Changes in storminess and surges in western France during the last century

French meteorological data provide some information on wind direction since 1890. Since 1951, wind speed and direction have been digitised every three hours in many stations. The Brest tide gauge has recorded hourly sea levels since 1860, thus enabling hourly surges to be calculated and studied. This set of data has been organised into a data base and analysed in order to recognize evolution and trends. Strong winds are increasing in frequency in the western part of Brittany and decreasing in Normandy and Pays de Loire. Surges associated with these winds do present a slight decreasing trend. A more precise analysis allows to distinguish between southerly winds, which are slightly decreasing, and strong winds from other directions, which display a more or less important increase in frequency. The analysis shows trends that may be linked to the global circulation pattern, and that result in a new spatial distribution of winds on western France.     

Matiere en suspension et phosphore en particules dans l'atlantique tropical sud le long de 4°W en Novembre 1971

Particles and particulate phosphorus (Pp) were studied along 4°W in the tropical south Atlantic, between Ivory Coast and 24°S. Seston concentrations in the upper layers (0–150 m) are about 0.350 mg/l; they decrease in the intermediate layers (0.252 mg/l between 150 and 500 m) and lower layers (0.215 mg/l below 500 m). Along 4°W it is possible to define some geographical zones; thus, the South equatorial counter current waters are shown to be characterized by very weak concentrations of particulate phosphorus (< 0.020 μg-at/l). Ratios between particulate phosphorus, suspended matter and chlorophyll a have been computed: Part/Pp is about 2; the relationship Chl-a-Pp is linear for Chl-a concentrations lower than 1.5 μg/l, increasing exponentially for Chl-a values increasing from 1.5 μg/l to 35 μg/l.

Résumé

L'étude de la répartition du seston et du phosphore particulaire (Pp) a été faite sur la radiale 4°W, dans l'Atlantique tropical, entre la Côte d'Ivoire et 24°S. Les concentrations de seston dans la couche superficielle (0–150 m) sont voisines de 0,350 mg/l; elles décroissent dans les eaux intermédiaires (0,252 mg/l de 150 à 500 m) et profondes (0,215 mg/l de 500 à 700 m). Le long du méridien 4°W, le Pp permet de caractériser des zones géographiques; le contre-courant sud équatorial est ainsi mis en évidence par des eaux particulièrement pauvres en Pp (< 0,020 μatg/l). Des relations entre le phosphore particulaire, la matière en suspension, et la chlorophylle ont été établies: le rapport Part/Pp est voisin de 2; la relation Chl-a-Pp, linéaire pour les concentrations en Chl-a inférieures à 1,5 μg/l, devient exponentielle pour des valeurs de Chl-a supérieures allant de 1,5 μg/l à 35 μg/l.     

Micro- to mesoscale evolution of beaches in response to climatic shift: observation and conceptual modelling (Brittany,France)

In the framework of the European project 'Climate change and coastal evolution in Europe', the microscale (< 1 year) morphological evolution of beaches in Brittany (W France) has been monitored for some years and linked to immediate forcings, the first one being tides (with a 13-m tidal range), and the second storm intensity. For the same sites, a mesoscale (decades) evolution was reconstructed from air photographs and historical data. The evolution is forced first of all by human activities, and secondly by climatic shift: wind direction changes and average wind velocity evolution. These results were used to build and calibrate a model of coastal behaviour. The model is based on a simple sediment cell (input – transit – output) and simulates the movements of volumes of sands under successive morphodynamic processes. Each of these processes is modelled (in terms of volume exchanges) and parameterized by the relevant atmospheric element. Thus, the model is not deterministic but simulates different probabilities for atmospheric events and morphological response. The model runs for one year and produces realistic sediment accumulation values and coastal retreat rates. When run for half a century, results are also quite comparable to the field data. Storm frequency changes appear to be an important element for the evolution, but the main and first forcing is the availability of sediment.     

Analyse des rapports C/N/P du seston dans la partie orientale de l'atlantique equatorial

A study of the C/N ratios of particulate matter in the eastern part of the Gulf of Guinea demonstrated a statistically significant spatial variation in these ratios. The variation in the ratios ranged between 7.2 and 9.5 and were due in part to differences in the composition of the suspended organic matter and in part to its age and history. Young nutrient rich waters had low ratios (generally less than 7), whereas older waters often had ratios greater than 10. A relation between Chlorophyl a and the C/N ratio was also observed. The distribution of C/P ratios with depth showed a linear correlation with the vertical distribution of oxygen (r = 0.74). C/P values in the neighbourhood of 100 at the surface, increased with depth to around 300 at 1000 m.