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Small pelagic fish species, such as sardine and anchovy, can exhibit dramatic decadal-scale shifts in abundance in response to climate variability. Understanding the mechanisms and the relationships among the different components of the food web through which environmental forcing can drive the observed fish variability remains a challenging problem. The modelling study described herein, focusing on the Canary Current System, implements and builds on earlier modelling efforts by Rose et al. (2015) and Fiechter et al. (2015) in the California Current System. This new application of the modelling framework to the Canary Current system provides an approach that bridges a comprehensive database with end-to-end (climate-to-fish) modelling, thereby enabling the investigation of the sources of variability of sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus). The aim of the modelling effort is to gain insights into the underlying mechanisms that drive the observed biological variability. Particular attention is given to the absence of regime shifts between sardine and anchovy in the Canary Current, which is a distinctive feature among the four major eastern boundary upwelling ecosystems. A model simulation for 1958–2007 was performed and analysed. The biological traits and behaviours prescribed for sardine and anchovy for the Canary Current give rise to different spatial distribution of their populations, and in contrast with other eastern boundary upwelling ecosystems, to synchronous (rather than asynchronous) variability of their abundance and biomass. Analyses of years with anomalously high increases and declines of the adult populations implicate food availability (instead of temperature or other environmental drivers) as the main factor determining recruitment for both sardine (via spawning and survival of feeding age-0 individuals) and anchovy (via survival of feeding age-0 individuals). The common dependence of sardine and anchovy on food, together with the domain-wide response of zooplankton to climate forcing generated in the model, provides a plausible explanation for the synchronization of the two populations. Our results also point at differences between sardine and anchovy; while the two species thrive under enhanced upwelling-favourable winds, anchovy larvae become particularly vulnerable to drift mortality, and thus do better than sardine under more moderate upwelling conditions.

The effects of tidal forcing on the exchange flow through the Strait of Gibraltar and the circulation in the near-field region are revisited with a regional numerical model. Also a basin-scale model run is conducted in a first attempt to assess the impact of these local processes on the Western Mediterranean thermohaline circulation. In the Strait of Gibraltar, tides are found to (1) increase the exchange flow volume transport, (2) modify the hydrological properties of Atlantic inflowing waters through the enhancement of mixing, and (3) facilitate the drainage of Mediterranean deep water. In the far-field, the model reveals that these local processes can favor deep convection in the Gulf of Lion. Some thoughts are provided offering possible explanations.

A primitive-equation model has been used to investigate the meteorologically-driven circulation of the Bay of Algeciras. It is shown that the mean circulation of Atlantic Water (AW) is characterized by an anticyclonic cell, while Mediterranean Water (MW) follows a preferred cyclonic pathway. Meteorological forcing distorts substantially the AW mean circulation pattern, and only modulates that of the MW. Winds drive a vertical circulation cell in the Atlantic layer consistent with Ekman dynamics, whereas the horizontal circulation pattern is markedly dependent on the swift Atlantic jet entering the Mediterranean and changes from clearly anticyclonic to cyclonic as the jet separates or approaches the strait’s northern shoreline. This occurs through atmospheric pressure-driven acceleration/deceleration of the jet, in agreement with internal hydraulics theory predictions. It is also found that the renewal of AW is largely modulated by tides, with meteorological forcing playing a secondary role. The opposite applies to the renewal of MW.

The stability of the mean state of the surface circulation of the Western Alboran Sea, characterized by a jet of Atlantic water (AJ) surrounding the Western Alboran Gyre (WAG), is investigated with a submesoscale-resolving operational ocean model. It is shown that this circulation state collapses through the interaction between the AJ-WAG system and a growing cyclonic eddy that arises close to the Spanish coast. This eddy develops as the WAG partially blocks the positive potential vorticity (PV) flux coming from the Strait of Gibraltar. It is found that tides increase the positive PV flux with respect to a non-tidal simulation, thus making the destabilization of the mean circulation state more likely. Atmospheric pressure driven flows are also shown to have the potential to destabilize the circulation by rising dramatically the PV flux within a time scale of some days. This provides an explanation as to why the circulation of the Western Alboran Sea exhibits more variability toward winter time, when meteorological fluctuations are enhanced.

An 18-month time series of temperature collected in the vicinity of the Guadalquivir estuary mouth has been analyzed to investigate tidal signals in the temperature records. Two sources of similar importance are acting jointly to produce these signals, the gravitational tide generating potential that acts through the tidal currents induced by the vertical tide at the estuary's mouth and the radiational potential whose origin is the sun's radiation. The most important signal is the radiational diurnal S1, which exhibits a seasonal modulation. The next one in importance is M2, introduced via the advective term, which shows semiannual modulation due to the change of sign of the horizontal temperature gradient. The S2 constituent is a mixture of radiational (via the first harmonic of S1) and gravitational contributions, the former being about twice the latter. The harmonic constants of all these constituents are time-dependent so that caution is needed to interpret results deduced from time series of few months., Partial financial funding from the Spanish Ministry of Science and Innovation through CTM2010-21229-C02-01, CTM2008-04150/E projects and also from Regional Andalucía Government P08-RNM-03738 project are acknowledged.

Recent observations collected at different places in the Strait of Gibraltar are used to investigate the temporal and spatial variability of tidal currents in this region. The analysis of a five-year long time series of velocity observations at the Espartel sill (western end of the strait) shows that harmonic constants fluctuate seasonally exhibiting smaller amplitude in winter. This fact, along with an increased subinertial flow during the winter induces a marked decrease in the relative importance of the tide to the total flow compared to the summer. New computations of tidal transport at the key sections of Espartel and Camarinal, together with historical information reported for the Eastern Exit of the strait, have been analyzed jointly to highlight the internal along-strait divergence of the tidal transport in each layer and the transfer of the tidal signal from one layer to the other. This study covers the whole length of the strait, thus extending previous results reported for the central-eastern strait. Most of the topographically forced divergence is accounted for by large vertical displacements of the interface, although velocity observations collected on the continental shelf of the northern strait suggest that coastal recirculation plays some role in the volume conservation at tidal frequencies.