Spatiotemporal evolution of upwelling regime along the western coast ...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, C07020, doi:10.1029/2008JC004744, 2008

Spatiotemporal evolution of upwelling regime along the western coast of the Iberian Peninsula Ines Alvarez,1,2 Moncho Gomez-Gesteira,1 Maite deCastro,1 and Joaõ Miguel Dias2 Received 21 January 2008; revised 14 March 2008; accepted 6 May 2008; published 17 July 2008.

[1] The spatial and temporal variability of upwelling regime along the western coast of

the Iberian Peninsula (IP) has been analyzed by means of two different but complementary databases: QuikSCAT satellite database, which is available from mid-1999 with a high spatial resolution (0.25° 0.25°); and Pacific Fisheries Environmental Laboratory (PFEL) database, with a coarser spatial resolution (1° 1°) but extending back to 1967. The combined use of both databases allows determination of the spatial variability of Ekman transport and pumping, and identification of temporal trends in upwelling signal. Upwelling-favorable conditions are observed during spring and summer. Apart from this well-known seasonality on upwelling regime along the western coast of the IP, some other features have been identified: (1) Upwelling occurs simultaneously along the entire western coast of the IP, but is undoubtedly more complex closer to particular features of the shoreline. (2) Positive Ekman pumping is more intense near Capes Sao Vicente and Rocha during spring-summer and near Cape Finisterre all over the year. (3) More favorable upwelling conditions have been observed in February and November during the last decade. This result is corroborated by Sea Level Pressure composites, which show the existence of abnormally high pressures close to the IP during the last decade compared to the historical records (1948–2006). (4) The decadal variability of UI is higher in autumn-winter than in spring-summer. (5) Monthly upwelling trends indicate a weakening in upwelling intensity during most of the year with the exception of February, June, and July. Citation: Alvarez, I., M. Gomez-Gesteira, M. deCastro, and J. M. Dias (2008), Spatiotemporal evolution of upwelling regime along the western coast of the Iberian Peninsula, J. Geophys. Res., 113, C07020, doi:10.1029/2008JC004744.

1. Introduction [2] Coastal upwelling along the western coast of the Iberian Peninsula (IP) (36°N to 44°N) is a frequent phenomenon during the spring-summer months [Wooster et al., 1976; Fraga, 1981; Tenore et al., 1984; Blanton et al., 1984; Alvarez-Salgado et al., 1993; Pe´rez et al., 1995; Go´mez-Gesteira et al., 2006]. Although downwelling favorable southerly winds prevail in wintertime [Wooster et al., 1976; McClain et al., 1986] upwelling occurrence can also be observed during this period [Santos et al., 2001; Alvarez et al., 2003; Borges et al., 2003; Santos et al., 2004; deCastro et al., 2006, 2008]. [3] During the last 2 decades, the study of the western coast of the IP has been mainly carried out splitting this region in two different domains: the western Galician coast (41.5° to 44°N) and the western Portuguese coast (36° to 41.5°N).

1 Grupo de Fı´sica de la Atmosfera y del Oceano, Facultad de Ciencias, Universidad de Vigo, Ourense, Spain. 2 Departamento de Fı´sica, CESAM, Universidade de Aveiro, Aveiro, Portugal.

Copyright 2008 by the American Geophysical Union. 0148-0227/08/2008JC004744

[4] The western Galician domain has been intensively studied owing to the location of four important estuaries along its coast, locally named as Rias Baixas. They are characterized by a great primary productivity, which supports an intense raft culture of mussels [Tenore et al., 1984; Blanton et al., 1987]. Considering the studies off the Galician Rias Baixas, basically at the adjacent shelf, the first which analyzed the process of upwelling was carried out by Fraga [1981]. This author described the upwelling off Galician coast using scattered measurements obtained from 1974 to 1977. Since this first study, upwelling processes have been extensively analyzed along the western Galician coast. Several studies have been carried out in terms of wind-driven upwelling. The wind field is far from homogeneous in the region so that wind observations at a single point, coastal or offshore, will not necessarily be representative of coastal conditions over a significant distance [Torres et al., 2003]. McClain et al. [1986] considered a grid of stations covering the continental shelf from Cape Finisterre to Ria de Vigo during April 1982. Using in situ measurements and numerical simulations, they analyzed the upwelling evolution and the induced coastal circulation in response to 2 wind events that occurred over a 10-day period. The simulations show that during these wind events, the highest upwelling occurred either at Cape Finisterre or along the northern coast. In fact, Cape Finisterre marks an

