) and P¶erez et al. (2000) found correla- and salinity compensating its density, whereas the tion between salinity at isopycnal level σθ = 27:1 variability in the ENACW is not so evident in the and atmospheric forcing. ENACW properties are θS diagram. As we shall see latter its changes are tightly correlated along the European margin (van mainly accounted by isopycnal downward displace- Aken, 2001; Huthnance et al., 2002; Lav¶³net al., ment (hence not a®ecting the θS structure). The 2006). main changes evident in ENACW are the increase In the ¯gure 2 it can be seen the θS diagram of in salinity from 2005 onwards and the interruption water mases at the southern Biscay from the San- of the θS straight line below the 27:1 isopycnal level tander Standard Section data set, the sequential in the last years. colour code also provides a ¯rst approach to the in- terannual variability. ENACW is found just below 2.1 Surface Waters and Mixed Layer the mixed layer (typically less than 200 dbar) and it Depth is described by a straight line which ends in a Salin- ity Minimum level located about 500 dbar. This Winter 2004-05 was very cold in south-western Eu- Salinity Minimum is influenced by water masses rope and particularly in the southern Bay of Biscay from di®erent sources and subjected to strong zonal with persistent northerly winds and ¯ve intense po- variations (Pollard et al., 1996). Below this level lar fronts reaching the Iberian Peninsula. Air tem- we progressively get into the Mediterranean Wa- peratures during February were nearly two stan- ter (MW) which has its core about 1000 dbar | dard deviations lower than the mean value for this ICES CM 2006/C:26 Gonz¶alez-Polaet. al. 4 month in the (1961-2005) time series. Those low values were only measured during the sixties in the 0 southern Bay of Biscay (Instituto Nacional de Me- teorolog¶³a, 2006). As a result, surface waters fell below 11:5±C in the oceanic stations, half a degree 100 below the previous coldest temperature recorded from 1992 and more than a degree below the cli- 200 matological mean (¯gure 3). 300 Surface Water temperature signal, Santander Station 6 23 All fitted seasonal signal 400 22 2005 21 500 20 19 Pressure (dbar) 600 18 17 700 Temperature 16 15 800 14 13 900 12 10.5 11 11.5 12 12.5 13 13.5 11 Temperature (ºC) J F M A My J Jl A S O N D J Month Figure 3: Surface temperatures (10 m) at San- Figure 4: All temperature pro¯les at Santander tander Station 6 (850 m depth), climatological Station 7 (2400 m depth). Pro¯le of march 2005 mean (black) and year 2005 (red). in red, pro¯le of march 2006 in blue and pro¯les from march 2005 to date in yellow. The consequence was that the mixed layer reached an extreme depth greater than 300 dbar, 2.2 Below the Mixed Layer Depth whereas in the whole series it seldom was reached more than 200 dbar (Gonzalez-Pola, 2006). The Changes on properties of water masses are com- water masses remained modi¯ed for the whole ex- monly split in two natural components. An e®ec- tent of the previous mixed layer depth when capped tive thermohaline water mass modi¯cation which in by the seasonal thermocline in summer 2005 sug- fact alters the θS relationship diagram is related to gesting that the anomaly had a spatial extension variations in heat and freshwater exchange in the enough to avoid a recovering of previous structure regions of formation, and the later diapycnal mix- through renewal.