Detection of Moroccan coastal upwelling using sea surface chlorophyll concentration

International audienceThe aim of this work is to automatically identify and extract the upwelling area in the coastal ocean of Morocco using the satellite observation of chlorophyll concentration. The algorithm starts by the application of FCM algorithm for the purpose of finding regions of homogeneous concentration of the chlorophyll, resulting in c-partitioned labeled images. A region­ growing algorithm is then used to filter out the noisy structures in the offshore waters not belonging to the upwelling regions. The proposed methodology has been validated by an oceanographer and tested over a database of 166 weekly Sea Surface chlorophyll data. The region of interst cover the southern part of Moroccan atlantic coast spanning from the years 2007 to 2012

Physical–biological interactions in the life history of small pelagic fish in the Western Iberia Upwelling Ecosystem.

This paper reviews relevant investigations conducted in the Western Iberia Upwelling Ecosystem (WIUE) on the dynamics of small pelagic fish (SPF), its relationship to climate changes and processes related to the ‘ocean triad’ (enrichment, concentration, transport/retention). In the last decades, a decline in the productivity of several SPF species (e.g., sardine and horse mackerel) was observed in the WIUE, which is partially explained by environmental variability (e.g., changes in coastal upwelling). The main mechanism proposed to explain this decline is the increased frequency and intensity of upwelling events during the spawning season (winter) of these species, which is typically a period when convergence conditions prevail. Thus, this ‘anomalous’ situation promotes egg and larval dispersal away from the favourable coastal habitat (the shelf) and consequently has a negative impact on their survival and recruitment. However, the variability of local features like the Western Iberia Buoyant Plume (WIBP) and the Iberian Poleward Current (IPC) introduce important fluctuations in the transport patterns of the region, and could modulate the impact of these winter upwelling events in the survival of larvae. The retention on the shelf of larval sardine along convergence zones formed by the interaction of these two features plays an important role in their survival. Furthermore, the WIBP is a suitable environment for the growth of phytoplankton and for larval fish survival. From these findings it is clear that simplified Ekman transport models could not explain larval fish transport/retention patterns in this region and more realistic biophysical models should be used to simulate the local oceanographic conditions to understand larval dynamics and the success of recruitment of SPF in the Western Iberia

Temporal Variability of Ocean Colour Derived Products in the European Seas

The ten-year record of ocean colour data provided by the SeaWiFS mission is an important asset for monitoring and research activities conducted on the optically-complex European seas. This study mainly makes use of the SeaWiFS data set of normalized water leaving radiances LWN to study the major characteristics of temporal variability associated with optical properties across the entire European domain. Specifically, the time series of LWN, band ratios, diffuse attenuation coefficient Kd(490) and concentration of chlorophyll a Chla are decomposed into terms representing a fixed seasonal cycle, irregular variations and trends, and the contribution of these components to the total variance is described for the various basins. The diversity of the European waters is fully reflected by the range of results varying with regions and wavelengths. Generally, the Mediterranean and Baltic seas appear as two end-members with, respectively, high and low contributions of the seasonal component to the total variance. The existence of linear trends affecting the satellite products is also explored for each basin. The interpretation of the trends observed for LWN and band ratios is not straightforward, but it circumvents the limitations resulting from the levels of uncertainty, very variable in space and often high, that characterize derived products such as Chla in European waters. Results for Kd(490) and Chla are also analyzed. Statistically significant, and in some cases large, trends are detected in the Atlantic Ocean west of the European western shelf, the central North Sea, the English Channel, the Black Sea, the northern Adriatic, and various regions of the Mediterranean Sea and the northern Baltic Sea, revealing changes in the concentrations of optically significant constituents in these regions.JRC.H.3-Global environement monitorin

Nitrous oxide and methane in the Atlantic Ocean between 50 degrees North and 52 degrees South: Latitudinal distribution and sea-to-air flux

