Journal of Marine Systems 129 (2014) 271–288

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Journal of Marine Systems

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Variability of temperature and chlorophyll of the Iberian Peninsula near costalecosystem during an upwelling event for the present climate and a future climatescenario

José Fortes Lopes a,b,⁎, Juan A. Ferreira c, Ana Cristina Cardoso a, Alfredo C. Rocha a,b

a CESAM, Universidade de Aveiro, 3810-193 Aveiro, Portugalb Departamento de Física, Universidade Aveiro, 3810-193 Aveiro, Portugalc CENSE, Department of Science Environmental Engineering, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal

⁎ Corresponding author at: Departamento de Física, UAveiro, Portugal. Tel.: +351 234370821; fax: +351 234

E-mail address: jflopes@fis.ua.pt (J.F. Lopes).

0924-7963/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.jmarsys.2013.07.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 January 2013Received in revised form 2 July 2013Accepted 3 July 2013Available online 10 July 2013

Keywords:Numerical modellingUpwellingTemperaturePhytoplanktonChlorophyll-aPortuguese coastClimate scenarios

Understanding the importance and the implication of the climate changes on coastal areas may be one of themajor issues for this and next centuries. Climate changes may, indeed, impact the nearshore marine ecosys-tem, as coastal areas are very sensitive to the strength and the variability of the meteorological forcings.The main purpose of this work is to study temperature and phytoplankton distributions along the Portuguesenear coastal zone during upwelling events in the present climate conditions and in a future climate scenario.The SRES-A2 IPCC scenario has been considered.We have used a three-dimensional model for coastal and shelf seas, including the following sub-models:hydrodynamical/physical, biological, sediment and contaminant. The forcings are provided by the interac-tions at the air–sea, considering the wind intensity and direction with the help of the WRF model (WeatherResearch and Forecast Model) and the coupled atmosphere–ocean model ECHAM5/MPI-OM.Results show that, for the future climate scenario, there is a reinforcement of the southward wind. The re-sponses of the coastal ecosystem corresponds to the reinforcement of both, the southward (up to 10 cm/s)and the westward (up to 6 cm/s) induced upwelling currents. This, in turn generates an enlargement ofthe near coast upwelled cold layer, extending up to 60 km, as well as the rise of the warm layer temperature(up to 2.0 °C) and the spreading of the phytoplankton offshore. Significant changes in both the Chl-a verticaland the horizontal distribution patterns have been observed, as the nutrient supply to the upper layers de-pends on the strength of the upwelling, the bottom topography and orography of the coastal. These resultsconfirm that changes in the strength and eventually the frequency of the upwelling favourable wind impactthe phytoplankton distribution, which can have significant effect in the distribution and population of theupper level of the trophic chain of the coastal ecosystem.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

The Atlantic Portuguese coast (Fig. 1) is situated in the North AtlanticUpwelling System of the western coast of the Iberian Peninsula,which extends from 10° N to about 44° N (Prego et al., 2007,Wooster et al., 1976). The wind and the current system present astrong seasonal variability, with the summer and the winter patternsbeing easily distinguished. From March to April the winds are typicallynorth-westerly and by May they become predominantly northerly,with the wind stresses near the coast of the order of 0.03 Pa, a patternwhich remains unchanged through summer. By September the windstresses reach the maximum value, of the order of 0.1 Pa (Reis andGonçalves, 1988), relaxing in strength by October until becoming

niversidade Aveiro, 3810-193424965.

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southerly, which becomes a typical situation of the autumn and winterseason. During the summer season significant changes occur inside anarrow layer of the coastal area (between 30 and 60 km (Coelho etal., 2002)). Due to the wind direction and stresses, a surface divergenceand geostrophy budget is setup near the coast corresponding of the up-welling of deep cold water, spreading and advected offshore, pushingthe warmer water far from the coast. This budget establishes a strongsouthward coastal jet, with the current intensity reaching values ofthe order of the order of 10 to 20 cm/s, in accordance with typical ob-served values (Coelho et al., 2002; Sauvaget et al., 2000). The horizontaltemperature distribution is, therefore, characterized by a nearshorecold layer (temperature b 18 °C), surrounded by an offshore warmlayer (temperature of the order of 20 °C). This event (McClain et al.,1986) favours nutrient input towards the surface waters, enhancingphytoplankton growth and primary productivity (Prego et al., 2007).

The area between Figueira da Foz and the Aveiro coasts is a partic-ular ecosystem controlled by the strength and the variability of the

Fig. 1. The study area: the near continental shelf of the Portuguese coast with the location of the vertical profile stations (Aveiro (S1) and Figueira da Foz (S2)).

272 J.F. Lopes et al. / Journal of Marine Systems 129 (2014) 271–288

upwelling. The continental shelf is relatively wide (~60 km) andgently sloping, with an edge defined by the 200-m isobath, wherethe Aveiro Canyon (40°42′N) is the most significant topographic fea-ture due to the fact that the slope gets very steep in just a fewkilometres (Peliz et al., 2002). At the northern and the southernedges of the canyon the filament activity is frequent. These filamentsare typical recurrent features observed off the Iberian Peninsula,probably due to flow instability resulting from the meandering ofsouthward flowing currents (Haynes et al., 1993; Peliz et al., 2002).The coastal water temperatures vary between 10 °C in January and20 °C in September (Moita, 2001; Peliz et al., 2002) but, during thesummer, due to the prevalence of the northerly upwelling winds,the nearshore water temperature may decrease by 2 to 3 °C relativelyto the offshore temperature (20 °C). The stratification tendency, dueto the sunstroke intensification, during this season, is thereforecounteracted by the upwelling vertical currents. In terms of the bio-geochemical properties, the summer upwelling system of the AtlanticIberian coast is, therefore, characterized by high nutrient availabilityand important phytoplankton biomass accumulation, with thechlorophyll concentrations varying between 1 and 10 mg m−3 orexceptionally between 10 and 25 mg m−3 (Moita, 2001).

Understanding the importance and the implication of the climatechanges on coastal areas of the Iberian Peninsula may be one of themajor issues of the century. Climate changes may, indeed, impact thenearshore marine ecosystem through the upwelling processes, as thecoastal areas are very sensitive to the strength and the variability ofthe meteorological forcings. It may result in major biological transfor-mation, as changes in the phytoplankton biomass distribution, intensi-ty and location, affect the productivity of the coastal areas, impactingthe entire coastal ecosystem. The ecological model previously imple-mented to a restricted area of the Aveiro coast zone (Lopes et al.,2009) is now generalized to the entire Portuguese coast, although theemphasis is still that area. The study presented here has the main pur-pose of contributing to realistically simulate the nearshore Portuguesecoastal water, as well as to better describe the physical and the lowtrophic states of the water column, namely the distribution of temper-ature and chlorophyll-a, during upwelling events in the present climateand for a future climate scenario. As the meteorological factors controlboth the timing and the strength of the upwelling processes, severalhypothetical wind forcing scenarios, which take into account changes

of the wind intensity and direction have been used to force themodel simulation of the coastal ecosystem relative to the presentclimate. The scenarios simulate the future climate change scenario asdefined by the IPCC for Climate Change, which represent a continuousincrease in the greenhouse gas emissions (Roeckner et al., 2006).

