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John Fyfe and Jacques Derome

Abstract

The stability of free and forced planetary waves in a β plane channel is investigated with a barotropic model. The equilibrium flows that are considered have the gravest possible scale in the meridional direction and a zonal wavenumber of either 1 or 2. The equilibrium-forced waves are the result of the interaction of a constant mean zonal wind over finite-amplitude surface orography.

The frequency of all possible small-amplitude perturbations to the equilibrium flows are calculated as a function of the strength of the mean zonal wind and of the amplitude of the orography. The forced zonal-wavenumber-1 flow is found to have three major regions of instability in parameter space, two of which have stationary growing perturbations. The free Rossby wave of that scale is stable for all amplitudes. The forced zonal-wavenumber-2 wave has two adjacent instability domains one on each side of the resonant mean zonal wind. The free wave becomes unstable for sufficiently large amplitudes. The results are interpreted through the use of a severely truncated spectral model and are related to those of previous studies with infinite β-planes. We also report the existence of a traveling subresonant topographic instability, which seems to have gone unnoticed in previous studies.

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Xiaohong Wang and John Fyfe

Abstract

A mechanism for the breakdown of vertically propagating edge waves in a Boussinesq fluid is investigated within the context of the destruction of the polar stratospheric vortex. Under inviscid, quasi-linear, and slowly varying conditions in a three-dimensional, quasigeostrophic contour dynamics model it is analytically predicted that planetary-scale edge wave breaking will occur if the zonal mean flow is decelerated by more than approximately one-half its initial value via a positive group-velocity–mean-flow feedback mechanism. Fully nonlinear model simulations confirm this “one-half rule” and detail the sequence of events leading to the breaking.

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Shiling Peng and John Fyfe

Abstract

Monthly variability of atmosphere-ocean interactions in the midlatitude North Atlantic during the winter months (November–April) is examined. Composite and singular value decomposition (SVD) analyses are applied to the observed sea level pressure (SLP) and sea surface temperature (SST) anomalies of each winter month for the period 1950–1987. The SLP anomaly composites (i.e., SLP differences between selected warm and cold SST months) are constructed based on the averaged SST anomalies over the RM region (60°–40°W and 50°–40°N). These composites shift from a positive monopole pattern in early winter to a dipole pattern in midwinter and then back to a monopole pattern in late winter.

A complementary SVD analysis reveals that the first SVD mode is dipole structured and especially dominant in midwinter. The second SVD mode is monopole featured and more dominant in early and late winter than in midwinter. By examining the spatial distributions of the SVD modes and especially their similarities to the patterns derived from other model simulations, two coupling processes are suggested. The dipole mode is suggested to be related to an atmosphere driving the ocean process and the monopole mode to an ocean forcing the atmosphere process. The month-dependency and the statistical significance of the SVD modes are subjected to two Monte Carlo tests. The results are used to further explain the shifts in the SLP anomaly composites and to indirectly estimate the predominance of the proposed coupling processes during each winter month.

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John C. Fyfe

Abstract

The three-dimensional structure and propagation characteristics of African easterly waves for the 1986–94 period are studied with June–August European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses, National Centers for Environmental Prediction (NCEP) reanalyses, and Canadian Centre for Climate Modelling and Analysis GCM output. Specific consideration is given to the differences in the time-mean circulation, synoptic-scale variance, covariance, and principal oscillation patterns. The African easterly waves derived from the ECMWF and NCEP analyses are very similar, with both providing a reasonably realistic depiction of African easterly waves—given the agreement with one another, earlier station data studies, and theory. Where significant differences exist between the results from the two data assimilation systems they expectably do so over the tropical Atlantic, and in fields that are not directly observed (such as vertical velocity).

The situation with the GCM is not as favorable but there are some encouraging areas of agreement—despite the GCM’s relatively coarse resolution and absence of observed data to constrain it. Selected points of agreement and disagreement between the GCM and the analyses include the following. (i) The GCM African easterly wave energetics are comparable with the analyses in terms of the sign and magnitude of the energy transfers from the time-mean state to the waves. (ii) The northern track of easterly waves seen in the analyses terminates prematurely in the GCM at the African coast. (iii) The southerly track of moist easterly waves seen in the analyses near the seasonal rainband is absent in the GCM. Possible reasons for the deficiencies are discussed.

The sensitivity of the GCM-simulated African easterly waves to CO2 doubling is investigated. Together with significant mean warming and moistening over the northern Sahara, the level of simulated African easterly wave activity increases with CO2 doubling.

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John C. Fyfe

Abstract

Changes in the naturally occurring modes of extratropical annual mean and zonal mean zonal wind variability are investigated using National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalyses and Canadian Centre for Climate Modelling and Analysis (CCCma) global climate model simulations. In the Northern Hemisphere, the first and second modes are primarily stratospheric and tropospheric in character, respectively. The surface pressure manifestations of these modes are intimately linked to the Arctic Oscillation (AO), and together suggest separate stratospheric and tropospheric origins for the AO. In the Southern Hemisphere, the first mode describes north–south shifts in the polar front jet accompanied by polar stratospheric jet fluctuations and Antarctic Oscillation (AAO)-like surface pressure anomalies. The second mode is primarily tropospheric and describes interannual changes in the strength and position of the polar front jet.

