Search Results
Abstract
Precipitation surges in the equatorial Pacific “dry zone” (as delineated in a study by Doberitz) reveal statistically significant teleconnections with midlatitude tropospheric flow patterns during all seasons. These connections are strongest, however, during winter and agree with linear numerical model results. There is evidence that deep troughs in the central North Pacific antecede precipitation surges in the equatorial dry zone during that season, suggesting the possibility of triggering of such surges by middle latitude planetary-wave perturbations. Nonlinear numerical model results also indicate that the midlatitude planetary-wave response is strongest when North Pacific cold sea-surface temperature anomalies and equatorial East Pacific warm anomalies are present simultaneously.
The planetary-wave pattern associated with equatorial Pacific precipitation surges appears to be in phase with orographically generated wave patterns, whereas an anomalous low-latitude heat source over the extreme equatorial west Pacific, the Bay of Bengal and India, corresponding to rainfall during an active phase of the Indian monsoon, tends to be associated with wave perturbations out-of-phase with orographic forcing.
Abstract
Precipitation surges in the equatorial Pacific “dry zone” (as delineated in a study by Doberitz) reveal statistically significant teleconnections with midlatitude tropospheric flow patterns during all seasons. These connections are strongest, however, during winter and agree with linear numerical model results. There is evidence that deep troughs in the central North Pacific antecede precipitation surges in the equatorial dry zone during that season, suggesting the possibility of triggering of such surges by middle latitude planetary-wave perturbations. Nonlinear numerical model results also indicate that the midlatitude planetary-wave response is strongest when North Pacific cold sea-surface temperature anomalies and equatorial East Pacific warm anomalies are present simultaneously.
The planetary-wave pattern associated with equatorial Pacific precipitation surges appears to be in phase with orographically generated wave patterns, whereas an anomalous low-latitude heat source over the extreme equatorial west Pacific, the Bay of Bengal and India, corresponding to rainfall during an active phase of the Indian monsoon, tends to be associated with wave perturbations out-of-phase with orographic forcing.
Abstract
Following the perturbation theory, the aircraft measurements of “Project Jet Stream” Flight No. 29 (4 April 1957) have been analyzed, applying a double-smoothing technique, thus treating the basic fields of flow and temperature separately from the superimposed mesoscale disturbances. Analyses of wind velocity and potential temperature lead to estimates of the distributions of vorticity, potential vorticity and divergence in a vertical cross-section. Several inferences are made as to the meso-structure of the atmosphere near the jet-stream core.
Abstract
Following the perturbation theory, the aircraft measurements of “Project Jet Stream” Flight No. 29 (4 April 1957) have been analyzed, applying a double-smoothing technique, thus treating the basic fields of flow and temperature separately from the superimposed mesoscale disturbances. Analyses of wind velocity and potential temperature lead to estimates of the distributions of vorticity, potential vorticity and divergence in a vertical cross-section. Several inferences are made as to the meso-structure of the atmosphere near the jet-stream core.
Abstract
The three-dimensional wind field in the vicinity of the jet stream is defined with the aid of four parameters: mean wind speed and direction, thickness, and mean altitude of the Layer of Maximum Wind (LMW). Routine methods of analysis of these quantities are discussed in view of the reliability of available wind data, and a statistical treatment of these quantities reveals the mean horizontal and vertical structure of the LMW. Forecasts of the LMW are attempted by means of a grid or “box” method, and a control-line method.
Abstract
The three-dimensional wind field in the vicinity of the jet stream is defined with the aid of four parameters: mean wind speed and direction, thickness, and mean altitude of the Layer of Maximum Wind (LMW). Routine methods of analysis of these quantities are discussed in view of the reliability of available wind data, and a statistical treatment of these quantities reveals the mean horizontal and vertical structure of the LMW. Forecasts of the LMW are attempted by means of a grid or “box” method, and a control-line method.