2. Material and methods

2.1. The models

2.1.1. The physical modelThe COHERENS model, previously setup by Lopes et al. (2009) for

a limited area, the Aveiro coastal zone, is now available for the entirewestern Portuguese coast, situated close to 9°W, between 37°N and42°N. The model consists in a three-dimensional hydrodynamic andecological model for coastal and shelf seas (Luyten, 1999, Proctor,1997), and constituted by four sub-models: the hydrodynamical/physical model, the biological model, the sediment model and thecontaminant model (Fig. 2). The physical model includes the mainprocesses, namely, the advective and the diffusive transport ofmomentum, as well as the interaction with the atmosphere, throughmomentum (wind stress at the surface) and radiative exchanges. Itcan be represented by a set of equations referent to the hydrodynam-ic and the transport equations, very similar to the Blumberg andMellor primitive equation models, which use the sigma coordinatesand an embedded turbulent closure scheme (Blumberg and Mellor,1987), that are greatly simplified by the introduction of a modifiedvertical σ-coordinate, varying between 0 at the bottom and 1 at thesurface (Phillips, 1957). The transport model is solved using a TVDscheme, a weighted average between the Upwind- and Lax-Wendrofffluxes in the horizontal and between the upwind and the centralfluxes in the vertical. This scheme is much recommended for situa-tions of strong shear, meandering and horizontal eddies, as is thecase of the Portuguese coast. The turbulence closure scheme used inthis work is the widely known level 2.5 turbulence closure of Mellorand Yamada (1982). The bathymetry of the study area has a 2 km res-olution in both the latitude and the longitude directions. It is repre-sented by a uniform horizontal grid composed by 53 × 180 cellsand a uniform 22 sigma levels distributed in vertical direction.

Fig. 2. The conceptual diagram of the COHERENS model.

273J.F. Lopes et al. / Journal of Marine Systems 129 (2014) 271–288

Each state variableΨ (represents any physical, biological or chem-ical properties, including sediments) is determined by solving thefollowing equation:

I þ Ah þ Av þ As−Dv−Dhð ÞΨ ¼ P Ψð Þ−S Ψð Þ ð1Þ

where the left side of the equation represents the time derivative I,the advective terms Ah and Av, the vertical sinking/upwelling termAs, and the diffusive terms, Dv and Dh. The right side of Eq. (1) in-cludes all the contribution of the source terms P(Ψ) and the sinkterms S(Ψ) for the chemical, biological and sediment processes.

The model uses a time splitting technique, which separates timesteps for the 2-D ‘external’ barotropic mode from the time step forthe 3-D ‘internal’ baroclinic mode. The time steps for the model inte-gration have to be small in order to guarantee the CFL criterion(Blumberg and Mellor, 1987), based on stability analysis for linearsurface-gravity waves (Deleersnijder et al., 1997; Luyten, 1999).This criterion implies a small time step for the 2-D mode (3 s),whereas for the 3-D mode time steps may be greater (90 s). As inthe last case, the wave speed of the surface-gravity waves dependson the reduced gravity. The von Newmann boundary conditions,relating gradients (of the horizontal currents or a scalar quantity) tothe respective fluxes, are applied at both the surface and the bottomboundaries. At the open sea, specific boundary conditions must besupplied for both 2-D and 3-D hydrodynamic modes. For the 2-Dmode a radiation condition based on the method of characteristic isapplied, using depth-averaged value of currents (Hedstrom, 1979;Røed and Cooper, 1987; Ruddick, 1995; Luyten, 1999). The boundaryconditions for the 3-D mode are then calculated in the form of a pre-scription of the deviation of the currents from its depth-averagedvalue (Deleersnijder, 1992; Luyten, 1999). As tides are the majorsource of turbulence in shelf and coastal seas, tidal forcing is impor-tant to properly simulate the nearshore currents: the sea surface ele-vation is imposed at the model southern boundary as a Kelvin wavewhich propagates along the coast from the south to the north. Theamplitude and phase of each tidal component are defined using the

Eastern North Atlantic data from Fanjul et al. (1997) and Sauvagetet al. (2000). The horizontal currents are determined at the openboundaries using a zero gradient condition. The momentum ex-changes between the atmosphere and the ocean are solved by calcu-lating the wind stress, using several standard formulations availablein the model, as Charnock (1955), Smith and Banke (1975) orGeernaert et al. (1986). The incident solar radiation flux Qrad used inthe model follows the Rosati and Miyakoda (1988) formulation,whereas the radiative exchanges between the atmosphere and theocean are solved by calculating the total incoming short-wave radia-tion flux at the ocean surface, using an empirical formula derivedfrom Reed (1977). The solar irradiance I within the water column isexpressed as the sum of an infrared and a short-wave component,whereas the non-solar heat flux (long wave emission, latent andsensible heats) lost by the surface is calculated using the well knownempirical bulk formulations (Gill, 1982; Blanc, 1985; Geernaert,1990). More details concerning the physical model can be found inLuyten (1999) and Lopes et al. (2009).

2.1.2. The biological modelThe biological sub-model is based on Tett conceptual model (Tett

and Droop, 1988; Tett and Grenz, 1994; Tett and Walne, 1995; Tettand Smith, 1997; Tett, 1998) following a common classification ofthe water column ecosystem distinguishing three main components:phytoplankton, zooplankton and detritus.

Themodel has the following structure: a) internal non-conservativebiological or chemical processes; b) photosynthesis by the absorptionof PAR; c) physical transport by advection and diffusion; d) verticalsinking; e) deposition and erosion via a “fluff” layer; and f) exchangesbetween the water column and the sediment layer. Its distinguishingfeatures are: (1) the use of a microplankton compartment to includethe biomasses and the microbial loop organism activity as well asthose for the phytoplankton, and (2) the use of variations in the chem-ical composition (especially, the nitrogen: carbon ratio) of the micro-plankton and the detrital components to control many of thebiological processes. Themodel considers only nitrogen as a potentially

274 J.F. Lopes et al. / Journal of Marine Systems 129 (2014) 271–288

limiting nutrient, and simulates changes in the concentration of ammo-nium (resulting directly from the zooplankton metabolic activity) andnitrate (deemed to include nitrite) formed by oxidation from ammoni-um. It contains eight state variables: micro-plankton carbon andnitrogen, detrital carbon and nitrogen, dissolved nutrients (nitrate, am-monium), dissolved oxygen and accumulated zooplankton nitrogen.The chlorophyll is not included as a state variable; it is derived fromthe microplankton carbon and nitrogen. One state variable for the inor-ganic particulate material is included in the sediment sub-model. Thezooplankton is not modelled but the meso-zooplankton grazing pres-sure is imposed by user.

The micro-plankton growth rate is calculated considering a mini-mum threshold limitation for both light intensity and nutrients:

μ ¼ min μ1 Ip� �

; μ2 Qð Þh i

ð2Þ

where Q = N / B is the nitrogen quota defined as the ration of the or-ganic nitrogen N (mmol N) and the microplankton carbon biomass B(mmol C), Ip is the photosynthetically active radiation (PAR) internal-ly calculated by the model. The growth rate is calculated from the cellquota threshold limitation (CQTL) theory (Droop et al., 1982), as theleast growth rate predicted from light or nutrient controlled growth.The light intensity controlled growth is calculated as the sum of thephotosynthetic production and the respiration loss, by a formalismdescribed by Droop et al. (1982), Tett and Droop (1988), Tett(1990), Tett and Grenz, (1994), Tett and Walne (1995) and Tett(1998), which takes into account the photosynthetic efficiency andthe ratio of chlorophyll to autotrophic carbon.

