The Bluelink ocean data assimilation system (BODAS)

Abstract

Bluelink is Australia’s contribution to the Global Ocean Data Assimilation Experiment (GODAE). The goals of GODAE include the development and application of eddy-resolving, data assimilating ocean forecast systems. In this paper, we describe several aspects of the Bluelink effort that are motivated by this goal. One of the main innovations of Bluelink is the development of the Bluelink ocean data assimilation system (BODAS). The initial test-bed for BODAS is the Bluelink ReANalysis (BRAN), a multi-year model integration with data assimilation. The Bluelink model is a global ocean general circulation model that is eddy-resolving in the Australian region. Observations that are assimilated into BRAN include satellite altimetry, sea-surface temperature and in situ temperature and salinity data from Argo, XBT, TAO and other sources. BODAS is an ensemble optimal interpolation system that uses an ensemble of intraseasonal anomalies from a free running model to estimate the background error covariances (BECs). The ensemble-based BECs are multivariate and inhomogeneous and are shown to reflect the length-scales, the anisotropy and the covariability of mesoscale oceanic processes. We evaluate the performance of BODAS and BRAN (version 1.5), spanning the period January 2003 to June 2006, by comparing reanalyzed fields to a range of satellite-derived and in situ observations. Specifically, we demonstrate that BRAN realistically reproduces the mesoscale circulation around Australia, representing both the broad-scale circulation and, in many instances, the relatively small-scale mesoscale features. Quantitatively, we show that reanalyzed fields in the region around Australia are typically within 6–12 cm of withheld altimetric observations, within 0.5–0.9° of observed sea-surface temperature and within 4–7 cm of observed coastal sea-level. Comparisons with Argo profiles and surface drifting buoys show that BRAN fields are within 1° of observed sub-surface temperature, within 0.15 psu of observed sub-surface salinity and within 0.2 m s −1 of near-surface currents. We identify initialisation as a key area in which the Bluelink system could be improved.