Francisco Plaza

Upwelling mechanisms in the northwestern Alboran Sea

Abstract From April 1996 to July 1997, a series of hydrographic surveys were carried out in the Northwestern part of the Alboran Sea to investigate the upwelling that is an almost permanent feature in this area. Simultaneously a mooring line was deployed in the north part of the eastern section of the Strait of Gibraltar to monitor the variability of the Atlantic Jet (AJ). Two mechanisms are shown to be relevant for the upwelling dynamic in the region: the southward drifting of the AJ and wind stress. A linear relation between the angle under which the Jet enters the Alboran Sea and the distance from the coastline to the front associated with the Jet has been found. This angle that has been estimated from the low passed time series of current velocity measured by the uppermost instrument of the moored line has been then used to identify the onshore–offshore excursions of the Jet. Both upwelling mechanisms are identified from hydrographic data, because each of them has associated a different type of water mass, and they take place in different locations. Wind-driven upwelling dominates in coastal zones, on the shelf, while upwelling associated with southward drifting of the AJ prevails further offshore. The amount of sub-surface water brought up to the surface by each one is of the same order. However, wind-driven upwelling contributes to the fertilization of this region in a major extent because water upwelled by wind is richer in nutrient concentration.

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.

Tide at the eastern section of the Strait of Gibraltar

From October 1995 to April 1996, three mooring lines were deployed at the eastern entrance of the Strait of Gibraltar. The spatial coverage of the mooring array allows for a good description of the tides. They exhibit a dominant semidiurnal nature and a noticeable baroclinic structure that matches the one of the mean exchange. Tidal currents in the upper layer are irregular and usually too weak to reverse the mean upper layer flow that keeps on flowing east. Lower layer flow reverses with semidiurnal periodicity because of the smallness of the mean flow and the appreciable amplitude of the regular semidiurnal oscillation of tidal currents in this layer. Tidal transports can be satisfactorily compared with previous estimates of Bryden et al. [1994] if we allow for strong internal divergences associated with the internal tide. No significant eddy flux of water transport (tidal rectification) is observed at the eastern section, contrary to the almost 50% of the total layer transport found by Bryden et al. [1994] in Camarinal Sill section. Time-dependent hydraulic theory provides a good scenario for interpreting these two independent sets of observations despite the fact that the composite Froude number does not reach the critical values predicted in the hydraulic models most of the time.

Low-frequency variability of the exchanged flows through the Strait of Gibraltar during CANIGO

Time series of the exchanged flows through the Strait of Gibraltar at the eastern section have been estimated from current-meter observations taken between October 1995 and May 1998 within the Canary Islands Azores Gibraltar Observations (CANIGO) project. The inflow exhibits a clear annual signal that peaks in late summer simultaneously with a deepening of the interface. The cycle seems to be driven by the seasonal signal of the density contrast between the surface Atlantic water that forms the inflow and the deep Mediterranean water of the outflow. The outflow and the depth of the interface have predominant semiannual signals and a smaller annual one whose phase agrees with that of the density contrast as well. Local wind stress and atmospheric pressure difference between the Atlantic and the Western Mediterranean to less extent have clear semiannual signal, so that the possibility that the semiannual cycle of the outflow and of the depth of the interface are forced by them was analyzed. The composite Froude number in this section is well below the critical value, suggesting submaximal exchange. Therefore, the conditions in the Alboran basin influence the exchange and some evidence that the size and location of the Western Alboran Gyre contribute to the observed signals, both annual and semiannual, is provided. r 2002 Elsevier Science Ltd. All rights reserved.

Physical–biological coupling in the Strait of Gibraltar

This study presents a joint analysis of the distributions of some biogeochemical variables and their relation to the hydrodynamics of Gibraltar Strait. It is a synthesis paper that brings together many results obtained during CANIGO project. We show the role of hydrodynamics as a forcing agent for the plankton community structure in the Strait, with emphasis on the two physical processes that we propose as key factors for the coupling: interface position and oscillations, and mixing processes along the Strait. As a general pattern, autotrophic plankton biomass increases at the Strait from southwest to northeast, a tendency that coincides with a gradual elevation of the interface depth in the same direction. The different mechanisms of mixing that take place in the Strait are briefly reviewed: The occurrence of the internal hydraulic jump is an important mechanism of mixing constrained to the spring tide situations, but other processes such as the generation of arrested internal waves of wavelength around 1 km are proposed as a complementary mixing mechanism, particularly during neap tides situations. Both mechanisms, the elevation of the pycnocline and these mixing events, can enhance biological productivity and biomass accumulation on the northeastern sector of the Strait, since phytoplankton cells are there packaged in a water mass with sufficient light and nutrients and smaller advective velocity. There is a clear north–south difference in the biological response to these upwelling episodes in the eastern section, with high nutrient and low chlorophyll in the south and the opposite in the north. The deeper interface and the greater water speed are the proposed reasons for this lower nutrient uptake on the southeastern sector. Finally, the temporal scales of variation of the mixing events, the influence of its periodicity on the productivity of the area and the influence of these upwelling episodes in the nearest Albor! an Sea are discussed. r 2002 Elsevier Science

