P. De Mey

Interannual long equatorial waves in the tropical Atlantic from a high‐resolution ocean general circulation model experiment in 1981–2000

We investigate the tropical Atlantic vertical structure variability (1981-2000) based on the CLIPPER ocean general circulation model (OGCM). We aim at determining to what extent the observed interannual variability can be explained by the low-frequency wave dynamics. The linear vertical modes of the OGCM climatological stratification are estimated along the equator. The baroclinic mode contributions to surface zonal current and sea level anomalies are calculated and analyzed at interannual timescales. The second baroclinic mode is the most energetic. The first (third) mode exhibits a variability peak in the west (east). The summed-up contribution of the high-order baroclinic modes (4-6) is as energetic as the gravest modes and is largest in the east. Wave components are then derived by projection onto the associated meridional structures. The effect of longitudinal boundaries near the equator is taken into consideration. Equatorial Kelvin and Rossby waves propagations, with phases speed close to the theory, are identified for the first three baroclinic modes. The comparison with a multimode linear simulation corroborates the propagating properties of the OGCM waves coefficients. An estimation of the meridional boundary reflection efficiency indicates that wave reflections take place at both boundaries. A 65% reflection efficiency is found at the eastern boundary. Our study suggests that low-frequency wave dynamics is to a large extent at work in the tropical Atlantic. On the basis of what is known on the Pacific El Nino-Southern Oscillation mode this may provide a guidance for investigating ocean-atmosphere mechanisms that can lead to the Atlantic zonal equatorial mode.

The eddy field associated with the Azores Front east of the Mid‐Atlantic Ridge as observed by the Geosat altimeter

Two years of Geosat data (November 1986 to December 1988) for the Azores-Madeira region of the North Atlantic are analyzed. The objective is to study the eddy field associated with the terminal retroflection of the Azores current. The rms variability of sea level anomaly is 4 to 8 cm, with maximum values between 33-degrees and 35-degrees-N being associated with the Azores current and located on its northern flank. Higher energy is observed during winter and during the year November 1987 to November 1988. Analysis of velocities at crossovers reveals anisotropy with larger meridional velocity variances. This is the signature of meanders and/or eddies elongated in the north-south direction. The eddy-mean flow interaction is studied through the analysis of the Reynolds tensor. Meridional and zonal accelerations of the mean by the eddy field are consistent with retroflection of the Azores Current to the south and to the west. Spectral analyses show that the dominant signals have periods and wavelengths larger than 100 days and 300 km, respectively. Propagation velocities are westward but slow (1.5 km/d). Synoptic mapping of the eddy field shows the wavelike structure of the variability on the northern flank of the current. It is likely that the wave generation is not directly due to an instability of the Azores current but is attributable to a cause more to the east. Once formed, the wave interacts with the current in winter and forms meanders. Seasonal variations of the surface circulation are finally analyzed from the 2-year-long time series of synoptic maps. There is a clear seasonality in the surface circulation which generally appears to be consistent with historical in situ measurements. In winter there is a strong meandering and a southward branching, while in summer the mean front is narrower and penetrates further to the east.

Assessing the impact of observations on ocean forecasts and reanalyses: Part 2, Regional applications

The value of global (e.g. altimetry, satellite sea-surface temperature, Argo) and regional (e.g. radars, gliders, instrumented mammals, airborne profiles and biogeochemical) observation-types for monitoring the mesoscale ocean circulation and biogeochemistry is demonstrated using a suite of global and regional prediction systems and remotely-sensed data. A range of techniques is used to demonstrate the value of different observation-types to regional systems and the benefit of high-resolution and adaptive sampling for monitoring the mesoscale circulation. The techniques include Observing System Experiments, Observing System Simulation Experiments, adjoint sensitivities, representer matrix spectrum, observation footprints and spectral analysis. It is shown that local errors in global and basin-scale systems can be significantly reduced when assimilating observations from regional observing systems.

Adaptive filtering: application to satellite data assimilation in oceanography

Abstract A new approach to assimilation of observations is proposed, which extends previous work on adaptive Kalman filtering. In the latter, the gain matrix of the filter was progressively determined without a priori explicit specification of the covariance matrices of the model or observation noise, so as to minimize the norm of the innovation vector. The new step taken here is to introduce a (relatively) low dimension parameterization of the gain matrix, thereby substantially decreasing the numerical cost of the filter. The reduced-order adaptive filter (ROAF) thus defined is tested on a simple diffusive-reactive equation, and implemented on the four-layer adiabatic Miami isopycnical coordinate ocean model (MICOM). In the latter case, the filter is used to assimilate synthetic observations of surface height. Both sets of experiments clearly show the efficiency of the proposed approach, and its superiority, in terms of the quality of the results, on Newtonian relaxation. In the case of the diffusive-reactive equation, the reduced-order adaptive filter is also superior to, and more economical than, a reduced non-adaptative Kalman filter.

A new reduced-order adaptive filter for state estimation in high-dimensional systems

A simple reduced-order adaptive filter, optimal in the sense of minimum prediction error, is proposed for estimating the state of high-dimensional systems in which the process and observation noise statistics are unknown. It is shown that implementation of this adaptive filter requires the solution of only two linear difference equations, the dimensions of which are the dimensions of the full and reduced states, respectively, and that no solution of either an algebraic Riccati equation or a Lyapunov equation is needed. In addition, substantial gain in computer memory and CPU time is obtained by parametrization of the filter gain in the form of the product of two matrices, one of which is a prescribed projection from the reduced space onto the full space. A twin experiment on data assimilation with a quasi-geostrophic ocean model shows the efficiency of the proposed approach.