The leading observed modes appear unchanged in strength since the 1950s except in the Northern Hemisphere where the second mode shows some evidence of increasing strength. The leading simulated modes appear unchanged in strength since the beginning of the twentieth century, and are predicted to remain so to the end of the twenty-first century. In all cases the leading modes are superimposed upon significant mean change, which when not properly accounted for can lead to erroneous conclusions.

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John C. Fyfe

Abstract

In concert with a poleward shift in baroclinicity, the synoptic environment south of 40°S appears to have changed significantly over recent decades. South of 40°S and north of the Antarctic Ocean the number of cyclones has dramatically decreased, while over the Antarctic Ocean a modest increase has occurred. A global climate model with anthropogenic forcing produces similar historical changes, and under a “business-as-usual” emissions scenario predicts that the number of sub-Antarctic Ocean cyclones will drop by over 30% between now and century's end.

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John Fyfe and Isaac M. Held

Abstract

A stationary Rossby wave, sinusoidal in longitude, is slowly switched on, and the meridional propagation of the resulting wave front through a shear flow is examined. Initially the flow is westerly everywhere and therefore free of critical layers. The transition from reversible to irreversible behavior as the wave amplitude is increased is described. It is shown that under slowly varying conditions in an inviscid quasi-linear model, a steady state is obtained if, and only if, the mean flow is decelerated by less than two-fifths of its initial value as a result of the passage of the wave front. If this passage causes a larger mean flow reduction, a pile-up of wave activity in the shear layer culminates in the generation of a critical layer, qualitatively as in Dunkerton's model of gravity wave–mean flow interaction. This qualitative picture is shown to be preserved in the quasi-linear model when the slowly varying assumption breaks down.

Fully nonlinear calculations show that these quasi-linear results are only part of the story. Once the mean flow is decelerated by two-fifths of its initial value in the fully nonlinear model, rapid wave breaking and irreversible mixing occur in the shear layer. But more slowly developing wave breaking also occurs for wave amplitudes that are too small to produce the two-fifths deceleration. Overturning of contours can be shown to occur in the quasi-linear slowly varying model once the mean flow has been decelerated by one-fifth of its initial value, and this appears to be the critical value for wave breaking to occur in the nonlinear integrations.

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Michael Sigmond and John C. Fyfe

Abstract

It has been suggested that the increase of Southern Hemisphere sea ice extent since the 1970s can be explained by ozone depletion in the Southern Hemisphere stratosphere. In a previous study, the authors have shown that in a coupled atmosphere–ocean–sea ice model the ozone hole does not lead to an increase but to a decrease in sea ice extent. Here, the robustness of this result is established through the analysis of models from phases 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5). Comparison of the mean sea ice trends in CMIP3 models with and without time-varying stratospheric ozone suggests that ozone depletion is associated with decreased sea ice extent, and ozone recovery acts to mitigate the future sea ice decrease associated with increasing greenhouse gases. All available historical simulations with CMIP5 models that were designed to isolate the effect of time-varying ozone concentrations show decreased sea ice extent in response to historical ozone trends. In most models, the historical sea ice extent trends are mainly driven by historical greenhouse gas forcing, with ozone forcing playing a secondary role.

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Adam H. Monahan and John C. Fyfe

Abstract

Analytic results are obtained for the mean and covariance structure of an idealized zonal jet that fluctuates in strength, position, and width. Through a systematic perturbation analysis, the leading empirical orthogonal functions (EOFs) and principal component (PC) time series are obtained. These EOFs are built of linear combinations of basic patterns corresponding to monopole, dipole, and tripole structures. The analytic results demonstrate that in general the individual EOF modes cannot be interpreted in terms of individual physical processes. In particular, while the dipole EOF (similar to the leading EOF of the midlatitude zonal mean zonal wind) describes fluctuations in jet position to leading order, its time series also contains contributions from fluctuations in strength and width. No simple interpretations of the other EOFs in terms of strength, position, or width fluctuations are possible. Implications of these results for the use of EOF analysis to diagnose physical processes of variability are discussed.

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Adam H. Monahan and John C. Fyfe

Abstract

Dipolar structures arise as empirical orthogonal functions (EOFs) of extratropical tropospheric zonal-mean zonal wind in observations, in idealized dynamical models, and in complex general circulation models. This study characterizes the conditions under which dipoles emerge as EOFs of a jet of fixed shape f (x), which takes a unique localized extremum and is smooth but is otherwise arbitrary, characterized by fluctuations in strength, position, and width of arbitrary distribution. It is shown that the factors that influence the extent to which a dipolelike structure will arise as an EOF are (i) the skewness of position fluctuations, (ii) the dependence of position fluctuations on strength and width fluctuations, and (iii) the relative strength of the position and width fluctuations. In particular, the leading EOF will be a dipole if jet position fluctuations are not strongly skewed, not strongly dependent on strength and width fluctuations, and sufficiently large relative to strength and width fluctuations. Because these conditions are generally satisfied to a good approximation by observed and simulated tropospheric eddy-driven jets, this analysis provides a simple explanation of the ubiquity of dipolar jet EOFs.

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