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Anthony R. Hansen

Abstract

Further results concerning the mean gates of the bimodal wavnumber 2 to 4 amplitude probability density distribution are presented followed by composites of transitions from one side of this distribution to the other. The data used are ECMWF analyses from the four winters from 1980/81 to 1983/84.

Cross sections of the mean states associated with the two modes reveal that both modes exhibit a baroclinic vertical structure, but that the difference between the two is more nearly equivalent barotropic. The composite transitions between the low-amplitude and high-amplitude states indicate that the transition time for the onset or decay of the large amplitude waves is about 4 days. The kinetic energy and available potential energy of wavenumbers 2 to 4 increases (or decreases) by 50 percent in this same time interval during the onset (or decay) of the large amplitude state. Nonlinear interaction with intermediate-scale waves is the only apparent source for the observed kinetic energy tendency during the transition from the low amplitude to the high amplitude mode. Thus, the growth of the 1arge amplitude events does not strictly resemble that of a classical baroclinic instability. During the decay of the large amplitude waves, nonlinear interaction between the wavenumber 2 to 4 ensemble and wavenumber 1 accounts for the decline in kinetic energy, while nonlinear interaction between wavenumbers 2 to 4 and smaller-scale waves accounts for the decline in available potential energy. Examples of individual cases are presented to corroborate the composite results.

Finally, a case study of the synoptic evolution of a large-amplitude event is presented to illustrate the event's life cycle.

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Anthony R. Hansen and Alfonso Sutera

Abstract

The effect of the zonally asymmetric forcing due to topography on the low-frequency variability of the large-scale flow is investigated for Northern Hemisphere winter conditions. Extended general circulation model integrations are used in which the topographic heights are reduced.

The effect of reduced topographic heights is to reduce the mean persistence of recurrent regimes identified from the amplitude of the planetary waves with spatial scales comparable to the topography. The number of episodes of large wave amplitude and their anomaly patterns are affected very little. The impact of topography on the total gridpoint height variance includes two components. Decreased topographic heights lead to increased high-frequency eastward-traveling variance and decreased low-frequency variance. In addition, the regionalization of the Pacific and Atlantic storm tracks found in the control simulation diminishes as the topographic heights are reduced.

From the results the authors conclude that the occurrence of persistent regimes in the large-scale flow is linked to the presence of topographic forcing of sufficient amplitude but that the amplification mechanism of the planetary waves is not directly linked to the topographic forcing. Therefore, it appears that the topography plays a catalytic role in permitting longer persistence of a large-scale, amplified planetary wave flow regime.

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Anthony R. Hansen and Alfonso Sutera

Abstract

Perpetual January and July simulations each 1200 days long of the NCAR Community Climate Model (CCM0B) are investigated for the existence of large scale, midlatitude weather regimes. Four realizations of the midlatitude circulation were considered: Northern Hemisphere (NH) winter, Northern Hemisphere summer, Southern Hemisphere (SH) winter, and Southern Hemisphere summer. Statistically significant bimodality appears in the planetary-wave amplitude probability density distributions in the former three cases that is very similar to that observed in the atmosphere. The probability density estimation for SH summer in the model is also similar to observations in general, but a hint of a second mode also appeared on the high amplitude tail of the distribution. The fact that the bimodality is present in a fixed external forcing simulation implies that it is not connected to changes in boundary conditions, but rather that it is internally generated.

The statistical flow regimes in physical space identified by the bimodal distributions are generally similar in the model to those in the atmosphere for NH winter and SH winter. Systematic errors in the model during NH summer preclude close comparisons to observations. Interestingly, the model's SH winter wavenumber 3 large-amplitude mode also shows bimodality in its phase. Partitioning the SH model days based on the amplitude and phase bimodality identifies three hemispheric-scale flow regimes in the SH during the perpetual July simulation. As with the observations, the time-mean circulation is not generally realized as a persistent weather regime in any of the bimodal CCM simulations.

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Anthony R. Hansen and Alfonso Sutera

Abstract

The transient eddy height variance in midlatitudes during Southern Hemisphere (SH) winter achieves its largest value at zonal wavenumber 3. The presence of two highly statistically significant modes in the probability density distribution of the zonal wavenumber 3 amplitude allows one to conclude that this variance is the result of the circulation switching between two statistical flow regimes. One regime is characterized by a predominantly wavenumber 1 pattern and the other by an amplified wavenumber 3 pattern. The probability density distributions of the duration of these two regimes can be roughly approximated as exponential functions with e-folding times of 6 to 12 days. Thus, on the intraseasonal time scale during SH winter, the time mean flow is not the most probable state of the circulation. In contrast, during SH summer no bimodality occurs and the wavenumber 3 probability density distribution is strongly skewed toward the winter low amplitude mode.

Simple energetics considerations suggest that in SH winter wave-wave interaction between intermediate-scale eddies (wavenumbers 5 to 7) and wavenumber 3 is a significant energy source to maintain the amplified wave pattern, while wave-mean flow interactions are a sink for wavenumber 3 kinetic energy. In contrast, during SH summer wave–wave coupling between intermediate-scale transients and wavenumber 3 is sharply reduced compared to winter. This suggests that wave–wave interaction is an important component in the mechanism of the SH bimodality.

