Search Results
Abstract
It is hypothesized that the interference of stationary and traveling waves of the same longitudinal can cause some of the observed time variations in the large-scale circulation. To explore this hypothesis the eight-winter average structure of a regularly occurring, westward propagating disturbance which we earlier called the “16-day wave” is further documented. Energy quantities are calculated as this 16-day wave moves in and out of phase with the stationary or time-mean wave. The resulting time variations are similar to some already reported in the literature. Eddy heat momentum transport associated with energy conversions have phase relationships between pressure levels that can be approximately predicted by a simple linear superposition of the observed stationary waves and traveling external Rossby waves. In further support of the hypothesis, cross-spectral results determined from independent data show a reasonable agreement with these predictions.
Abstract
It is hypothesized that the interference of stationary and traveling waves of the same longitudinal can cause some of the observed time variations in the large-scale circulation. To explore this hypothesis the eight-winter average structure of a regularly occurring, westward propagating disturbance which we earlier called the “16-day wave” is further documented. Energy quantities are calculated as this 16-day wave moves in and out of phase with the stationary or time-mean wave. The resulting time variations are similar to some already reported in the literature. Eddy heat momentum transport associated with energy conversions have phase relationships between pressure levels that can be approximately predicted by a simple linear superposition of the observed stationary waves and traveling external Rossby waves. In further support of the hypothesis, cross-spectral results determined from independent data show a reasonable agreement with these predictions.
Abstract
Daily rawinsonde data from 19 near-equatorial stations are examined to learn more about annual variations of the 40–50 day oscillations. Lengths of the available time series range from 5 to 28 years. A technique is devised to isolate spectral and cross-spectral quantities as a function of season. It is determined that a variance of the zonal wind in a relatively broad band centered on 47-day periods generally exceeds that in adjacent lower and higher frequency bands by the largest amount during December January and February (DJF) and at stations in the Indian and western Pacific Oceans during all seasons. The coherence between lower-and upper-tropospheric zonal winds tends to be largest in the summer hemisphere for stations in the Indian and western Pacific Oceans. Upper tropospheric zonal and meridional winds are coherent and out of (in) phase at several stations there during DJF [June, July and August (JJA) These results. coupled with composited wind and outgoing longwave radiation data, lead us to conclude that in the Indian and western Pacific Oceans the eastwardd-waving regions of enhanced convection associated with the 40-50 day oscillation force a Kelvin-like wave to the east and anticyclonic, Rossby-like waves to the west. The anticyclonic eddies are found in the summer hemisphere during solstice seasons and cause local surges in upper-level southeasterlies (northeasterlies) during DJF (JJA).
Abstract
Daily rawinsonde data from 19 near-equatorial stations are examined to learn more about annual variations of the 40–50 day oscillations. Lengths of the available time series range from 5 to 28 years. A technique is devised to isolate spectral and cross-spectral quantities as a function of season. It is determined that a variance of the zonal wind in a relatively broad band centered on 47-day periods generally exceeds that in adjacent lower and higher frequency bands by the largest amount during December January and February (DJF) and at stations in the Indian and western Pacific Oceans during all seasons. The coherence between lower-and upper-tropospheric zonal winds tends to be largest in the summer hemisphere for stations in the Indian and western Pacific Oceans. Upper tropospheric zonal and meridional winds are coherent and out of (in) phase at several stations there during DJF [June, July and August (JJA) These results. coupled with composited wind and outgoing longwave radiation data, lead us to conclude that in the Indian and western Pacific Oceans the eastwardd-waving regions of enhanced convection associated with the 40-50 day oscillation force a Kelvin-like wave to the east and anticyclonic, Rossby-like waves to the west. The anticyclonic eddies are found in the summer hemisphere during solstice seasons and cause local surges in upper-level southeasterlies (northeasterlies) during DJF (JJA).
Abstract
Evidence of regularly propagating, large-scale waves is found in a 73-year record of Northern Hemisphere sea-level pressure data and in a nine-year record of upper air data. Cross-spectrum analyses indicate that south of 50°N, in all seasons, a zonal wavenumber 1 disturbance moves westward around the world in 5 days. In addition, north of 50°N a zonal wavenumber 1 disturbance moves westward around the world in one to three weeks with an average period near 16 days. This disturbance appears to be strongest in winter and spring. The structure of the 16-day wave during winter is studied in detail, and it is shown to be consistent, in many respects, with that of a theoretically predicted free planetary wave, or wave of the second class. A similar conclusion can be made concerning the 5-day wave.
Abstract
Evidence of regularly propagating, large-scale waves is found in a 73-year record of Northern Hemisphere sea-level pressure data and in a nine-year record of upper air data. Cross-spectrum analyses indicate that south of 50°N, in all seasons, a zonal wavenumber 1 disturbance moves westward around the world in 5 days. In addition, north of 50°N a zonal wavenumber 1 disturbance moves westward around the world in one to three weeks with an average period near 16 days. This disturbance appears to be strongest in winter and spring. The structure of the 16-day wave during winter is studied in detail, and it is shown to be consistent, in many respects, with that of a theoretically predicted free planetary wave, or wave of the second class. A similar conclusion can be made concerning the 5-day wave.
