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 effect of the severe winter 2005

C´esarGonz´alez-Pola*†, Alicia Lav´ın‡, Raquel Somavilla‡ and Manuel Vargas-Y´a˜nez§ Instituto Espa˜nolde Oceanograf´ıa † C.O. de Gij´on, ‡ C.O. de Santander, § C.O. de M´alaga * Avda Pr´ıncipe 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 effective modification 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 modified remaining in summer 2006 completely different 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 figure 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 profiles covering the intermediate wa- haline structure of shallower levels probably linked ters (down to 1000 m depth) at the same fixed 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- ified from Gonz´alez-Polaet al. (2005). The Santander Standard Section (43◦30.000N–43◦54.000N, 003◦47.000W) is magnified 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 effects, 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 figure 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´ınet al., ment (hence not affecting the θS structure). The 2006). main changes evident in ENACW are the increase In the figure 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 first 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 different sources and subjected to strong zonal with persistent northerly winds and five 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´ıa, 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 (figure 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 profiles at Santander tander Station 6 (850 m depth), climatological Station 7 (2400 m depth). Profile of march 2005 mean (black) and year 2005 (red). in red, profile of march 2006 in blue and profiles 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 modified for the whole ex- monly split in two natural components. An effec- tent of the previous mixed layer depth when capped tive thermohaline water mass modification 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. The existence of a very low strat- ing by diffusion processes. On the other hand, there ified water column below the seasonal thermocline can be vertical displacements of isothermal and/or favoured the formation of a very deep mixed layer isohalines, and hence of isopycnal levels if no mod- again in the winter 2005-06 (figure 4). ification of θS relationship is present. This second type of modification can be produced by dynami- cal processes, for example, changes on the slope of the currents or the displacement of an established permanent gyre, but also a variation on renewal ICES CM 2006/C:26 Gonz´alez-Polaet. al. 5

σ σ θ =27.1 θ =27.2 0 300

350 100 400 200 450 300 500

400 550 1992 1994 1996 1998 2000 2002 2004 2006 1992 1994 1996 1998 2000 2002 2004 2006

σ σ θ =27.3 θ =27.4 500 600

550 650

600 700

650 750

700 800 1992 1994 1996 1998 2000 2002 2004 2006 1992 1994 1996 1998 2000 2002 2004 2006

σ σ θ =27.5 θ =27.6 750 900

800 950

850 1000

900 1050 1992 1994 1996 1998 2000 2002 2004 2006 1992 1994 1996 1998 2000 2002 2004 2006

Figure 5: Pressure of isopycnals at Santander Station 7, red dots are raw data and blue line is the linear fit (same station and codes for figures 6, 7 and 8).

rates of water masses formation. Based on the pi- In the work of Gonz´alez-Polaet al. (2005) the oneering work of Bindoff and McDougall (1994), equation 1 was applied to the Santander Section who established linear relations within changes of hydrographical timeseries for the period 1992-2003. potential temperature and salinity over isopycnals The procedure consisted in taking σθ values from and isobaric levels through the temporal variation 27.1 to 27.6 on intervals of 0.1 and constructing the of isopycnal depths, Arbic and Owens (2001) inter- time series for the sinking of each isopycnal and preted isobaric changes through the expression (for variations of the thermohaline average properties potential temperature) in the branches defined in between. During this ¯ ¯ ¯ period it was found that ENACW warmed through ¯ ¯ ¯ dθ ¯ dθ ¯ dp¯ ∂θ pure heave (mainly because of the progressive sink- ¯ = ¯ − ¯ . (1) dt p dt n dt n ∂p ing of isopycnals 27.1 and 27.2) and there were vari- ability in salinity probably related to Precipitation where terms are named (from left to right) isobaric minus Evaporation regime, the level of the Salin- change, isopycnal change and change due to isopy- ity Minimum warmed only by pure heave with no cnal displacement (pure heave). This relation as- changes over isopycnals and MW warmed by isopy- sumes that vertical gradients of temperature are cnal change (hence this level got saltier). constant in time. ICES CM 2006/C:26 Gonz´alez-Polaet. al. 6