Coastal Ocean Forecasting: science foundation and user benefits

The advancement of Coastal Ocean Forecasting Systems (COFS) requires the support of continuous scientific progress addressing: (a) the primary mechanisms driving coastal circulation; (b) methods to achieve fully integrated coastal systems (observations and models), that are dynamically embedded in larger scale systems; and (c) methods to adequately represent air-sea and biophysical interactions. Issues of downscaling, data assimilation, atmosphere-wave-ocean couplings and ecosystem dynamics in the coastal ocean are discussed. These science topics are fundamental for successful COFS, which are connected to evolving downstream applications, dictated by the socioeconomic needs of rapidly increasing coastal populations.

Coastal Ocean Forecasting: system integration and evaluation

Recent advances in Coastal Ocean Forecasting Systems (COFS) are discussed. Emphasis is given to the integration of the observational and modeling components, each developed in the context of monitoring and forecasting in the coastal seas. These integrated systems must be linked to larger scale systems toward seamless data sets, nowcasts and forecasts (from the global ocean, through the continental shelf and to the nearshore regions). Emerging capabilities include: methods to optimize coastal/regional observational networks; and probabilistic approaches to address both science and applications related to COFS. International collaboration is essential to exchange best practices, achieve common frameworks and establish standards.

Isopycnal empirical orthogonal functions (EOFs) in the North and tropical Atlantic and their use in estimation problems

We investigated the vertical variability of the thermal/dynamical structure of the North and tropical Atlantic ocean from a set of historical hydrographic data [Reynaud et al., 1998]. The analysis was performed in 10° geographic bins in terms of vertical empirical orthogonal functions (EOFs) in isopycnal coordinates as first proposed by Gavart and De Mey [1997] (hereinafter referred to as GDM97). In the isopycnal coordinate system the state vector is made up of isopycnal displacements (η) and the compensated potential temperature along isopycnals (θ). GDM97 showed that in the Azores-Madeira region the isopycnal EOFs consistently help to better separate physical processes in the water column and to better project the dynamic height anomaly downward in comparison with the classical EOF analysis in depth coordinates. In the present study, we extended their work geographically and the connection with state estimation and data assimilation was more formalized. From our results, smooth spatial variations of isopycnal EOFs were found in the North Atlantic and confirmed the large-scale relevance of isopycnal EOFs in the ocean. North of 20°N the dominant isopycnal EOF was associated with a quasihomogeneous vertical displacement of the isopycnal surfaces (η ≃ 1) with a quasiconservation of compensated potential temperature on those surfaces (θ ≃ 0). That mode was strongly modified by thermocline-intensified baroclinic effects along the path of the Gulf Stream up to 40°W. Some physical interpretations of the vertical EOFs were also suggested in several areas as far as the isopycnal formulation was concerned (e.g., some thermal fronts, the Mediterranean outflow, water lenses, or meddies). This work confirmed the validity of the ideas of GDM97 over the entire North Atlantic ocean. The potential use of the isopycnal EOFs in state estimation problems was discussed. They are possible candidates to reduce the order of state estimation problems provided that they have good observability properties. These observability properties were studied in the case of satellite altimetry; that is, we examined how the dominant isopycnal EOFs were connected to sea-level anomalies. In most bins it was found that the order of an estimation problem could be truncated to one (primarily north of 20°N) or two modes. The dominant EOF was found to account for most of the surface dynamic height variability with a rather normally distributed error of only a few centimeters. Dynamically more robust results were also presented, performing a univariate isopycnal analysis in η, leaving coherent thermal effects out. As a result, the dominant character of mode 1 was further enhanced everywhere. These univariate isopycnal EOFs can be used for purely dynamical state estimation. More generally, we believe that our results can be used to set up reduced-order data assimilation schemes in the North Atlantic or in other regions with good historical data.

Characterisation of errors of a regional model of the Bay of Biscay in response to wind uncertainties: a first step toward a data assimilation system suitable for coastal sea domains

This study is aimed at exploring the errors of a regional model of the Bay of Biscay, a regional zoom of the IBI configuration of the ocean model NEMO, with the ultimate objective of guiding the choice and implementation of a data assimilation system in that region. An ensemble experiment was carried out by randomly perturbing winds along a base of EOFs with the aim to mimic a potential source of error in the model forecasts. A characterisation was attempted with proxy forecast errors by using statistical moments of order 1 to 4. The temporal variability of model correction patterns in a hypothetical data assimilation system was also illustrated. Significant departures from linear/Gaussian response were found, as well as well-marked non-stationarities in the error patterns. Within the limits of the experimental protocol, this could be technically applicable to other coastal areas as the study illustrates the likely limits of stationary/Gaussian data assimilation approaches in the Bay of Biscay.

The EuroGOOS Mediterranean Test Case: science and implementation plan

This paper is a synthesis of the scientific and technological plans to develop a Mediterranean Forecasting System. The overall aim of the plan is to provide the conceptual framework for the design of the observational and modeling systems which could exploit the potential predictability of the ecosystem fluctuations in the Mediterranean coastal regions.