Extratropical Atmosphere Research Articles

Circulation regime fluctuations and their effect on intraseasonal variability in the ECHAM climate model

The interannual variability of the observed extratropical atmosphere is known to be arranged in well-defined spatial patterns (teleconnection patterns). The representation of these patterns in multi-year runs with the ECHAM2/T21 general circulation model is investigated for Northern Hemisphere winter, applying the techniques of teleconnectivity analysis and EOF-analysis. Simulations either including or neglecting the forcing effect of interannual sea surface temperature variability (SST) are considered. Prominent modes of interannual atmospheric variability like the Atlantic oscillation, the West Pacific pattern, and the Pacific/North American pattern are all reproduced by the model. If interannual SST variations are included, the simulation of the patterns is far more realistic. The features of the Atlantic Oscillation appear to be particularly sensitive in this respect. The fluctuations of the interannual variability patterns (or regimes) have a substantial influence on the strength and the spatial structure of intraseasonal transient activity, both on the cyclonic and the low-frequency (weekly) time scale. This is consistently pointed out by conventional transient eddy statistics and by consideration of individual synoptic events. The simulated relationship between interannual variability and intraseasonal variability compares favourably with observational evidence. Hence, the results suggest that methods of deducing local climate changes from large-scale response patterns of the model may be successful. DOI: 10.1034/j.1600-0870.1994.t01-2-00003.x

Large scale perturbations in extratropical atmosphere—Part II: On geostrophic waves

We have examined, in Part I, the propagation mechanism and geostrophic property of classical Rossby waves in a non-divergent barotropic atmosphere. As we found that the non-divergent Rossby waves do not propagate in a hydrostatically equilibrium atmosphere, and do not manifest a good geostrophic property, an alternative large scale circulation pattern of geostrophic waves has been proposed (McHall, 1991a). The propagation mechanism and geostrophic property of these waves are examined in the present study.

Large scale perturbations in extratropical atmosphere—Part I: On rossby waves

This study reexamines the propagation mechanism and geostrophic property of the classical two dimensional Rossby waves in a non-divergent barotropic atmosphere. It will be found that propagation of large scale atmospheric waves depends crucially on horizontal divergence. A small Rossby number in Rossby waves is not sufficient for the waves to have a small ageostrophic component, because the two dimensional classical Rossby waves do not manifest the geostrophic balance as good as observed in the atmosphere.

Baroclinic adjustment in a zonally varying flow

AbstractIn agreement with previous published results, based on a 2‐layer model measure of baroclinicity, the extratropical northern hemisphere zonal mean atmosphere is found to be close to marginal stability in both winter and summer of the FGGE year. In winter, however, considerable zonal variations are present. Results of some numerical model runs, with both zonally symmetric and varying thermal forcing, are presented, and it is found that for sufficiently short forcing time scales a state close to marginal stability, and a storm‐tracklike region, can be produced. Large steady eddies do not seem to result so readily from the thermal forcing. Time‐lagged correlations provide evidence of forcing of the high frequency (periods less than 10 days) eddy heat flux by the baroclinicity, and it is suggested that the low frequency eddy heat flux may be a result of a nonlinear cascade from the baroclinic scales.

On the Role of Ozone in the Stability of Rossby Normal Modes

The role of ozone in the linear stability of Rossby normal modes is examined in a continuously stratified, extratropical baroclinic atmosphere. The flow is described by coupled equations for the quasi-geostraphic potential vorticity and ozone volume mixing ratio. A perturbation analysis is carried out under the assumption of weak diabatic heating, which is generated by Newtonian cooling and dynamics-ozone interaction. An expression for the propagation and growth characteristics is obtained analytically in terms of the vertically averaged wave activity, which depends explicitly on the wave spatial structure, photochemistry, and basic state distributions of wind, temperature, and ozone mixing ratio. Calculations show that stationary internal modes, whose amplitudes are largest in the stratosphre, are destabilized by dynamics-ozone interaction and Newtonian cooling, with e-folding times on the order of 20-40 days.

The EOFs of the Geopotential Eddies at 500 mb in Winter and Their Probability Density Distributions

In this paper we examine the empirical orthogonal functions (EOFs) of the geopotential eddies at 500 mb in the northern extratropical atmosphere during the cold season, and discuss the time variability of the EOF coefficients and their probability density distributions. Our data sample includes 1152 5-day mean fields derived from 32 years of NMC and ECMWF analyses. The first EOF is representative of the wintertime stationary eddies: the second and the fourth show good similarity to the PNA and WA teleconnection patterns, respectively. The pattern of the fifth EOF resembles a zonal wavenumber-3 with a latitudinally confined amplitude. The probability density distributions, estimated by means of nonparametric statistical techniques, have been computed both for the time-dependent amplitudes of individual EOFs and for indices of the spatial variance explained by more than one pattern. The EOF-5 appeared to be particularly relevant in this analysis; its amplitude shows a bimodal distribution, and bimodality is also found in the distribution of the combined amplitude of the first few EOFs when the fifth is included. The difference between the composite map computed averaging the fields classified in the high-amplitude mode (according to EOFs 1–5) and the corresponding map for the low-amplitude mode shows a large-scale pattern with positive maxima in regions known to be preferred locations for blocking centers.