σ σ θ =27.1 − 27.2 θ =27.2 − 27.3 11.7 11.2

11.6 11.1

11.5 11

11.4 10.9

11.3 10.8

11.2 10.7 1992 1994 1996 1998 2000 2002 2004 2006 1992 1994 1996 1998 2000 2002 2004 2006 σ σ θ =27.3 − 27.4 θ =27.4 − 27.5 10.6 10.5

10.55 10.4 10.5 10.3 10.45 10.2 10.4

10.35 10.1 1992 1994 1996 1998 2000 2002 2004 2006 1992 1994 1996 1998 2000 2002 2004 2006 σ θ =27.5 − 27.6 10.3

10.2

10.1

10

9.9

9.8 1992 1994 1996 1998 2000 2002 2004 2006

Figure 6: Potential temperature θ(◦C) at isopycnal bounded branches.

In figures 5–8 it is shown the update of these (Salinity Minimum) which had maintained its ther- timeseries to date. Figure 5 shows how the isopyc- mohaline properties stable (this level was warm- nal level 27.1 which sunk continuously from 1992 ing by pure heave) began to warm by isopycnal to 2005 from about 200 dbar to more than 300 change (hence getting saltier) from 2005 onwards. dbar reaches the surface in winter 2005 and is found MW continue its warming/salt-increase tendency just below the seasonal thermocline since then. It but the shallower level with MW influence (27.3-4) is not clear whether the isopycnal 27.2 which also seem also to accelerate this processes so it may be sunk about 100 dbar during the previous period related for the first time in the series with mixing may present signs of shallowing by 2006, deeper from above. isopycnals remain with no trend. Figure 7 provides the temperature changes at Figure 6 presents the potential temperature θ fixed isobars (depths) showing that only the level at fixed isopycnal levels, the variation of salinity 200-300 dbar is from year 2005 in a process of cool- between density levels presents the same pattern ing, which have compensated in two years the 0.5◦C (linked by density) and it is not shown. The pe- gained in the period 1994-2004. It is not clear riod of progressive freshening from 1998 to 2003 whether the level 300-400 dbar have interrupted its in ENACW shifted to a recovery trend in salinity warming either but all levels below continue the from the beginning of 2004 to date, reaching again warming trend of 0.020–0.030◦C yr−1. The warm- the values of 1998. However, the level 27.2-27.3 ing at MW levels continues to be caused by isopyc- ICES CM 2006/C:26 Gonz´alez-Polaet. al. 7

12.5 12

12 11.5 11.5 200 300 layer 300 400 layer 11 11 1992 1994 1996 1998 2000 2002 2004 2006 1992 1994 1996 1998 2000 2002 2004 2006

11.5 11.5

11 11 10.5 400 500 layer 500 600 layer 10.5 10 1992 1994 1996 1998 2000 2002 2004 2006 1992 1994 1996 1998 2000 2002 2004 2006

10.8 10.6

10.6 10.4

10.4 10.2 600 700 layer 700 800 layer 10.2 10 1992 1994 1996 1998 2000 2002 2004 2006 1992 1994 1996 1998 2000 2002 2004 2006

10.4 10.2

10.2 10

10 9.8 800 900 layer 9.8 900 1000 layer 9.6 1992 1994 1996 1998 2000 2002 2004 2006 1992 1994 1996 1998 2000 2002 2004 2006

Figure 7: Potential temperature θ(◦C) at isobaric bounded branches. ICES CM 2006/C:26 Gonz´alez-Polaet. al. 8

35.7 35.7

35.65 35.65

35.6 35.6

200 300 layer 35.55 300 400 layer 35.55 1992 1994 1996 1998 2000 2002 2004 2006 1992 1994 1996 1998 2000 2002 2004 2006

35.65 35.62

35.6 35.6 35.58 35.55 35.56

400 500 layer 35.5 500 600 layer 35.54 1992 1994 1996 1998 2000 2002 2004 2006 1992 1994 1996 1998 2000 2002 2004 2006

35.7 35.75

35.65 35.7

35.6 35.65

600 700 layer 35.55 700 800 layer 35.6 1992 1994 1996 1998 2000 2002 2004 2006 1992 1994 1996 1998 2000 2002 2004 2006

