Anthony R. Hansen

On the Probability Density Distribution of Planetary-Scale Atmospheric Wave Amplitude

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.

The Late Quaternary Glaciations as the Response of a Three-Component Feedback System to Earth-Orbital Forcing

Abstract A climatic feedback system previously described, consisting of three prognostic nonlinear equations governing the mass of ice sheets ζ, the mass of marine and continental marginal ice χ, and the mean ocean temperature θ is forced by a representation of the effects of external earth-orbital variations. With reasonable amplitudes for the eccentricity, obliquity, and precession forcing, the free oscillatory solutions of major period near 100 kyr can be modified in a way that substantially agrees with the δ18O-derived observations of ice mass evolution. In particular, a proper structure, variance spectrum, and “phase lock” of the major variations are obtained over the last 400 kyr. An analysis of the sensitivity of these results to variations in the model parameters and to random perturbations shows that the solution is robust for small changes in all but a few of the equation coefficients. Concomitant variability in the marine ice mass, ocean temperature and net radiation at the top of the atmospher...

A Spectral Energetics Analysis of Atmospheric Blocking

Abstract The spectral energetics of two blocking case studies from the winter of 1978–79 are calculated. One case occurred over the North Atlantic and the other over the North Pacific. The temporal evolution and geographical distribution of the energetics of the planetary-scale (zonal harmonic wavenumbers 1–4) and the intermediate-scale (wavenumbers 5–10) wave ensembles are examined to identify the physical mechanisms important in the blocking process. The Atlantic block was forced by the nonlinear interaction of intense baroclinic cyclone-scale waves with barotropic, ultralong waves. The Pacific blocking resulted from the baroclinic amplification of planetary-scale waves. In addition, at least two necessary conditions are required for the development of blocking. First, one of the forcing mechanisms must be in operation, and second, the phase of the antecedent planetary-scale waves must be such that the forcing mechanism can amplify them into a blocking pattern. In each of the cases studied, the blocking...

A Possible Marine Mechanism for Internally Generated Long-Period Climate Cycles

Abstract By reinterpreting the “sea-ice extent” discussed in a recently proposed model of auto-oscillatory climatic change (Saltzman et al., 1981) to represent high-inertia grounded and shelf sea ice forms, instead of thinner marine ice forms, it is possible for the model to yield a much longer period of auto-oscillation, perhaps as much as 100 000 years (the dominant period of ice volume variations revealed by deep-sea ocean cores). The physical basis of the model can, in principle, be tested by future evidence regarding the long-term variations of bulk ocean temperature and atmospheric CO2 concentration.

The Probability Density Distribution of the Planetary-Scale Atmospheric Wave Amplitude Revisited

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.

Midtropospheric Flow Regimes and Persistent Wintertime Anomalies of Surface-Layer Pressure and Temperature

Abstract The effects on surface-layer temperature, pressure, and circulation of flow regimes defined from parameters related to the midtropospheric flow are examined for Northern Hemisphere (NH) winter. Interregime contrasts identified from modes in the probability density distribution of midlalitude planetary-scale wave amplitude are compared and contrasted with those associated with conventionally defined blocking patterns and those associated with the Pacific/North American (PNA) teleconnection index. The wave amplitude regimes lead to surface pressure contrasts of up to 16 mb, which accompany enhanced meridional surface flow. This meridional flow appears linked to significant low-layer (100 and and 850 mb) temperature contrasts of 4°C to 10°C. Comparison of them results to similar results for blocking and the PNA teleconnection index indicates that all three lead to effects of comparable magnitude in the NH winter low-layer temperature field. These effects are comparable to or greater than the local c...

Planetary wave amplitude bimodality in the Southern Hemisphere

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...

Weather Regimes in a General Circulation Model

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 identifi...

The Probability Density Distribution of the Speed and Horizontal and Vertical Shear of the Zonal-Mean Flow

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.

Structure in the phase space of a general circulation model deduced from empirical orthogonal functions

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 (CCMO) 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. 18 refs., 16 figs.