Impact of satellite data assimilation in a coupled physical-biogeochemical model of the North Atlantic

The general objective of this work is to examine how the assimilation of data in a circulation model can improve the biological response simulated by a coupled physical-ecosystem model. In this work, the focus will be on the impact of altimetric, SST and SSS data assimilation in an eddy-permitting coupled model of the North Atlantic. The physical model is a z-coordinate, rigid lid, primitive-equation model based on the OPA code [Madec et al, 1998]. The horizontal resolution is 1/3° and there are 43 vertical levels with refinement near the surface. The biogeochemical model is the P3ZD biogeochemical model [Aumont et al., 1998] that describes the cycling of carbon, silica and calcium. The simulations are performed using realistic forcings during 1998. The assimilation method is based on a Kalman filter with reduced order error covariance matrix, known as the SEEK filter [ Pham et al., 1998]. The sequential scheme has been modified recently using the concept of "incremental analysis update" to enforce temporal continuity of the assimilation run. In order to evaluate how the assimilation can improve the representation of the biological fields, comparisons are made between free runs and simulations with assimilation. A first comparison with the assimilation run obtained using the scheme developed by Testut et al. [2003] indicates the excessive supply of nutrients in the euphotic zone through spurious mixing and advection mechanisms. This can be partly attributed to several factors, e.g. the statistical method which is unable to maintain the model constraint of hydrostatic stability, the discontinuous nature of the sequential algorithm, or the lack of consistent corrections between the physical and biological components of the state vector. Several variants of the assimilation algorithm are implemented in order to improve the representation of the model dynamics and its subsequent impact on the biological variables. A comparison between the assimilation runs obtained with these variants will be presented and discussed. References Aumont O., 1998. Etude du cycle naturel du carbone dans un modèle 3D de l'océan mondial. Ph.D. thesis, Univ. Pierre et Marie Curie, Paris, 1998 Madec, G. Delecluse, P., Imbard M., Levy. C, 1998. OPA8.1 Ocean General Circulation Model Reference manual. Notes du pôle de modélisation, Tech. rep., Institut Pierre-Simon Laplace (IPSL). 91p. Pham D.T., Verron J. et Roubaud M.C., 1998:, Singular evolutive extended kalman filter with EOF initialization for data assimilation in oceanography, J. Mar. Systems., 16, 323-340. Testut, C-E., Brasseur, P., J-M., Brankart., Verron, J. 2003, Assimilation of sea-surface temperature and altimetric observations during 1992-1993 into an eddy-permiting primitive equation model of the North Atlantic Ocean. J. Mar. Systems., 40-41, 291-316