I. Alvarez

Hydrographic characterization of a winter-upwelling event in the Ria of Pontevedra (NW Spain)

An unusual winter-upwelling event was observed in the Ria of Pontevedra during a cruise carried out on 27 January 1998. Upwelled water masses inside the estuary are observed to depend on the season due to the annual variation of water circulation at the adjacent shelf. The winter upwelled water corresponds to the water mass transported by the poleward current, which is saltier and warmer, but less dense than the Eastern North Atlantic Central Water observed in spring and summer.

Has upwelling strengthened along worldwide coasts over 1982-2010?

Changes in coastal upwelling strength have been widely studied since 1990 when Bakun proposed that global warming can induce the intensification of upwelling in coastal areas. Whether present wind trends support this hypothesis remains controversial, as results of previous studies seem to depend on the study area, the length of the time series, the season, and even the database used. In this study, temporal and spatial trends in the coastal upwelling regime worldwide were investigated during upwelling seasons from 1982 to 2010 using a single wind database (Climate Forecast System Reanalysis) with high spatial resolution (0.3°). Of the major upwelling systems, increasing trends were only observed in the coastal areas of Benguela, Peru, Canary, and northern California. A tendency for an increase in upwelling-favourable winds was also identified along several less studied regions, such as the western Australian and southern Caribbean coasts.

Spatio-temporal Upwelling Trends along the Canary Upwelling System (1967–2006)

Spatio‐temporal trends in upwelling patterns were studied along the Canary Upwelling System for the period 1967–2006. The northwestern coast of African from lat 20°N to 32°N is observed to be under a permanent upwelling regime characterized by coastal sea surface temperatures (SST) colder than the oceanic ones at the same latitude. The temperature difference is termed the temperature upwelling index (UISST). This regime is consistent with the wind‐derived Ekman transport (UIW), which is observed near the coast and is directed offshore. This index shows the existence of upwelling‐favorable conditions all year but has an annual cycle characterized by more upwelling‐favorable conditions from April to September, with a maximum in July, and less upwelling‐favorable conditions from October to March, with a minimum in December to January. Although both indices can be used to characterize the phenomenon, only UIW values were used to quantify upwelling change during the four decades under review because this index is less sensitive to external factors compared to UISST. A strong decrease in upwelling intensity has been observed in all seasons. In particular, the summer (winter) decrease is on the order of 45% (20%) of the mean amplitude of the upwelling cycle.

Ekman transport patterns in the area close to the Galician coast (NW, Spain)

A 40 year time series of daily and monthly Ekman transport is analysed in an oceanic area 150 km offshore Cape Finisterre (43°N, 11°W). The periodicity of the signal showed an annual component (365 days), a seasonal fluctuation (50 days) and a time scale related to passing storms (20 days). The Ekman transport showed different seasonal patterns. Thus, the summer pattern is characterized by transport pointing seaward (upwelling favourable) practically perpendicular to the shoreline, while the winter pattern is characterized by transport pointing landward (downwelling favourable). Although the most favourable upwelling conditions were observed during summer, for the last 15 years the frequency of observed winter upwelling events is higher due to the prevalence of northerly winds.

Assessment of Wind Pattern Accuracy from the QuikSCAT Satellite and the WRF Model along the Galician Coast (Northwest Iberian Peninsula)

Surface wind along the Galician coast is a key factor allowing the analysis of important oceanographic features that are related to the great primary production in this area, as upwelling events. A comparative analysis between surface winds obtained from the Quick Scatterometer (QuikSCAT), the Weather Research and Forecasting (WRF) Model, and in situ observations from buoys along the Galician coast is carried out from November 2008 to October 2009. This comparison evaluates the accuracy of satellite and modeled data. The results show that the wind speeds derived from QuikSCAT and the WRF Model are similar along the coast, with errors ranging from 1.5 to 2 m s 21 . However, QuikSCAT tends to overestimate wind speeds when compared to the buoys measurements. Regarding the wind direction, the RMSE values are about 358 for the stations under analysis. The bias presents a similar pattern between satellite and modeled data, with positive values at the western coast and negative values at the middle and northern coasts, the satellite data always being lower in absolute value than the modeled data. A spatial comparison between QuikSCAT and WRF data is also performed over the whole Galician coast to evaluate the differences between the two datasets. This comparison shows that the modeled wind speed tends to be lower than satellite winds over the entire domain, with the highest RMSE and bias values found for the wind speed and direction observed near the shoreline.

Variability of coastal and ocean water temperature in the upper 700 m along the Western Iberian Peninsula from 1975 to 2006.

Temperature is observed to have different trends at coastal and ocean locations along the western Iberian Peninsula from 1975 to 2006, which corresponds to the last warming period in the area under study. The analysis was carried out by means of the Simple Ocean Data Assimilation (SODA). Reanalysis data are available at monthly scale with a horizontal resolution of 0.5°×0.5° and a vertical resolution of 40 levels, which allows obtaining information beneath the sea surface. Only the first 21 vertical levels (from 5.0 m to 729.35 m) were considered here, since the most important changes in heat content observed for the world ocean during the last decades, correspond to the upper 700 m. Warming was observed to be considerably higher at ocean locations than at coastal ones. Ocean warming ranged from values on the order of 0.3°C dec−1 near surface to less than 0.1°C dec−1 at 500 m, while coastal warming showed values close to 0.2°C dec−1 near surface, decreasing rapidly below 0.1°C dec−1 for depths on the order of 50 m. The heat content anomaly for the upper 700 m, showed a sharp increase from coast (0.46 Wm−2) to ocean (1.59 Wm−2). The difference between coastal and ocean values was related to the presence of coastal upwelling, which partially inhibits the warming from surface of near shore water.