We discuss nitrous oxide (N2O) and methane (CH4) distributions in 49 vertical profiles covering the upper 300 m of the water column along two 13,500 km transects between 50°N and 52°S during the Atlantic Meridional Transect (AMT) programme (AMT cruises 12 and 13). Vertical N2O profiles were amenable to analysis on the basis of common features coincident with Longhurst provinces. In contrast, CH4 showed no such pattern. The most striking feature of the latitudinal depth distributions was a well-defined “plume” of exceptionally high N2O concentrations coincident with very low levels of CH4, located between 23.5°N and 23.5°S; this feature reflects the upwelling of deep waters containing N2O derived from nitrification, as identified by an analysis of N2O, apparent oxygen utilization (AOU) and NO3-, and presumably depleted in CH4 by bacterial oxidation. Sea-to-air emissions fluxes for a region equivalent to 42% of the Atlantic Ocean surface area were in the range 0.40–0.68 Tg N2O yr-1 and 0.81–1.43 Tg CH4 yr-1. Based on contemporary estimates of the global ocean source strengths of atmospheric N2O and CH4, the Atlantic Ocean could account for 6–15% and 4–13%, respectively, of these source totals. Given that the Atlantic Ocean accounts for around 20% of the global ocean surface, on unit area basis it appears that the Atlantic may be a slightly weaker source of atmospheric N2O than other ocean regions but it could make a somewhat larger contribution to marine-derived atmospheric CH4 than previously thought

Hydrographic conditions of a continental shelf region in the Northwest African Upwelling System from in-situ data

This study investigates the hydrographic and dynamic properties of the continental shelf region between Cape Juby (28.5 °N) and Agadir (30.4 °N) within the Moroccan Sub-region of the North West African Upwelling System. Data came from two cruises conducted in June (beginning of summer) and November (end of autumn) 2013. Coastal upwelling was obvious in both cruises in the in-situ temperature and salinity data as well as in remotely sensed sea surface temperature maps. ADCP data showed the presence of a strong jet like current associated with enhanced upwelling off Cape Juby. This strong quasi-permanent upwelling center was observed during both cruises. It results from the orientation of this portion of coast which is aligned with the dominant wind direction, as well as a wind intensification near the cape. The presence of a secondary upwelling front was also observed near the shelf break. It was accompanied by an intense baroclinic jet. The EK60 data showed evidence of internal waves as well as small and mesoscale turbulence that were probably strongly interacting with the mean upwelling circulation and made a straightforward interpretation of the data quite challenging. However, this study revealed the main physical processes of this poorly studied region, as well as their seasonal variability

Harmful Algal Blooms (HAB) in a changing world : the case of S and W Iberian Bays

This thesis aimed to investigate, in two wide-open sheltered bays (Lisbon and Lagos) influenced by upwelling, how the meteorological and oceanographic (MetOc) setting may affect phytoplankton communities. Results of a 9-year time series data showed a high interannual variability of phytoplankton biomass, estimated as chlorophyll a (Chl-a). Nevertheless, the Chl-a sinusoidal model showed different temporal variability patterns in each bay: a uni-modal pattern with a short peak and low Chl-a concentrations in Lagos, and a weak bi-modal pattern with a long period of high Chl-a concentrations in Lisbon. Cross-correlation analyses performed for Chl-a and different MetOc variables indicated that PAR contributed most to Chl-a in winter/early-spring, while upwelling and SST were the main drivers in late-spring/summer. Analysis performed during 1-year showed significant spatial differences in phytoplankton assemblages between the bays. On a temporal scale, significant differences were observed on phytoplankton communities in both bays in the 4-meteorological seasons. However, results from a nearshore station studied in Lagos only indicated the occurrence of 3-biological seasons, with no significant differences between summer and autumn communities. This study suggests that Lagos region has a higher probability for the occurrence of HABs (in higher cell concentrations and persistence). The ecology of the benthic genus Ostreopsis was studied based on 7-years of water samples. Two species were identified reaching maximum cell densities in late-summer/early-autumn: Ostreopsis cf. ovata restricted to the south coast and Ostreopsis cf. siamensis present in both Portuguese coasts. Ostreopsis was much more abundant in Lagos (nearshore) and maxima concentrations were related to positive SST anomalies. High densities in the plankton were often recorded after a period of more than 2-weeks of low sea state, followed by short-time events of onshore wind and moderate waves. In Lisbon, O. cf. siamensis was seldom recorded in the plankton and no clear relationship could be established with the studied MetOc drivers. The recent records of Ostreopsis in this bay are interpreted as an early colonization stage of an invasion process. The present work highlights the relevance of the peculiarities of regional setting in determining phytoplankton dynamics in wide-open coastal bays influenced by upwelling, even at short latitudinal distance.Instituto Português do Mar e da Atmosfera (MAR2020-P02M01-1490P)Centro de Ciências do Mar e do Ambiente da Faculdade de Ciências da Universidade de Lisboa (UIDB/04292/2020)Centro de Ciências do Mar da Universidade do Algarve (UID/Multi/04326/2020

Nutrient mineralization patterns in shelf waters of the Western Iberian upwelling