The autotrophs take up both nitrate, NO3, and ammonium, NH3,but it is assumed that nitrate uptake can be inhibited by the presenceof ammonium as well as by a high cell quota, excluding excretion.

The biological model cycles the concentrations of organic carbonand nitrogen through the microplankton and the detrital compart-ments with associated changes in dissolved concentrations of nitrate,ammonium and oxygen. Nitrate and oxygen are closely linked, as theoxidation of ammonium consumes oxygen. On the other hand ammo-nium is directly used by microplankton, leading to an additional oxy-gen production. The nitrates' budget of the model (not presented) isdependent on the ammonium nitrification and nitrate microplanktonuptake, whereas the ammonium one is dependent on the ammoniumdenitrification, the ammonium microplankton uptake, the amount ofassimilated nitrogen excreted as ammonium due tomicroplankton car-bon and nitrogen assimilated by zooplankton and of detrital nitrogenremineralisation. As the nitrite species is not considered in the model,the nitrification rate is modelled as a single first-order process andthe parameters chosen to reflect the slower oxidation of ammoniumto nitrite. The oxygen budget is calculated as the function of the photo-synthetic respiration quotients for respectively, the microplanktoncarbon growth, the nitrate uptake, the detrital respiration and thenitrification. The oxygen is lost across the air–sea interface if the nearsurface waters become saturated in oxygen and used during the detri-tal degradation.

Several other processes are included in the model, namely the phy-toplanktonic grazing by micro- and meso-zooplankton, the detritusand the remineralisation processes as well as the seabed processes.As the model is essentially a tool for water column microbiology, itdoes not explicitly represent the benthos, the animals and microorgan-isms living on and in the seabed, nor does it describe the dynamics ofthe larger planktonic animals (mesozooplankton and others). Thechlorophyll concentration is not directly simulated, but is derivedalgebraically from the microplankton carbon and the nitrogen concen-trations. The chlorophyll concentration X (mg Chl m−3) is supposed tobe proportional to the microplankton carbon biomass B: X = χb B. Theproportionality factor is named ratio of chlorophyll to microplanktoncarbon and was defined constant. In the model X is calculated directly

from the nitrogen concentration N: X = χα N where χα is thechlorophyll-a to nitrogen ratio for autrotrophs (see Table 1). Zooplank-ton is not directly modelled but mesozooplankton grazing pressure isimposed. The ecosystem described by the model is therefore “open”at the second trophic level. The zooplankton nitrogen variable accumu-lates potential losses at this level. The sediment model determines thetime evolution and the transport of inorganic particulate material.

In order to account for the fate of themicroplankton and detritus, asthey sink to the seabed and accumulate there, as well as for the ex-changes between the water column and the seabed, a loosely-packedlayer named as “fluff” layer was defined in the microplankton anddetritus compartments and in the sediment model. It is an unconsoli-dated layer of sediment, of few mm thick, situated at the bases of thewater column, immediately above the sediment surface, which canbe easily resuspended by the action of the bottom stress. Theremineralisation of the organic matter contained in this layer enrichesthe lower water column with nutrients.

More details concerning these processes can be found in Luyten(1999) and Lopes et al. (2009).

2.1.3. Configuring the climate scenariosPrevious studies (e.g. Lopes et al., 2009) have consisted in the im-

plementation of the ecological model to a restricted domain, the Aveirocoast, consisting in the modelling of the temperature and the phyto-plankton biomass spatial distributions, along this coastal ecosystem.The present study intends to generalize the study to the Portuguesecoast, using the same approach, without changing fundamentally thepreviously setup model parameterisation. The study focuses on typicalsummer upwelling situations for the western Iberian coast. The modelwas initialised with realistic data concerning temperature as well aschemical and biological variables (Moita, 2001; Moita et al., 2003).The forcings are provided by the interactions at the air–sea interfaceusing formulations for the surface fluxes of momentum, tides, heatand salinity. The wind intensity and direction were taken from theWRF model (Weather Research and Forecast Model) a nesting meso-scale and assimilation forecast model (Michalakes et al., 2005;Skamarock and Klemp, 2007), which allows a mesh refinementenabling a high resolution output.

Here, we estimate first the climate change of near surface windalong the Portuguese west coast. For this, we have obtained data sim-ulated by the coupled atmosphere–ocean model ECHAM5/MPI-OM.This has been done for the summer months between 1971 and2000, representing the present climate, which run with greenhousegases forcing as observed through the 20th century (Roeckner,2005) and for the period 2071–2100, which represents the futureclimate change scenario SRES-A2 defined by the IPCC. This scenariorepresents a continuous increase in greenhouse gas emissions andglobal population (Roeckner et al., 2006).

Next, we have obtained wind data for closest grid point to Aveiroand calculated the PDFs of zonal and meridional wind in the region,for the two climates. Differences in the PDFs served as a basis todefine wind forcing conditions that represent climate change.

Two simulations have been performed. The first of these, namedREF (for reference), represents a summer upwelling event typical ofthe present climate. For the second simulation, named CC (for ClimateChange), several wind forcing regimes which represent climatechange have been used to force the model. These wind forcingshave been added to the WRF wind (Skamarock and Klemp, 2007)used to simulate _REF. Finally, the responses of the coastal ecosystemto the imposed wind forcings have been evaluated by comparing theCC simulation against the REF simulation.

2.1.4. The upwelling strength index (UI)In order to characterize the upwelling strength at the study area,

an upwelling index (UI) was defined as minus the zonal componentof the Ekman transport, supposing the shoreline macroscopically

Table 1Key model parameters and values.

Symbol Units Referencevalue

Sensivity values

ϕ nmol C μE−1 40.0 Photosynthetic quantum yield used for the calculation of photosyntheticefficiency α

α mmol C(mg Cloa)−1 day−1

(W/m2)−10.23 Photosynthetic efficiency for the light-controlled growth rate

χα mg Cloa (mmol N)−1 3.0 2.0/4.0 Chlorophyll-a to nitrogen ratio for autrotrophsroa day−1 0.1 0.05/0.3 Basal respiration rate for autotrophsba – 0.5 0.5/2.0 Slope of the respiration–growth relationship for autotrophsQmin mmol N (mmol C)−1 0.09 Minimum value for the nitrogen to carbon quota used in the formulations

for nutrient uptake and microplankton sinking ratesQmax mmol N (mmol C)−1 0.19 Maximum value for the nitrogen to carbon quota used in the formulations

for nutrient uptake and microplankton sinking ratesμmax day−1 2.33 Maximum value for the nutrient controlled growth rate of autotrophs at 20 °CqO,NO, qO,NH mmol O(mmol N)−1 1.0 0.5/2.0 Photosynthetic and respiratory quotient for nitrate uptake and nitrificationqO,B,qO,C mmol O(mmol N)−1 1.0 0.5/1.0 Photosynthetic and respiratory quotient for microplankton carbon growth