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ALVAREZ ET AL.: UPWELLING ALONG WESTERN IBERIAN COAST

abrupt change in coastline orientation and may induce important wind stress variations. Torres et al. [2003] used 2 years of wind data from the QuikSCAT satellite to describe the Galician upwelling region from July 1999 to May 2001. They found that summer and winter wind fields present a diminutive number of dominant patterns characteristics of the typical upwelling and downwelling distributions off the Galician coast. Nevertheless, the mean long-term wind field’s summer and winter patterns are not necessarily representative of particular years when summer-like upwelling patterns may also dominate in winter. They also found that major features of upwelling around Finisterre are related to strong spatial wind field structures. Finisterre is frequently the site of a stationary upwelling maximum [Blanton et al., 1984; Castro et al., 1994] and of a recurrent upwelling filament [Haynes et al., 1993]. Barton et al. [2001] analyzed the complex features of an upwelling filament located near 42°N off Galicia during a period of upwelling-favorable winds in August 1998. Torres et al. [2003] also showed a coincidence between strong Finisterre filaments and specific patterns of wind curl or Ekman pumping in July 1999. In addition to wind-driven upwelling studies, recent research analyzed the upwelling conditions along the western Galician coast using Ekman transport data. Go´mez-Gesteira et al. [2006] analyzed Ekman transport close to the Galician coast using forecasted winds from November 2001 to October 2004. They found that the upwelling at western coast reaches a maximum probability of 60% during summer and a minimum probability of around 15% in December – January. These results were corroborated by Alvarez et al. [2007] using QuikSCAT satellite data from November 1999 to October 2005. [5] The transition from the downwelling winter regimen to the upwelling summer regime along the Galician coast was also studied by Torres and Barton [2007] analyzing a set of observations, including satellite, cruise and mooring data for the period of May – July 1997. They found a rather complex circulation where poleward flow over the slope coexists with coastal upwelling and strong outflow from the Rias. [6] Most of the studies were focused on the springsummer months, characterized by upwelling-favorable northerly winds along the coast. Nevertheless, recent studies revealed the existence of upwelling events during autumnwinter months. In fact, Alvarez et al. [2003] characterized an unusual winter upwelling event in January 1998, where seawater driven by the poleward current was pumped into the Rias Baixas. The mechanism driving this winter upwelling was similar to the one observed in summer, but the thermohaline properties of the upwelled water were completely different. A more complete analysis was carried out along the Galician western coast by deCastro et al. [2008] from 2000 to 2005. These authors found six upwelling events during the wet season (November to February) with features similar to the ones observed in summer (analogous wind-forcing and upwelled water). [7] Several studies about the upwelling processes along the western coast of Portugal were also carried out during the last years. As well as the Galician coast, the Portuguese coast has an important primary productivity related to sardine and horse mackerel. In consequence, the evolution of physical and chemical parameters related to upwelling

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processes has been extensively analyzed. Sousa and Bricaud [1992] studied the correlation between upwelling and the space-time distribution of phytoplankton from July 1981 to September 1983. The temporal variability of the pigment patterns was compared with the wind-induced offshore Ekman transport calculated from measurements made at 2 meteorological stations. A significant relationship was found: well-developed phytoplankton structures were generally related to moderate or intense offshore transport, whereas the absence of plumes corresponded to either weak offshore transport or coastal convergence. [8] According to the analysis (1959 – 1969) conducted by Fiuza et al. [1982] wind-induced upwelling conditions tend to be more stable, frequent and intense in June– August, and upwelling indices have a tendency to be higher between July and September. Following this work, Fiuza [1983] found that the strength and extension of upwelling are connected to local winds and to the geophysical surrounding properties. However, the analysis of the upwelling evolution off the western coast of Portugal performed by Lemos and Pires [2004] using SST images and the meridional wind component between 1941 and 2000, revealed a clear evidence of a progressive weakening of the upwelling regime in the last 60 years (1941– 2000), especially in the warm season (April – September). They also found that in March, the v-wind component displayed negative trends. Hence, toward the end of the 20th century, the onset of the upwelling season has occurred progressively earlier. In fact, as at the western Galician coast, winter upwelling events have also been identified along the Portuguese coast [Borges et al., 2003]. Santos et al. [2001] found that the decreasing trends in the recruitment of sardine and horse mackerel observed during the 1990s in the nursery grounds off the west coast of Portugal are linked with the increase of upwelling events in winter, i.e., during the spawning season of these species. [9] Along the Portuguese coast upwelling responds quickly to northerly winds, particularly south of capes, appearing first along the coastline and then spreading offshore as the event progresses producing upwelling filaments [Fiuza et al., 1982]. Peliz et al. [2002] observed the existence of some filaments off northern Portugal at the end of the upwelling season (September 1998) after a short relaxation event. Using satellite imagery, Relvas and Barton [2005] also described a recurrent filament of cold, upwelled water which extends offshore from Cape Sao Vicente during periods of persistent northerly winds, while the rapid development of a warm coastal countercurrent accompanies relaxation of upwelling-favorable winds. [10] Upwelling was also analyzed in the entire western coast of the IP. Bakun [1990] found that northerly, upwelling-favorable winds had become stronger along the western coast of the IP from the late 1940s onward, using 6 month (April to September) averages of the meridional wind stress component derived from ship reports. These results are in contradiction with those of Lemos and Pires [2004], who found a progressive weakening of the upwelling regime in the last 60 years along the Portuguese coast. [11] Upwelling filaments have also been found off the Atlantic coast of the IP by means of advanced very high resolution radiometer infrared imagery [Haynes et al., 1993]. The analysis performed by Haynes et al. [1993]

