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Abstract
Ten years of global tropospheric data from the European Centre for Medium-Range Weather Forecasts analyses were used to obtain a climatology of quasi-stationary waves and transient normal-mode Rossby waves. The data were split up into a mean annual cycle, reflecting the forced fields, and a transient part, containing the traveling waves. Data were then projected onto Hough normal modes, yielding a mean annual behavior of the quasi-stationary fields and time series of expansion coefficients for the transient waves. The latter were analyzed by a space-time spectral method independently for each of the four seasons. The Hough normal modes with low zonal wavenumber and low meridional index show clear peaks in the power spectra at theoretically predicted frequencies. Some modes have a strong seasonality.
Abstract
Ten years of global tropospheric data from the European Centre for Medium-Range Weather Forecasts analyses were used to obtain a climatology of quasi-stationary waves and transient normal-mode Rossby waves. The data were split up into a mean annual cycle, reflecting the forced fields, and a transient part, containing the traveling waves. Data were then projected onto Hough normal modes, yielding a mean annual behavior of the quasi-stationary fields and time series of expansion coefficients for the transient waves. The latter were analyzed by a space-time spectral method independently for each of the four seasons. The Hough normal modes with low zonal wavenumber and low meridional index show clear peaks in the power spectra at theoretically predicted frequencies. Some modes have a strong seasonality.
Abstract
Seasonal and geographical variations in tropical intraseasonal wind variance are described using bandpass filtered 850 and 150 mb wind time series derived from rawinsonde observations. Three bandpass filters, with central response periods of 31, 47, and 99 days, are applied to the daily time series. The intermediate filter is designed to isolate variance associated with the “40–50 day oscillation.” The spatial coherence of the bandpass filtered wind fluctuations is examined using complex eigenvector analysis.
Comparisons are made of u and v variance and large-scale structure of filtered wind anomalies for each season and frequency band, with emphasis on the u component. At stations across the western Pacific the 47-day filtered u 150 variance is nearly constant with season. The largest seasonal variability in 47-day filtered zonal wind variance is at 150 mb at stations along and to the north of the equator between Africa and Southeast Asia, and in the central Pacific. Compared to the u 150 variance over the western Pacific, the variance at these stations is much larger in the boreal winter and much smaller in the boreal summer. Large variance at 850 mb is found in each frequency band from the central Indian Ocean eastward to the dateline, with u 850 and u 150 fluctuating out-of-phase and the largest u 850 variance in the summer hemisphere. Eastward propagation of u 150 anomalies is found in each season and frequency band. A longitudinally varying wavenumber structure fits the eigenvectors reasonably well. Across the western Pacific, the u 150 anomalies have a wavenumber 2 structure, consistent with the leading pattern of large-scale convection anomalies. From the dateline eastward across Africa the scale of the u 150 anomalies is broader, closer to a wavenumber 1 scale.
The results suggest that the 40–50 day oscillation in the global tropics has a “two-regime” character. Across the eastern Indian and western Pacific Oceans (the “convective regime”) the 40–50 day oscillation occurs year-round and its spatial structure indicates that it is closely coupled to convection. Elsewhere (the “dry regime”) the oscillation is clearly evident only in the upper troposphere and is subject to strong seasonal modulation.
Abstract
Seasonal and geographical variations in tropical intraseasonal wind variance are described using bandpass filtered 850 and 150 mb wind time series derived from rawinsonde observations. Three bandpass filters, with central response periods of 31, 47, and 99 days, are applied to the daily time series. The intermediate filter is designed to isolate variance associated with the “40–50 day oscillation.” The spatial coherence of the bandpass filtered wind fluctuations is examined using complex eigenvector analysis.
Comparisons are made of u and v variance and large-scale structure of filtered wind anomalies for each season and frequency band, with emphasis on the u component. At stations across the western Pacific the 47-day filtered u 150 variance is nearly constant with season. The largest seasonal variability in 47-day filtered zonal wind variance is at 150 mb at stations along and to the north of the equator between Africa and Southeast Asia, and in the central Pacific. Compared to the u 150 variance over the western Pacific, the variance at these stations is much larger in the boreal winter and much smaller in the boreal summer. Large variance at 850 mb is found in each frequency band from the central Indian Ocean eastward to the dateline, with u 850 and u 150 fluctuating out-of-phase and the largest u 850 variance in the summer hemisphere. Eastward propagation of u 150 anomalies is found in each season and frequency band. A longitudinally varying wavenumber structure fits the eigenvectors reasonably well. Across the western Pacific, the u 150 anomalies have a wavenumber 2 structure, consistent with the leading pattern of large-scale convection anomalies. From the dateline eastward across Africa the scale of the u 150 anomalies is broader, closer to a wavenumber 1 scale.