and detrital respirationO1/2,nit mmol O·m−3 30.0 Half-saturation constant in the oxygen dependence of nitrification at 20 °CkNOS mmol N·m−3 0.32 Half-saturation constant for nitrate uptakerCmax day−1 0.06 Maximum detrital carbon remineralisation rate at 20 °CrMmax day−1 0.08 Maximum rate of detrital nitrogen remineralisation at 20 °CrNHmax day−1 0.1 Maximum rate of nitrification at 20 °CqT (°C)−1 0.07 Temperature growth rateG day−1 0.06 0.05/0.10 Grazing pressureγ – 0.8 Fraction of grazed microplankton carbon and nitrogen assimilated by

zooplanktone – 0.5 Portion of assimilated nitrogen excreted as ammoniumqh mmol N (mmol C)−1 0.18 Nitrogen to carbon quota for heterotrophsroh day−1 0.1 0.02/0.15 Basal respiration rate for heterotrophsbh – 2.5 0.5/3.0 Slope of the respiration–growth relationship for heterotrophsη – 0.3 0.2/0.4 Ratio of heterotroph to microplanktonΕA m2·g−1 0.1 Diffuse PAR attenuation cross-section for inorganic suspended matterΕC m2 (mmol C)−1 0.002 Diffuse PAR attenuation cross-section for detritical carbonΕX m2 (mg Cloa)−1 0.016 Diffuse PAR attenuation cross-section for chlorophyllIp W/m2 10 5/20 Photosynthetically active radiation (PAR)Δopt m 0.71 – Thickness of the optical surface layer for hyper-exponential PAR decayR1 – 0.4 0.2/0.8 The infrared fraction of solar irradianceR2 – 10.0 5/15 Near surface attenuation factor for hyper-exponential decayk1 m−1 2.06 2.06 The optical attenuation coefficient for infrared radiationk20w m−1 The fresh water value of optical attenuation coefficient k2 for

monochromatic PARz0 m 5.0 × 10−03 2.4× 10−03/7.4× 10−03 Uniform bottom roughness length (quadratic bottom friction law)Cm0 – 0.1 0.1/0.2 Coefficient in the Smagorinsky formulation for the horizontal diffusion

coefficient νH for momentumCs0 – 0.1 0.1/0.2 Coefficient in the Smagorinsky formulation for the horizontal diffusion

coefficient λH

for scalars.νH m2 s−1 200 Uniform horizontal diffusion coefficient νH for momentumλH m2 s−1 200 Uniform horizontal diffusion coefficient λH for scalarskω NHumaxa s/m 5.0× 10−08 5.0× 10−09/5.0× 10−07 Parameter used to determine the wind dependence in the formulation for

the surface oxygen fluxNHumax mmol N (mmol C)−1 day−1 1.5 Maximum value for the ammonium uptake rate of autotrophs at 20 °CNOumax mmol N (mmol C)−1 day−1 0.35 Maximum value for the nitrate uptake rate of autotrophs at 20 °CqT (°C)−1 0.07 Growth rate coefficient in the temperature growth factor

275J.F. Lopes et al. / Journal of Marine Systems 129 (2014) 271–288

perpendicular to the Equator, along the western Iberian Peninsula(Pardo et al., 2011; Santos et al., 2011a, b):

UI ¼ −Qx ¼ −τyρw f

: ð3Þ

ρw is the seawater density (1025 kg m−3) and f the Coriolis parameter(f = 2Ωsenλ), with Ω the Earth's angular velocity and λ the latitude.The meridional wind stress, τy, can be defined as:

τy ¼ ρaCd W2x þW2

y

� �1=2Wy ð4Þ

where Cd is a dimensionless drag coefficient (1.4 × 10−3), ρa is theair density (1.22 kg m−3) and W is the wind speed near the watersurface.

Using this definition, time series representing the values for UI(m3 s−1 km−1) for the same area and the same period of time were

built. From Eq. (3) it can be concluded that negative values of τy cor-responds to positive values of UI and therefore favourable upwelling.Following Alvarez et al. (2011), favourable conditions for the upwell-ing occurrence at the western coast of the Iberian Peninsula can bedefined by considering the mean number of days per month underupwelling, as UI N 16 m3 s−1 km−1. They found as well that thehighest number of days under favourable conditions is observedduring the spring–summer months and is of the order of 20–28 daysper month. Figs. 3 and 4 show, respectively, time series for dailymean wind velocity and daily mean upwelling index, for the summerupwelling season (June–September) of 1989, for the Aveiro coastalzone. It can be observed that winds are almost northerly, UI frequentlyexceeds the threshold limit and the number of days under the upwell-ing conditions is within the interval predicted by Alvarez et al. (2011).Therefore, summer 1989 has been chosen since it representsfavourable conditions for the upwelling occurrence in the region. Thisseason has been simulated by the model.

Fig. 4. Daily mean values of the upwelling index measured at the Aveiro coast, fromJune to September 1989.

276 J.F. Lopes et al. / Journal of Marine Systems 129 (2014) 271–288

3. Results

3.1. Physical response of an upwelling event (REF)

Fig. 5 shows snapshots of the surface temperature distributions ofthe reference situation (named REF), corresponding, respectively, toan early summer upwelling situation (first row) and to a typicalmid-summer upwelling situation (second row). In each case it hasbeen considered four stages of the development of the upwelling:(a and e) the pre-upwelling situation; (b and f) an early stage of theupwelling; (c and g) a developing stage of the upwelling; (d and h)a mature stage and ending of the upwelling. Surface currents arealso shown in these figures.

Concerning the early summer situation it can be observed for thepre-upwelling situation (Fig. 5a) the setup, close to the coast, of awarm surface layer, with a temperature of the order 16–17 °C, andthe beginning of the formation of a southward coastal current. Thesituation becomes clearer in the early stage of the upwelling(Fig. 5b), for which it can be observed that the warm layer hasmoved offshore and becomes warmer, while a cold upwelled water(~14–15 °C) has emerged and set up along the coast. At this stagethe southward current sets up.

Concerning the mid-summer situation it can be observed that thetemperature patterns are now clearly formed. During the pre-upwelling stage (Fig. 5e) the southward current becomes veryestablished, reaching speed up to 20 cm/s, and an extended layer ofwarm water is formed along the coast, with the temperature reachingmaximum values (~20.5 °C) close to the Aveiro coast. During the earlystage, (Fig. 5f), the currents reverse, becoming south-westward and thesurface temperature distribution is now characterized by a cold layer(15 °C), near the coast, and a warm layer offshore (~20.5 °C). Duringthe developing stage, (Fig. 5g), a very typical upwelling surfacetemperature distribution is set up characterized by a wide cold layerlocated near the coast, whereas the warm layer was pushed offshore,up to 40 km away from the coast. At this stage, even though thecurrents show some southward or south-westward patterns, it can beobserved a reversing situation with northward currents beginning toset up. At the mature stage and ending of this upwelling cycle, thewarm layer approaches again the coast and mixes, with the coldupwelled water, rising, progressively, the temperature to values of theorder of 16–19 °C (Fig. 5h). According to observations and satellite im-ages (Cardoso, 2007; Lopes et al., 2009;Moita, 2001) these results satis-factorily simulate the temperature distribution along the Portuguesecoast during a typical summer upwelling event.

Fig. 3. Daily mean values of thewind velocity at the Aveiro coast, from June to September1989.

