Q. J. R. Meteorol. Soc. (2005), 131, pp. 3301–3311 doi: 10.1256/qj.05.95 Ensemble data assimilation for an eddy-resolving ocean model of the Australian region By P. R. OKE 1 ∗ , A. SCHILLER 1 , D. A. GRIFFIN 1 and G. B. BRASSINGTON 2 1 CSIRO Marine and Atmospheric Research, Hobart, Tasmania, Australia 2 Bureau of Meteorology Research Centre, Melbourne, Victoria, Australia (Received 23 May 2005; revised 8 November 2005) SUMMARY The Bluelink Ocean Data Assimilation System (BODAS) is an ensemble optimal interpolation system applied to a global ocean general-circulation model with 10 km resolution around Australia. BODAS derives estimates of forecast-error covariances (FECs) from a stationary 72-member ensemble of intraseasonal model anomalies. The FECs are localized around each observation to reduce the negative effects of sampling error and to increase the rank of the ensemble. The FECs have characteristics that reflect the length-scales and the anisotropy of the ocean circulation in different regions. BODAS assimilates in situ and satellite-derived observations of temperature, salinity and sea-level anomaly. Results from a 13-year ocean re-analysis demonstrate that the re- analysed fields are often in very good agreement with withheld observations, and provide a good synoptic representation of the eddy field around Australia. KEYWORDS: Ocean prediction 1. I NTRODUCTION Bluelink is an Australian partnership between the Commonwealth Scientific and Industrial Research Organisation (CSIRO), the Bureau of Meteorology and the Royal Australian Navy. The primary objective of Bluelink is to develop a forecast system for the mesoscale ocean circulation in the Australian region. To this end, the Ocean Forecasting Australia Model (OFAM), a global ocean general-circulation model, has been configured and the Bluelink Ocean Data Assimilation System (BODAS) has been developed. BODAS is an ensemble optimal interpolation (EnOI) system, similar to that introduced by Oke et al. (2002) and Evensen (2003). The purpose of this paper is to describe recent developments in ocean data assimilation in the Australian oceanographic community. This paper includes a description of BODAS, examples of its ensemble- based estimates of forecast-error covariances (FECs) and some results from a global application of BODAS. 2. MODEL OFAM is based on version 4.0 of the Modular Ocean Model (Griffies et al. 2004), with local enhancements including the hybrid mixed-layer model described by Chen et al. (1994), an improved parametrization for the penetration of solar radiation and vector optimization for an NEC SX6. OFAM is intended to be used for re-analyses and short-range prediction. The horizontal grid has 1191 and 968 points in the zonal and meridional directions respectively, with 0.1 ◦ horizontal resolution around Australia between 90 ◦ E and 180 ◦ and between Antarctica and 16 ◦ N. Outside of this domain, the horizontal resolution decreases to 2 ◦ in the North Atlantic Ocean. OFAM has 47 vertical levels, with 10 m resolution down to 200 m depth. The topography for OFAM is a composite of a range of different topography sources. Horizontal diffusion ∗ Corresponding author: CSIRO Marine and Atmospheric Research, Hobart 7001, Australia. e-mail: peter.oke@csiro.au c  Royal Meteorological Society, 2005. 3301