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Murry L. Salby and Patrick F. Callaghan

Abstract

The 2-day wave is a prominent feature of the middle and upper atmosphere, amplifying twice-yearly around solstice. Its period, structure, and reproducibility have led to its association with the gravest planetary normal mode of zonal wavenumber 3, the so-called Rossby-gravity mode. On the other hand, its amplification around solstice has also led to its association with baroclinic instability of summer easterlies. To explore the relationship between the Rossby-gravity mode and instability, calculations are performed with the linearized primitive equations that have been generalized to account for that mode's interaction with a generally unstable mean flow u. The mode's eigenfrequency is then complex, the imaginary component representing amplification and decay. For mean states representative of solstice and equinox, the normal mode is calculated and then compared to observed behavior in terms of its period, structure, and amplification.

The behavior recovered, including structural differences between solstice and equinox, is consistent with major features of the 2-day wave. Under solstitial conditions, the Rossby-gravity mode amplifies by extracting energy from the mean flow, with e-folding times as short as 5 days. Even though its eigenfrequency is then complex, the mode's period remains close to the theoretical value, consistently lying at westward periods of 2.0–2.2 days. Equally robust is its eigenstructure, which extends into both hemispheres. It mirrors the modal structure isolated earlier in the response over real frequency. In contrast, the mode's amplification depends sensitively upon details of the zonal-mean state. Changes of u that are modest, in some instances subtle, are sufficient to remove instability. Those changes of mean flow sharply alter the mode's growth rate, but have little effect on its eigenperiod and structure.

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Harry H. Hendon and Murry L. Salby

Abstract

Interhemispheric differences of the Madden–Julian oscillation (MJO) are investigated in a linearized primitive equation model. Heating is prescribed from the observed life cycle of the MJO, in which anomalous convection is concentrated in the Eastern Hemisphere. The dynamical response in the Eastern Hemisphere has the form of a forced disturbance that involves Kelvin and Rossby components. These dynamical components propagate eastward along with the prescribed heating and have zonal wavenumber-2 structure, in accord with observed behavior. The behavior in the Eastern Hemisphere also resembles that emerging from frictional wave-CISK, in which heating follows autonomously from the circulation. When the prescribed heating collapses near the date line, the Rossby component dissipates. The Kelvin component, however, continues to advance across the Western Hemisphere, where it propagates at more than twice the speed of the disturbance in the Eastern Hemisphere. The disturbance in the Western Hemisphere, which likewise is in accord with the observed behavior, can be understood as the radiating response to transient heating confined to the Eastern Hemisphere.

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Murry L. Salby and Rolando R. Garcia

Abstract

We explore the interference Pattern Produced when a traveling planetary wave propagates over a stationary forced wave. The interference signature is examined in a variety of diagnostics, ranging from instantaneous local wave amplitudes to cross sections of Eliassen–Palm flux and synoptic maps of Ertel potential vorticity. Results capture the salient characteristics of quasi-Periodic disturbances in the stratosphere reported by Madden and others.

The interference process results in a modulation of all the transport Properties of the stationary wave, even if the traveling component is purely barotropic as is typical of transient planetary waves in the troposphere and lower stratosphere. Locally, the Eliassen–Palm flux involves a transient vector which orbits about the time-mean component, causing the instantaneous flux of wave activity to vary in both magnitude and direction. For stationary and traveling waves of comparable amplitude, the EP flux vector F can readily be driven through zero, completely altering its strength and direction. This temporal fluctuation actually arises out of the spatial modulation of the wave field and the migration of the resulting pattern with time. In this manner, the steady uniform stream of wave activity associated with the stationary wave is organized into a series of capsules or wavepackets which propagate upward and equatorward. Consequently, the signature at a particular location emerges as a series of bursts in wave activity.

Because of rising values of F at the leading edge of each of these wavepackets and opposite behavior at the trailing edge, the mean flow experiences an alternating succession of eddy forcing. This gives rise to a fluctuating response in the zonal-mean which, under typical amplitudes, may be considerable. Greatest influence is exerted in the polar stratosphere due to the veering of F to and away from the pole. Convergence of meridians at high latitudes leads to repeated focusing and spreading of wave activity and thereby a magnified response in the mean flow. For amplitudes representative of January 1979, a mean wind reversal occurs in the polar stratosphere and mesosphere, attended by substantial warming over the polar cap and out-of-phase behavior in the tropics and mesosphere.

