Assessing ocean-model sensitivity to wind forcing uncertainties

International audienceIn this paper, we assess the short-term forecast error of a mesoscale primitive-equation open-ocean model, induced by uncertainties in wind forcing. Statistics calculated from an ensemble of ocean states show that temperature forecast error is strongest at the top of the ensemble-mean thermocline, as a consequence of vertical displacement of the mixed-layer base around its ensemble mean. Horizontal pattern of the temperature error in the mixed-layer is mainly explained by horizontal advection and surface heat flux fluctuations. These two mechanisms and entrainment through the mixed-layer bottom are presented as the three processes responsible for thermal forecast error growth in the modeled upper ocean

Equatorial upper-ocean dynamics and their interaction with the West African monsoon

Zonal wind anomalies in the western equatorial Atlantic during late boreal winter to early summer precondition boreal summer cold/warm events in the eastern equatorial Atlantic (EEA) that manifest in a strong interannual Atlantic cold tongue (ACT) variability. Local intraseasonal wind fluctuations, linked to the St. Helena anticyclone, contribute to the variability of cold tongue onset and strength, particularly during years with preconditioned shallow thermoclines. The impact of cold tongue sea surface temperature (SST) anomalies on the wind field in the Gulf of Guinea is assessed. It contributes to the northward migration of humidity and convection and possibly the West African monsoon (WAM) jump. Copyright @ 2010 Royal Meteorological Societ

Amazon Plume Salinity Response to Ocean Teleconnections

Pacific and Atlantic sea surface temperature (SST) variability strongly influences rainfall changes in the Amazon River basin, which impacts on the river discharge and consequently the sea surface salinity (SSS) in the Amazon plume. An Empirical Orthogonal Function (EOF) analysis was performed using 46 years of SST, rainfall, and SSS datasets, in order to establish the relationship between these variables. The first three modes of SST/rainfall explained 87.83% of the total covariance. Pacific and Atlantic SSTs led Amazon basin rainfall events by 4 months. The resultant SSS in the western tropical North Atlantic (WTNA) lagged behind basin rainfall by 3 months, with 75.04% of the total covariance corresponding to the first four EOF modes. The first EOF mode indicated a strong SSS pattern along the coast that was connected to negative rainfall anomalies covering the Amazon basin, linked to El Niño events. A second pattern also presented positive SSS anomalies, when the rainfall was predominantly over the northwestern part of the Amazon basin, with low rainfall around the Amazon River mouth. The pattern with negative SSS anomalies in the WTNA was associated with the fourth mode, when positive rainfall anomalies were concentrated in the northwest part of South America. The spatial rainfall structure of this fourth mode was associated with the spatial rainfall distribution found in the third EOF mode of SST vs. rainfall, which was a response to La Niña Modoki events. A statistical analysis for the 46 year period and monthly anomaly composites for 2008 and 2009 indicated that La Niña Modoki events can be used for the prediction of low SSS patterns in the WNTA

Role of the spatial resolution on the oceanic subduction rate

International audienceSubduction is the process by which water masses irreversibly escape from the mixed layer in which they were in contact with the atmosphere, into the main thermocline. Diagnosing the annual subduction rates plays a key role in numerical models because the rate of deep water formation largely depends of the properties and realism of the simulated top mixed layers. In this study, annual subduction rates are evaluated with the kinematically defined instantaneous rate method of Valdivieso and al. (2004) and applied to academic simulations of the North Atlantic basin which only differ by their spatial resolution (respectively 1°, 1/3°, 1/9°, 1/27° and 1/54°). The comparison of the simulations allows to evaluate the role of the resolution on subduction mechanisms. A second kind of comparison is provided with simulations obtained from the degradations of the stronger resolution simulation (1/54°). This second kind of comparisons allows to directly evaluate the role of the resolution on subduction rate diagnostics

Advanced insights into sources of vertical velocity in the ocean

Estimating vertical velocity in the oceanic upper layers is a key issue for understanding ocean dynamics and the transport of biogeochemical elements. This paper aims to identify the physical sources of vertical velocity associated with sub-mesoscale dynamics (fronts, eddies) and mixed-layer depth (MLD) structures, using (a) an ocean adaptation of the generalized Q-vector form of the omega-equation deduced from a primitive equation system which takes into account the turbulent buoyancy and momentum fluxes and (b) an application of this diagnostic method for an ocean simulation of the Programme Ocean Multidisciplinaire Meso Echelle (POMME) field experiment in the North-Eastern Atlantic. The approach indicates that w-sources can play a significant role in the ocean dynamics and strongly depend on the dynamical structure (anticyclonic eddy, front, MLD, etc.). Our results stress the important contribution of the ageostrophic forcing, even under quasi-geostrophic conditions. The turbulent w-forcing was split into two components associated with the spatial variability of (a) the buoyancy and momentum (Ekman pumping) surface fluxes and (b) the MLD. Process (b) represents the trapping of the buoyancy and momentum surface energy into the MLD structure and is identified as an atmosphere/oceanic mixed-layer coupling. The momentum-trapping process is 10 to 100 times stronger than the Ekman pumping and is at least 1,000 times stronger than the buoyancy w-sources. When this decomposition is applied to a filamentary mixed-layer structure simulated during the POMME experiment, we find that the associated vertical velocity is created by trapping the surface wind-stress energy into this structure and not by Ekman pumping

