Jesús García-Lafuente

About the seasonal variability of the Alboran Sea Circulation

Data from a mooring line deployed midway between the Alboran Island and Cape Tres Forcas are used to study the time variability of the Alboran Sea from May 1997 to May 1998. The upper layer salinity and zonal velocity present annual and semiannual cycles characterised by a minimum in spring and autumn and a maximum in summer and winter. Temperature has the opposite behaviour to that of salinity indicating changes in the presence of the Atlantic water within the Alboran Passage. A large set of SST images is used to study these cycles. The decrease of salinity and velocity in our mooring location in spring and autumn seems to be related to the eastward drifting of the Western Alboran Gyre (WAG). The increase of salinity and velocity is caused by the Atlantic current flowing south of the Alboran Island and its associated thermohaline front. Conductivity– temperature–depth (CTD) data from two cruises along the 3jW are coherent with current meters and SST interpretations. During the period analysed, summer months are characterised by the stability of the two-gyre system, while in winter, the circulation is characterised by a coastal jet flowing close to the African shore. We use sea level differences across the Strait of Gibraltar for studying the variability of the Atlantic inflow. We discuss the changes in the Alboran Sea circulation and its relation with the variability of the inertial radius of the Atlantic inflow. Though our results are speculative, we find a possible relation between the disappearance of the two-gyre system and a reversal of the circulation in Gibraltar. Longer time series are needed to conclude, but comparison with previous works makes us think that the seasonal cycle described from May 1997 to May 1998 could be the most likely one for the Alboran Sea upper layer. D 2002 Elsevier Science B.V. All rights reserved.

Seasonal and wind-induced variability of Sea Surface Temperature patterns in the Gulf of Cadiz

The evolution of thermal structures in the Gulf of Cadiz is analysed with a set of 325 weekly composite Sea Surface Temperature (SST) images derived from NOAA-AVHRR sensor, and covering a time span of 7 years, from 1993 to 1999. A spatial Empirical Orthogonal Function (EOF) analysis has been performed in order to identify the main SSTspatial patterns. The first EOF mode explains 60% of the temperature variance of the images, and shows a quasi-permanently warmer than the mean region in the southern part of the area of study. The second mode (13% of variance), has a strong temporal variability, and is the main responsible for the cooling and warming of the shelf waters in southwestern Iberia. These two modes explain together most of the seasonal variability of SSTover the basin, particularly the variation and strength of the upwelling area located southeast of Portugal. The third mode explains 6% of variance and is well correlated with the local zonal wind. Two wind-induced upwelling can be clearly identified in this mode. The first one, located at the southwestern end of the Strait of Gibraltar, takes place during easterlies events. The second one, related to westerlies, is located to the east of Cape Santa Maro´a, and is associated with a southeastward transport of cold surface waters from that Cape. D 2002 Elsevier Science B.V. All rights reserved.

Transport estimates at the western section of the Strait of Gibraltar: A combined experimental and numerical modeling study

[1] Three-yearlong time series of Acoustic Doppler Current Profiler (ADCP) observations at a single station in Espartel Sill (Strait of Gibraltar) were used to compute an outflow of Q2 = � 0.82 Sv through the main channel. The cross-strait structure of the velocity field or the outflow through a secondary channel north of the submarine ridge of Majuan in Espartel section is not captured by observations so that an improved version of a numerical model (CEPOM) has been used to fill the observational gap. Previously, the model performance has been checked against historical data sets by comparing harmonic constants of the main diurnal and semidiurnal constituents from observed and modeled data at different sites of the strait. Considering the great complexity of tidal dynamics in the area, the comparison is quite satisfactory and validates the model to infer the exchange at longer timescales. Using a ‘‘climatological’’ April in the simulation, extracting a ‘‘single station’’ from the model at the same position as the monitoring station and processing the data similarly, the model gives an outflow through the southern channel 13% higher than observations. The inclusion of the cross-strait structure of velocity reduces the computed outflow through the southern channel, whereas the contribution of the northern channel brings the total outflow close to that computed using a single station (5% smaller). If the same correction is applied to observations, the total outflow would reduce to Q2 = � 0.78 Sv. The paper also assesses the importance of eddy fluxes to the total outflow, their contribution being negligible (� 5%).

