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- Author or Editor: Roland A. Madden x
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Abstract
The average structure of westward traveling disturbances that contribute to relative maxima found in space-time spectra from 13–32 days at northern latitudes is determined for each season. A compositing method used employs a minimum of space and time filtering in order to avoid biasing the results. The average latitudinal structure is “global” in that it is discernible in the Southern Hemisphere during December–February and September–November. It is primarily confined to northern latitudes during March–August. In all seasons the disturbance is out of phase between northern high latitudes and subtropical and tropical latitudes. The longitudinal structure is primarily zonal wavenumber 1 in all seasons. Further work is suggested to confirm the structures determined here and to learn if they reflect the superposition of a number of occasionally excited Rossby normal modes.
Abstract
The average structure of westward traveling disturbances that contribute to relative maxima found in space-time spectra from 13–32 days at northern latitudes is determined for each season. A compositing method used employs a minimum of space and time filtering in order to avoid biasing the results. The average latitudinal structure is “global” in that it is discernible in the Southern Hemisphere during December–February and September–November. It is primarily confined to northern latitudes during March–August. In all seasons the disturbance is out of phase between northern high latitudes and subtropical and tropical latitudes. The longitudinal structure is primarily zonal wavenumber 1 in all seasons. Further work is suggested to confirm the structures determined here and to learn if they reflect the superposition of a number of occasionally excited Rossby normal modes.
Abstract
Planetary-scale free Rossby waves present in the earth’s atmosphere propagate toward the west. Pressure torques varying in time then arise as a consequence of unequal pressure on the eastern and western sides of mountains and small-scale topographic features. These torques, referred to as mountain torques, have an influence on the exchange of angular momentum between the atmosphere and the earth.
The authors investigated the impact of all identified planetary-scale free Rossby waves on atmospheric angular momentum by computing the contribution from mountain torques to the rate of change of total atmospheric angular momentum for each wave.
Comparing contributions from individual waves, the authors found that for the average wave amplitudes the maximum torque for a particular wave is around 2 Hadleys, and that considering all meridional wavenumbers, zonal wavenumber 2 causes the largest global torques. Changes in angular momentum depend on both the amplitude of the changing torque and on its period. As a result zonal wavenumbers 1 and 2 cause the largest angular momentum anomalies with peak-to-trough amplitudes of 2–5 × 1023 kg m2 s−1. The 16-day wave produces the largest amplitude, 4.9 × 1023 kg m2 s−1. These values refer to average amplitudes reported in the literature. Individual waves may cause anomalies five times as big.
Abstract
Planetary-scale free Rossby waves present in the earth’s atmosphere propagate toward the west. Pressure torques varying in time then arise as a consequence of unequal pressure on the eastern and western sides of mountains and small-scale topographic features. These torques, referred to as mountain torques, have an influence on the exchange of angular momentum between the atmosphere and the earth.
The authors investigated the impact of all identified planetary-scale free Rossby waves on atmospheric angular momentum by computing the contribution from mountain torques to the rate of change of total atmospheric angular momentum for each wave.
Comparing contributions from individual waves, the authors found that for the average wave amplitudes the maximum torque for a particular wave is around 2 Hadleys, and that considering all meridional wavenumbers, zonal wavenumber 2 causes the largest global torques. Changes in angular momentum depend on both the amplitude of the changing torque and on its period. As a result zonal wavenumbers 1 and 2 cause the largest angular momentum anomalies with peak-to-trough amplitudes of 2–5 × 1023 kg m2 s−1. The 16-day wave produces the largest amplitude, 4.9 × 1023 kg m2 s−1. These values refer to average amplitudes reported in the literature. Individual waves may cause anomalies five times as big.
Abstract
The structure of the 33-h Kelvin wave, a normal mode of the atmosphere, is examined in 6-hourly station and NCEP–NCAR reanalysis data. Cross-spectral analysis of 6 yr (1993–98) of tropical station pressure data shows a peak in coherence in a narrow frequency band centered near 0.74 cycles per day, corresponding to a period of approximately 33 h. The phase angles are consistent with an eastward-propagating zonal-wavenumber-1 structure, implying an equatorial phase speed of approximately 340 m s−1. The global structure of the mode is revealed by empirical orthogonal function and regression analysis of 31 yr (1968–98) of reanalysis data. The horizontal structure shows a zonal-wavenumber-1 equatorial Kelvin wave with an equatorial trapping scale of approximately 34° lat. The vertical structure has zero phase change. The amplitude of the wave is approximately constant in the troposphere with an equatorial geopotential height perturbation of 0.9 m, and then increases exponentially with height in the stratosphere. Cross-spectral analysis between the station and reanalysis data shows that the results from the two datasets are consistent. No evidence can be found for forcing of the wave by deep tropical convection, which is is examined using a twice-daily outgoing longwave radiation dataset.