Role of the spatial resolution on the oceanic subduction rate

International audienceSubduction is the process by which water masses irreversibly escape from the mixed layer in which they were in contact with the atmosphere, into the main thermocline. Diagnosing the annual subduction rates plays a key role in numerical models because the rate of deep water formation largely depends of the properties and realism of the simulated top mixed layers. In this study, annual subduction rates are evaluated with the kinematically defined instantaneous rate method of Valdivieso and al. (2004) and applied to academic simulations of the North Atlantic basin which only differ by their spatial resolution (respectively 1°, 1/3°, 1/9°, 1/27° and 1/54°). The comparison of the simulations allows to evaluate the role of the resolution on subduction mechanisms. A second kind of comparison is provided with simulations obtained from the degradations of the stronger resolution simulation (1/54°). This second kind of comparisons allows to directly evaluate the role of the resolution on subduction rate diagnostics

Role of the spatial resolution on the oceanic subduction rate

International audienceSubduction is the process by which water masses irreversibly escape from the mixed layer in which they were in contact with the atmosphere, into the main thermocline. Diagnosing the annual subduction rates plays a key role in numerical models because the rate of deep water formation largely depends of the properties and realism of the simulated top mixed layers. In this study, annual subduction rates are evaluated with the kinematically defined instantaneous rate method of Valdivieso and al. (2004) and applied to academic simulations of the North Atlantic basin which only differ by their spatial resolution (respectively 1°, 1/3°, 1/9°, 1/27° and 1/54°). The comparison of the simulations allows to evaluate the role of the resolution on subduction mechanisms. A second kind of comparison is provided with simulations obtained from the degradations of the stronger resolution simulation (1/54°). This second kind of comparisons allows to directly evaluate the role of the resolution on subduction rate diagnostics

Variabilité de la température de la couche de mélange océanique en Atlantique équatorial aux échelles saisonnières à interannuelles, à l'aide de simulations numériques

TOULOUSE3-BU Sciences (315552104) / SudocTOULOUSE-Observ. Midi Pyréné (315552299) / SudocSudocFranceF

Argo float observations of basin-scale deep convection in the Irminger sea during winter 2011-2012

Analysis of Argo data obtained during winter 2011–2012 revealed the presence over the Irminger Basin of an exceptionally large number of profiles (41) with mixed layer depths (MLD) exceeding 700 m, which was deep enough to reach the pool of the intermediate Labrador Sea Water located in the Irminger Sea. Four of these profiles exhibited an MLD of 1000 m, which was the maximum value observed for the winter in question. The Argo sampling in the Irminger Sea during that winter, which was 3 to 4 times greater than for the preceding winters, enabled the different phases of the mixed layer deepening down to 1000 m, together with their spatial extents, to be observed for the first time. Two intense convective periods occurred: in late January south of Cape Farewell and in late February-early March east of Greenland. A final deepening period was observed in mid-March, during which the deepest mixed layers were observed. This long deepening period occurred in large regional areas and was followed by a rapid restratification phase. The temporal evolution of oxygen profiles from one Argo float testifies to the local and rapid ventilation of the mixed layer by the deep convection. A mixed layer heat budget along the trajectories of the 4 floats that sampled the deepest mixed layers showed that heat loss at the air-sea interface was mainly responsible for heat content variations in the mixed layer. Greenland Tip Jets were of primary importance for the development of deep convection in the Irminger Sea in the winter of 2011–2012. They enhanced the winter heat loss and two long (more than 24 hours), intense late events close together in time pushed the mixed layer deepening down to 1000 m. Net air-sea fluxes, the number of Greenland Tip Jets, the stratification of the water column, the NAO index and the Ekman-induced heat flux are pertinent indicators to assess conditions that are favorable for the development of deep convection in the Irminger Sea. By considering each of those indicators, it was concluded that the 2011-2012 event was not significantly different from the three other documented occurrences of deep convection in the Irminger Sea