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Vertical structure variability in a seasonal simulation of a medium-resolution regional model of the Eastern South Pacific.

Boris Dewitte, Marcel Ramos, Vincent Echevin, Oscar Pizarro and Yves duPenhoat

The oceanic variability of the South-Eastern Pacific is peculiar in that the coastal variability off Peru and Chile connects the equatorial Kelvin waves to the extra-tropical Rossby waves at a variety of timescales, from intraseasonal to interannual. Here, we present the results of a medium resolution, eddy permitting regional model simulation for the eastern south Pacific forced by climatological fields both at the air-sea interface and open boundaries. Our objective is to investigate the intraseasonal to annual propagating signal in the model and understand how the energy is transmitted from the open boundaries along the coast to the coastal zone and from the coastal zone to the off-shore ocean as extra-tropical Rossby waves.
Comparison with available observations indicates that the main aspects of the model mean state and variability is reasonably realistic and allows for further investigation of the characteristics of the propagating variability along the coast and off-shore. In particular, despite mesoscale features not fully resolved by the model, the simulated sea level annual cycle has a similar off-shore propagating pattern than the altimetric data. A vertical mode decomposition of the model mean stratification is carried out and the baroclinic mode contributions to sea level are estimated. Results indicate that the momentum forcing project preferentially on the first gravest modes in the inner basin and on the higher-order modes along the coast. Due to the peculiarities of the wind stress curl and boundary forcing, the distribution of energy on the baroclinic modes indicates however that the gravest mode is dominant over most of the domain and that annual and residual variability are on average twice as large as the semi-annual variability which is confined near the coast for all the modes. The first baroclinic mode contribution of the annual cycle exhibits a clear westward propagation north of the critical latitude, which, consistently with the theory of Clarke and Shi (1991), supports that the energy of equatorial origin transmitted by the coastally trapped waves propagates offshore in the form of Rossby waves equatorward of this meridonal limit. The higher-order modes only contribute significantly near the coast where they are associated to vertically propagating energy at annual period. The residual variability, which consists in the energy at all timescales other than the annual and semi-annual periods, is the weakest just off-shore between 27°S and 17°S. This confirms that Rossby waves at these frequencies can be triggered by coastal variability in this latitude range. However, residual energy peaks in the inner basin between 33°S and 15°S, suggesting the generation of Rossby waves by interactions with the mean flow.

Results of sensitivity experiments to the atmospheric and boundary forcing indicate that the annual extra-tropical Rossby waves results from both local wind stress and boundary forcing. The residual variability is more ubiquitous and apparently results from the non-linear interaction between the mean circulation and the first baroclinic seasonal extra-tropical Rossby waves. Overall the study
extends the classical analysis based on the linear theory of the extra-tropical sea level variability in the South Eastern Pacific.

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