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between 1982 and 1990 using SST images, revealed the presence of five or six fully developed filaments off the IP late in the upwelling season. Most of these are associated with major topographic features of the region, as the large capes. However, some filaments observed along the more regular northern coastline can be formed by flow instabilities, possibly initiated by the large capes of northern IP. [12] Recently, Santos et al. [2005] also used satellitederived SST data to study the variability of the Canary Upwelling Ecosystem from 1960 to 2000, considering the western coast of the IP (37°N – 42°N) as a subarea within the entire area under study (12°N–42°N). These authors observed that the Iberian coast (37°N – 42°N) was experiencing weak but persistent winter upwelling events during the strong phase of the upwelling regime (1992 – 2001). The strong phase started with the development of the strong negative meridional wind anomaly which occurred in the IP on 1992. [13] The aim of this study is to describe the wind-forcing over the area extending from 36°N to 44°N with a spatial resolution high enough to detect some effect of coastal morphology on wind patterns and with a time resolution long enough to observe trends in upwelling patterns. The upwelling spatial and temporal variability along the western coast of the IP was studied using high-resolution wind data supplied by QuikSCAT satellite from 2000 to 2006. This database allows determining the effect of coastal topographic features on wind variability leading to Ekman pumping, which is especially important near capes. Satellite observations were completed by upwelling index (UI) data provided by the Pacific Fisheries Environmental Laboratory (PFEL) which extend back to 1967, although with a coarser spatial resolution. This database allows the analysis of monthly and seasonal trends in upwelling regime.

2. Data Used for Analysis [14 ] Surface wind fields were obtained from the QuikSCAT satellite, which is available from July 1999. Wind data were retrieved from the Jet Propulsion Laboratory web site (http://podaac.jpl.nasa.gov/quikscat/qscat_data. html). The data set consists of global grid values of meridional and zonal components of wind measured twice daily on an approximately 0.25° 0.25° grid with global coverage. QuikSCAT data are given in an ascending and descending pass. Data corresponding to one pass present numerous shadow areas, therefore, an average between both passes was considered to increase the coverage. Wind speed measurements range from 3 to 20 m s1, with an accuracy of 2 m s1 and 20° in direction. The reference height of wind data is 10 m. In addition, it is necessary to take into account that wind data close to coast (25 km) are not available owing to the existence of a small coast mask. This mask makes the study difficult in the inner 75 km and prevents the analysis of orographic effects nearer to shoreline. Nevertheless, a recent study carried out by Go´mezGesteira et al. [2006] using modeled wind data around the Galician coast, showed that along the western coast (41°N – 43°N) from the shoreline to around 75 km the wind direction presents a constant pattern, with small differences in amplitude values. On the other hand, the lack of real wind measurements (e.g., buoy data and meteorological stations)

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simultaneously all over the coast makes the analysis of wind patterns difficult near the shoreline. [15] The precision of QuikSCAT data was previously analyzed along the Galician coast for the period 2002– 2005. A statistical comparison between satellite wind measurements and high-resolution numerical models was carried out [Penabad et al., 2007], revealing similar results between models and satellite data. A statistical analysis based on mean errors, root mean square errors and complex correlation was performed on spatial, temporal and directional scales. High-resolution numerical models results compared to satellite wind estimations showed behavior within the limits of confidence of the satellite scatterometer. [16] Ekman transport was calculated using the wind speed at the 10 m level, W, the seawater density, rw = 1025 kg m3, a dimensionless drag coefficient, Cd = 1.4 103, and the air density, ra = 1.22 kg m3, by means of Qx ¼

