Isopycnal Empirical orthogonal Functions (EOFs) in the North and Tropical Atlantic and their use in estimation problems.

Pascal Faucher (1) , Michel Gavart (1) , Pierre de Mey (2)

1 LEGOS - Laboratoire d'études en Géophysique et océanographie spatiales
2 ECOLA - Echanges Côte-Large

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.

Mots clés: Atlantic Ocean Islands; Atlantic Ocean; North Atlantic; Madeira; Azores; Tropical Atlantic Ocean; Depth profile; Sea level; Altimetry; Anticyclonic eddy; Thermal front; Gulf Stream; Thermocline; Surface temperature; Spatial variation; Data assimilation; Anomaly; Dynamic height; Potential temperature; Coordinate system; Thermal variation; Empirical orthogonal function

Domaines: Océanographie

Pascal Faucher, Michel Gavart, Pierre de Mey. Isopycnal Empirical orthogonal Functions (EOFs) in the North and Tropical Atlantic and their use in estimation problems.. Journal of Geophysical Research. Oceans, 2002, 107, pp.2146-2157. ⟨10.1029/2000JC000690⟩. ⟨hal-00764343⟩