Effect of Minho estuarine plume on Rias Baixas: numerical modeling approach

ABSTRACT Sousa, M.C., Vaz, N., Alvarez, I. and Dias, J.M., 2013. Effect of Minho estuarine plume on Rias Baixas: numerical modeling approach. The Minho River, situated 30 km south from the Rias Baixas, is the most important freshwater source flowing into the Western Galician coast. This discharge is particularly important in driving the circulation and hydrography of this coastal region. To study this important issue, numerical modeling may constitute an important tool being used to understand the coastal plume effects under different conditions. The main purpose of this study is to implement and validate a marine model able to reproduce the propagation of the Minho estuarine plume. The chosen period for the validation was the spring of 1998, because a high Minho River discharge was reported as well as favorable wind patterns to spread the river plume towards the Rias Baixas. The numerical model MOHID was used through a downscaling approach with a three-level one-way nested scheme. The numerical predictions show good agreement with the observed water level in the entire domain. Also, the measured components of the velocity are well represented by the model, as well as the observed pycnocline, which is predicted for the observed depth. According to the model results, a buoyancy intrusion caused by the Minho river reverses the normal estuarine salinity longitudinal gradient of the Rias de Vigo and Pontevedra. Otherwise, this pattern is not observed in the Ria de Arousa. All these patterns are corroborated by in situ measurements. In summary, the validation results show that the model adequately reproduces the hydrodynamic and thermohaline patterns of the Western Galician coast.

Unusual Circulation Patterns of the Rias Baixas Induced by Minho Freshwater Intrusion (NW of the Iberian Peninsula)

The Minho River, situated 30 km south of the Rias Baixas, is the most important freshwater source flowing into the Western Galician coast (NW of the Iberian Peninsula). The buoyancy generated by the Minho estuarine plume can reverse the normal circulation pattern inside the Rias Baixas affecting the exchange between the Rias and the ocean, changing the input of nutrients. Nevertheless, this inversion of the circulation patterns is not a well-monitored phenomenon. The only published results based on in situ data related to the presence of the Minho River plume inside the Rias de Vigo and Pontevedra correspond to an event measured on spring 1998. In this case unexpectedly higher inflow surface current velocities were found at the Ria de Pontevedra, located further away from Minho River. Thus, the main aim of this study is to research the main factors inducing this unusual pattern on the circulation of the Rias de Vigo and Pontevedra. A numerical model implementation of MOHID previously developed, calibrated, and validated for this coastal area was used. Several scenarios were performed in order to explain the individual effect of the Minho River, rivers discharging into each Rias, and estuarine morphology changes. According to the model results, the Minho River discharge is a key factor in the establishment of the negative circulation, while small rivers inside the Rias slightly attenuate this circulation. The negative circulation was stronger in Ria de Pontevedra independently of the distance of this coastal system from the Minho River mouth, showing that morphologic estuarine features are the main factor justifying the different local circulation patterns.

Why coastal upwelling is expected to increase along the western Iberian Peninsula over the next century

Former studies about coastal upwelling along the Western Iberian Peninsula (WIP) using historical data indicated contradictory results, showing either its strengthening or reduction, while previous studies using Global Climate Models (GCMs) indicated that global warming is likely to intensify this phenomenon although predicting different rates and not justifying the patterns found. Taking advantage of the recent high spatial resolution Regional Climate Models (RCMs) projections from EURO-CORDEX project (Representative Concentration Pathway, RCP 8.5), detailed higher accuracy estimations of the spatio-temporal trends of Upwelling Index (UI) along the WIP coast were performed in this study, integrating the coastal mesoscale effects within the framework of climate change. Additionally, this research brings new insights about the origin of the WIP coastal upwelling intensification over the next century. These new projections clarified the upwelling strengthening rates predicted along the coast of the WIP from 2006 to 2099 revealing more prominent changes in the northern limit of the region (25-30m3s-1km-1 per decade between 41.5 and 42.5°N). Trends observed at high latitudes of the region were found to be induced by the displacement of the Azores High, which will intensify (0.03hPa per decade) and drift northeastward (10km per decade) during the 21st century.

The role of stochastic forcing on the behavior of thermohaline circulation.

The nonlinear nature of the climate system suggests that its reactions to unexpected perturbations could be different from the expected ones. In nonlinear science it is recognized as a promising paradigm that stochastic fluctuations can generate order or other counterintuitive effects. Thus, noise sources, adequately coupled to a nonlinear system, may give rise to a rich new phenomenology not present in a deterministic noiseless scenario. In this review we focus attention on thermohaline circulation (THC). THC presents two modes of operation; one state shows active THC and the other inactive. Previous episodes of transitions between both states of THC observed in paleoclimatic records and the influence of this circulation on climate have resulted in detailed investigations on the dynamics of the THC. A weakening or a collapse of this current could trigger the onset of a new Younger Dryas. In this review the introduction of stochastic forcing in key parameters, both in a simple box model and in an earth model of intermediate complexity, provokes a weakening and even a shutdown of the THC. The consequences of this weakening are observed in different variables. The surface air temperature and the sea surface temperature are dominated by cooling of the Northern Hemisphere. Changes in the position of the Intertropical Convergence Zone and in precipitation are observed. There is also an intensification of the North Atlantic Oscillation values during winter. These results reinforce the necessity to consider stochastic sources in climate models to improve our understanding of the climate.