A singular evolutive interpolated Kalman filter for efficient data assimilation in a 3-D complex physical–biogeochemical model of the Cretan Sea G. Triantafyllou a, * , I. Hoteit b , G. Petihakis a a IMBC, P.O. Box 2214, Iraklio, 71003 Crete, Greece b LMC/IMAG, B.P. 53X, 38041 Grenoble, France Received 19 October 2001; accepted 2 December 2002 Abstract A singular evolutive interpolated Kalman (SEIK) filter is used to assimilate pseudo-observations via twin simulation experiments in a complex three-dimensional coupled physical – biogeochemical model of the Cretan Sea. The simulation system comprises two on-line coupled sub-models: the three-dimensional Princeton Model and the European Regional Seas Ecosystem Model (ERSEM). In the SEIK filter, the estimation error is represented by an ensemble of state vectors, which are drawn randomly at every filtering step. In the twin experiments performed the predictions of the coupled model were corrected every 2 days using synthetic measurements extracted from a model reference run according to a network of 23 stations in the Cretan Sea. The filter is shown to be very efficient, with the assimilation results exhibiting a continuous decrease of the estimation error during the experimental period. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Data assimilation; Coupled biogeochemical models; Kalman filter 1. Introduction The Mediterranean Sea is an elongated basin with significant water exchanges only at the western part the Gibraltar strait. The Sicily strait marks two sub- basins, decoupling hydrodynamics and ecological conditions. The high evaporation, low rainfall and river runoff, in conjunction with the out flow of the bottom layer at Gibraltar, result in an oligotrophic gradient toward the east. The observed oligotrophy is thought to be due to low phosphorus concentrations (decreasing from west to east) limiting phytoplankton and bacterial growth (Krom et al., 1992). The Cretan Sea has an average depth of 1000 m and is linked with the Levantine basin and the Ionian Sea through the eastern and western straits of the Cretan Arc. It is dominated by multiple scale circulation patterns with a complex hydrological structure characterized by mesoscale variability. The system is characterized by a seasonal thermocline (60–80 m) from spring to autumn separating the water column into two layers 0924-7963/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-7963(03)00019-8 * Corresponding author. Tel.: +30-81-242022; fax: +30-81- 241882. E-mail address: gt@imbc.gr (G. Triantafyllou). www.elsevier.com/locate/jmarsys Journal of Marine Systems 40 – 41 (2003) 213 – 231