1=2 1=2 ra Cd 2 r Cd 2 Wy and Qy ¼ a Wx ; Wx þ Wy2 Wx þ Wy2 rw f rw f

where f is the Coriolis parameter defined as twice the vertical component of the Earth’s angular velocity, W, about the local vertical given by f = 2Wsin(q) at latitude q. Finally, x subscript corresponds to the zonal component and the y subscript to the meridional one. In addition to the global analysis of transport patterns corresponding to the whole area under scope, a discrete set of 28 points placed 75 km far from the coast was also considered (Figure 1, circles). Discretization effects were smoothed by calculating Ekman transport values at each point as the average of its nearest neighbors in latitude. The obtained discrete series covers from 36°N to 43°N, in such a way that the distance between adjacent points is on the order of 18 km. [17] The UI was calculated from Ekman transport as the fraction of the Ekman transport perpendicular to the coast [Nykjaer and Van Camp, 1994]. UI can be defined as the Ekman transport component in the direction perpendicular to the shoreline by means of UI = Q? = sin(q)Qx + cos(q)Qy where q = p/2 + 8 and 8 is the angle of the unitary vector perpendicular to the shoreline pointing landward [Go´mez-Gesteira et al., 2006]. Although the shoreline angle along the western coast of the IP changes slightly from the northern to the southern limits, macroscopically it can be considered approximately 90° relative to the equator. Positive (negative) UI values mean upwelling-favorable (unfavorable) conditions. [18] The Pacific Fisheries Environmental Laboratory (PFEL) (www.pfel.noaa.gov) distributes environmental index products and time series databases to cooperating researchers, taking advantage of its long association with the U.S. Navy’s Fleet Numerical Meteorology and Oceanography Centre (FNMOC). FNMOC produces operational forecasts of the atmosphere and the ocean state several times daily and maintains archives of several physical variables. For our purposes monthly data of UI were considered at 6 points located in front of the IP coast (Figure 1, crosses) on an approximately 1° 1° grid from 1967 to 2006. Although monthly data are not adequate considering the storm frequency, it can be accurate enough to study seasonal fluctuations.

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Figure 1. Area under scope, corresponding to the western coast of the Iberian Peninsula. Circles represent the 28 points placed along the coast 75 km from the shoreline. Black circles represent the seven points considered to analyze the interannual differences of Ekman transport. Crosses represent the six points where monthly data of UI from the PFEL database were considered. [19] Sea Level Pressure (SLP) data were obtained from reanalysis data from a cooperating project between the National Centers for Environmental Prediction (NCEP) and the National Centre of Atmospheric Research (NCAR) (www.cdc.noaa.gov/cdc/reanalysis/reanalysis.html). The goal of this joint effort is to produce new atmospheric analyses using historical data (1948 onward) additionally to produce analyses of the current atmospheric state (Climate Data Assimilation System, CDAS). The data spatial coverage is a global grid with a spatial resolution of 2.5° 2.5°. Monthly data from 1948 to 2006 on a 20°N–60°N, 40°W – 5°E grid were selected in the present study. [20] These different databases afford a detailed spatial and temporal analysis of upwelling. Thus, QuikSCAT provides data with high spatial resolution (0.25° 0.25°), although they are available only from 1999 on. The PFEL and NCEP-NCAR data lie on a coarser spatial grid (1° 1° and 2.5° 2.5°, respectively), although with a longer temporal extent (from 1967 and 1948, respectively).

3. Results and Discussion 3.1. Ekman Transport Patterns [21] The monthly mean Ekman transport was calculated by averaging daily values provided by the QuikSCAT satellite at each grid point from January 2000 to December 2006 (Figure 2). A common pattern between all the months

is observed, both in direction and amplitude, over the entire area for all the period under study. Analyzing each month individually, Ekman transport direction reveals the same trend from Cape Finisterre (43°N) to Cape Sao Vicente (37°N) and transport amplitude tends to be higher in the southern region. [22] Important differences are also observed between the various months. From January to March two different patterns are detected. In January and in March the pattern is mainly characterized by southward transport over the entire region. The intensity of transport increases from north to south, with a significant amplitude change around 39°N– 40°N and reaching the maximum value (300– 400 m3 s1 km1) between 36 and 38°N. In February the pattern is completely different. Transport is directed westward and its intensity is considerably higher than those observed during the previous and the next month, with maximum values of 700 m3 s1 km1 at the southern region. [23] During spring and summer seasons (April–September) a pattern similar to that observed in February is observed, with the transport pointing westward along the entire coast. In this case, the intensity of transport is higher, ranging from 300 m3 s1 km1 at 44°N to 1100– 1200 m3 s1 km1 at 36°N. In September, although the transport direction is westward, an important intensity decrease is observed in the entire region, with maximum values between 400 and

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Figure 2. Monthly Ekman transport (m3 s1 km1) along the western coast of the Iberian Peninsula obtained by averaging each month’s values from 2000 to 2006. 5 of 14

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Figure 3. Monthly Ekman transport (m3 s1 km1) from 2000 to 2006. Each monthly pattern was represented independently by means of arrows which indicate transport direction and amplitude for each year. 700 m3 s1 km1. This westward transport direction is related to favorable upwelling conditions. [24] Ekman transport pattern is less uniform from October to December. In October the transport direction is mainly southward and its amplitude ranges from 300 m3 s1 km1 at the northern region to 550 m3 s1 km1 at the southern one. In contrast, in November the pattern is similar to that observed in spring and summer seasons, although with lower intensity values (minimum 300 m3 s1 km1, maximum 700 m3 s1 km1). Finally, in December the transport

direction does not present a clear trend, and its intensity values were found the lowest of the year (100–200 m3 s1 km1) over the region. [25] To analyze interannual variability, monthly mean patterns of Ekman transport were calculated for each month at some control points (Figure 3). Only 7 control points were considered for the sake of clarity (Figure 1, black circles). These points are located at an approximate distance 75 km far from the nearest shoreline. In average, this is the minimum distance to the coast where reliable data can be