24 páginas, 10 figuras, 2 tablasA set of hydrographicdata collectedduring severalcruisesto theNWIberian upwelling systemhas been used to assess the patterns of nutrient mineralizationover the shelf. Mineralized nutrient ratios (N/P and N/Si)in the lowerwatercohrmnare clearlybelowthe ratios characteristicof freshly upwelled oceanic waters. Nitrogen mineralization, in comparison to phosphorus mineralization, is incomplete,in agreementwith phosphate’sfaster regenerationrate and the low residence time of water, which precludes complete mineralization. Silicate shows the highest enrichmentas a consequenceof the selectiveconcentrationof particulate biogenicsilica in the near bottom waters. Diffusionof nutrients mineralizedin the sedimentsseemsto representan important contribution.TheIowestmineralizationof the surveyedarea is foundto the north of CapeFinisterre, especiallywith regard to silicate.To the South, the Rias Baixas(four large embayments)return as reducingparticulate organicmatter (POM)a smallfraction of the upwellednutrientsimportedfrom the shelf. However,the out-flowingfreshwatercontributesto stratificationof coastal waters, which favoursPOMproduction,accumulationand sedimentation.In addition,highlevelsofnewdissolved organic matter (DOM) in the outflowcan also enhanceprimary productionin shelfsurface waters. Finally, highest mineralization was found to the south of the River Miiio, where there are no embayments and the shelf is wider. Mineralization tends to: (1) increase the potential primary productionof thisecosystembyupto 50%,(nitrogenIimitation)orup to 80%(siliconlimitation);(2) favour the developmentof phytoplanktonassemblagesdominatedbydiatoms(Sienrichmentdouble that ofN and P); and (3)bufferthe largespatial and temporaldifferencesintroducedbythe advected nutrientsSupport for this work came from EU projects contract No. MAST-CT90-0017and MAS2- CT93-006Peer reviewe

A Simple Tool for Automatic Extraction of Moroccan Coastal Upwelling from Sea Surface Temperature Images

International audienceThis work aims at automatically identify and ex- tract the region covered by the upwelling waters in the costal ocean of Morocco using the well known region-growing segmen- tation algorithm. The later consists in coarse segmentation of upwelling area which characterized by cold and usually nutrient- rich water near the coast. The complete system has been validated by an oceanographer over a database of 30 Sea Surface Tem- perature (SST) satellite images of the year 2007 obtained from Advanced Very High Resolution Radiometer (AVHRR) sensor onboard NOAA-18 satellite serie, demonstrating its capability and effectiveness to reproduce the shape of upwelling area

Detection of Moroccan Coastal Upwelling Fronts in SST Images using the Microcanonical Multiscale Formalism

International audienceNonlinear signal processing using the Microcanonical Multiscale Formalism (MMF) is used to the problem of detecting and extracting the upwelling fronts in coastal region of Morocco using Sea Sur- face Temperature (SST) satellite images. The algorithm makes use of the Singularity Exponents (SE), computed in a microcanonical framework, to detect and analyze the critical transitions in oceano- graphic satellite data. The objective of the proposed study is to develop a helpful preprocessor that transforms SST images into clean and simple line drawing of upwelling fronts as an input to a subse- quent step in the analysis of SST images of the ocean. The method is validated by an oceanographer and it is shown to be superior to that of an automatic algorithm commonly used to locate edges in satellite oceanographic images. The proposed approach is applied over a collection of 92 SST images, covering the southern Moroccan Atlantic coast of the years 2006 and 2007. The results indicate that the approach is promising and reliable for a wide variety of oceanographic conditions

Surface mixing and biological activity in the North-West African upwelling

International audienceNear-shore water along the NorthWest African margin is one of the world's major upwelling regions. It is associated with physical structures of oceanic fronts which influence the biological productivity. The study of these coherent structures in connection with chlorophyll concentration data is of fundamental importance for understanding the spatial distributions of the plankton. In this work, we study the horizontal stirring and mixing in different upwelling areas using Lagrangian coherent structures (LCSs). These LCSs are calculated using the recent geodesic theory of LCSs. We use these LCSs to study the link between the chlorophyll fronts concentrations and surface mixing, based on 10 years of satellite data. These LCSs move with the flow as material lines, thus the horizontal mixing is calculated from the intersection of these LCSs with the finite time Lyapunov exponents (FTLEs) maps. We compare our results with those of a recent study conducted over the same area, but based on Finite Size Lyapunov Exponents (FSLEs) whose output is a plot of scalar distributions. We discuss the differences between FSLE and geodesic theory of LCS. The latter yields analytical solutions of LCSs, while FSLEs can only provide LCSs for sharp enough ridges of nearly constant height