Abstract
AbstractVariability in the tropical atmosphere is concentrated at wavenumber-frequency combinations where linear theory indicates wave-modes can freely propagate, but with substantial power in between. This study demonstrates that such a power spectrum can arise from small scale convection triggering large scale waves via wave-wave interactions in a moderately turbulent fluid. Two key pieces of evidence are provided for this interpretation of tropical dynamics using a nonlinear rotating shallow water model: a parameter sweep experiment in which the amplitude of an external forcing is gradually ramped up, and also an external forcing in which only symmetric or only anti-symmetric modes are forced. These experiments do not support a commonly accepted mechanism involving the forcing projecting directly onto the wave-modes with a strong response, yet still simulate a power spectrum resembling that observed, though the linear projection mechanism could still complement the mechanism proposed here in observations. Interpreting the observed tropical power spectrum using turbulence offers a simple explanation as to why power should be concentrated at the theoretical wave-modes, and also provides a solid footing for the common assumption that the back-ground spectrum is red, even as it clarifies why there is no expectation for a turbulent cascade with a specific, theoretically derived slope such as -5/3. However it does explain why the cascade should be towards lower wavenumbers, that is an inverse energy cascade, similar to the midlatitudes even as compressible wave-modes are important for tropical dynamics.
Highlights
In the tropics, the upward motion of air at small scales generates clouds and local precipitation, and impacts the large-scale flow
In addition to the power concentrated along the dispersion curves of linear waves, there is substantial power evident in between these curves which is referred to as the background spectrum
Variability in the tropical atmosphere occurs on spatial scales that are separated by several orders of magnitude
Summary
The upward motion of air at small scales generates clouds and local precipitation, and impacts the large-scale flow. Variations in outgoing longwave radiation (OLR), precipitation, and zonal winds in the tropics peak at wavenumber–frequency combinations given by the theoretical dispersion curves of longitudinally propagating equatorial waves (Matsuno 1966), as compared to the background spectrum in between these dispersion curves. Even though the characteristic length scale of individual clouds is tens of kilometers or less, this convection appears to launch global-scale waves with wavelength of order thousands of kilometers, which propagate away and affect the global atmosphere (Garcia and Salby 1987). The precise mechanism whereby the small-scale clouds launch these global-scale waves has been discussed by at least.
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