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Figure 4. Monthly Ekman pumping velocity (m d1) along the western coast of the Iberian Peninsula obtained by averaging each month results from 2000 to 2006. 7 of 14

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Figure 5. (a) The 6-year mean (2000 –2006) of the annual evolution of Ekman pumping velocity (m d1) at 28 points (Figure 1, circles) situated along the coast at an approximate distance 75 km far from the nearest shoreline. (b) Time averaging of monthly Ekman pumping velocity from 2000 to 2006. obtained, since QuikSCAT data have a mask of at least 0.25°. Each monthly pattern was represented independently by arrows indicating transport direction and amplitude for each month under study (Figure 3). In addition, a reference arrow showing the maximum amplitude value obtained for each case was represented inside each frame. [26] In January and March are observed marked variations in amplitude and direction at the same point, showing that transport direction fluctuates over the years. The highest amplitudes occurred in 2001, with the maximum values at the northern region (40°N–43°N) corresponding to a southeastward component. The February case shows a westward component at each point independently of the year. Transport amplitude is also similar at each point, except in 2005 when the highest-amplitude values were observed in the northwestward direction. From April to September, Ekman transport shows approximately the same direction and amplitude at each point independently of the year. Transport is directed westward over the region, in accordance with the patterns observed in Figure 2 (April – September). In October some years with southwestward and southeastward transport were observed, with the maximum amplitude values found around the northern area. In November, the highest-amplitude values are observed in 2001, showing a northwestward direction at all points.

Finally, in December transport shows important changes in amplitude and direction all over the region, in accordance with the pattern found in Figure 2 (December). In view of the described behavior during the whole period, it is verified that fall-winter months show great wind variability while spring-summer months practically present constant patterns independently of the year. This behavior is in accordance with the results obtained by Torres et al. [2003] along the Galician coast using QuikSCAT satellite data. They found that the mean summer and winter long-term patterns of wind field’s are not necessarily representative of particular years, when summer upwelling patterns may also dominate in winter. These results were also corroborated by Go´mezGesteira et al. [2006] and by Alvarez et al. [2007] along the Galician coast using modeled and satellite data. 3.2. Ekman Pumping Velocity [27] The wind spatial variation induced by the coastal features produces an analogous variation in the resulting Ekman transport, causing convergence in some places and divergence in other. This leads to Ekman pumping. The Ekman pumping velocity (wE) over the area under study was calculated from wind vectors provided by the QuikSCAT satellite by the following relation wE = rf1 k (r t), where k, t, f and r are vertical unit vector, wind stress,

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Figure 6. (a) Interannual evolution of UI from 2000 to 2006. (b) The 6-year (2000 – 2006) mean of the annual evolution of UI. (c) Interannual evolution of the meridional average of UI from 2000 to 2006. (d) Annual cycle calculated by meridional and time averaging of UI from 2000 to 2006. Units of UI are m3 s1 km1. Coriolis parameter and water density, respectively [Enriquez and Friehe, 1995; Torres et al., 2003]. [28] Figure 4 shows the monthly Ekman pumping velocity from 2000 to 2006. From January to March the Ekman pumping velocity is positive at the northern region with the highest values (0.2 m d1) in front of the Galician coast. In March, these positive values cover a larger area, from the shoreline to 11°W. In January and in February it are also observed positive values (0.1 – 0.15 m d1) at the southern region, approximately at 36°N and 38°N. During spring and summer seasons (April – September) positive Ekman pumping velocities are observed along the entire coast in an area between 9°W – 10°W. Depending on the area, some nearshore regions are obscured by the existence of a small coastal mask in QuikSCAT data. Maximum values (0.25 m d1) are found at the northern and southern region, around 43°N and 36°N. The pattern is slightly different in September, when these positive values in front of the Galician coast extend for a larger area. From October to December it is not observed a clear trend in Ekman pumping velocity, with positive and negative values dispersed over the region. [29] The monthly evolution of Ekman pumping velocity was also considered at 28 points (Figure 1, circles) situated along the coast at an approximate distance 75 km far from the nearest shoreline (Figure 5a). The highest positive values are observed from April to September around points 5 (42.5°N), 21 (38°N) and 26 (36°N), with the maximum value (0.4 – 0.5 m d1) found in June – July. These are the latitudes of the location of Cape Finisterre (Galician coast), Cape Rocha and Cape Sao Vicente