Fig. 6 presents panels of the temperature vertical cross sectionsalong the Aveiro transect, representing, respectively, four different in-stants of REF, corresponding to an early summer situation (first row)and a mid-summer situations (second row), for the same instants asFig. 5. A typical upwelling structure is beginning to set up in theearly upwelling stage of the early summer (Fig. 6a), correspondingby a stratified pattern, characterized by a warm mixed layer(16–17 °C) and a cold water (13–14 °C) underneath. A thermoclineis set up and almost reaches the surface near the coast. At the nextstages (early, developing and the mature stages) (Fig. 6b–d), thewarm mixed layer is pushed offshore, the thermocline deepens off-shore to near 20 m below the surface, while the near coastal watercools to temperatures of the order or below 16 °C. The upwelling pat-tern is strengthened throughout the mid-summer (Fig. 6e–h), withthe increasing of the temperature of the mixing layer (up to 19 °C).

3.2. Biological response of an upwelling event (REF)

Fig. 7 shows snapshots of Chl-a surface distributions of REF corre-sponding, respectively, to an early summer upwelling situation and toa typical mid-summer upwelling situation. As for Fig. 5, in each caseit has been considered four stages of the developing upwelling:(a and e) the pre-upwelling situation; (b and f) an early stage of theupwelling; (c and g) a developing stage of the upwelling; (d and h) amature stage and ending of the upwelling. Concerning the early sum-mer situation, it can be observed, during the pre-upwelling situation(Fig. 7a), the setup of a typical situation of phytoplankton accumula-tion, along a narrow band of the coast, covering the areas ofVila-do-Conde (A), Porto (B), Aveiro (C) and Figueira da Foz (D), aswell as onshore of the Cabo da Roca (E),. The highest Chl-a concentra-tion values (~3 mg Chl m−3) are located along the northern coast,more precisely between A and C. The situation becomes more clearfor the early stage of the upwelling (Fig. 7b), for which it can beobserved a very established coastal layer of relatively high concentrat-ed Chl-a (~6 mg Chl m−3) along the entire coast. This surface layerexpands offshore throughout the developing stage (Fig. 7c) and themature stage (Fig. 7d) of the upwelling. Helped by the nutrients avail-able through the upwelling process and the advective transport by thesurface currents, the bloom intensifies and spreads offshore, extendinghorizontally up to 40 km from the coast, as well as alongshore. A typi-cal upwelling inshore–offshore surface Chl-a gradient is, therefore, setup.

Fig. 5. Surface temperature distributions at different instants of REF corresponding, respectively to an early summer upwelling situation and to a typical mid-summer upwellingsituation: (a and e) a pre-upwelling situation; (b and f) an early stage of the upwelling; (c and g) a developing stage of the upwelling; (d and h) a mature stage and ending ofthe upwelling. Surface currents are as well shown in these figures (A—Vila-do-Conde; B—Porto; C—Aveiro; D—Figueira da Foz; E—Cabo da Roca).

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Concerning the mid-summer upwelling situation, it is importantto observe that the surface bloom is now located mainly betweenthe coastal area between Figueira da Foz (D) and the Aveiro (C), asituation which reflects the bottom topography feature for these loca-tions, as referred in Section 1. The concentration values are relativelyhigher than that for the early summer situation, showing maximumvalues of the order of 11 mg Chl m−3. The high values of the surfaceChl-a gradient evidence the strength of the upwelling as well as thenutrient accumulation along the coast during this stage of theupwelling.

Again, referring to observations and satellite images presented inprevious papers (Cardoso, 2007; Lopes et al., 2009; Moita, 2001), itcan be observed that the results satisfactorily simulates the Chl-a dis-tribution along the Portuguese coast.

Fig. 8a, b, c, d presents panels representing the Chl-a vertical crosssections at Aveiro corresponding to, respectively, an early summerupwelling situation (first row) and a typical mid-summer upwelling(second row) for two different instants of REF: (a, c) an early stageof the upwelling; (b, d) a mature stage of the upwelling. Fig. 8e, f, g,h presents the same situations for Figueira da Foz.

Fig. 6. Surface Chl-a distributions at four different instants of REF, corresponding, respectively, to an early summer upwelling situation (first row) and to a typical mid-summer up-welling situation (second row): (a and e) the pre-upwelling situation; (b and f) an early stage of the upwelling; (c and g) a developing stage of the upwelling; (d and h) a maturestage and ending of the upwelling.

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It can be observed that Aveiro presents, in overall, higher Chl-a con-centration values than Figueira da Foz. In the early summer and earlystage of the upwelling (Fig. 8a), the maximum chlorophyll concentra-tion (8 mg Chl m−3) is situated near 10 m depth, deepening offshoreto depth below 20 m, for Aveiro. In the mature stage of the upwelling

Fig. 7. Temperature vertical cross section along the Aveiro transect, corresponding, respectupwelling situation (second row): (a and e) the pre-upwelling situation; (b and f) an early sstage and ending of the upwelling.

(Fig. 8b) the maximum chlorophyll concentration (10 mg Chl m−3) issituated close to the surface deepening, as well as offshore to depthsbelow 20 m. This pattern remains throughout the mid-summer situa-tion (Fig. 8c, d). The Figueira da Foz coast is deeper than the Aveirocoast, which enables the Chl-a to adjust to light and nutrient conditions

ively, to an early summer upwelling situation (first row) and to a typical mid-summertage of the upwelling; (c and g) a developing stage of the upwelling; (d and h) a mature

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Fig. 9. Probability density functions, of the zonal (U) andmeridional (V) components ofthe wind speed at 10 m, respectively, for the ECHAM5/MPI-OM grid point nearestAveiro, estimated through the kernel density smoothing method (left panels). Meanwind vector for the southward component greater than 1 m/s (right panel). Bluelines and arrow stand for the present climate (1971–2000) and red lines and arroware for the future climate scenario (2071–2100).

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and, therefore, to establish a typical vertical distribution. In the earlysummer and early stage of the upwelling (Fig. 8e) the maximum chlo-rophyll concentration (4 mg Chl m−3) is situated near 20 m depth,deepening offshore to depths of the order of 50 m. In the maturestage of the upwelling (Fig. 8f) it can be observed an increase of theChl-a concentration and dispersion between the surface and themid-layer, as well as the intensification of its maximum concentrationvalues. The maximum chlorophyll concentration (7 mg Chl m−3) isvery well established and positioned at the mid-layer of the water col-umn, extending farther offshore near the same depth as in the previoussituation. This pattern remains the same throughout the mid-summer(Fig. 8g, h).

3.3. Physical and biological responses to a climate change scenario

Fig. 9 (left panels) shows the probability density functions (PDF)estimated through the kernel density smoothing method, of thezonal and meridional wind at 10 m, respectively, simulated by theECHAM5/MPI-OM at its grid point nearest Aveiro. The PDF of themeridional wind component for the future climate scenario shifts to-wards more negative values when compared to the present climate.For the zonal wind component, a small shift of the PDF to positivevalues is observed. This means that the westward wind componentwill become weaker and the southward wind component will be-come stronger. It results in an anti-clockwise rotation of the windvector, as shown in Fig. 9 (right panel) representing the mean windvectors for present and future climate scenario, when the southwardwind component is greater than 1 m/s. Furthermore, there is a highprobability of the intensity of v component of the wind speed to in-crease within the 4 to 6 m/s interval. The following wind forcingswere, therefore, chosen: (a) the intensification of the southwardwind component (2 m/s), a value which falls into the interval ofvalues suggested by the climate change scenario used here; (b) the

Fig. 8. Chl-a vertical cross sections at Aveiro corresponding to, respectively, an early summertwo different instants of REF: (a, c) an early stage of the upwelling; (b, d) a mature stage o

intensification of the westward wind component (1 m/s); and(c) the intensification of the eastward wind component (1 m/s).