Abstract
A space-time spectral analysis of a long time series of observed geopotential heights for each season at several levels and latitudes of the Northern Hemisphere was performed as part of a continuing investigation of large-scale traveling waves. The data set that is analyzed consists of the first six zonal wavenumbers. A discussion emphasizes westward traveling wave 1 with periods near 16 and 5 days which we argue are consistent with external Rossby warm. An additional outstanding feature is an eastward propagating wave 6 which may result from baroclinic instability.
Abstract
A space-time spectral analysis of a long time series of observed geopotential heights for each season at several levels and latitudes of the Northern Hemisphere was performed as part of a continuing investigation of large-scale traveling waves. The data set that is analyzed consists of the first six zonal wavenumbers. A discussion emphasizes westward traveling wave 1 with periods near 16 and 5 days which we argue are consistent with external Rossby warm. An additional outstanding feature is an eastward propagating wave 6 which may result from baroclinic instability.
Abstract
Eastward phase propagation, at speed faster than 30 m s−1, of a signal in the equatorial troposphere of the Eastern Pacific is detected, first in historical meteorological observations and then in more recent data. A first baroclinic mode vertical structure is identified with this signal in separate analyses based on linear theory and complex empirical orthogonal functions, respectively. This rapid, eastward signal is conceptualized as a far-field dispersion product of strong convection associated with the intraseasonal tropical oscillation in the Indian Ocean and Western Pacific.
Abstract
Eastward phase propagation, at speed faster than 30 m s−1, of a signal in the equatorial troposphere of the Eastern Pacific is detected, first in historical meteorological observations and then in more recent data. A first baroclinic mode vertical structure is identified with this signal in separate analyses based on linear theory and complex empirical orthogonal functions, respectively. This rapid, eastward signal is conceptualized as a far-field dispersion product of strong convection associated with the intraseasonal tropical oscillation in the Indian Ocean and Western Pacific.
Abstract
Seasonally varying spectral and cross-spectral calculations are carried out on multiyear time series of vertically and zonally averaged daily zonal wind fields to describe the seasonal cycle of the 40–50-day oscillation of atmospheric angular momentum. Intraseasonal variability (including 40–50-day fluctuations) of global momentum is largest in late boreal winter and smallest in boreal autumn; however, the 40–50-day spectral peak is most pronounced in boreal summer when lower-frequency intraseasonal variance is depressed. The 40–50-day spectral peak in global momentum is much less pronounced and apparently is restricted to a narrower frequency band, than corresponding peaks in zonal wind spectra from individual tropical rawinsonde stations. Contributions to global momentum fluctuations from three near-equal-area latitude bands (tropics, Northern Hemisphere, and Southern Hemisphere) are compared, confirming that intraseasonal momentum fluctuations are tropical in origin. The variance of extratropical momentum at this time scale is about an order of magnitude less than the tropical momentum variability. Coherent tropical–extratropical interactions are found principally in boreal winter, with the highest coherence between the tropics and Northern Hemisphere. The corresponding phase difference between tropical and Northern Hemisphere momentum is suggestive of poleward propagation of momentum out of the tropics.
Abstract
Seasonally varying spectral and cross-spectral calculations are carried out on multiyear time series of vertically and zonally averaged daily zonal wind fields to describe the seasonal cycle of the 40–50-day oscillation of atmospheric angular momentum. Intraseasonal variability (including 40–50-day fluctuations) of global momentum is largest in late boreal winter and smallest in boreal autumn; however, the 40–50-day spectral peak is most pronounced in boreal summer when lower-frequency intraseasonal variance is depressed. The 40–50-day spectral peak in global momentum is much less pronounced and apparently is restricted to a narrower frequency band, than corresponding peaks in zonal wind spectra from individual tropical rawinsonde stations. Contributions to global momentum fluctuations from three near-equal-area latitude bands (tropics, Northern Hemisphere, and Southern Hemisphere) are compared, confirming that intraseasonal momentum fluctuations are tropical in origin. The variance of extratropical momentum at this time scale is about an order of magnitude less than the tropical momentum variability. Coherent tropical–extratropical interactions are found principally in boreal winter, with the highest coherence between the tropics and Northern Hemisphere. The corresponding phase difference between tropical and Northern Hemisphere momentum is suggestive of poleward propagation of momentum out of the tropics.
