Tropical 40–50- and 25–30-Day Oscillations Appearing in Realistic and Idealized GFDL Climate Models and the ECMWF Dataset

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Abstract
To clarify differences between the tropical 40–50- and 25–30-day oscillations and to evaluate simulations and various theories, space-time spectrum and filter analyses were performed on a nine-year dataset taken from the nine-level R30 spectral general circulation model and the nine-year (1979–1987) ECMWF four-dimensional analysis dataset. In addition, the 40-level SKYHI model was analyzed to examine the effect of increased vertical resolution, while an ocean-surface perpetual January R30 model was analyzed to examine the effects of the absence of geographical and seasonal variations. The R30 model results indicate that the relative amplitude of the wavenumber-one component of the 40–50- and 25–30-day oscillations varies greatly from year to year. For the nine-year average, the simulated 40–50-day zonal velocity oscillations are as strong as observed, while the simulated 25–30-day zonal velocity oscillations are much stronger than observed. Although 40–50- and 25–30-day oscillations have similar structures, the 25–30-day oscillations exhibit a greater increase with height in their tropospheric amplitudes than the 40–50-day oscillations, resulting in different relative magnitudes at different levels. The time variance of the two oscillations has similar longitudinal distributions, implying that the two periods are not due to differences in local phase speeds. They appear to grow and decay independently without any coherent phase relationship, implying that the two periods are not a result of the seasonal modulation of an intrinsic 30–40-day period. The SKYHI model indicates that 25–30-day oscillations still appear too strong. Nevertheless, this model reveals a longer vertical wavelength, a higher penetration of the 25–30-day amplitude above the level of convective heating, and a slightly greater height of the convective-heating amplitude, which cannot be detected in the R30 model. This implies that the two oscillations differ in their intrinsic vertical wavelengths. The ocean-surface perpetual January R30 model indicates that not only the 25–30-day mode but also the 40–50-day mode can be simulated in the absence of geographical and seasonal modulations, while the wave CISK and evaporation-wind feedback theories cannot explain the 40–50-day mode. Both R30 models indicate that daily precipitation is almost always associated with upward motion, being consistent with theoretical conditional heating. A comparison between the two R30 models suggests that the sea surface temperature geographically modulates the intrinsically eastward-moving wavenumber-one precipitation oscillations, resulting in their major Pacific and minor Atlantic local amplitudes. This in turn causes planetary-scale eastward-moving zonal-velocity oscillations and standing geopotential oscillations.

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