Fig. 10 presents the surface temperature differences between thewind forcing scenarios (named a, b and c, above) and the REF.Superimposed to the maps, are plotted the respective surface currentdifferences that are also shown in each figure. Henceforth, the term‘cooler’ used in the text is referred to a water layer for which the tem-perature, for a given scenario, is lower than that of REF.

As expected, the intensification of the southward wind component(Fig. 10a) enhanced the upwelling with a significant increase of theintensity of the southward coastal current (up to 10 cm/s) as well asthe offshore westward current (up to 6 cm/s). The cooler layer nearthe coast is wide, extending up to 60 km offshore from the coast andpushing the warm layer away from the coast. The cold front moves far-ther offshore replacing the warm layer there. It results in significantsurface temperature drops (of the order of 2.0 °C) near the coast. Theintensification of the westward wind component (Fig. 10b) did not re-sult in significant changes concerning the temperature surface distribu-tion. Indeed, the westward current intensifies slightly (up to 4 cm/s),while the cold upwelled water generated between Aveiro (C) andCabo da Roca (E) (only 0.5 °C lower than the reference situation) hasexpanded offshore, pushing warm layers offshore (only 0.5 °C higherthan the reference situation). The intensification of the eastwardwind component (Fig. 10c) generates surface currents towards thecoast, weakening the upwelling surface currents. In this case the induc-tion of an eastward current (up to 4 cm/s), pushes the water towardsthe coast, increasing the temperature of the coastal waters by 0.5 °C,inducing a dropping of the same order offshore.

Fig. 11 presents the surface Chl-a concentration differencesbetween the wind forcing scenarios (a, b and c, above) and REF. Therespective surface current differences are also shown in each figure.

It can be observed that the intensification of the southward windcomponent (Fig. 11a) has induced an increase of the Chl-a concentra-tion offshore and pushed away from the coast, the surface layer ofmaximum Chl-a concentration, which was located closer to thecoast in REF. A consequence of this westward displacement is, there-fore, an increase of concentration between 1 and 5 mg Chl m−3 off-shore, and a drop of the same order near the coast.

The intensification of the westward wind component (Fig. 11b)shows a similar result as the previous case, between Aveiro andFigueira da Foz (D), even though with weaker intensity. The Chl-aconcentration front has moved westward, and a concentrationincrease, between 1 and 2 mg Chl m−3 can be observed offshore,between Aveiro and Figueira da Foz as well in front of Cabo daRoca, whereas a drop of the order of 1 mg Chl m−3 is shown nearthe coast.

Finally, the intensification of the eastward wind component(Fig. 11c), results in a concentration drop of the order of 1 mg Chl m−3

along the coast from Aveiro to northern coastal areas coveringVila-do-Conde (A) as well as in front of Figueira da Foz and of Cabo daRoca, confirming the negative impact of an eastwardwind in the upwell-ing process and the phytoplankton growth.

Fig. 12 shows the Chl-a vertical profiles for each of the three thewind forcing scenarios, for the Aveiro station, S1, (first row) and forthe Figueira da Foz station, S2 (see Fig. 1) (second row), for a devel-oping stage of upwelling (June). Fig. 13 shows the same as Fig. 12but for a mature stage of the upwelling (July). Each plot also showsChl-a for REF.

It can be observed that Aveiro shows, in general, greater values ofthe maximum Chl-a concentration (9 mg Chl m−3) than Figueira daFoz (3 mg Chl m−3). Concerning the developing stage of the upwelling(Fig. 12), Chl-a shows typical vertical profiles with a clear maximum

upwelling situation (first row) and a typical mid-summer upwelling (second row), forf the upwelling. Plots e, f, g, and h present the same situations for Figueira da Foz.

Fig. 10. Surface temperatures differences between the wind forcing scenarios ((a) the intensification of the southward wind component (2 m/s); (b) the intensification of thewestward wind component (1 m/s); (c) the intensification of the eastward wind component (1 m/s)) and the REF. Superimposed to the maps, are plotted the respective surfacecurrent differences are also shown in each figure.

Fig. 11. Surface Chl-a concentration differences between the wind forcing scenarios ((a) the intensification of the southward wind component (2 m/s); (b) the intensification of thewestward wind component (1 m/s); (c) the intensification of the eastward wind component (1 m/s)) and REF. The respective surface current differences are also shown in eachfigure.

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Fig. 12. Chl-a vertical profiles for each of the wind forcing scenarios ((a) the intensification of the southward wind component (2 m/s); (b) the intensification of the westward windcomponent (1 m/s); (c) the intensification of the eastward wind component (1 m/s)), for the Aveiro station, S1, (first row) and for the Figueira da Foz station, S2 (second row), for adeveloping stage of upwelling (June). Each plot shows Chl-a for REF.

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concentration (1–3 mg Chl m−3) below the water surface, between 20and 40 m, as observed by Moita (2001), while the profiles for the ma-ture stage of the upwelling (Fig. 13) show secondary and more intensemaximum concentrations values (3–9 mg Chlm−3), located within the

first 10 m from the surface. Considering the developing stage of the up-welling, when comparing the scenarios to REF, (Fig. 12a, b, c), it can beobserved, in general, relatively small changes in both the concentrationand the values of the depth of maximum concentration, even though

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Fig. 13. Chl-a vertical profiles for each of the wind forcing scenarios ((a) the intensification of the southward wind component (2 m/s); (b) the intensification of the westward windcomponent (1 m/s); (c) the intensification of the eastward wind component (1 m/s)), for the Aveiro station, S1, (first row) and for the Figueira da Foz station, S2 (second row), for amature stage of upwelling (July). Each plot shows Chl-a for REF.

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some details worth to be point out. Indeed, the intensification of thesouthward wind component (Fig. 12a) shows some reduction of thedepth of the maximum concentration (from the interval 40–30 m tothe interval 30–20 m), as well as a decrease of the values of the Chl-aconcentration relatively to REF, below this depth. A similar behaviouris observed for the intensification of the eastward wind component(Fig. 12c1). On the other hand, concerning the intensification of thewestward wind component, the depth of the maximum concentrationat Aveiro is reached at a depth below 30 m (Fig. 12b1). Figueira da Fozshows no significant changes in the Chl-a vertical profile for the inten-sifications of the eastward and the westward wind components(Fig. 12b2, c2), while for the southward wind component (Fig. 12a2)it can be observed some similarity with Aveiro (Fig. 12a1).

Considering themature stage of the upwelling, when comparing thewind forcing scenario to REF, it can be observed that, while the intensi-fication of the eastward and the westward wind components (Fig. 13b,c) shows no significant changes (small concentration difference of theorder or smaller than 0.5 mg Chl m−3), the intensification of the south-ward component (Fig. 13a1, a2) induces significant changes in the Chl-avertical distribution patterns and reinforces the particularities of theAveiro and Figueira da Foz vertical distributions. It can be observedthat Aveiro shows higher concentration values than Figueira da Foz(Fig. 13a1, a2), namely near the surface, where the former reaches rel-atively high concentration values, 9 mg Chl m−3, when compared to5 mg Chl m−3 reached by the latter. While the scenario shows higherconcentration values than REF, above 50 m, for both Figueira da Fozand Aveiro, the last one shows an inflexion in the profile near 20 mdepth, abovewhich the concentration remains nearly constant, forminga mixing layer. Near the surface (above 20 m depth), the concentrationfor REF for Aveiro becomes higher than the one for the scenario(Fig. 13a1), in accordance with Fig. 11.