35.8 35.85

35.75 35.8

35.7 35.75 800 900 layer 35.65 900 1000 layer 35.7 1992 1994 1996 1998 2000 2002 2004 2006 1992 1994 1996 1998 2000 2002 2004 2006

Figure 8: Salinity at isobaric bounded branches. ICES CM 2006/C:26 Gonz´alez-Polaet. al. 9 nal changes (i.e. accompanied by salt-increase) and the main formation area of ENACW, under the in- the warming at the Salinity Minimum has shifted fluence of high pressure the whole year. This at- (at least partially) from pure heave to isopycnal mospheric pattern supposed also very crude condi- change. Figure 8 presents the salinity at fixed iso- tions at the Western Mediterranean causing snow bars helping to understand the interpretation. records in north Africa (World Meteorological Or- ganization, 2006). In order to quantify the anomaly in the air-sea 3 Air-Sea interaction and heat heat fluxes and Precipitation minus Evaporation fluxes (P-E) rates we have used the NCEP Reanalysis data provided by the NOAA-CIRES Climate Di- The changes observed in the local water masses in agnostics Center and available since 1948 Kalnay Santander should be linked to air-sea interaction et al. (1996). We have extracted winter heat fluxes variability at their formation areas (and they may for some cells representing the main formation area be also related to changes in circulation). The most of ENACW, the local Cantabrian Sea (southern significant change in ENACW affecting even the Bay of Biscay) and the so-called MEDOC area (for- deepest levels of this water mass occurred after the mation zone of Western Mediterranean Deep Wa- winter 2004-05, characterised by the already com- ter, MEDOC Group, 1970) to serve as a comparison mented very cold conditions in all south-western (figure 10). Europe.

NCEP grid and selected cells for ENACW and WMDW

56oN

48oN

40oN

32oN

15oW 0o 15oE 30oE

Figure 9: NCEP/NCAR Reanalysis Sea Level Pres- Figure 10: Selected cells for heat flux anomaly sure (mb) composite. Anomaly of winter 2005 over computation representing ENACW main formation the North Atlantic respect to the 1968-1996 clima- area (green), local conditions at the southern Bay tology. of Biscay (red) and the MEDOC area (blue) The cause of this crude winter was an anomalous The atmospheric anomaly caused very dry condi- atmospheric pattern with a strong anticyclone lo- tions at the formation area of ENACW which seems cated west of the British Islands during the whole to be correlated to the salinity increase found in winter (figure 9) which produced intense and per- this water mass (figure 11). It is interesting to note sistent cold and dry northerly winds over south- that ENACW is a mode water formed by winter ern Biscay. The typical atmospheric pattern has deep convection in the wide area roughly depicted the anticyclone situated further south and domi- by the green cells in figure 10. Though the win- nance of south-westerly winds over Biscay area in ter heat-loss was average for the whole formation wintertime. This anomalous situation also affected area (figure 12) it increases notably in the south- ICES CM 2006/C:26 Gonz´alez-Polaet. al. 10

5

0 35.75

) −5 −1 −10 35.7 −15 month −2 −20 35.65 −25

35.6 P−E (mm m −30 −35 Sal ENACW (isotherm 11.75ºC)

35.55 −40 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Figure 11: P-E at the ENACW formation area (green cells in figure 10) and characteristic salinity for ENACW in station 6. The salinity is taken at a fixed isotherm to dwell with Huthnance et al. (2002, figure 4) but the patter is the same for salinity between isopycnal or isobars representative of ENACW (figures 6 and 8).

Winter air−sea Flux (Watt m−2)