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Anthony R. Hansen and Alfonso Sutera

Abstract

The probability density distribution of the speed, horizontal shear and vertical shear of the zonal-mean wind are computed from a 16-winter NMC dataset in a study designed to complement an earlier study of the planetary-scale wave amplitude probability density distribution. The speed of the zonal wind is found to have a probability density distribution that is unimodal both within the zone of the largest planetary wave amplitude (45°–70°N) and when averaged over the entire midlatitudes (25°–75°N). Unimodal distributions are also found for both the north–south shear of the zonal mean wind and the vertical shear of the zonal mean flow between 300 and 850 mb.

Lagged correlations between the zonal mean wind parameters and the wavenumber 2–4 amplitude are computed and found to be small at all lags. This suggests that the temporal variability of the planetary waves in the midlatitudes of the troposphere is not directly connected to the variability in the strength or shear of the zonal mean flow.

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Anthony R. Hansen and Alfonso Sutera

Abstract

A preliminary study of the probability density distribution of the wavenumber 3 amplitude in midlatitudes of the Southern Hemisphere is undertaken with 4.25 years of 500 mb height data compiled by the European Centre for Medium-Range Weather Forecasts. The wavenumber 3 amplitude probability density appears to be bimodal during winter. Analogous results during the Southern Hemisphere summer reveal a unimodal wavenumber 3 amplitude probability density distribution with the single summer mode corresponding in value to the winter low amplitude mode.

The physical implications of the winter bimodality are examined to gain confidence in the result. Partitioning the data based on the two modes leads to a consistent picture of the large amplitude events in physical space. Synoptically, the large amplitude mode corresponds to “amplified waves” of broad extent. It is suggested that the SH winter time-mean eddies for wavenumbers greater than one are the statistical residue of the intermittent large-amplitude events. Individual Southern Hemisphere large amplitude events exhibit grid point height departures from zonal symmetry comparable to their Northern Hemisphere counterparts. In addition, the summer mean eddy field is very similar to the winter low amplitude mode's mean eddy field.

Examination of the time series of the wave 3 amplitude and phase during winter reveals that the more persistent of the individual large amplitude events exhibit relatively stationary phase lines, although different large amplitude events may have different phases. A remarkable feature of these time series is that certain of the more persistent events exhibit a sudden phase shift while the amplitude remains large with the phase being steady both before and after the shift.

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Anthony R. Hansen and Alfonso Sutera

Abstract

Recent studies of low-frequency variability have shown that at least two planetary-scale statistical flow regimes exist in the Northern Hemisphere winter circulation both in observations and in a general circulation model. This result was obtained from an analysis of a large-scale circulation index based on planetary-wave amplitude. In this paper, a 1200-day integration of the NCAR Community Climate Model (CCM0) in perpetual January mode is used as a case study to show that similar results in terms of multiple flow regimes can also be obtained from an empirical orthogonal function (EOF) analysis. Two modes are found in the probability density distribution in the subspace formed from the leading two EOFs of the model. There is an apparent correspondence between these modes and the two modes deduced from the previous wave-amplitude analysis.

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Anthony R. Hansen and Alfonso Sutera

Abstract

The composite of large amplitude flow anomalies identified from extremely large amplitudes of the planetary-scale waves is examined in terms of the temporal and spatial evolution of both the large-scale flow and the storm tracks. The characteristic spatial patterns, growth and decay rates, and persistence characteristics that the individual large amplitude anomaly cases share come out naturally in the analysis.

The composite anomaly's growth and decay are very rapid, taking an average of only 4 days to develop local anomalies of 200–300 m. The spatial evolution of the flow suggests a rapidly growing standing wave over the North Pacific Ocean and North America. After a persistence of random duration (averaging 8.4 days), the composite anomaly's decay is accompanied by simultaneous retrogression of the pattern from western North America to eastern Asia and eastward progression of the pattern over Europe and western Asia. Substantial disruption of the Pacific storm track and enhancement of the Atlantic storm track accompanies the life cycle of this flow regime. A residual effect of the regime life cycle is a reduction in the low-level meridional temperature gradient, particularly over eastern Asia in the entry region of the Pacific jet stream.

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Anthony R. Hansen and Alfonso Sutera

Abstract

The investigation for evidence of multiple statistical flow regimes in the amplitude of the planetary-scale waves presented earlier by Hansen and Sutera is revisited. A methodology for generating Monte Carlo tests of the statistical significance of univariate probability density estimates of temporally correlated time series is proposed and applied to a 42-winter time series of the amplitude of the planetary-scale waves. The statistical significance of bimodality in the planetary wave amplitude can be established from a 28-winter dataset as well as the 42-winter dataset, but not from the 16-winter dataset originally used by Hansen and Sutera. Corroboration of this result is found from comparable analysis of an extended general circulation model integration and from examination of the results of other independent observational studies. Several analysis schemes find flow regimes with a similar pattern that features large amplitudes in the planetary-scale waves.

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Anthony R. Hansen and Alfonso Sutera

Abstract

The statistical properties of a measure of planetary-scale wave activity are investigated in a 16 winter NMC dataset. The probability density distribution of the wavenumber 2 to 4 amplitude is found to be bimodal, confirming earlier results from a smaller dataset. The statistical significance of this result is established empirically with statistical simulations. It is also shown that the bimodality is not connected with any periodicity in the time series.

Partitioning the data based on the density estimation reveals two statistical flow regimes in physical space. One corresponds to an amplified planetary-scale wave pattern and the other to a predominantly zonal flow. Both regimes exhibit a baroclinic vertical structure but the difference between them is equivalent barotropic. These differences extend through the depth of the troposphere and appear to be of hemispheric extent.

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