Abstract
Evidence is derived from observational data that the trade wind circulations in both hemispheres over the Pacific are related to the sea surface temperature (SST) anomalies in the North Pacific. The equatorward component of the surface trade winds leads the North Pacific SST anomalies by approximately 20 months, equivalent to the half-rotation time of the Pacific gyre.
Long-term trends in the Pacific trade wind regime appear to have an influence on hemispheric and global mean temperatures. The trade wind anomalies also seem to be tied to meridional atmospheric pressure- and temperature-gradient anomalies which, in turn, appear to respond to anomalies in the meridional SST gradients. Thus a closed feedback loop is demonstrated, in which SST anomalies in the North Pacific generate trade wind anomalies which promote the development of North Pacific SST anomalies of opposite sign. The trade wind surges also are related to El Niño through a feedback involving the hydrological cycle and upwelling of cold water forced by Ekman pumping.
Abstract
Evidence is derived from observational data that the trade wind circulations in both hemispheres over the Pacific are related to the sea surface temperature (SST) anomalies in the North Pacific. The equatorward component of the surface trade winds leads the North Pacific SST anomalies by approximately 20 months, equivalent to the half-rotation time of the Pacific gyre.
Long-term trends in the Pacific trade wind regime appear to have an influence on hemispheric and global mean temperatures. The trade wind anomalies also seem to be tied to meridional atmospheric pressure- and temperature-gradient anomalies which, in turn, appear to respond to anomalies in the meridional SST gradients. Thus a closed feedback loop is demonstrated, in which SST anomalies in the North Pacific generate trade wind anomalies which promote the development of North Pacific SST anomalies of opposite sign. The trade wind surges also are related to El Niño through a feedback involving the hydrological cycle and upwelling of cold water forced by Ekman pumping.
Abstract
Gridded NMC data for 500 mb geopotential height, and 300–500 mb and 500–700 mb thickness for the period 1951–78 wore subjected to linear trend analyses. These analyses were performed for each calendar month. Significant geographical and seasonal distributions of cooling and warming patterns emerged. An atmospheric cooling trend over the North Pacific during the winter months appears in a region where oceanic cooling has also been observed, but planetary-wave adjustments rather than ocean-atmosphere feedback mechanisms appear to dominate the atmospheric cooling on climatic time scales. Consistently large temperature trends also appear over the Asian continent. Comparisons between thickness trends in the layer 300–500 mb with those in the. layer 500–700 mb reveal well-pronounced patterns of stabilization and destabilization.
Abstract
Gridded NMC data for 500 mb geopotential height, and 300–500 mb and 500–700 mb thickness for the period 1951–78 wore subjected to linear trend analyses. These analyses were performed for each calendar month. Significant geographical and seasonal distributions of cooling and warming patterns emerged. An atmospheric cooling trend over the North Pacific during the winter months appears in a region where oceanic cooling has also been observed, but planetary-wave adjustments rather than ocean-atmosphere feedback mechanisms appear to dominate the atmospheric cooling on climatic time scales. Consistently large temperature trends also appear over the Asian continent. Comparisons between thickness trends in the layer 300–500 mb with those in the. layer 500–700 mb reveal well-pronounced patterns of stabilization and destabilization.
Abstract
Ultralong and long planetary waves are analyzed at the 500 mb level in terms of their amplitudes, phases and stationarity characteristics, the latter described in terms of a stationarity index SI n . This index consists of the ratio between planetary-wave amplitudes computed from time-averaged 500 mb height patterns, and the wave amplitudes computed on a daily basis and averaged for the same time interval irrespective of their phase angles. The index assumes the value 1 for completely stationary waves and 0 for randomly variable waves.