Tidal motions and tidally induced fluxes through La Línea submarine canyon, western Alboran Sea

Detailed observations from two mooring lines deployed in La Linea submarine canyon, western Alboran Sea, are presented. This is a narrow canyon in the sense that its width is always less than the internal radius of deformation. Tidal currents within the canyon are polarized in the along-canyon direction according to its narrow nature. They have considerable amplitude (values of around 0.5 m/s are often observed) and are forced by the internal pressure gradients associated with the baroclinic tide that is generated in the surroundings. Subsequent amplification of onshore baroclinic currents within the canyon accounts for the large amplitude observed. Cross-shelf exchange through the canyon due to tidal motions is different from zero despite the close to zero mean of tidal currents. The explanation is based on the asymmetry of water properties flowing up-canyon and down-canyon (some sort of tidal rectification). Regarding the energy flux, the canyon seems to be an adequate conduit to carry energy to the shore. Estimations made from our observations indicate that energy input onto the shelf per unit length parallel to the shore at the canyon head is enough to maintain mixing on the shelf at intermediate depths.

Deep and intermediate layer warming in the western Mediterranean: Water mass changes and heaving

[1] Temperature and salinity have increased in the Western Mediterranean Deep Water during the second half of the 20th century, but surprisingly, the two water masses contributing to its formation (Atlantic Water and Levantine Intermediate Water) show no clear warming trends, at least for the same period of time. Previous studies concerned with long term TS property changes in the Western Mediterranean had dealt with changes on isobaric surfaces. In this study we decompose these changes into their two natural contributions: Changes on isopycnal surfaces and changes induced by heaving (vertical displacements of density surfaces). This decomposition shows that LIW temperature and salinity have increased when changes on density surfaces are analyzed. The upward movement of deep layers has induced cooling on isobaric levels masking LIW temperature trends and underestimating deep layers temperature trends.

Mass transport in the Bay of Biscay from an inverse box model

[1] Mass transports in the Bay of Biscay for thermocline, intermediate and deep water masses are estimated from a box inverse model applied to hydrographic data from a quasi-synoptic survey carried out in August 2005. The model obeys conservation statements for mass in a volume of ocean bounded by 43 full depth CTD stations in the horizontal, and neutral density layers in the vertical. It permits advective exchange between layers and an adjustment of the Ekman transport and the freshwater flux divergences. Bay of Biscay has a thermocline transport of -8.0 ± 0.7 Sv with an anticyclonic recirculation of +6.9 ± 0.6 Sv. An equatorward flow of +0.5 ± 0.4 Sv is attributed to the slope current close to the northwestern Spanish coast. The mean wind stress curl for the region corresponds with a clockwise upper layer flow which produces a downwelling of -0.6 ± 0.4 Sv. The surface circulation pattern agrees with the obtained with the Jason-1 altimeter. Intermediate waters follow the same surface pattern of the thermocline waters as a subsurface eastern boundary current with an eastward flow of -7.9 ± 1.0 Sv and a southwestward flow of +8.0 ± 1.0 Sv. Finally, our results show that deep waters also flow into the box with a net transport of -3.6 ± 2.0 Sv and flow out of the box with a net transport of 2.3 ± 2.0 Sv.

Rising temperature and salinity fields to the north of the Almeria-Oran Front during the past decade

[1] Time series of temperature and salinity profiles in the South Western Mediterranean Sea are analyzed from June 1996 to November 2006. Previous studies have addressed the detection of changes on isobaric levels. In this paper we decompose thermohaline changes into those caused by vertical displacements of the isopycnals, more likely linked to changes in the dynamics of the region and dominant winds, and those changes on isopycnal surfaces. The latter give a more accurate idea about changes in water mass properties due to ocean-atmosphere interaction. Our analysis reveals that there has been a warming and salting on isopycnal surfaces. The warming during this 10-year period is quite intense, around 0.032°C yr -1 , and the salinity increase is around 0.009 yr -1 . The depth of isopycnals shows an upward trend, rising 45 m over the study period, compensating for the warming on isobaric surfaces.