Wireless Communications for Marine Sensor Networks

Peer Reviewe

Biogeochemical patterns in the Atlantic Inflow through the Strait of Gibraltar

The effects of tidal forcing on the biogeochemical patterns of surface water masses flowing through the Strait of Gibraltar are studied by monitoring the Atlantic Inflow (AI) during both spring and neap tides. Three main phenomena are defined depending on the strength of the outflowing phase predicted over the Camarinal Sill: non-wave events (a very frequent phenomenon during the whole year); type I Internal wave events (a very energetic event, occurring during spring tides); and type II Internal wave events (less intense, occurring during neap tides). During neap tides, a non-wave event comprising oligotrophic open-ocean water from the Gulf of Cádiz is the most frequent and clearly dominant flow through the Strait. In this tidal condition, the inflow of North Atlantic Central Water (NACW) provides the main nutrient input to the surface layer of the Alboran Sea, supplying almost 70% of total annual nitrate transport to the Mediterranean basin. A low percentage of active and large phytoplankton cells and low average concentrations of chlorophyll (0.3–0.4 mg m−3) are found in this tidal phase. Around 50% of total annual phytoplankton biomass transport into the Mediterranean Sea through the Strait presents these oligotrophic characteristics. In contrast, during spring tides, patches of water with high chlorophyll levels (0.7–1 mg m−3) arrive intermittently, and these are recorded concurrently with the passage of internal waves coming from the Camarinal Sill (type I internal wave events). When large internal waves are arrested over the Camarinal Sill this implies strong interfacial mixing and the probable concurrent injection of coastal waters into the main channel of the Strait. These processes result in a mixed water column in the AI and can account for around 30% of total annual nitrate transport into the Mediterranean basin. Associated with type I internal wave events there is a regular inflow of large and active phytoplankton cells, transported in waters with relatively high nutrient concentrations, which constitutes a significant supply of planktonic resources to the pelagic ecosystem of the Alboran Sea (almost 30% of total annual phytoplankton biomass transport)

Meteorologically forced subinertial flows and internal wave generation at the main sill of the Strait of Gibraltar

The generation of large-amplitude internal waves in the Strait of Gibraltar is a widely known phenomenon. Those waves are produced by the interaction of barotropic tidal flow with the main sill (Camarinal Sill) topography and the stratified water column. That interaction primarily causes internal tides that evolve, by non-linear processes, into large-amplitude (more than 100m) internal waves exhibiting much shorter oscillation periods than those related to the basic tidal variability. Recent observations have shown that on many occasions large-amplitude internal wave generation is dependent on the state of the subinertial flows, which are basically driven by the atmospheric pressure fluctuations over the Mediterranean. Therefore, depending on the meteorological situation over the Mediterranean, internal wave events may be inhibited or activated. (c) 2008 Elsevier Ltd. All rights reserved

Internal waves and short-scale distribution patterns of chlorophyll in the Strait of Gibraltar and Alboran Sea

A selection of ASAR images have been analyzed, together with instantaneous images of surface chlorophyll recorded with MERIS and MODIS, in order to study the relationship between the physical and biological processes associated with internal waves in the Strait of Gibraltar and Alborán Sea. The images show peak levels of chlorophyll at the coastal edges to the north and south of the Camarinal Sill (CS) during the generation of internal waves, and peak levels of chlorophyll associated with the wave fronts as they travel into the Alborán Sea. The images have been compared with in-situ data. The results seem to indicate that, during the generation of the internal waves, a suction process takes place by which coastal waters rich in chlorophyll are drawn towards the center of the channel and then accompany the internal waves as they move towards the Alborán Sea

Baroclinic M2 circulation in Algeciras Bay and its implications for the water exchange with the Strait of Gibraltar: Observational and 3-D model results

The M2 tidal circulation in Algeciras Bay (Strait of Gibraltar) is analyzed using a 3-D, nonlinear, baroclinic, hydrodynamic model, in conjunction with observed data series. Results show the influence of the density stratification on the vertical structure of the M2 currents in Algeciras Bay, although its tidal dynamics shows major differences with respect to the Strait of Gibraltar. Whereas the M2 currents in the Strait present mainly barotropic behavior, the baroclinic effects prevail in Algeciras Bay. A notable finding is the presence of a tidal M2 counter-current system between the upper Atlantic and the lower Mediterranean water layers within the Bay, with amplitudes of up to 25 cm s−1. The interface between the two layers oscillates in antiphase relation with respect to the free-surface elevation, with amplitudes of almost 20 m. The presence of the submarine Algeciras Canyon was found to be determinant in the three-dimensional structure of tidal currents within the Bay, strengthening the baroclinic tidal regime of currents. This situation has quantitative consequences for the flow-exchange processes between Algeciras Bay and the outer Strait, with rates 20 times higher than those obtained when considering only the barotropic behavior, as well as inflow/outflow lateral recirculation volumes during half a tidal cycle that account for more than 20% of the net accumulated volume. This flow-exchange system was found to be affected by the nonlinear interaction processes between the first baroclinic period of resonance of Algeciras Bay and the M2 tide

Seasonal variability of intermediate water masses in the Gulf of Cádiz: implications of the Antarctic and subarctic seesaw