Mediterranean Sea level and barotropic flow through the Strait of Gibraltar for the period 1958–2001 and reconstructed since 1659

Sea level values from a two-dimensional model of the Mediterranean Sea forced by atmospheric pressure and wind are used to estimate the barotropic flow through the Strait of Gibraltar for the period 1958–2001. The Mediterranean mean sea level derived from the model ranges between ±20 cm with a standard deviation of 5.5 cm and is correlated to the North Atlantic Oscillation (NAO) index. Thus NAO historical data and reconstructions are used to derive the Mediterranean Sea level variability from 1659 until 2001. The accuracy of the reconstruction is estimated in 2.7 cm for monthly mean values, 0.41 cm for annual mean values, and 0.22 cm for decadal mean values (0.48 cm for decadal winter mean sea level). The barotropic flow through the strait is computed from the model output as the time derivative of the total volume of the basin. During the period 1958–2001 the estimated daily flow ranges between ±2.7 Sv, with a standard deviation of 0.56 Sv. The dominant periodicities are in between 1 and 2 weeks. At these scales the model successfully reproduces previously published flow estimates based on current meter observations, which confirms that atmospheric pressure and wind dominate the intraseasonal variability of the flow. For the annual cycle the variability of the atmospherically induced flow is similar to the variability of the flow induced by the evaporation-precipitation (E-P) budget (±0.025 Sv), though absolute values of the first are about a third of the latter. At longer timescales the atmospheric contribution is much smaller than the E-P induced flow.

On the steric and mass‐induced contributions to the annual sea level variations in the Mediterranean Sea

The authors thank NASA Pathfinder project and Andrew Au for assistance in data processing. The altimeter products were produced by the CLS Space Oceanography Division as part of the Environment and Climate EU ENACT project (EVK2-CT2001-00117) and with support from CNES. GRACE data are provided by the NASA/DLR GRACE Project via the Center for Space Research website in the University of Texas at Austin. The ECCO model is a contribution of the Consortium for Estimating the Circulation and Climate of the Ocean funded by the National Oceanographic Partnership Program. This work, as part of the lead authors Ph.D. dissertation, is supported by the Spanish Science and Technology Ministry Projects ESP2001-4533/PE, ESP2005-02212 and REN2003- 01608/MAR, by the Valencian regional government grant ACOMP06-120, and by NASA’s Physical Oceanography Program.

Circulation of the Mediterranean Sea and its variability

The aim of this chapter is to introduce the main topics in the Mediterranean circulation research and to give a general description of the Mediterranean Sea variability. Its scope is to provide information that is important for a comprehensive view of the most relevant issues concerning climate, which is a coupling between atmosphere and sea. The chapter describes data, research results, and key findings that have been reported in the most recent literature about the Mediterranean Sea circulation. The issue about sea level is considered in Chapter 4. The chapter also includes, in its concluding section, a discussion of the main open issues in Mediterranean Sea research.

Small-scale temporal variations in biogeochemical features in the Strait of Gibraltar, Mediterranean side—the role of NACW and the interface oscillation

On the Mediterranean side of the Strait of Gibraltar, the distribution of physical, chemical and biological variables Ž . temperature, salinity, nutrients, chlorophyll a, lipids, particles size and plankton abundance was examined. Sampling was carried out between the surface and 150 m at a fixed station over a 24-h time series. The patterns observed were related to the overlaying of different processes. The Atlantic-Mediterranean interface acts as a strong pycnocline and its vertical oscillation accounts for the gross distribution of nutrients, particles, and living biomass. Injection of North Atlantic Central Ž. Ž . Water NACW into the upper layer occurs at the sill each semidiurnal tidal cycle every 12 h . As a consequence, in the upper Atlantic layer the NACW was observed every 12 h in the trough of the interface oscillation, whereas Surface Atlantic Ž. Water SAW dominated in the crest at the fixed station. The initially nutrient-rich NACW was associated with eutrophic signatures such as high chlorophyll, large cells and low turbidity; The nutrient depleted SAW was associated with oligotrophic signatures such as low chlorophyll, small cells and high turbidity. The distribution of lipid biotracers at the Ž. depth of the chlorophyll maxima 10-40 m depicted a similar trend with abundant chloroplast lipids and a low lipolysis index in NACW-enriched waters, and a high lipolysis index and abundant zooplankton tracers in SAW especially at night. During the eastward advection of Atlantic water, the nutrient content of NACW is likely to be assimilated by phytoplankton. A scenario is proposed for explaining changes in phytoplankton maxima composition during the time series observations, taking into account the timing of the NACW injection at the sill, the diurnal cycle and zooplankton grazing. Although more studies over a longer temporal scale are necessary to validate this scenario, our observations show the scale of daily variations in the physicalrbiological coupling in the Strait and the implications for nutrient and matter exchanges between the Atlantic and Mediterranean. q 2001 Elsevier Science B.V. All rights reserved.

