Wave Structure of the Tropical Stratosphere over the Marshall Islands Area during 1 April–1 July 1958

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  • 1 Center for Earth and Planetary Physics, Harvard University, Cambridge, Mass. 02138
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Abstract

Stratospheric wind data for the Marshall Islands region during 1 April–1 July, 1958, are analyzed for contributions in the 4–6 day period range. It is shown that, excluding waves with vertical wavelengths <2 km from the data, 4–6 day power in the equatorial stratosphere during this period must be due to some combination of Kelvin, mixed gravity-Rossby, and n=1 Rossby waves with zonal wavenumbers <7–10. It is further shown that a theoretical model wherein each of the above three wave types is associated with a zonal wavenumber of either 3 or 4 is consistent with the data. The resulting observationally calibrated model is used to calculate the acceleration of the mean flow by wave absorption, which is then compared with the observed acceleration. In general, the waves satisfactorily account for accelerations above 23 km. Below 23 km there is a need for an additional source of easterly momentum with a specific vertical distribution which we show could be provided by an n=1 easterly gravity wave whose vertical wavelength, however, would be too short for the wave to be seen in radiosonde data. We also show that if a mean flow together with 4–6 day waves is spectral analyzed, there will be power at periods >4–6 days due to the acceleration of the mean flow by waves, and there may also be power at periods <4–6 days due to the modification of the waves by the changing mean flow. We finally examine what the theory suggests is happening at levels where wave absorption is altering the mean flow, and show, some of the difficulties in relating such behavior to data averaged over a three-month period.

Abstract

Stratospheric wind data for the Marshall Islands region during 1 April–1 July, 1958, are analyzed for contributions in the 4–6 day period range. It is shown that, excluding waves with vertical wavelengths <2 km from the data, 4–6 day power in the equatorial stratosphere during this period must be due to some combination of Kelvin, mixed gravity-Rossby, and n=1 Rossby waves with zonal wavenumbers <7–10. It is further shown that a theoretical model wherein each of the above three wave types is associated with a zonal wavenumber of either 3 or 4 is consistent with the data. The resulting observationally calibrated model is used to calculate the acceleration of the mean flow by wave absorption, which is then compared with the observed acceleration. In general, the waves satisfactorily account for accelerations above 23 km. Below 23 km there is a need for an additional source of easterly momentum with a specific vertical distribution which we show could be provided by an n=1 easterly gravity wave whose vertical wavelength, however, would be too short for the wave to be seen in radiosonde data. We also show that if a mean flow together with 4–6 day waves is spectral analyzed, there will be power at periods >4–6 days due to the acceleration of the mean flow by waves, and there may also be power at periods <4–6 days due to the modification of the waves by the changing mean flow. We finally examine what the theory suggests is happening at levels where wave absorption is altering the mean flow, and show, some of the difficulties in relating such behavior to data averaged over a three-month period.

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