Validación biogeoquímica de una simulación interanual del modelo acoplado ROMS-PISCES en el Pacífico Sudeste

Currently biogeochemical models are used to understand and quantify key biogeochemical processes in the ocean. The objective of the present study was to validate predictive ability of a regional configuration of the PISCES biogeochemical model on main biogeochemical variables in Humboldt Current Large Marine Ecosystem (HCLME). The statistical indicators used to evaluate the model were the bias, root-mean-square error, correlation coefficient and, graphically, the Taylor’s diagram. The results showed that the model reproduces the dynamics of the main biogeochemical variables (chlorophyll, dissolved oxygen and nutrients); in particular, the impact of El Niño 1997-1998 in the chlorophyll (decrease) and oxygen minimum zone depth (increase). However, it is necessary to carry out sensitivity studies of the PISCES model with different key parameters values to obtain a more accurate representation of the properties of the Ocean.Los modelos biogeoquímicos en la actualidad son utilizados para entender y cuantificar los principales procesos biogeoquímicos que suceden en el océano. El objetivo del presente estudio es validar estadísticamente la habilidad predictiva de una simulación del modelo biogeoquímico PISCES en reproducir la dinámica de las principales variables biogeoquímicas del Ecosistema de la Corriente de Humboldt (ECH). Para evaluar el modelo se utilizaron indicadores estadísticos: sesgo, error de la raíz del cuadrado medio, coeficiente de correlación y gráficamente el diagrama de Taylor. Los resultados muestran que el modelo es capaz de reproducir la dinámica de las principales variables biogeoquímicas (clorofila, oxígeno disuelto y nutriente), captando bien el impacto que tiene El Niño 1997-1998 en la clorofila (disminución) y profundidad de la zona mínima de oxígeno (incremento). Es necesario llevar a cabo estudios de sensibilidad del modelo PISCES usando diferentes valores de los principales parámetrospara obtener una mejor representación de las propiedades biogeoquímicas del océano

Oxygen Variability During ENSO in the Tropical South Eastern Pacific

The Oxygen Minimum Zone (OMZ) of the Tropical South Eastern Pacific (TSEP) is one of the most intensely deoxygenated water masses of the global ocean. It is strongly affected at interannual time scales by El Niño (EN) and La Niña (LN) due to its proximity to the equatorial Pacific. In this work, the physical and biogeochemical processes associated with the subsurface oxygen variability during EN and LN in the period 1958–2008 were studied using a regional coupled physical-biogeochemical model and in situ observations. The passage of intense remotely forced coastal trapped waves caused a strong deepening (shoaling) of the OMZ upper limit during EN (LN). A close correlation between the OMZ upper limit and thermocline depths was found close to the coast, highlighting the role of physical processes. The subsurface waters over the shelf and slope off central Peru had different origins depending on ENSO conditions. Offshore of the upwelling region (near 88°W), negative and positive oxygen subsurface anomalies were caused by Equatorial zonal circulation changes during LN and EN, respectively. The altered properties were then transported to the shelf and slope (above 200 m) by the Peru-Chile undercurrent. The source of nearshore oxygenated waters was located at 3°S−4°S during neutral periods, further north (1°S−1°N) during EN and further south (4°S−5°S) during LN. The offshore deeper (<200–300 m) OMZ was ventilated by waters originating from ~8°S during EN and LN. Enhanced mesoscale variability during EN also impacted OMZ ventilation through horizontal and vertical eddy fluxes. The vertical eddy flux decreased due to the reduced vertical gradient of oxygen in the surface layer, whereas horizontal eddy fluxes injected more oxygen into the OMZ through its meridional boundaries. In subsurface layers, remineralization of organic matter, the main biogeochemical sink of oxygen, was higher during EN than during LN due to oxygenation of the surface layer. Sensitivity experiments highlighted the larger impact of equatorial remote forcing with respect to local wind forcing during EN and LN

Competition between baroclinic instability and Ekman transport under varying buoyancy forcings in upwelling systems: An idealized analog to the Southern Ocean

International audienceCoastal upwelling rates are classically determined by the intensity of the upper-ocean offshore Ekman transport. But (sub)mesoscale turbulence modulates offshore transport, hence the net upwelling rate. Eddy effects generally oppose the Ekman circulation, resulting in so-called “eddy cancellation,” a process well studied in the Southern Ocean. Here we investigate how air–sea heat/buoyancy fluxes modulate eddy cancellation in an idealized upwelling model. We run CROCO simulations with constant winds but varying heat fluxes with and without submesoscale-rich turbulence. Eddy cancellation is consistently evaluated with three different methods that all account for the quasi-isopycnal nature of ocean circulation away from the surface. For zero heat fluxes the release of available potential energy by baroclinic instabilities is strongest and leads, near the coast, to nearly full cancellation of the Ekman cross-shore circulation by eddy effects, i.e., zero net mean upwelling flow. With increasing heat fluxes eddy cancellation is reduced and the transverse flow progressively approaches the classical Ekman circulation. Sensitivity of the eddy circulation to synoptic changes in air–sea heat fluxes is felt down to 125-m depth despite short experiments of tens of days. Mesoscale dynamics dominate the cancellation effect in our simulations which might also hold for the real ocean as the relevant processes act below the surface boundary layer. Although the idealized setting overemphasizes the role of eddies and thus studies with more realistic settings should follow, our findings have important implications for the overall understanding of upwelling system dynamics

Seasonal and intraseasonal surface chlorophyll-a variability along the northwest African coast