The synoptic signature of wavenumber 1 interference consists of two basic elements: (i) displacement and wobbling of the vortex about the pole, as a ridge builds in from below; (ii) distortion of the vortex into a comma-like shape, its axis spiraling anticyclonically and equatorward about the ridge. Both features are widely documented in observations. They are introduced here by the eddy field as a capsule of wave activity propagates through a given level. Potential vorticity on isentropic surfaces exhibits similar but exaggerated behavior. The results suggest an alternate, perhaps complementary, interpretation to planetary wave breaking of the evolution of potential vorticity during January 1979.

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Murry L. Salby and Harry H. Hendon

Abstract

The spectral character of tropical convection is investigated in an 11-yr record of outgoing longwave radiation from the Advanced Very High Resolution Radiometer to identify interaction with the tropical circulation. Along the equator in the eastern hemisphere, the space–time spectrum of convection possesses a broad peak at wave-numbers 1–3 and eastward periods of 35–95 days. Significantly broader than the dynamical signal of the Madden–Julian oscillation (MJO), this quasi-discrete convective signal is associated with a large-scale anomaly that propagates across and modulates time mean or “climatological convection” over the equatorial Indian Ocean and western Pacific. Outside that region the convective signal is small, even though, under amplified conditions, coherence can be found east of the date line and in the subtropics. Having a zonal scale of approximately wavenumber 2, anomalous convection propagates eastward at some 5 m s−1 and suppresses as well as reinforces climatological convection in the eastern hemisphere. The convective signal amplifies to a seasonal maximum near vernal equinox and, to a weaker degree, again near autumnal equinox, when climatological convection and warm SST cross the equator.

Contemporaneous records of motion from ECMWF analyses and tropospheric-mean temperature from Microwave Sounding Unit reveal an anomalous component of the tropical circulation that coexists with the convective signal and embodies many of the established properties of the MJO. Unlike anomalous convection, that dynamical signal extends globally around the Tropics. The anomalous circulation differs fundamentally between the eastern and western hemispheres. In the eastern hemisphere, subtropical Rossby gyres and zonal Kelvin structure along the equator flank the convective anomaly as it tracks eastward, giving the anomalous circulation the form of a “forced response.” In the western hemisphere, the dynamical signal is composed chiefly of wavenumber−1 Kelvin structure, which has the form of a “propagating response” that is excited in and radiates away from anomalous convection at some 10 m s−1. Kelvin structure comprising the propagating response appears in 850-mb and 200-mb zonal winds even when the convective signal is absent, albeit with much smaller amplitude. In contrast, the signal in 1000-mb convergence appears only when accompanied by anomalous convection, which suggests that convergence in the boundary layer is instrumental in achieving strong interaction with the convective pattern.

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Murry L. Salby and Patrick F. Callaghan

Abstract

Northern Hemisphere ozone underwent a monotonic decline during the 1980s and 1990s. Systematic changes associated with that trend are shown to have a close relationship to random changes of ozone. These two components of interannual variability share a common structure. In it, ozone changes at high latitude are compensated at low latitude by changes of opposite sign. The out-of-phase relationship between ozone changes at high and low latitudes is consistent with a change of the residual mean circulation of the stratosphere, and so is the seasonality of systematic changes. Compensating trends at high and low latitudes amplify simultaneously—during winter, when the polar-night vortex is disturbed by planetary waves that force residual motion. Analogous relationships are obeyed by Northern Hemisphere temperature. The strong resemblance between systematic and random changes of Northern Hemisphere ozone implies that a major portion of its decline during the 1980s and 1990s involved a systematic weakening of the residual circulation.

Anomalous forcing of the residual circulation is strongly correlated to random changes of ozone, which in turn have the same structure as systematic changes. The magnitude and structure of the ozone trend are broadly consistent with the climate sensitivity of ozone with respect to a change of the residual circulation. Derived from random changes over a large population of winters, the climate sensitivity implies an ozone trend quite similar to the observed trend, but with about two-thirds of its magnitude. When account is taken of both the anomalous residual circulation and anomalous photochemistry, the climate sensitivity of ozone reproduces the major structure as well as the magnitude of the observed trend.