140

120

100

80

60

ENACW formation area 40 1950 1960 1970 1980 1990 2000

140

120

100

80

60 Cantabrian area 40 1950 1960 1970 1980 1990 2000

250

200

150

100

WMDW formation area 50 1950 1960 1970 1980 1990 2000

Figure 12: Time series of the Heat Flux loss (sea to air) from the National Centers for Environmental Prediction (NCEP) Reanalysis data, averaged from December to March at the areas shown in figure 10. Colour codes are the same as in the map (Green for ENACW main formation area, red for the southern Bay of Biscay and blue for the MEDOC area. ICES CM 2006/C:26 Gonz´alez-Polaet. al. 11 ern Bay of Biscay (above the standard deviation) Colas, F., Circulation et dispersion lagrangien- were the northerly winds were more evident and nes en Atlantique Nord-Est, Thesis, Universit`e probably enhanced locally the modification of the de Bretagne Occidentale. Num`erode ordre 943, water column structure. This atmospheric anomaly 2003. caused the greatest winter heat-loss at the MEDOC area causing an unprecedented modification of wa- Gonz´alez-Pola, C., A. Lav´ın,and M. Vargas-Y´a˜nez, ter column structure in the deep Western Mediter- Intense warming and salinity modification of in- ranean (L´opez-Jurado et al., 2005). termediate water masses in the southeastern cor- The new structure in the water column which ner of the Bay of Biscay for the period 1992-2003, emerged after the anomalous winter 2004-05 re- J. Geophys. Res., 110 , C05020, 2005. mained to date, suggesting that changes may have Gonzalez-Pola, C., Variabilidad clim´aticaoce´anica had an extension big enough to avoid being blown en la regi´onsureste del Golfo de Vizcaya, Thesis, away by the renewal of water at our sampling site. University of Oviedo, Spain, 2006. Besides the analysis of interannual variability in re- lation to air-sea fluxes, the analysis of large-scale Huthnance, J. M., H. M. van Aken, M. White, E. D. cruises at the Bay of Biscay before and after this Barton, B. Le Cann, E. F. Coelho, E. A. Fan- special winter, in addition to the presented results jul, P. Miller, and J. Vitorino, Ocean margin ex- based on timeseries at fixed stations, should give us change - water flux estimates, J. Mar. Sys., 32 , valuable information about the circulation patterns 107–137, 2002. at this level in the Bay of Biscay. Instituto Nacional de Meteorolog´ıa,Resumen An- ual climatol´ogicodel a˜no2005, Tech. rep., 2006. Acknowledgements Kalnay, E., M. Kanamitsu, R. Kistler, W. Collins, We thank Dr. Luis Valdes, coordinator of the Span- D. Deaven, L. Gandin, M. Iredell, S. Saha, ish Oceanographic Institute (IEO) time-series pro- G. White, J. Woollen, Y. Zhu, M. Chelliah, gram during most of the period of the data collec- W. Ebisuzaki, W. Higgins, J. Janowiak, K. C. tion, for the Santander Section support. We also Mo, C. Ropelewski, J. Wang, A. Leetmaa, thank the crew of the RV Jose Rioja and IEO staff R. Reynolds, R. Jenne, and D. Joseph, The involved in the sampling for maintaining this time NCEP/NCAR 40-year reanalysis project, Bul- series, often under difficult circumstances. The letin of the American Meteorological Society, 77 , analysis of this data set has been performed in re- 437–471, 1996. lationship with projects RADIALES and VACLAN Lav´ın, A., L. Vald´es, F. S´anchez, P. Abaunza, (REN2003-08193-C03-01/MAR). The NCEP Re- J. Forest, P. Boucher, P. Lazure, and A. M. analysis data and figure 9 has been provided by the J´egou,The Bay of Biscay. The encountering of NOAA-CIRES Climate Diagnostics Center, Boul- the Ocean and the shelf, in The Seas, edited by der, Colorado, USA, http://www.cdc.noaa.gov Robinson and Brink, vol. 14, book chapter 24, pp. 933–1001, Harvard Press, 2006. References L´opez-Jurado, J. L., C. Gonz´alez-Pola, and P. V´elez-Belch´ı, Observation of an abrupt dis- Arbic, B. K., and W. B. Owens, Climatic warm- ruption of the long-term warming trend at the ing of Atlantic intermediate waters, J. Clim., 14 , Balearic Sea, western Mediterranean Sea, in 4091–4108, 2001. summer 2005, Geophys. Res. Lett., 32 , L24606, Bindoff, N. L., and T. J. McDougall, Diagnosing 2005. climate change and ocean ventilation using hy- MEDOC Group, Observation of formation of deep drographic data, J. Phys. Oceanogr., 24 , 1137– water in Mediterranean-Sea, 1969, Nature, 227 , 1152, 1994. 1037, 1970. ICES CM 2006/C:26 Gonz´alez-Polaet. al. 12

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