(southern Portuguese coast). Along the rest of the coast, positive values are also observed during spring and summer seasons, although with a lower magnitude (around 0.2 m d1). In February and in November the Ekman pumping velocity also tends to be positive along the entire coast while for the rest of the year are observed negative or practically null values. Between 42°N and 43°N (points from 4 to 6) positive velocities are observed basically all over the year. This behavior could be understood taking into account the location of Cape Finisterre, where a stationary upwelling maximum and a recurrent upwelling filament is frequently observed [Blanton et al., 1984; Castro et al., 1994]. [30] Figure 5b shows the time averaging of the monthly Ekman pumping velocity from 2000 to 2006 (Figure 5a). These results show the existence of three maxima. The first maximum (0.25 m d1) is found around point 5, the second maximum (0.15 m d1) is located around point 21 and the last one (0.12 m d1) is situated around point 26. These points correspond to the location of the most important capes along the coast of the IP, Cape Finisterre, Cape Rocha and Cape Sao Vicente. Capes may induce important wind stress variations [Enriquez and Friehe, 1995], producing localized upwelling maxima and upwelling filaments [Barth and Brink, 1987; Haynes et al., 1993; Freudenthal et al., 2002; Torres et al., 2003; Barton et al., 2004; Relvas and Barton, 2005]. In fact, Haynes et al. [1993] observed that most of the filaments off the IP are associated with major topographic features of the region, in

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Figure 7. (a) Interannual evolution of UI from 1967 to 2006. (b) The 6-year (1967 – 2006) mean of the annual evolution of UI. (c) Interannual evolution of the meridional average of UI from 1967 to 2006. (d) Annual cycle calculated by meridional and time averaging of UI from 1967 to 2006. Units of UI are m3 s1 km1. particular the large capes which are common to the northern and southern coast of the IP. 3.3. Upwelling Evolution [31] Figure 6 shows the time evolution of UI from 2000 to 2006 along the coast of the IP taking into account the 25 points considered along the western coast of the IP (Figure 1, circles). The interannual evolution of UI (Figure 6a) shows a marked annual cycle, with maximum values (upwelling-favorable conditions) in July – August and minimum values in December –January. This periodicity disappears from 2005 on, when it are observed maxima at the beginning and end of each year. This behavior is in accordance with the results obtained by Alvarez et al. [2007] along the Galician coast using QuikSCAT satellite data. [32] Monthly average values of UI from 2000 to 2006 are shown in Figure 6b. The highest positive values (upwellingfavorable conditions) are observed from May to August, with a maximum value of 1400 m3 s1 km1 in July between 36°N and 38°N. These results coincide with previous studies carried out along the western coast of the IP, which showed that upwelling events occur during summer season [McClain et al., 1986; Blanton et al., 1987; Tilstone et al., 1994; Santos et al., 2005]. In February and in November positive values are also identified along the entire coast, although with a lower magnitude (500 – 800 m3 s1 km1). Negative or practically null values are observed for the rest of the year. These positive upwelling values in February and November indicate upwellingfavorable conditions during fall-winter seasons. This

situation is in accordance with the pattern observed in Figure 2 for February and November. The monthly average of Ekman transport during these months shows that transport is directed westward over the region under study, with high-intensity values from Cape Finisterre to Cape Sao Vicente (600 –700 m3 s1 km1). [33] Figure 6c shows the interannual evolution of the meridional average of UI. Meridional averaging could be a source of errors in areas with high latitudinal gradients. However, in the present study, the small latitudinal changes described in the previous figures make the meridional averaging appropriate. The signal is displaced toward positive values, showing maxima in July – August and minima in December – January. Remarkable differences can be observed among the different years. The amplitude of the signal is higher from the beginning of the period to 2004. From 2004 on, the amplitude decreases and is observed the existence of two maxima between the end and the beginning of 2004 – 2005 and 2005– 2006. Furthermore, the minima amplitude is much higher at the end of 2000 and at the beginning of 2001. This situation is related to the weather conditions, which depend on each particular year. Thus, this period was characterized by intense southerly winds which can originate high rainfall events [Alvarez et al., 2005]. [34] The meridional and time averaging of UI (Figure 6d) shows strong positive values (favorable upwelling conditions) higher than 600 m3 s1 km1 from April to September, with the maximum value in July (around 1000 m3 s1 km1). This behavior is in accordance with the results of the study

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Figure 8. (a) Decadal variability of the UI (m3 s1 km1) from 1967 to 2006. Period (a) 1967– 1976, (b) 1977 – 1986, (c) 1987– 1996, and (d) 1997 – 2006. carried out by Cabanas and Alvarez [2005] using Ekman transport at the ocean (point 43°N, 349°E) for a 40-year period (1966 – 2005). These authors found that the zonal Ekman transport (Qx) showed the lowest values in July, corresponding to the upwelling season. Two important positive peaks are also observed in February and November, with values close to 600 and 400 m3 s1 km1, respectively, indicating a situation favorable to the occurrence of upwelling events during fall-winter seasons. UI values are lower than 300 m3 s1 km1 for the rest of the year. The increase in winter upwelling events has been observed along the western coast of the IP during the last decade [Alvarez et al., 2003; Borges et al., 2003; Santos et al., 2004; deCastro et al., 2006; Prego et al., 2007]. Actually, six upwelling events were characterized during the wet season (November – February) from 2000 to 2005 by deCastro et al. [2008] in the Ria of Pontevedra, in the western Galician coast. The mechanism driving these fall-winter upwelling events were found similar to those observed in summer season (similar wind-forcing and upwelled water). Thus, Figures 2 and 6 suggest that the fall-winter upwelling events described by deCastro et al. [2008] could also occur along the Portuguese coast.