Abstract
The structure of the 33-h Kelvin wave, a normal mode of the atmosphere, is examined in 6-hourly station and NCEP–NCAR reanalysis data. Cross-spectral analysis of 6 yr (1993–98) of tropical station pressure data shows a peak in coherence in a narrow frequency band centered near 0.74 cycles per day, corresponding to a period of approximately 33 h. The phase angles are consistent with an eastward-propagating zonal-wavenumber-1 structure, implying an equatorial phase speed of approximately 340 m s−1. The global structure of the mode is revealed by empirical orthogonal function and regression analysis of 31 yr (1968–98) of reanalysis data. The horizontal structure shows a zonal-wavenumber-1 equatorial Kelvin wave with an equatorial trapping scale of approximately 34° lat. The vertical structure has zero phase change. The amplitude of the wave is approximately constant in the troposphere with an equatorial geopotential height perturbation of 0.9 m, and then increases exponentially with height in the stratosphere. Cross-spectral analysis between the station and reanalysis data shows that the results from the two datasets are consistent. No evidence can be found for forcing of the wave by deep tropical convection, which is is examined using a twice-daily outgoing longwave radiation dataset.
Abstract
Diurnal and semidiurnal variations in the budget of atmospheric angular momentum are evident in a simulation by the NCAR Community Climate Model (CCM2). These variations depicted with 20-min time resolution (each time step) are used as guides to study similar variations determined from 6-hourly NCEP/NCAR Reanalysis data. A semidiurnal variation in relative angular momentum and in angular momentum related to solid body rotation of the atmosphere is found in the reanalysis. Although there is evidence that both frictional and gravity-wave drag torques play roles, the effects of semidiurnal variations in mountain torque, resulting from the migrating semidiurnal pressure wave, are most thoroughly documented.
Abstract
Diurnal and semidiurnal variations in the budget of atmospheric angular momentum are evident in a simulation by the NCAR Community Climate Model (CCM2). These variations depicted with 20-min time resolution (each time step) are used as guides to study similar variations determined from 6-hourly NCEP/NCAR Reanalysis data. A semidiurnal variation in relative angular momentum and in angular momentum related to solid body rotation of the atmosphere is found in the reanalysis. Although there is evidence that both frictional and gravity-wave drag torques play roles, the effects of semidiurnal variations in mountain torque, resulting from the migrating semidiurnal pressure wave, are most thoroughly documented.
Abstract
Upper tropospheric and lower stratospheric wind data spanning 31 years from 1964 to 1994 were analyzed at rawinsonde stations in the central/western Pacific. Traditional spectral and cross-spectral analysis led to the conclusion that there is a significant signal with periods between 3 and 4.5 days, which the authors link with the dominant antisymmetric waves predicted by theory to have these periods, mixed Rossby–gravity waves, and equatorial Rossby waves. Then the authors applied the seasonally varying spectral analysis method developed by Madden to study the average seasonal variation of these waves. The seasonally varying analysis suggested that there are significant twice-yearly maxima in equatorial wave activity throughout the upper troposphere and lower stratosphere, with peaks occurring in late winter–spring and in late summer–fall. The twice-yearly signal was most prominent at the 70-hPa and 100-hPa levels. Similar and consistent results were also shown by an autoregressive cyclic spectral analysis. The cyclic spectral analysis suggested that the frequency characteristics of the υ-wind wave power are different during the two maxima at some stations. In addition, the seasonally varying squared coherence between the u and υ winds and the associated phase implied that there is horizontal momentum flux associated with these waves and that the sign of the flux is different during the two maxima. The differences in wave characteristics during the maxima periods may be related to different wave modes, seasonal variation of the basic zonal state, or possibly to different equatorial wave forcing mechanisms (i.e., convective versus lateral excitations).
Abstract
Upper tropospheric and lower stratospheric wind data spanning 31 years from 1964 to 1994 were analyzed at rawinsonde stations in the central/western Pacific. Traditional spectral and cross-spectral analysis led to the conclusion that there is a significant signal with periods between 3 and 4.5 days, which the authors link with the dominant antisymmetric waves predicted by theory to have these periods, mixed Rossby–gravity waves, and equatorial Rossby waves. Then the authors applied the seasonally varying spectral analysis method developed by Madden to study the average seasonal variation of these waves. The seasonally varying analysis suggested that there are significant twice-yearly maxima in equatorial wave activity throughout the upper troposphere and lower stratosphere, with peaks occurring in late winter–spring and in late summer–fall. The twice-yearly signal was most prominent at the 70-hPa and 100-hPa levels. Similar and consistent results were also shown by an autoregressive cyclic spectral analysis. The cyclic spectral analysis suggested that the frequency characteristics of the υ-wind wave power are different during the two maxima at some stations. In addition, the seasonally varying squared coherence between the u and υ winds and the associated phase implied that there is horizontal momentum flux associated with these waves and that the sign of the flux is different during the two maxima. The differences in wave characteristics during the maxima periods may be related to different wave modes, seasonal variation of the basic zonal state, or possibly to different equatorial wave forcing mechanisms (i.e., convective versus lateral excitations).
