José Juan Alonso

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

Non-linear interaction between tidal and subinertial barotropic flows in the Strait of Gibraltar

Abstract The present work establishes clear relationships of the amplitude and phase variation of the barotropic M 2 signal in the velocity of the current with the barotropic subinertial flow in the Strait of Gibraltar. The analytical procedure is applied on data from Gibraltar Experiment in order to obtain barotropic subinertial series and the amplitude and phase variation of the M 2 signal series involves harmonic analysis, empirical orthogonal function analysis and complex demodulation. In addition, cross-spectral analysis has been applied to study these relationships, concerning which non-linear interaction between M 2 and the subinertial oscillation is proposed as the responsible physical mechanism. An analytical solution characterizing this type of non-linear interaction is offered in explanation of the experimental results.

Vertical structure of the semidiurnal tidal currents at Camarinal Sill, the strait of Gibraltar

Abstract The dynamical mode decomposition (DMD) technique is applied to the data of currentmeter and CTD measurements taken during the 1985–1986 Gibraltar Experiment and the 1989 survey so as to clarify features of the vertical structure of the M2 and S2 tidal currents at the Camarinal Sill. It is shown that in conformity with the inference made on the basis of the empirical orthogonal function (EOF) decomposition technique, these currents are mainly due to the M2 and S2 barotropic modes. At the same time the first three baroclinic modes are responsible not only for the vertical variability of the tidal currents but also for the velocity and density amplitude variances at semidiurnal frequencies. Certain quantitative discrepancies between the values of barotropic tidal current characteristics as deduced from DMD and EOF decomposition techniques are revealed. In order to eliminate these, new currentmeter data are required with a finer vertical resolution than those which are available.

Precise determination of HPGe detector efficiency for gamma spectrometry measurements of environmental samples with variable geometry and density

Abstract A methodology to determine the full energy peak efficiency (FEPE) for precise gamma spectrometry measurements of environmental samples with high-purity germanium (HPGe) detector, valid when this efficiency depends on the energy of the radiation E, the height of the cylindrical sample H, and its density ρ, is introduced. The methodology consists of an initial calibration as a function of E and H and the application of a self-attenuation factor, depending on the density of the sample ρ, in order to correct for the different attenuation of the generic sample in relation to the measured standard. The obtained efficiency can be used in the whole range of interest studied, E = 120–2000 keV, H = 1–5 cm, and ρ = 0.8–1.7 g/cm3, being its uncertainty below 5%. The efficiency has been checked by the measurement of standards, resulting in a good agreement between experimental and expected activities. The described methodology can be extended to similar situations when samples show geometric and compaction differences.

Lake Izabal (Guatemala) shoreline detection and inundated area estimation from ENVISAT ASAR images

The surface extent of a lake reflects its water storage variations. This information has important hydrological and operational applications. However, there is a lack of information regarding this subject because the traditional methodologies for this purposes (ground surveys, aerial photos) requires high resources investments. Remote sensing techniques (optical/radar sensors) permit a low cost, constant and accurate monitoring of this parameter. The objective of this study was to determine the surface variations of Lake Izabal, the largest one in Guatemala. The lake is located close to the Caribbean Sea coastline. The climate in the region is predominantly cloudy and rainy, being the Synthetic Aperture Radar (SAR) the best suited sensor for this purpose. Although several studies have successfully used SAR products in detecting land-water boundaries, all of them highlighted some sensor limitations. These limitations are mainly caused by roughened water surfaces caused by strong winds which are frequent in Lake Izabal. The ESAs ASAR data products were used. From the set of 9 ASAR images used, all of them have wind-roughened ashore waters in several levels. Here, a chain of image processing steps were applied in order to extract a reliable shoreline. The shoreline detection is the key task for the surface estimation. After the shoreline extraction, the inundated area of the lake was estimated. In-situ lake level measurements were used for validation. The results showed good agreement between the inundated areas estimations and the lake level gauges.

Fractal dimension and altimeter data

The radar altimeter data sets are used to study several dynamical characteristic of the Worlds Oceans because the artificial satellites have a global coverage. One of the most important applications is related to the Mean Sea Level changes. The authors take into account the TOPEX/Poseidon data for the Mediterranean Sea and the Iberian Atlantic in order to compute the trend of the Mean Sea Level by means of two different procedures: the linear regression and fractal geometry. The first one leads to very well known results but the errors in the estimation are quite large and the second leads to more reliable results.

Theoretical variability of s1 in the tide generating potential

The S1 tidal wave arising from the second order tidal potential occurs in any geophysical data record. Since the frequency of S1 is the same as that of earth’s rotation, many distortions are observed in real data sets. To isolate S1 from its powerful neighbours,K1 and P1 at least one-year hourly data series are required. Studies onS1 on an experimental basis, have been aimed mainly at a precise determination of its origin and mechanisms of variability. There are many hypotheses on its variability. In physical oceanography it is commonly accepted that the variability of S1 is due to air-sea interaction processes. The authors review most existing hypotheses on the variability of S1 They also show that the distortions of real data must be expected due to additional contributions from the second and third-order lunar potentials. An analysis is made to detect the sources of the additional contributions to the S1 frequency.