Mean 500 mb height and planetary wave characteristics have been developed by calendar day. using NMC data between the years 1946 and 1979. Various significant singularities (i.e., departures from a low-order harmonic seasonal trend) in planetary-wave behavior are described. An index, SI n (d), computed by calendar day, was developed as a measure of probability that a certain planetary wave with number n would achieve its long-term mean phase angle on a certain date. This index, too, shows significant patterns with latitude and season, suggesting the presence of relatively short periods within the seasonal cycles of planetary-wave behavior, during which these waves may be extra sensitive to anomalies in their forcing parameters.
It is suggested that perturbation statistics derived from numerical general circulation models should be compared with the results presented in this study to permit an improved judgement on the veracity of such models.
Abstract
Ultralong and long planetary waves are analyzed at the 500 mb level in terms of their amplitudes, phases and stationarity characteristics, the latter described in terms of a stationarity index SI n . This index consists of the ratio between planetary-wave amplitudes computed from time-averaged 500 mb height patterns, and the wave amplitudes computed on a daily basis and averaged for the same time interval irrespective of their phase angles. The index assumes the value 1 for completely stationary waves and 0 for randomly variable waves.
Mean 500 mb height and planetary wave characteristics have been developed by calendar day. using NMC data between the years 1946 and 1979. Various significant singularities (i.e., departures from a low-order harmonic seasonal trend) in planetary-wave behavior are described. An index, SI n (d), computed by calendar day, was developed as a measure of probability that a certain planetary wave with number n would achieve its long-term mean phase angle on a certain date. This index, too, shows significant patterns with latitude and season, suggesting the presence of relatively short periods within the seasonal cycles of planetary-wave behavior, during which these waves may be extra sensitive to anomalies in their forcing parameters.
It is suggested that perturbation statistics derived from numerical general circulation models should be compared with the results presented in this study to permit an improved judgement on the veracity of such models.
Abstract
A strong, persistent and significant oscillation of about 24-day periodicity is observed in hemispheric-scale energy parameters during the winter season. The characteristics of this cycle are defined using 9.5 years of daily NMC gridded height and temperature fields. Analysis of northward eddy heat flux strongly suggests that baroclinic processes are involved in this oscillation. The vacillation acts primarily on the planetary-wave scale and some of its synoptic characteristics are investigated. A decomposition of eddy available potential energy by zone wavenumber shows that the vacillation tends to favor different wavenumbers in different years. The concept of a vacillation in orientation of trough and ridge line tilt is not substantiated.
Abstract
A strong, persistent and significant oscillation of about 24-day periodicity is observed in hemispheric-scale energy parameters during the winter season. The characteristics of this cycle are defined using 9.5 years of daily NMC gridded height and temperature fields. Analysis of northward eddy heat flux strongly suggests that baroclinic processes are involved in this oscillation. The vacillation acts primarily on the planetary-wave scale and some of its synoptic characteristics are investigated. A decomposition of eddy available potential energy by zone wavenumber shows that the vacillation tends to favor different wavenumbers in different years. The concept of a vacillation in orientation of trough and ridge line tilt is not substantiated.
Abstract
Measurements of clear-air turbulence spectra conducted by a Canberra aircraft over Australia between July and October 1963 reveal the existence of a wavelength region from somewhat less than 1000 ft to 4000 ft, in which the atmosphere receives turbulent energy. It is suggested that this energy stems from gravitational shearing waves which break up into turbulent eddies below a critical wavelength. The energies of these turbulent eddies seem to he well represented by a proportionality to k −5/3, characteristic of the inertial subrange of turbulence.
Abstract
Measurements of clear-air turbulence spectra conducted by a Canberra aircraft over Australia between July and October 1963 reveal the existence of a wavelength region from somewhat less than 1000 ft to 4000 ft, in which the atmosphere receives turbulent energy. It is suggested that this energy stems from gravitational shearing waves which break up into turbulent eddies below a critical wavelength. The energies of these turbulent eddies seem to he well represented by a proportionality to k −5/3, characteristic of the inertial subrange of turbulence.