4. Discussion and conclusions

Summer is a typical season for the occurrence of upwelling events inthewestern Iberian Peninsula coastal system. Using an upwelling index(UI) for the Aveiro coast, which is a measure of the strength of the up-welling, the summer of 1989 was selected for study since the UI waswell above the threshold value of 16 m3s−1 km−1 suggested byAlvarez et al. (2011). This result can be easily generalized for the Portu-guese west coast as the wind intensity and direction are very similarthroughout almost the entire coastal zone during this period. Duringan upwelling event, the geostrophy budget establishes a strong south-ward coastal jet, with the intensity of surface currents reaching valuesof the order of 10 to 20 cm/s (Fig. 5), which are in accordance withthe typical observed values (Coelho et al., 2002; Sauvaget et al., 2000).A typical mature upwelling regime at the study area is characterizedby a nearshore cold layer (temperature b 18 °C) surrounded by an off-shorewarm layer (temperature of the order of 20 °C), a situationwhichcontinuously evolves in time and within a horizontal band as large as40 km, as the cold upwelled water generated near the coast is spreadand advected offshore, pushing the warmer water far from the coast.A temperature cross-section vertical structure (Fig. 6) shows that dur-ing this situation the water column becomes thermally stratified, pre-senting a thin surface layer of warm water (b20 °C) and a deep coldlayer underneath, separated by a thermocline whose depth increasesfrom less than 5 m, onshore, to near 20 m, offshore. As the upwelling ig-nites and strengthens, during the developing and the latter stages, thethermocline may break into the surface, bringing more deep water tothe surface and creating a large cold front along the coast. The modelsimulates, therefore, a thermocline depth varying between 5 and20 m. The positioning of the thermocline is very important for phyto-plankton growth and accumulation. This is so, since it provides a refugefor phytoplankton avoiding vertical turbulence typical of the mixedlayer. Under these circumstances a region where the phytoplanktongrowth and accumulation may develop, provided that nutrients and

light are in sufficient supply, and grazing does not decimate standingstock (Fasham et al., 1993; Vandervelde et al., 1987, Probyn et al., 1995).

Sanchez et al. (2008) observing rich nutrients upwelling filamentsoff southwest Iberia at 150 m depth, estimated the rate of upwellingnutrients of the order of 15 m/d, and a time of about 10 days for a richnutrient filament to emerge the near surface water, which is in agree-ment with the estimation for duration of the phytoplankton bloom.That is, the maximum Chl-a does not, in general, coincide with theminimum temperature, but are lagged in time, by several days,confirming that the most favourable conditions for the phytoplanktongrowth are not those corresponding to intense upwelling and turbu-lence near the coast but rather, to the relaxation situations for whichthe upwelling turbulence is weakening. This is a common pattern ofmany upwelling systems in the world and is related to the reactiontime needed by phytoplankton to react to the increase in nutrientconcentration (Macías et al., 2012, Yokomizo et al., 2010). FollowingHuete-Ortega et al. (2010), which studied the phytoplankton abun-dance in coastal shelf waters of the NW Iberian Peninsula duringthe peak of the upwelling event, when upward water velocitieswere highest, the relatively strong turbulence and offshore wash-out of cells, together with the physiological time lag required for com-munities to adjust to the new conditions, prevent the accumulation oflarge amounts of phytoplankton. The upwelling relaxation is morefavourable for high nutrient concentrations which, combined with in-creased water-column stability and reduced dispersion, allows theonset of the phytoplankton blooms. On the other hand, it is wellknown that wind relaxation favours nutrient accumulation enabling,therefore, more efficient phytoplankton uptake and growth. Indeed,the importance to productivity of the strength of the wind as wellas its temporal variability was emphasized by Yokomizo et al.(2010). They stated that periods of strong upwelling favourablewinds are interrupted by frequent (synoptic scale, i.e., daily to week-ly) periods of relaxation of upwelling winds, a period which is themost benefit to the productivity near shore as it allows additionaltime on the shelf for upwelled nutrients to be converted to primaryand secondary production. The bottom topography and orographyfeature, is therefore, one of the major features influencing the upwell-ing enriched nutrient waters into the surface. The vertical Chl-a struc-ture of the study area clearly evidences that the subsurfacechlorophyll maximum is located at about 40 m below the surface, atFigueira da Foz, slightly deepening offshore whereas, at Aveiro, it isshallower, even coinciding with the surface in situations of a matureor a strong upwelling situation. Indeed, Vandervelde et al. (1987)studied the vertical distributions of Chl-a and hydrodynamic proper-ties of the Gulf of St. Lawrence and observed that higher vertical sta-bility at depth, favoured shade adaptation of the phytoplankton in thelayer of maximum stability, as compared to the more light-adaptedcells of the upper well-mixed layer. Therefore, the observed sub-surface chlorophyll maximum does not only result from favourableenvironmental conditions for phytoplankton accumulation andgrowth, but also from active photosynthetic responses of phytoplank-ton. They concluded that the fact that the subsurface chlorophyllmaximum was continuously observed in the lower part of the 20 mof the photic layer was related to a depth corresponding to maximumvertical stability of the water column, just above the nutricline and toa maximum of phytoplankton production. Situations correspondingto a depletion of the surface nutrients are, in general, caused by a con-tinuous nutrient uptake by phytoplankton.

Several hypothetical wind forcing scenarios have been used to forcethe model simulation of the coastal ecosystem. These wind forcingscenarios were obtained from climate change simulations performedby the ECHAM5/MPI-OM climate model, for the summer months,between 1971 and 2000. Relative to the present climate, future windforcing scenarios represent, for average conditions, a) an intensificationof the southerly wind component, b) stronger westward wind compo-nent and c) weaker (stronger) westward (eastward) wind component.

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Although the climate scenario shows a tendency for strongersouthward wind to develop in the region, we have also consideredwind forcing scenarios which represent modest changes in thezonal wind component.

For the wind forcing scenario where the southward wind compo-nent is significantly reinforced, the model shows a significant increaseof the intensity of both, southward (up to 10 cm/s) and westward (upto 6 cm/s) induced currents, resulting in strong upwelling events. Thisresults in a significant offshore spread of the near coast upwelled coolerlayer, extending up to 60 km and pushing the warm layer away fromthe coast. Also, a significant rise of the temperature of the warm layer(up to 2.0 °C) is simulated (Fig. 10). During the mature stage of areinforced upwelling situation, it is expected that the nitraclineshoaling close to the coast brings more nutrients to the surface,enabling phytoplankton growth and spread offshore. Indeed, signifi-cant changes in the Chl-a vertical distribution patterns have been sim-ulated when comparing this scenario with REF. Also, changes areevident between different coastal locations (e.g. differences betweenAveiro and Figueira da Foz). Indeed, although the position of thenitracline depends on the strength of the upwelling, it cruciallydepends on the bottom topography and orography of a given area.This is particularly so for the Aveiro coastal area, as evident on boththe horizontal and vertical Chl-a structures (Figs. 8 and 13).

