Contribution de P.-A. BOUTTIER, E. BLAYO & J. VERRON:

Impact of Non-Linearities on Incremental 4D-VAR Data Assimilation Method in A High Resolution Ocean Model

The purpose of this poster is to explore the behaviour of variational data assimilation methods in a non-linear ocean model. In an eddy-permitting or eddy-resolving ocean model, controlling mesoscale eddies activity is crucial for data assimilation methods. Our goal is to highlight the impact of these non-linearities on the assimilation system. To illustrate this, test experiments are performed with a double-gyre NEMO configuration at different resolutions (1/4, 1/12) which mimics Gulfstream-like behaviour in term of eddy system and an incremental 4D-VAR formulation for the assimilation system. First, we are mainly interested in observing the impact of the length of assimilation window on the quality of the analyzed trajectory. Twin experiments are performed with the 1/4 model, using simulated altimeter data, for different lengths of the assimilation window. Helped by diagnoses on error scales, we also attempt to link the non-linear phenomena with error structures observed after assimilation quantitavely and qualitatively. Then, by increasing the model resolution and consequently the mesoscale eddy activity, we bring to light the sensitivity of our assimilation system to non-linearity by repeating the same types of experiments and diagnoses. To conclude, we suggest different assimilation strategies to reduce the error in the analyzed trajectory due to the mesoscale eddy activity.

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