Assimilation of thermal data into an ocean model near the equator often results in a dynamically unbalanced state with unrealistic deep overturning circulations. The way in which these circulations arise from errors in the model or its forcing and the equatorial dynamics is discussed. A scheme is proposed to calculate a state with a better balance by using the observational increments to the model to update slowly evolving bias fields. These bias fields augment the model state and affect the model's pressure gradients. The properties of the augmented shallow-water equations are examined. When forced by steady incorrect wind stresses, solutions of the augmented equations assimilating full fields of surface height data converge to the correct surface height and vertical velocity fields. © Royal Meteorological Society, 2004. The contributions of M. J. Bell and M. J. Martin are Crown copyright.
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