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On improving the accuracy of the barotropic tides embedded in a high-resolution global ocean circulation model

James G. Richman, Hans E. Ngodock, Innocent Souopgui, Alan J. Wallcraft, Brian K. Arbic
Naval Research Laboratory
(Abstract received 04/14/2015 for session X)
ABSTRACT

The ocean tidal velocity and displacement can be estimated concurrently with the ocean circulation by adding the astronomical gravitational forcing and parameterized damping to the general circulation physics. However, the accuracy of these tidal estimates doesn’t match the best data-assimilative barotropic tidal models. This talk presents an application of a data assimilation technique, Augmented State Ensemble Kalman Filter (ASEnKF), to improve the accuracy of barotropic tides embedded in a 1/12° three-dimensional ocean general circulation model. The ASEnKF is an alternative to the techniques typically used with the linearized tide-only models, which can be applied to the embedded tides in a nonlinear eddying circulation. An extra term, meant to correct for errors in the tide model due to, for instance, imperfectly known topography and damping terms, is introduced into the tidal forcing. Ensembles of the model are performed with stochastically generated fields of the model forcing correction term and the model discrepancies with TPXO, an existing data-assimilative tide model, are computed. The ASEnKF method yields an optimal estimate of the model forcing correction terms which minimizes resultant RMS tidal sea surface elevation error with respect to TPXO, as well as an estimate of the tidal elevation. The global area-averaged RMS sea surface elevation error of the principal lunar semidiurnal tide M2 is reduced from 4.4 cm in a best-case non-assimilative solution to 2.6 cm. The largest elevation errors in both the non-assimilative and ASEnKF solutions are in the North Atlantic, a highly resonant basin. Possible pathways for achieving further reductions in the RMS error are discussed.

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2015 LOM Workshop, Copenhagen, Denmark June 2nd - 4th, 2015