Abstract
A two-level, global, spectral model is used to study the response of the atmosphere to sea surface temperature anomalies. Two sea surface temperature anomaly patterns are investigated. The first, called the El Niño pattern (Experiment 1), represents a warm anomaly in the equatorial Pacific, whereas the second pattern (Experiment 2) represents coupled midlatitude (cold)/ equatorial (warm) sea surface temperature anomalies in the pacific Ocean.
The results demonstrate that both of these sea surface temperature anomaly patterns produce statistically significant midtropospheric geopotential responses in middle latitudes. However, the geopotential response forced by the coupled sea surface temperature anomaly is qualitatively more similar to the geopotential height pattern which is observed in association with the negative phase of the Southern Oscillation (Horel and Wallace). Analysis of the differences (anomaly minus control) of the meridional transports of momentum. sensible heat and latent heat indicates that the coupled pattern tends to largely enhance the northward transports of momentum and sensible heat, especially for the transient and stationary eddy components. The maximum difference in the total (transient, stationary eddies and mean meridional circulation) transport of momentum is nearly double that revealed by the El Niño experiment.
Abstract
A two-level, global, spectral model is used to study the response of the atmosphere to sea surface temperature anomalies. Two sea surface temperature anomaly patterns are investigated. The first, called the El Niño pattern (Experiment 1), represents a warm anomaly in the equatorial Pacific, whereas the second pattern (Experiment 2) represents coupled midlatitude (cold)/ equatorial (warm) sea surface temperature anomalies in the pacific Ocean.
The results demonstrate that both of these sea surface temperature anomaly patterns produce statistically significant midtropospheric geopotential responses in middle latitudes. However, the geopotential response forced by the coupled sea surface temperature anomaly is qualitatively more similar to the geopotential height pattern which is observed in association with the negative phase of the Southern Oscillation (Horel and Wallace). Analysis of the differences (anomaly minus control) of the meridional transports of momentum. sensible heat and latent heat indicates that the coupled pattern tends to largely enhance the northward transports of momentum and sensible heat, especially for the transient and stationary eddy components. The maximum difference in the total (transient, stationary eddies and mean meridional circulation) transport of momentum is nearly double that revealed by the El Niño experiment.
Abstract
The influence of sensible heating from the earth's surface on the development of summertime vortices over the Tibetan Plateau was investigated using a numerical model. It was found that sensible heating could cause local intensification of vortices over high elevations and sometimes act in combination with topography to block intrusions of cold air. Sensible heating can play an important role, not indicated by its magnitude, when it is combined with topography and the proper synoptic situation. Sensible heating had a greater impact over higher elevations, areas with strong cold advection, and areas under the upper-tropospheric jet stream. Sensible heating tends to destabilize an air column, permitting downward transfer of westerly momentum in the vicinity of the jet stream and causing an increase in cyclonic vorticity in the lower troposphere north of the upper-level jet. During the premonsoon period, when the upper-level jet was located over the southern plateau, sensible heating acted to intensify plateau vortices. After the transition into the summer monsoon period, the jet was north of the plateau and sensible heating had only localized and gradual effects on plateau vortices.
Abstract
The influence of sensible heating from the earth's surface on the development of summertime vortices over the Tibetan Plateau was investigated using a numerical model. It was found that sensible heating could cause local intensification of vortices over high elevations and sometimes act in combination with topography to block intrusions of cold air. Sensible heating can play an important role, not indicated by its magnitude, when it is combined with topography and the proper synoptic situation. Sensible heating had a greater impact over higher elevations, areas with strong cold advection, and areas under the upper-tropospheric jet stream. Sensible heating tends to destabilize an air column, permitting downward transfer of westerly momentum in the vicinity of the jet stream and causing an increase in cyclonic vorticity in the lower troposphere north of the upper-level jet. During the premonsoon period, when the upper-level jet was located over the southern plateau, sensible heating acted to intensify plateau vortices. After the transition into the summer monsoon period, the jet was north of the plateau and sensible heating had only localized and gradual effects on plateau vortices.