Global circulation of intermediate water masses has been extensively studied; however, its regional and local circulation along continental margins and variability and implications on sea floor morphologies are still not well known. In this study the intermediate water mass variability in the Gulf of Cádiz (GoC) and adjacent areas has been analysed and its implications discussed. Remarkable seasonal variations of the Antarctic Intermediate Water (AAIW) and the Subarctic Intermediate Water (SAIW) are determined. During autumn a greater presence of the AAIW seems to be related to a reduction in the presence of SAIW and Eastern North Atlantic Central Water (ENACW). This interaction also affects the Mediterranean Water (MW), which is pushed by the AAIW toward the upper continental slope. In the rest of the seasons, the SAIW is the predominant water mass reducing the presence of the AAIW. This seasonal variability for the predominance of these intermediate water masses is explained in terms of the concatenation of several wind-driven processes acting during the different seasons. Our finding is important for a better understanding of regional intermediate water mass variability with implications in the Atlantic Meridional Overturning Circulation (AMOC), but further research is needed in order to decode their changes during the geological past and their role, especially related to the AAIW, in controlling both the morphology and the sedimentation along the continental slopes

Meteorologically-induced mesoscale variability of the North-western Alboran Sea (southern Spain) and related biological patterns

Hydrographic mesoscale structures in the North-western Alboran Sea show a high variability induced by a number of different factors. One of the most important is the differences in atmospheric pressure over the Mediterranean basin when compared to the Gulf of Cadiz. This difference modulates the zonal wind field in the Alboran Sea and the intensity of the Atlantic inflow through the Strait of Gibraltar, also affecting the formation and extension of the Western Alboran Gyre (WAG). When westerly winds are dominant, lower atmospheric pressure in the Mediterranean enhances the inflow of Atlantic waters causing the Atlantic Jet to be located in the vicinity of the Spanish shore, creating a well-defined frontal zone in front of Estepona Cove. In this situation, the coastal upwelling is enhanced, leading to a minimum in sea surface temperature and a maximum of surface nutrient concentrations located in the coastal area. The vertical position of the chlorophyll maximum found in these circumstances appeared to be controlled by the nutrient availability. On the other hand, when easterly winds prevail, higher atmospheric pressure in the Mediterranean leads to a reduced inflow and the oceanographic and biological structures are clearly different. The Atlantic Jet moves southward flowing in a south-eastern direction, changing the structure of the currents, resulting in an enhanced cyclonic circulation extending throughout the North-western Alboran Sea basin. These physical alterations also induce changes in the distribution of biogeochemical variables. Maximum nutrient and chlorophyll concentrations are located further off the coast in the central area of the newly created cyclonic gyre. During these easterlies periods coastal upwelling stops and the distribution of phytoplankton cells seems to be mainly controlled by physical processes such as advection of coastal waters to the open sea. (C) 2007 Elsevier Ltd. All rights reserved

The Use of Sentinel-3 Altimetry Data to Assess Wind Speed from the Weather Research and Forecasting (WRF) Model: Application over the Gulf of Cadiz

This work presents the quality performance and the capabilities of altimetry derived wind speed (WS) retrievals from the altimeters on-board Copernicus satellites Sentinel-3A/B (S3A/B) for the spatial assessment of WS outputs from the weather research and forecasting (WRF) model over the complex area of the Gulf of Cádiz (GoC), Spain. In order to assess the applicability of the altimetry data for this purpose, comparisons between three different WS data sources over the area were evaluated: in situ measurements, S3A/B 20 Hz altimetry data, and WRF model outputs. Sentinel- 3A/B WS data were compared against two different moored buoys to guarantee the quality of the data over the GoC, resulting in satisfying scores (average results: RMSE = 1.21 m/s, r = 0.93 for S3A and RMSE = 1.36 m/s, r = 0.89 for S3B). Second, the WRF model was validated with in situ data from four different stations to ensure the correct performance over the area. Finally, the spatial variability of the WS derived from the WRF model was compared with the along-track altimetry-derived WS. The analysis was carried out under different wind synoptic conditions. Qualitative and quantitative results (average RMSE < 1.0 m/s) show agreement between both data sets under low/high wind regimes, proving that the spatial coverage of satellite altimetry enables the spatial assessment of high-resolution numerical weather prediction models in complex water-covered zones

Submesoscale, tidally-induced biogeochemical patterns in the Strait of Gibraltar

Tidal forcing and its fortnightly variation are known to be one of the main regulating agents of physical and biogeochemical signatures in the Strait of Gibraltar and surrounding areas. Samples obtained during spring and neap tides in the region were analyzed to determine the influence of this tidal variation on the submesoscale distribution of water masses and biological elements. During spring tides, strong and intermittent mixing processes between Mediterranean and Atlantic waters occur in the vicinity of the Camarinal Sill together with an eastward advection of those mixed waters into the Alboran Sea. Furthermore, the intense suction of surface coastal waters into the main channel of the strait was detected as chlorophyll patches in the Alboran Sea during spring tides. In contrast, the most characteristic phenomenon during neap tides was the arrival of pulses of relatively nutrient-rich North Atlantic Central Waters to the surface regions of the Alboran Sea. In addition, traces of the suction of coastal waters were observed for the first time during neap tides. Therefore, our results show that submesoscale processes are crucial in the dynamics of the Strait of Gibraltar, and they must be considered for the correct description of the biogeochemical features of Alboran Sea, especially during an inactive period of the coastal upwelling