For wind forcing scenarios where the westward or the eastwardwind components are modestly reinforced no significant changes inboth temperature (b0.5 °C) and Chl-a patterns during an upwelling sit-uation were simulated. Nevertheless, the Chl-a concentration increasesbetween 1 and 2 mg Chl m−3 (Fig. 11b), due to the westward currentintensification (up to 4 cm/s), offshore, mainly between Aveiro andCabo da Roca, while in the case of a stronger eastward wind compo-nent (Fig. 11c), a concentration drop of the order of 1 mg Chl m−3

along the coast and between Aveiro and Cabo da Roca is simulated,denoting the negative impact of an eastward wind situation in theupwelling process.

Our approach has focused on the study of the impact of the cli-mate scenarios on the upwelling events occurring in the study area.Since the alongshore southward winds are upwelling favourable, sce-narios considering changes in strength or frequency of these windsare the most interesting for the purposes of this study, and are,therefore, of particular interest for the understanding of the hypotheticchanges that can undergo the coastal ecosystem. The results evidencethe intensification of the alongshore southward current (up to 10 cm/s)and the offshore westward current (up to 4 cm/s). These values arenon negligible when compared to the order of magnitude of the along-shore southward current (up to 20 cm/s, Coelho et al., 2002; Sauvagetet al., 2000) for a typical upwelling event. It is, therefore, expected forthe scenario an increasing, of the overall offshore transport within thesurface layer. This situation has significant impacts in both the temper-ature and phytoplankton distribution patterns. Indeed, it was observeda widening of the upwelled surface cold layer located near the coast,driving farther offshore thewarm layer: a significant drop of the surfacetemperature coastal layer and of the rising temperature of the offshorewarm layer is not surprisingly and may constitute typical signatures ofthe scenario. The nutrient enrichment of the upper layers, namely thesurface layer, is another consequence, which explains the phytoplank-ton biomass accumulation offshore. Indeed, the idea that the upwellingintensity is likely to increase with global warming was initially pro-posed by Bakun (1990). Miranda et al. (2013) in the frame of the globalwarming scenario studied the response of the ocean circulation near theIberian coast to atmospheric forcing and demonstrated that the upwell-ing intensity increase occurs as a direct response to changes in thecoastal wind, which has wide regional impacts. They found as wellthat the upwelled cooler waters are exported westwards throughoutthe extended summer period, cooling surface waters in a regionextending up to more than 200 km from the coast. Another worthnote result is the increase of the offshore chlorophyll concentration

(between 1 and 5 mg Chl m−3), reflecting, therefore, the accumulationof phytoplankton biomass near the frontal zone separating the cold andwarm waters generated by the upwelling event itself. It is thereforeexpected in this scenario a net reinforcement of the primary productionand high productivity. These results seems to be comforted by Samo etal. (2012) and Li et al. (2012) which studied the setup of a strongtemperature front separating water masses with distinct hydrographicand biogeochemical characteristics of the upwelling California CurrentEcosystem. The front was characterized by a typical surface boundarybetween the warmer and low-chlorophyll southern/offshore watersand the colder, high-chlorophyll northern/coastal waters. They haveassessed the influence of frontal hydrography on several microbial pa-rameters and quantified how the front-induced physical mixinginfluenced the production, grazing and transport of phytoplankton car-bon. They found that enhanced diffusive diapycnal fluxes of nutrientsstimulated phytoplankton primary production at the front; this effect,togetherwith reducedmicrozooplankton grazing, increased net growthof the phytoplankton community leading to locally enhanced biomassof large phytoplankton, such as diatoms, in the frontal zone. Theyfound aswell that the fronts can facilitate intense biological transforma-tion and physical transport of organic matter, in sharp contrast toadjacent low productivity waters, and harbor dynamic microbial popu-lations that influence nutrient cycling. Acha et al. (2004), discussed aswell the role played by marine fronts formed at the continental shelvesin ecological processes, and emphasized the fact that fronts allow highbiological production, offering feeding and/or reproductive habitatsfor fishes, squids, and birds; acting as retention areas for larvae of ben-thic species and promoting establishment of benthic invertebrates thatbenefit from the organic production in the frontal area. The importanceand implication of climate changes in fisheries can be better understoodthrough Rykaczewski and Checkley (2008) review. Indeed, they arguedthat environmental variability is thought to be the major cause of thedecadal-scale biomass fluctuation characteristic of fish populations inan ecosystem like the California Current Ecosystem, but the mecha-nisms relating atmospheric physics to fish production remain un-explained. Although the alongshore wind stress, resulting in rapidupwelling (with high vertical velocity, w) is very important, wind-stress curl, resulting in slower upwelling (low w) may play, as well,important role in the production of Pacific sardine. The same authorspredicted as well that the type of biological production resulting fromcoastal and curl-driven upwelling to differ, with high vertical velocity(w) resulting in larger phytoplankters and low w favouring smallerphytoplankters. The demand for nutrients by a phytoplankton cell istypically a function of cell volume (Clark et al., 2013), whereas themaximal uptake rate is a function of the cell's surface area (Fiksen etal., 2013). For this reason, smaller cells, with higher surface-area-to-volume ratios, have a competitive advantage in nutrient-limitedenvironments. The increased nutrient concentrations in vigorously up-welling waters (high w) reduces nutrient limitation and the competi-tive advantage of small cells, allowing populations of large cells withlower surface area-to-volume ratios to develop. Given that prey sizecorrelates positively with predator size, larger zooplankters arefavoured in areas with larger phytoplankters and higher w. As the sizestructure of plankton assemblages is related to the rate of wind-forcedupwelling, sardine can, for instance, benefit of a situation of slowerupwelling by feeding efficiently on small plankters which can prosperduring this event. Upwelling rate is, therefore, a fundamental determi-nant of the biological structure and production in coastal pelagic ecosys-tems, and future changes in themagnitude and spatial gradient of windstress may have important and differing effects on these ecosystems.Finkel et al. (2010) suggests that cell size and elemental stoichiometryare promising ecophysiological traits for modelling and trackingchanges in phytoplankton community structure in response to climatechange. In turn, these changes are expected to have further impactson phytoplankton community structure through as yet poorly under-stood secondary processes associated with trophic dynamics. The

287J.F. Lopes et al. / Journal of Marine Systems 129 (2014) 271–288

understanding of the biological mechanisms relating fisheries produc-tion to environmental variability is, therefore, essential for sustainablemanagement of marine resources under a changing climate.

It can be concluded that the impact of the climate scenarios in theecosystem of study area does not only affect the main physical and bi-ological processes occurring in the low trophic level of the marineecosystem but also may alter the equilibrium among species feedingon phytoplankton as well as on each other. Climate change may,therefore, impact the nearshore marine ecosystem through the up-welling processes as they are very sensitive to its strength and vari-ability. As meteorological factors control both the timing and thestrength of the upwelling processes, any significant changes in thestrength and frequency of this upwelling favourable wind are likelyto affect the phytoplankton population, which is the main source offood for the population of the upper level of the marine trophicchain. A better understanding of the climate change is of outstandingimportance in order to anticipate the potential changes in the coastalecosystem of the study area. It is, therefore, important to better pre-dict the climate scenarios for the study area in order to control andmitigate their impacts on the marine ecosystems. Furthermore thePortuguese coastal areas have a huge importance in the economy ofthe country.

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