International audienceFive years of SeaWiFS ocean color data are used to characterize the variability of surface chlorophyll (SCHL) over seasonal and intraseasonal timescales along the northwest African coast (between 10°N and 33°N). This variability is interpreted in regards of remotely sensed wind stress and sea surface temperature and of climatological surface nitrate concentration. Three regions with fairly different behaviors are identified: the region of the subtropical gyre (24°N-33°N) is characterized by a weak seasonality and chlorophyll confined at the coast. The inter-gyre region off Cape Blanc (19°N-24°N) is characterized by a weak seasonality and a persistent large offshore extension of chlorophyll. The region of the recirculation gyre (10°N-19°N) is characterized by a strong seasonality and a large offshore extension of chlorophyll from February to May followed by an abrupt chlorophyll drop that propagates northward from May to June. The seasonal variability is well explained by the seasonal variability in wind-forcing. Nutrient limitation is the key factor that explains the weak offshore extension of chlorophyll in the North. The chlorophyll drop in the South is attributed to the weakening of the wind-forcing and the simultaneous advection of warm water from the South by a coastal and seasonal branch of the North Equatorial Counter Current (NECC). Intraseasonal variability is present in all regions. The cases of good correlation between the intraseasonal variability of the chlorophyll and of the wind-forcing are found to be associated with weak chlorophyll offshore extension and large nutrient limitation

Impact of eddy-driven vertical fluxes on phytoplankton abundance in the euphotic layer

International audienceThis paper shows with a simple analytical model, that phytoplankton concentration decreases in response to sub-mesoscale vertical circulation only in regions of strong permanent upwelling

Upper ocean subduction in an idealized eastern boundary upwelling model

International audienceIn the traditional view the newly upwelled water is transported offshore in the surface layer via Ekman transport in upwelling systems. However eddies and filaments are responsible for major spatio-temporal modulations of the transport of productive waters offshore. In fact a fraction of recently upwelled waters undergo expeditious frontal subduction and return back into the thermocline. Here we study this subduction process in an idealized upwelling model using CROCO (Coastal and Regional Ocean COmmunity model, www.croco-ocean.org). The horizontal resolution ranges from mesoscale resolving (8 km) to submesoscale rich (200 m). A simple tracer model used to study the subduction of phytoplancton like tracer. The model ability to produce the submesoscale turbulence typical of eastern boundary systems is highly sensitive to resolution but our lowest resolution configurations are able to generate substantial subduction

Seasonal and intraseasonal surface chlorophyll-a variability along the northwest African coast - art. no. C05007

Five years of SeaWiFS ocean color data are used to characterize the variability of surface chlorophyll (SCHL) over seasonal and intraseasonal timescales along the northwest African coast (between 10 degrees N and 33 degrees N). This variability is interpreted in regards of remotely sensed wind stress and sea surface temperature and of climatological surface nitrate concentration. Three regions with fairly different behaviors are identified: the region of the subtropical gyre (24 degrees N-33 degrees N) is characterized by a weak seasonality and chlorophyll confined at the coast. The inter-gyre region off Cape Blanc (19 degrees N-24 degrees N) is characterized by a weak seasonality and a persistent large offshore extension of chlorophyll. The region of the recirculation gyre (10 degrees N-19 degrees N) is characterized by a strong seasonality and a large offshore extension of chlorophyll from February to May followed by an abrupt chlorophyll drop that propagates northward from May to June. The seasonal variability is well explained by the seasonal variability in wind-forcing. Nutrient limitation is the key factor that explains the weak offshore extension of chlorophyll in the North. The chlorophyll drop in the South is attributed to the weakening of the wind-forcing and the simultaneous advection of warm water from the South by a coastal and seasonal branch of the North Equatorial Counter Current (NECC). Intraseasonal variability is present in all regions. The cases of good correlation between the intraseasonal variability of the chlorophyll and of the wind-forcing are found to be associated with weak chlorophyll offshore extension and large nutrient limitation

ON THE CONTRIBUTIONS OF EKMAN TRANSPORT AND PUMPING TO THE DYNAMICS OF COASTAL UPWELLING IN THE SOUTH-EAST PACIFIC

We investigate the oceanic circulation in the South-East Pacific (SEP) and Peru-Chile coastal upwelling, using satellite data and a medium resolution, eddy permitting regional model. The data consist in ERS 1 and 2 derived winds and AVHRR SST. Ekman pumping and transport are first calculated from wind data and analysed. Results are compared to the estimate of the zonal extension of upwelling cells obtained from SST data. The time variability of these parameters confirms that the northern and southern parts of the region have different characteristics. Namely, in the northern part, the seasonal cycles of SST and Ekman pumping are not correlated whereas in the southern part, the Ekman transport, pumping and the zonal extension of SST fluctuations exhibit a large coherency. In particular, the maximum in zonal extension takes place during the upwelling favourable conditions (spring to early fall). In an intermediate region satellite derived wind data were not representative of coastal conditions as shown by data from land stations. These observations are compared to model outputs: relations between seasonal and interannual variability of the upwelling with model current variability are derive

Upper ocean subduction in an idealized eastern boundary upwelling model

International audienceIn the traditional view the newly upwelled water is transported offshore in the surface layer via Ekman transport in upwelling systems. However eddies and filaments are responsible for major spatio-temporal modulations of the transport of productive waters offshore. In fact a fraction of recently upwelled waters undergo expeditious frontal subduction and return back into the thermocline. Here we study this subduction process in an idealized upwelling model using CROCO (Coastal and Regional Ocean COmmunity model, www.croco-ocean.org). The horizontal resolution ranges from mesoscale resolving (8 km) to submesoscale rich (200 m). A simple tracer model used to study the subduction of phytoplancton like tracer. The model ability to produce the submesoscale turbulence typical of eastern boundary systems is highly sensitive to resolution but our lowest resolution configurations are able to generate substantial subduction