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Akira Kasahara and Pedro L. da Silva Dias

Abstract

The response of planetary waves to stationary tropical heating in a stratified global atmosphere linearized with respect to a basic zonal mean flow is investigated. The basic zonal wind has meridional and vertical shear. The basic equations are solved by using the method of three-dimensional normal-mode expansion. Forced solutions to a prescribed tropospheric equatorial heating distribution with a specific wavenumber in longitude are examined.

Without the basic zonal flow, the internal vertical modes whose equivalent depths are on the order of a few hundred meters are favorably excited, but the response of the external mode (“barotropic” mode) is relatively small. With the inclusion of a zonal flow, the vertical shear of the zonal wind permits the coupling of the external mode with the internal vertical modes. As a result of the coupling, a significant response occurs in the external mode due to the excitation of the “baroclinic” internal modes by tropical heating. The meridional structures of internal vertical modes are equatorially trapped and their intensities are less affected by the basic zonal flow. Since the meridional structures of the external mode is global, a significant response of the external mode to tropospheric tropical heating is no longer confined to the tropics. The direction of the basic zonal flow and its meridional shear have a profound influence on the intensity of planetary waves in the mid- to higher latitudes generated by stationary tropical heating. The present findings may fill in a missing link in the dynamical theory of atmospheric teleconnections.

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Enver Ramirez, Pedro L. da Silva Dias, and Carlos F. M. Raupp

Abstract

In the present study a simplified multiscale atmosphere–ocean coupled model for the tropical interactions among synoptic, intraseasonal, and interannual scales is developed. Two nonlinear equatorial β-plane shallow-water equations are considered: one for the ocean and the other for the atmosphere. The nonlinear terms are the intrinsic advective nonlinearity and the air–sea coupling fluxes. To mimic the main differences between the fast atmosphere and the slow ocean, suitable anisotropic multispace/multitime scalings are applied, yielding a balanced synoptic–intraseasonal–interannual–El Niño (SInEN) regime. In this distinguished balanced regime, the synoptic scale is the fastest atmospheric time scale, the intraseasonal scale is the intermediate air–sea coupling time scale (common to both fluid flows), and El Niño refers to the slowest interannual ocean time scale. The asymptotic SInEN equations reveal that the slow wave amplitude evolution depends on both types of nonlinearities. Analytic solutions of the reduced SInEN equations for a single atmosphere–ocean resonant triad illustrate the potential of the model to understand slow-frequency variability in the tropics. The resonant nonlinear wind stress allows a mechanism for the synoptic-scale atmospheric waves to force intraseasonal variability in the ocean. The intraseasonal ocean temperature anomaly coupled with the atmosphere through evaporation forces synoptic and intraseasonal atmospheric variability. The wave–convection coupling provides another source for higher-order atmospheric variability. Nonlinear interactions of intraseasonal ocean perturbations may also force interannual oceanic variability. The constrains that determine the establishment of the atmosphere–ocean resonant coupling can be viewed as selection rules for the excitation of intraseasonal variability (MJO) or even slower interannual variability (El Niño).

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Ariane Frassoni, Dayana Castilho, Michel Rixen, Enver Ramirez, João Gerd Z. de Mattos, Paulo Kubota, Alan James Peixoto Calheiros, Kevin A. Reed, Maria Assunção F. da Silva Dias, Pedro L. da Silva Dias, Haroldo Fraga de Campos Velho, Stephan R. de Roode, Francisco Doblas-Reyes, Denis Eiras, Michael Ek, Silvio N. Figueroa, Richard Forbes, Saulo R. Freitas, Georg A. Grell, Dirceu L. Herdies, Peter H. Lauritzen, Luiz Augusto T. Machado, Antonio O. Manzi, Guilherme Martins, Gilvan S. Oliveira, Nilton E. Rosário, Domingo C. Sales, Nils Wedi, and Bárbara Yamada
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