Rita Nolasco

Magnetotelluric imaging of the Society Islands hotspot

In April-June 1989, seafloor magnetotelluric data across and along the leading edge of the Tahiti hotspot were obtained. The magnetotelluric response functions were found to be strongly influenced by bathymetric and island effects, and a new procedure for modeling and removing this distortion using a thin sheet approach combined with the measured water depths is introduced. The corrected response functions are consistent with a two-dimensional structure. Inversion of the data shows a slightly higher conductivity (relative to a reference site located away from the hotspot) down to 130 km depth beneath the active area southeast of Tahiti underlain by a more resistive structure. There is a suggestion for a change in conductivity in the 400–450 km depth range, which is consistent with elevated temperatures. This result is consistent with a mantle plume of limited extent (less than 150 km radius) located near the leading edge of the Tahiti hotspot. The magnetotelluric data provide no evidence for lithospheric thinning or for a strong thermal influence over a large area, as would be required by a superswell model.

Electromagnetic imaging of a transpressional tectonics in SW Iberia

Forty-one magnetotelluric soundings were carried out along a 200 km-long profile (approximately NNE-SSW) across the three major geotectonic units in SW Iberia. A model obtained from two-dimensional inversion of the magnetotelluric data set reveals high conductivity zones in the middle-lower crust (10–30 km). Two of these zones correspond to the transition between the main geotectonic units: one between the South Portuguese Zone and the Ossa Morena Zone, interpreted as having been caused by metasediments, and the other between the Ossa Morena Zone and the Central Iberia Zone associated with a shear zone and metasediments. Another high conductivity anomaly related to black shales with major graphite impregnation was detected within the Ossa Morena Zone. The resistive features, located preferentially in the upper crust (1–10 km), coincide with gabbroic and granitic complexes.

Short Note: Application of the Mohid-2D model to a mesotidal temperate coastal lagoon

1. IntroductionA mathematical model can be considered as anapproximate reconstruction of a real phenomenon.All parameterizations and approximations used inmodels lead to deviations of the model results fromnature. It is an accepted requirement that anumerical model of estuarine hydrodynamicsshould be verified, calibrated and validated beforeused in a practical application. However, theprocedures to perform these tasks are not widelyaccepted (Cheng et al., 1991). Calibration andvalidation methods appear in several forms, de-pending on data availability, water mass character-istics and researchers’ opinion (Hsu et al., 1999).In this work, the Mohid-2D model implementa-tion for the Ria de Aveiro lagoon is presented,describing its assessment through calibration andvalidation against several different data sets. Due tothe lagoon complex geometry and the large numberof calibration stations used, this goal constitutes avery challenging task.The model is calibrated using as a first approach aqualitative comparison of the temporal evolution ofsea surface elevation (SSE) data measured in 1987/1988 at several locations. When a good match isobtained for all stations, the model’s accuracy isevaluated through the determination of the rootmean square (RMS) error and also through thecomparison between amplitude and phase of themain tidal constituents determined from harmonicanalysis of the observed and computed data. Thevalidation procedure is performed using two in-dependent data sets, which includes observations ofcurrent velocities and SSE values (1997 data) andmeasured water fluxes at the lagoon’s inlet for theperiod of October 2002.2. The study areaRia de Aveiro (Fig. 1) is a shallow mesotidallagoon located in the Northwest coast of Portugalð40 38

Model-derived dispersal pathways from multiple source populations explain variability of invertebrate larval supply.

Background Predicting the spatial and temporal patterns of marine larval dispersal and supply is a challenging task due to the small size of the larvae and the variability of oceanographic processes. Addressing this problem requires the use of novel approaches capable of capturing the inherent variability in the mechanisms involved. Methodology/Principal Findings In this study we test whether dispersal and connectivity patterns generated from a bio-physical model of larval dispersal of the crab Carcinus maenas, along the west coast of the Iberian Peninsula, can predict the highly variable daily pattern of wind-driven larval supply to an estuary observed during the peak reproductive season (March–June) in 2006 and 2007. Cross-correlations between observed and predicted supply were significant (p<0.05) and strong, ranging from 0.34 to 0.81 at time lags of −6 to +5 d. Importantly, the model correctly predicted observed cross-shelf distributions (Pearson r = 0.82, p<0.001, and r = 0.79, p<0.01, in 2006 and 2007) and indicated that all supply events were comprised of larvae that had been retained within the inner shelf; larvae transported to the outer shelf and beyond never recruited. Estimated average dispersal distances ranged from 57 to 198 km and were only marginally affected by mortality. Conclusions/Significance The high degree of predicted demographic connectivity over relatively large geographic scales is consistent with the lack of genetic structuring in C. maenas along the Iberian Peninsula. These findings indicate that the dynamic nature of larval dispersal can be captured by mechanistic biophysical models, which can be used to provide meaningful predictions of the patterns and causes of fine-scale variability in larval supply to marine populations.

A hydrogeological investigation using EM34 and SP surveys

EM34 and SP surveys were used to delineate shallow structures associated with the mineral water springs in the Vilarelho da Raia area located NE Portugal. This spring is part of a set of CO2-rich mineral (hot and cold) waters connected to the main Hercynian NNE-SSW fault systems. The EM34 survey was interpreted using a quasi-three-dimensional inversion approach based on a smooth-regularisation algorithm. The model put into evidence the conductive overburden as well as the fractured granitic formation. A preliminary estimation of the aquifer porosity is made based on the EM34 model. The SP anomalies were interpreted considering as having their sources on the aquifer interfaces. The modelling of two SP profiles allow the characterisation of a fault connected to the spring.