Fueling plankton production by a meandering frontal jet: a case study for the Alboran Sea (Western Mediterranean).

A three dimensional biophysical model was employed to illustrate the biological impacts of a meandering frontal jet, in terms of efficiency and persistency of the autotrophic frontal production, in marginal and semi-enclosed seas. We used the Alboran Sea of the Western Mediterranean as a case study. Here, a frontal jet with a width of 15–20 km, characterized by the relatively low density Atlantic water mass, flows eastward within the upper 100 m as a marked meandering current around the western and the eastern anticyclonic gyres prior to its attachment to the North African shelf/slope topography of the Algerian basin. Its inherent nonlinearity leads to the development of a strong ageostrophic cross-frontal circulation that supplies nutrients into the nutrient-starved euphotic layer and stimulates phytoplankton growth along the jet. Biological production is larger in the western part of the basin and decreases eastwards with the gradual weakening of the jet. The higher production at the subsurface levels suggests that the Alboran Sea is likely more productive than predicted by the satellite chlorophyll data. The Mediterranean water mass away from the jet and the interiors of the western and eastern anticyclonic gyres remain unproductive.

Exchange of planktonic biomass through the Strait of Gibraltar in late summer conditions

In order to estimate plankton biomass transport through the Gibraltar Strait, plankton biomass and velocity profiles were measured at three stations located in the eastern side of the Strait as a part of the CANIGO project. Abundance and biomass measurements were carried out for autotrophic (Prochlorococcus, Synechococcus and eukaryotic piconano- and microplankton) and heterotrophic (bacteria and nanoflagellates) organisms, in September 1997. Biomass and velocity decreased from the surface to deeper water. Highest biomass concentration was observed at the northern station ð0:12 g C m � 3 ), whereas maximum mean velocities ð80 cm s � 1 ) were found at the central and southern stations. Biomass transport is estimated with a approach with a 10 -m resolution in the vertical and three subareas of approximately 5 km in the horizontal direction. Estimate of plankton biomass transports towards the Mediterranean and the Atlantic are 5570 and 1140 tonnes C day � 1 ; respectively. The former is co-dominated by heterotrophic bacteria (37%) and autotrophic nanoplankton (42%), while the latter is dominated by heterotrophic organisms like bacteria (75%) and heterotrophic nanoflagellates (14%). The variation during a one-day period of the biomass transport estimate at the central part of the Strait was explored. Also, in order to estimate the influence of spatial distribution of both biomass and velocity in the transport estimates, a comparison of our results with other possible estimates performed with less spatial resolution is carried out. The results confirms that both temporal and spatial resolution are key factors for transport estimates of inhomogeneous distributed variables through the Strait. r 2002 Elsevier Science Ltd. All rights reserved.

Trends of pH decrease in the Mediterranean Sea through high frequency observational data: indication of ocean acidification in the basin.

A significant fraction of anthropogenic carbon dioxide (CO2) released to the atmosphere is absorbed by the oceans, leading to a range of chemical changes and causing ocean acidification (OA). Assessing the impact of OA on marine ecosystems requires the accurate detection of the rate of seawater pH change. This work reports the results of nearly 3 years of continuous pH measurements in the Mediterranean Sea at the Strait of Gibraltar GIFT time series station. We document a remarkable decreasing annual trend of −0.0044 ± 0.00006 in the Mediterranean pH, which can be interpreted as an indicator of acidification in the basin based on high frequency records. Modeling pH data of the Mediterranean outflow allowed to discriminate between the pH values of its two main constituent water masses, the Levantine Intermediate Water (LIW) and the Western Mediterranean Deep Water (WMDW). Both water masses also exhibited a decline in pH with time, particularly the WMDW, which can be related to their different biogeochemical nature and processes occurring during transit time from formation sites to the Strait of Gibraltar.