Abstract
Nearly ten years of daily rawinsonde data for Canton Island (3S, 172W) have been subjected to spectrum and cross-spectrum analysis. In the course of this analysis a very pronounced maximum was noted in the co-spectrum of the 850- and 150-mb zonal wind components in the frequency range 0.0245–0.0190 day−1 (41–53 days period). Application of a posteriori sampling theory resulted in a significance level of ∼6% (0.1% prior confidence level). This type of significance test is appropriate because no prior evidence or reason existed for expecting such a spectral feature. Subsequent analysis revealed the following structure of the oscillation. Peaks in the variance spectra of the zonal wind are strong in the low troposphere, are weak or non-existent in the 700–400 mb layer, and are strong again in the upper troposphere. No evidence of this feature could be found above 80 mb, or in any of the spectra of the meridional component. The spectrum of station pressure possesses a peak in this frequency range and the oscillation is in phase with the low tropospheric zonal wind oscillation, and out of phase with that in the upper troposphere. The tropospheric temperatures exhibit a similar peak and are highly coherent with the station pressure oscillation; positive station pressure anomalies are associated with negative temperature anomalies throughout the troposphere. Thus, the lower-middle troposphere appears to be a nodal surface with u and P oscillating in phase but 180° out of phase above and below this surface. Evidence for this phenomenon was found in shorter records at Kwajalein (9N, 168E) but not at Singapore (1N, 104E) or Balboa, Canal Zone (9N, 79w). We speculate that the oscillation is a large circulation cell oriented in zonal planes and centered in the mid-Pacific.
Abstract
Nearly ten years of daily rawinsonde data for Canton Island (3S, 172W) have been subjected to spectrum and cross-spectrum analysis. In the course of this analysis a very pronounced maximum was noted in the co-spectrum of the 850- and 150-mb zonal wind components in the frequency range 0.0245–0.0190 day−1 (41–53 days period). Application of a posteriori sampling theory resulted in a significance level of ∼6% (0.1% prior confidence level). This type of significance test is appropriate because no prior evidence or reason existed for expecting such a spectral feature. Subsequent analysis revealed the following structure of the oscillation. Peaks in the variance spectra of the zonal wind are strong in the low troposphere, are weak or non-existent in the 700–400 mb layer, and are strong again in the upper troposphere. No evidence of this feature could be found above 80 mb, or in any of the spectra of the meridional component. The spectrum of station pressure possesses a peak in this frequency range and the oscillation is in phase with the low tropospheric zonal wind oscillation, and out of phase with that in the upper troposphere. The tropospheric temperatures exhibit a similar peak and are highly coherent with the station pressure oscillation; positive station pressure anomalies are associated with negative temperature anomalies throughout the troposphere. Thus, the lower-middle troposphere appears to be a nodal surface with u and P oscillating in phase but 180° out of phase above and below this surface. Evidence for this phenomenon was found in shorter records at Kwajalein (9N, 168E) but not at Singapore (1N, 104E) or Balboa, Canal Zone (9N, 79w). We speculate that the oscillation is a large circulation cell oriented in zonal planes and centered in the mid-Pacific.
Abstract
Long time series (5–10 years) of station pressure and upper air data from stations located in the tropics are subjected to spectral and cross-spectral analysis to investigate the spatial extent of a previously detected oscillation in various variables with a period range of 40–50 days. In addition, time series of station pressure from two tropical stations for the 1890's are examined and indicate that the oscillation is a stationary feature. The cross-spectral analysis suggests that the oscillation is of global scale but restricted to the tropics: it possesses features of an eastward-moving wave whose characteristics change with time. A mean wave disturbance, constructed with data from the IGY, provides additional descriptive material on the spatial and temporal behavior of the oscillation. The manifestation in station pressure consists of anomalies which appear between 10N and 10S in the Indian Ocean region and propagate eastward to the Eastern Pacific. Zonal winds participate in the oscillation and, in general, are out-of-phase between the upper and lower troposphere. Mixing ratios and temperatures are also investigated. The sum total of evidence indicates that the oscillation is the result of an eastward movement of large-scale circulation cells oriented in the equatorial (zonal) plane.
Abstract
Long time series (5–10 years) of station pressure and upper air data from stations located in the tropics are subjected to spectral and cross-spectral analysis to investigate the spatial extent of a previously detected oscillation in various variables with a period range of 40–50 days. In addition, time series of station pressure from two tropical stations for the 1890's are examined and indicate that the oscillation is a stationary feature. The cross-spectral analysis suggests that the oscillation is of global scale but restricted to the tropics: it possesses features of an eastward-moving wave whose characteristics change with time. A mean wave disturbance, constructed with data from the IGY, provides additional descriptive material on the spatial and temporal behavior of the oscillation. The manifestation in station pressure consists of anomalies which appear between 10N and 10S in the Indian Ocean region and propagate eastward to the Eastern Pacific. Zonal winds participate in the oscillation and, in general, are out-of-phase between the upper and lower troposphere. Mixing ratios and temperatures are also investigated. The sum total of evidence indicates that the oscillation is the result of an eastward movement of large-scale circulation cells oriented in the equatorial (zonal) plane.
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.
