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Interaction between near-annual and ENSO modes in a CGCM simulation: Role of the equatorial background mean state

Boris Dewitte, Carole Cibot, Claire Périgaud, Soon-Il An and Laurent Terray

A 260-yr long coupled general circulation model (CGCM) simulation is used to investigate the interaction between ENSO mode and near-annual variability and its sensitivity to the equatorial background mean stratification and seasonal cycles. Although the thermocline mean vertical structure of the model favors the high-order baroclinic modes that are associated to the slow timescales of the coupled variability, the simulated ENSO oscillates at a dominant a quasi-biennial frequency. Biases of the climatological velocity field are favorable to the dominance of the zonal advective feedback over the thermocline feedback, the model exhibiting overenergetic westward seasonal zonal current in the central-western equatorial Pacific and an upwelling rate that is about half the observations. This sets the conditions for the enhancement of a near-annual mode that is observed to oscillate at an 8-month period in the model. Using an intermediate coupled model of the tropical Pacific where the climatological fields are prescribed to the ones derived from the CGCM, it is demonstrated that the quasi-biennial ENSO variability simulated by the CGCM is mostly due to the to the biases in the climatological currents of the CGCM. These biases favor the dominance of the fast ‘zonal advective feedback’ over the slow ‘thermocline feedback’ in the coupled system and enhance a fast coupled basin mode. This fast mode differs from the theoretical Pacific Ocean Basin mode in that, besides mean temperature advection by the zonal current anomalies, it is also driven by anomalous temperature advection by the total current. Results suggest that the near-annual mode destabilizes the ENSO mode to produce overenergetic quasi-biennial oscillations in the model. It also participates to the ENSO asymmetry and the cold bias of the CGCM mean state by non-linear accumulation of temperature zonal advection, that works towards cold in the western Pacific more than warm in the east. It is suggested that the model equilibrium results from the interaction between ENSO mode, near-annual mode and mean state.

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