Towards Operational Modeling and Forecasting of the Iberian Shelves Ecosystem

There is a growing interest on physical and biogeochemical oceanic hindcasts and forecasts from a wide range of users and businesses. In this contribution we present an operational biogeochemical forecast system for the Portuguese and Galician oceanographic regions, where atmospheric, hydrodynamic and biogeochemical variables are integrated. The ocean model ROMS, with a horizontal resolution of 3 km, is forced by the atmospheric model WRF and includes a Nutrients-Phytoplankton-Zooplankton-Detritus biogeochemical module (NPZD). In addition to oceanographic variables, the system predicts the concentration of nitrate, phytoplankton, zooplankton and detritus (mmol N m−3). Model results are compared against radar currents and remote sensed SST and chlorophyll. Quantitative skill assessment during a summer upwelling period shows that our modelling system adequately represents the surface circulation over the shelf including the observed spatial variability and trends of temperature and chlorophyll concentration. Additionally, the skill assessment also shows some deficiencies like the overestimation of upwelling circulation and consequently, of the duration and intensity of the phytoplankton blooms. These and other departures from the observations are discussed, their origins identified and future improvements suggested. The forecast system is the first of its kind in the region and provides free online distribution of model input and output, as well as comparisons of model results with satellite imagery for qualitative operational assessment of model skill.

Voltage measurements over the CAM-1 submarine cable between Madeira Island and Portugal mainland

Data of electrical voltage measured between the ends of the CAM-1 cable, during fifteen months, have been analysed in order to determine the electrical stability of the cable and to obtain some preliminary information related to water transport. The monthly average of the potential measured during days with low geomagnetic activity suggests a periodicity of 120 days that was tentatively interpreted as having origin in water flow. These results show a small trend that is compatible with no drift in the cable. The mean electric field estimated from quiet days is 0.206 ± 0.022 mV/km. If the variations of this potential are attributable to ocean water transport a total water flow of ±0.09 cm/s is estimated. This value represents a maximum value for temporal variations of the spatially averaged water-flow velocity.

Numerical modelling of the phytoplankton patterns in an upwelling event off the NW Iberian Margin

ABSTRACT Rocha, C., Cordeiro, N., Nolasco R., Dubert J., 2013. Numerical modelling of the phytoplankton patterns in an upwelling event off the NW Iberian Margin. An extreme event in September 2007, presenting a strong upwelling core detaching from the Galician NW coast and extending towards west-northwest from the Cape Finisterre - Cape Ortegal zone, with the formation of several filaments and a consequent strong response in chlorophyll concentration values is analyzed. To do so, a NPZD biogeochemical module coupled to a ROMS_Agrif configuration is used. The model response was satisfactory and an analysis of the event, including its forcing, was made. The particular wind direction and intensity along with the specific coastal orientation of the study area were identified as important characteristics for the development of the event. The analysis of the behavior and evolution of this phytoplankton bloom may give further insight in the relations between atmospheric forcing, the consequent characteristic coastal ocean processes, and their conditioning in phytoplankton distribution and patterns in the study area.

Filaments on the Western Iberian Margin: A modeling study

Coastal upwelling filaments off the Western Iberian Margin, detected in AVHRR satellite imagery and in a realistic ROMS simulation of sea surface temperature, were studied in the upwelling seasons (May–October) of 2001–2010. Sea surface temperature data were retrieved from AVHRR satellite imagery and from a realistic ROMS numerical simulation. The development and variability of the observed filaments were characterized and analyzed during each upwelling season of the 10 year period. Filaments were generally found anchored to the main bathymetric and coastal features but off the more regular northern coast of the Western Iberian Margin their locations were more variable. The results from the modeling analysis reproduced well the general features of filament development. Moreover results of model and observation showed very similar characteristics as those found in the earlier study of Haynes et al. (1993). The model output was used to relate filament patterns, eddy activity, and wind forcing. There was a clear relation between upwelling-favorable wind strength and number and length of filaments, although the relation was weaker in the north of the region. Model filaments were clearly related to eddies only during periods of weak winds. The filament detection method was also applied to a climatologically forced ROMS simulation, which reproduced only gross features of the observed and interannually forced model filament development. This suggests that direct wind forcing and its spatial structure are highly important.

Assessing future climate change in the Iberian Upwelling System

ABSTRACT Cordeiro Pires, A., Nolasco, R., Rocha, A. and Dubert, J., 2013. Assessing future climate change in the Iberian Upwelling System The Western Iberian Margin is the northern limit of the Canary Upwelling System, a region of strong mesoscale activity, seasonal variability and thus very likely to be sensitive to climate change. Using a regional ocean model and data from several coupled global climate models (CGCM), climatological simulations were set up for present and for a future scenario. Forcing is obtained from averaging the outputs of an ensemble of CGCM provided by the Intergovernmental Panel for Climate Change (IPCC) A2 emission scenario. Results are focused on the continental shelf (~200 m). In general, the sea surface temperature (SST) seasonal evolution shows, for the future, an increase of about 1°C during the upwelling season (April to September) and 2°C in the rest of the year, while sea surface salinity (SSS) shows a freshening of about −0.2. These results agree with a general increase in air temperature and in fresh water input resulting from ice melting in the North Pole, which characterize this future scenario. However, differences depend on latitude and distance from the coast (higher differences to the south and more offshore, respectively). Also, SSS undergoes a shift of its minimum from July to May or September. Cross-shore sections show that SST and SSS differences are mainly observed in the upper 200 m. In winter, the typical upper slope poleward flow undergoes a slight weakening and shallowing. In summer, while the upwelling jet intensifies at the surface, it is also more restricted in both width and depth.