[35] In order to analyze the increase in winter upwelling events observed along the coast of the IP during the last decade, a more complete temporal analysis was performed considering the UI evolution at 6 points located in front of the IP coast (Figure 1, crosses) from 1967 to 2006 (Figure 7) (see description of PFEL database given above). Figure 7a shows the interannual evolution of UI, which follows a pattern similar to that observed in Figure 6a. [36] Monthly average of UI from 1967 to 2006 (Figure 7b) shows positive values from March to October, with maximum values in July (800 m3 s1 km1). These maximum values are observed at latitudes from 37°N to 41°N, although UI also shows high positive values (600 m3 s1 km1) at the northern part of the IP coast (41°N– 42.5°N). For the rest of the year UI shows negative or practically null values. [37] The interannual evolution of the meridional average of UI (Figure 7c) shows a pattern similar to that observed for the period 2000 – 2006 (Figure 6c), with maximum values in July – August and minimum values in December – January. The signal tends to be displaced toward positive values, although it is possible to observe some years with strong negative values, which reveals significant differences among years. Thus, the minima amplitude is much higher

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Table 1. Monthly Upwelling Trends Corresponding to the Period 1967 – 2006 Month

UI

Confidence Level

Jan Feb March April May June July Aug Sept Oct Nov Dec

4.2 9.8 11.1 4.3 1.0 2.0 4.7 1.3 1.3 11.1 4.5 12.6

85% 99% 99% 99% — 99% 99% 95% 99% 99% 99% 99%

at the end of 2000 and at the beginning of 2001 than for the rest of the years, in agreement with Figure 6c. [38] The meridional and time averaging of UI is represented in Figure 7d. Positive values are observed from

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March to September, with the maximum value close to 700 m3 s1 km1 in July. Negative values can be observed for the rest of the year, showing unfavorable upwelling conditions during pffiffiffiffi winter. The error bars were calculated using s(UIW)/ N , where s(UIW) is the standard deviation of the monthly data and N the number of years (40). Error bars are observed to be negligible compared to the amplitude of the annual cycle, which ranges from 0 to 700 m3 s1 km1. [39] To investigate interdecadal variations in upwelling conditions, the meridional and time averaging of UI was analyzed considering decadal periods from 1967 to 2006 (Figure 8). The error bars for each decade are also negligible compared to the amplitude of each annual cycle. During the first decade (1967– 1976) (Figure 8a) are observed positive values from March to September, with a maximum value (the most favorable upwelling conditions) of 600 m3 s1 km1 in July. In January and February UI shows negative values, while from October to December values are close to zero.

Figure 9. SLP composites for a 58-year period (1948 – 2006) in (a) February and (b) November. SLP composites for a 7-year period (2000 – 2006) in (c) February and (d) November. (e) SLP difference composite calculated as the difference of composites From Figures 9a and 9c. (f) SLP difference composite calculated as the difference of composites from Figures 9b and 9d. 12 of 14

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[40] The results for the second decade (1977 – 1986) (Figure 8b) also show positive values from March to October, with the maximum being also observed in July (600 m3 s1 km1). Furthermore, in February, October and December are found negative values, while the January result is practically zero. [41] The results for the third decade (1987 – 1996) (Figure 8c) reveal a more typical annual variation, with positive values during spring-summer seasons and negative values during autumn-winter seasons (similar to Figure 7d). The maximum positive value is once more observed in July (600 m3 s1 km1). [42] The results for the last decade (1997 – 2006) (Figure 8d) show a pattern similar to that observed in Figure 6d, although present slight differences in magnitude. Positive values are observed from April to September, with the maximum value in July (800 m3 s1 km1). For the rest of the year UI values tend to be negative, although increasing in February and in November regarding the previous and the next months. [43] Considering the results for the various decades, it is observed that the upwelling-favorable conditions tend to be permanent during spring-summer and highly variable during autumn-winter. [44] The trend of monthly UI over the area under study was calculated from 1967 to 2006 (Table 1). Positive (negative) values indicate more favorable (less favorable) upwelling conditions. The results show statistical significance for most of the months, except for January and May which confidence level is lower than 90%. In general, monthly upwelling trends indicate a weakening in upwelling intensity during most of the year, with the exception of February, June and July. The highest positive value is observed in February (9.8 m3 s1 km1), followed by that observed in July (4.7 m3 s1 km1). On the other hand, the highest negative value corresponds to December (12.6 m3 s1 km1), followed by the values observed in October and March (11.1 m3 s1 km1). From the analysis of Table 1 it is not observed a clear seasonal trend in UI. During spring-summer months this trend fluctuate with the month, indicating an upwelling strengthening in June and July and a weakening trend for the rest of the months. For autumn-winter months, an upwelling weakening was observed, with the exception of February. [45] The unusual winter upwelling conditions observed in February and November along the western coast of the IP during the last decade were analyzed in terms of SLP composites. Figure 9 shows the SLP composites in February and November for a 58-year period (1948 – 2006) (Figures 9a and 9b) and for the period under study (2000 – 2006) (Figures 9c and 9d). For the 58-year period case it is observed a high-pressure center in front of the western coast of the IP (Figures 9a and 9b), near 35°N, 25°W for both months. These centers originate northerly winds blowing at shelf, generating favorable conditions to upwelling events along the entire coast of the IP. During the 7-year period (Figures 9c and 9d) a high-pressure center located in front of the western coast of the IP (Figures 9c and 9d) is also observed, although in February it is closer to the IP (40°N, 20°W).