The results suggest that the 40–50 day oscillation in the global tropics has a “two-regime” character. Across the eastern Indian and western Pacific Oceans (the “convective regime”) the 40–50 day oscillation occurs year-round and its spatial structure indicates that it is closely coupled to convection. Elsewhere (the “dry regime”) the oscillation is clearly evident only in the upper troposphere and is subject to strong seasonal modulation.
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
Optimal averaging is a method to estimate some area mean of datasets with imperfect spatial sampling. The accuracy of the method is tested by application to time series of January temperature fields simulated by the NCAR Community Climate Model. Some restrictions to the application of optimal averaging are given. It is demonstrated that the proper choice of a spatial correlation model is crucial. It is shown that the optimal averaging procedures provide a better approximation to the true mean of a region than simple area-weight averaging does. The inclusion of measurement errors of realistic size at each observation location hardly changes the value of the optimal average nor does it substantially alter the sampling error. of the optimal average.
Abstract
Optimal averaging is a method to estimate some area mean of datasets with imperfect spatial sampling. The accuracy of the method is tested by application to time series of January temperature fields simulated by the NCAR Community Climate Model. Some restrictions to the application of optimal averaging are given. It is demonstrated that the proper choice of a spatial correlation model is crucial. It is shown that the optimal averaging procedures provide a better approximation to the true mean of a region than simple area-weight averaging does. The inclusion of measurement errors of realistic size at each observation location hardly changes the value of the optimal average nor does it substantially alter the sampling error. of the optimal average.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
Using more than three times as many stations and time series of daily data that am generally 1.5–3.0 times longer than those in a previous study, estimates of the natural variability, also known as climate noise, of surface air temperatures are extended over most North America. The potential for long-range prediction of monthly means is determined by comparing the actual interannual variability of monthly means with the climate noise that is assumed to be unpredictable at long range. The climate noise estimates am typically larger during winter than during the other seasons. Nonetheless, the potential for long-range prediction is, generally, greatest for January and least for April. During January, temperatures nearest the oceans am more predictable than those for the central portions of North America.
The low-frequency white-noise statistical model that is used to estimate the unpredictable climate noise is compared with time series of (near) surface temperatures from a general circulation model to confirm its credibility. The estimates of the potential for prediction are tested further to establish their sensitivities to a critical parameter of the statistical model and to spatial averaging.
Abstract
Using more than three times as many stations and time series of daily data that am generally 1.5–3.0 times longer than those in a previous study, estimates of the natural variability, also known as climate noise, of surface air temperatures are extended over most North America. The potential for long-range prediction of monthly means is determined by comparing the actual interannual variability of monthly means with the climate noise that is assumed to be unpredictable at long range. The climate noise estimates am typically larger during winter than during the other seasons. Nonetheless, the potential for long-range prediction is, generally, greatest for January and least for April. During January, temperatures nearest the oceans am more predictable than those for the central portions of North America.
The low-frequency white-noise statistical model that is used to estimate the unpredictable climate noise is compared with time series of (near) surface temperatures from a general circulation model to confirm its credibility. The estimates of the potential for prediction are tested further to establish their sensitivities to a critical parameter of the statistical model and to spatial averaging.
Abstract
Observational aspects of the 40–50-day oscillation are reviewed. The oscillation is the result of large-scale circulation cells oriented in the equatorial plane that move eastward from at least the Indian Ocean to the central Pacific. Anomalies in zonal winds and the velocity potential in the upper troposphere often propagate the full circumference of the globe. Related, complex convective regions also show an eastward movement. There is a zonally symmetric component to the oscillation. It is manifest in changes in surface pressure and in the relative atmospheric angular momentum. The oscillation is an important factor in the timing of active and break phases of the Indian and Australian monsoons. It affects ocean waves, currents, and air-sea interaction. The oscillation was particularly active during the First GARP (Global Atmospheric Research Program) Global Experiment year, and some features that were evident during the Monsoon Experiment are described.
Abstract
Observational aspects of the 40–50-day oscillation are reviewed. The oscillation is the result of large-scale circulation cells oriented in the equatorial plane that move eastward from at least the Indian Ocean to the central Pacific. Anomalies in zonal winds and the velocity potential in the upper troposphere often propagate the full circumference of the globe. Related, complex convective regions also show an eastward movement. There is a zonally symmetric component to the oscillation. It is manifest in changes in surface pressure and in the relative atmospheric angular momentum. The oscillation is an important factor in the timing of active and break phases of the Indian and Australian monsoons. It affects ocean waves, currents, and air-sea interaction. The oscillation was particularly active during the First GARP (Global Atmospheric Research Program) Global Experiment year, and some features that were evident during the Monsoon Experiment are described.