Cumulus Heating and CISK in the Extratropical Atmosphere. Part I: Polar Lows and Comma Clouds

Abstract The effects of cumulus heating and the possibility of CISK in extratropical latitudes are explored by combining the Eady model of baroclinic instability with wave-CISK. For perturbations about a state of rest, there is no intrinsic time scale, however the equations can be solved for geopotential tendency. It was found that CISK disturbances could not grow unless the heating exceeded a threshold value. This value is reduced when the boundary layer moisture content is increased, the static stability decreased, or the heating distributed lower in the troposphere. Perturbations about a baroclinic basic state, for small heating rates resemble a baroclinic wave with heating acting mainly to reduce the effective static stability, resulting in faster growth and shorter wavelengths. However, as the heating is increased, the instability gradually takes on the characteristics of the pure CISK disturbance. The transition level shows the same dependencies on model parameters as for the threshold in the CISK c...

On the extended-range predictability of large-scale quasi-stationary patterns in the atmosphere

A principal objective of this article is to establish theoretically the result that, contrary to some previously held beliefs, internal nonlinear processes (wave-wave and wave-mean interactions) among the large scales not likely to be the dominant mechanisms that lead to unpredictability of the low-frequency portion of the large-scale waves in the extra-tropical atmosphere. It is further argued that theoretically quasi-stationary long waves can probably be predicted at extended ranges if boundary forcings and tropical influence can be treated as known during the range of prediction. The problem of long-range predictability of quasi-stationary long waves is studied here using a three-dimensional quasi-geostrophic channel model of the atmosphere, with lateral and lower boundary forcings prescribed. The low-frequency component of the flow is isolated by taking the running-time mean with an averaging period taken to be longer than the typical lifecycle of synoptic-scale baroclinic waves. When this period is chosen to be 10 days or longer, it is found that the evolution of the large-scale flow is not governed by a regular initial-value problem and detailed information of the initial condition is not “remembered‘ by the quasi-stationary waves beyond the predictability limit for the atmosphere as a whole. Instead, it is found that the extended-range evolution of the stationary waves responds to the corresponding evolution of the zonal index, which measures the total zonal geostrophic angular momentum in the channel. Some issues of theoretical predictability, such as the effect of small errors in the initial condition and of small uncertainties in our knowledge of the transient eddy fluxes, are addressed. It is argued that, since the evolution of the zonal index is found to be governed by a remarkably simple first order nonlinear differential equation of a single scalar function of time, whose solution has predictable properties, the quasi-stationary long waves, whose evolution is determined by the prognostic information contained in the zonal index, are perhaps also predictable beyond the “predictability limit”. DOI: 10.1111/j.1600-0870.1986.tb00419.x

The extra-tropical atmospheric response to El Niño events-a multivariate significance analysis

Up to now, a well-defined and consistent image of the extra-tropical atmosphere's response to intense El Nino warmings of the sea surface was missing—in contrast to the tropical area near the forcing source. We attribute this to the currently usual inadequate analysis methods which lead to a misinterpretation of multidimensional signals in the presence of large random fluctuations. In this paper, we demonstrate how the multi-variate significance test procedures, reviewed herein, serve to detect the El Nino signal in the GCM atmosphere of the ECMWF as well as in the observed circulation in 1982/83. The results of the rigorous statistical tests are illustrated by stability and univariate considerations and are “synoptically” compared with respective response patterns generated by a few other GCMs. It is concluded that a stable and highly significant extra-tropical response in terms of 500 mbar height to an El Nino warming exists. It is found that the January 1983 observed and the GCM generated patterns are highly coherent. This signal covers the 270° sector of longitude from the North-East Pacific to Eurasia. The model response to a cold SST anomaly is marginally significant, less stable, and different in structure, so that a strong non-linearity outside the Pacific region is evident. Furthermore, we hypothesize that an intense El Nino event induces an intensification of the north-westerly Atlantic daily variability. DOI: 10.1111/j.1600-0870.1985.tb00433.x

The extra-tropical atmospheric response to El Niño events-a multivariate significance analysis

Up to now, a well-defined and consistent image of the extra-tropical atmosphere's response to intense El Niño warmings of the sea surface was missing—in contrast to the tropical area near the forcing source. We attribute this to the currently usual inadequate analysis methods which lead to a misinterpretation of multidimensional signals in the presence of large random fluctuations. In this paper, we demonstrate how the multi-variate significance test procedures, reviewed herein, serve to detect the El Niño signal in the GCM atmosphere of the ECMWF as well as in the observed circulation in 1982/83. The results of the rigorous statistical tests are illustrated by stability and univariate considerations and are “synoptically” compared with respective response patterns generated by a few other GCMs. It is concluded that a stable and highly significant extra-tropical response in terms of 500 mbar height to an El Niño warming exists. It is found that the January 1983 observed and the GCM generated patterns are highly coherent. This signal covers the 270° sector of longitude from the North-East Pacific to Eurasia. The model response to a cold SST anomaly is marginally significant, less stable, and different in structure, so that a strong non-linearity outside the Pacific region is evident. Furthermore, we hypothesize that an intense El Niño event induces an intensification of the north-westerly Atlantic daily variability.

An Analysis of Interactions between Geostrophic and Ageostrophic Modes in a Simple Model

Abstract We examine some properties of the extratropical atmosphere which act to maintain quasi-geostrophic balance. A nonlinear, f-plane, primitive equation, two-layer model is used. The momentum and temperature fields are described in terms of normal modes of the system given by the model's linear terms. These modes are classified as either geostrophic or ageostrophic depending on their associated eigenvalues. The original nonlinear equations are transformed into a system in which the modulations of the modal amplitudes by nonlinear effects are explicitly expressed in terms of the modal amplitudes themselves. This transformation facilitates a multiple-time-scale analysis. The stability of simple finite-amplitude geostrophic solutions with respect to infinitesimal perturbations in other modes is investigated. Results are discussed for nondimensional unperturbed-state amplitudes of magnitude ϵ < 1. These geostrophic solutions may be unstable with respect to further geostrophic perturbations, with growth r...