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[46] SLP differences of previous composites were also calculated in February (Figure 9e) and in November (Figure 9f), as the difference between the SLP composite from 1948 to 2006 and from 2000 to 2006. For both months are observed positive values in the surrounding area of the IP, showing that the SLP pattern for the last decade has higher values than the averaged pattern (1948 – 2006). Both SLP patterns show a high-pressure center located near the IP. In February (Figure 9e), this center is located northwest of the IP, near 50°N, 15°W – 20°W, with maximum values around 4 mbar. In November (Figure 9b), the high-pressure center is located in front of the western coast of the IP, near 40°N, 30°W, with maximum values around 2 mbar.

4. Summary [47] The spatial and temporal variability of upwelling regime along the western coast of the Iberian Peninsula was analyzed using data from two different, but complementary databases. On the one hand, QuikSCAT satellite database is used for the period 2000 –2006 with a high spatial resolution (0.25° 0.25°). On the other hand, PFEL database provides longer records (1967 –2006), although with a coarser spatial resolution (1° 1°). The combined use of both databases permits the calculation of the Ekman transport and pumping spatial variability, and the identification of temporal trends in upwelling signal. In addition, the observed trends are discussed in terms of SLP data provided by NCEP-NCAR from 1948 to 2006. The upwelling events are found along the full western coast of the IP, although are definitely more complex off particular coastal features (e.g., capes). Spatial changes in upwelling were found to occur owing to the interaction between the macroscopic wind regime and coastal orography. In particular, positive Ekman pumping was observed near Capes Sao Vicente (37°N) and Rocha (38°N) during spring-summer and near Cape Finisterre (43°N) all over the year. It is a well-known fact that Capes may induce important wind stress variations, producing localized upwelling maxima and upwelling filaments. Thus, the described maxima provided support to the suggestions of some previous works [Blanton et al., 1984; Haynes et al. 1993; Castro et al., 1994]. [48] In average, there is a strong seasonality characterized by upwelling-favorable conditions from April to September and unfavorable conditions from October to March. Nevertheless, abnormally favorable upwelling conditions have been observed in February and November during the last decade. The observed changes in upwelling regime are in good agreement with the presence of several winter upwelling events previously described by the same authors [Alvarez et al., 2003; deCastro et al., 2006, 2008] during the last decade. This finding is corroborated by SLP composites, which show the existence of abnormally high pressures close to the IP in February and November during the last decade compared to historical records (1948 – 2006). [49] A decadal analysis of UI evolution from 1967 to 2006 showed that the upwelling conditions are highly variable during autumn-winter, without a clear seasonal trend. This variability is considerably lower during springsummer. In fact, monthly upwelling trends indicate a

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weakening in upwelling intensity during most of the year, with the exception of February, June and July. [50] Acknowledgments. This work is supported by the Ministerio de Educacion y Ciencia under project CTM2007-62546-C03-03/MAR and by Xunta de Galicia under projects 517PGIDIT06PXIB383285PR and PGIDIT06PXIB383288PR.518. The first author of this work has been supported by the Fundaçaõ para a Cieˆncia e a Tecnologia through a postdoctoral grant (SFRH/BPD/38292/2007).

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I. Alvarez, M. deCastro, and M. Gomez-Gesteira, Grupo de Fı´sica de la Atmosfera y del Oceano, Facultad de Ciencias, Campus de Ourense, Universidad de Vigo, 32004 Ourense, Spain. ([email protected]; mdecastro@ uvigo.es; [email protected]) J. M. Dias, Departamento de Fı´sica, CESAM, Universidade de Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal. ([email protected])

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