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Murry L. Salby and Rolando R. Garcia

Abstract

The dynamical response to localized, unsteady tropical heating is studied in a stochastic framework. Spectral statistics of the random wave response are derived from those of tropical convection using the primitive equations for a spherical baroclinic atmosphere.

Short-time near-field behavior emerges in the form of a transient wavepacket which disperses away the source region. Two principal components characterize the response: 1) a projection response which matches the vertical scale of the heating and 2) a barotropic response involving Rossby normal modes. The projection response consists of a continuum of frequencies and vertical scales centered about vertical wavelengths twice the effective depth of the heating. This scale discrimination is shown to be insensitive to variations in the heating distribution. The associated disturbance is trapped laterally about the equator but radiates vertically away from the source region. It corresponds to the tropical waves traditionally studied on the equatorial beta-plane. The barotropic component, on the other hand, radiates latitudinally into middle and high latitudes but is vertically trapped. This component of the response corresponds to planetary Rossby waves usually developed with the barotropic vorticity equation on the sphere. Because of the complementary nature of these two components, far-field tropospheric behavior is dominated by the barotropic contribution.

These elements of the response are presented in both local and more conventional modal descriptions. Vertical radiation and dispersion are evaluated for several modes. The wavepacket associated with the Kelvin mode completes less than one circuit around the globe Before propagating completely out of the troposphere. Higher frequency components of the projection continuum radiate out of the source region even more quickly.

For short-term heating fluctuations, typical of tropical convection, the response at tropopause level is in accord with classical observations of the Wallace and Kousky Kelvin wave. The fast and ultra-fast Kelvin waves are secondary ingredients of the initial wave spectrum. In the case of slow transitional heating, e.g., the seasonal drift in monsoon activity between hemispheres the Kelvin response assumes the form of a damped transient Walker circulation. This eastward migrating cell captures the salient characteristics of Madden and Julian's composite of the 40-day wave in the tropical Pacific Ocean.

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Murry L. Salby and Patrick F. Callaghan

Abstract

Interannual changes of dynamical structure and ozone are investigated in the Tropics and Southern Hemisphere over the 1980s and 1990s. Changes of dynamical structure over the winter hemisphere are accompanied by coherent changes over the summer hemisphere, but of opposite sign. They are most noticeable during northern winter, when amplified planetary waves of the Northern Hemisphere drive strong downwelling in the Arctic stratosphere that penetrates well into the troposphere. Changes over the summer hemisphere operate coherently and in phase with weaker changes over the Tropics. Coherent changes appear even inside the tropical troposphere, where they coincide with regions of deep convection. Changes in the summer hemisphere and Tropics both operate coherently but out of phase with changes over the Arctic, which in turn operate coherently with anomalous forcing of the residual mean circulation. Anomalous summertime structure modulates the polar low in the upper troposphere and lowermost stratosphere. It modifies the wintertime spinup of westerlies and the storm track of the Southern Hemisphere.

Very similar changes are found in total ozone. Like dynamical structure, anomalous ozone over the summer hemisphere operates coherently with anomalous ozone in the Tropics. Both are out of phase with anomalous ozone over the Arctic, which in turn operates coherently with anomalous forcing of the residual circulation. Anomalous ozone has the same basic structure as anomalous temperature. The two are consistent with anomalous upwelling over the Tropics and Southern Hemisphere that compensates anomalous downwelling over the Arctic. Compensation is also evident in systematic changes of ozone during the 1980s and 1990s.

Interannual changes over the Southern Hemisphere during southern winter are weaker than changes over the Northern Hemisphere during northern winter. However, they have the same character. They operate coherently with anomalous forcing of the residual circulation, resembling the Southern Hemisphere counterpart of the Arctic Oscillation. Accompanying changes of ozone, which are as large as 50–100 DU, cover a wide area of the Southern Hemisphere. When mixed with chemically depleted polar air that is released during the spring breakdown of the vortex, they can make a significant perturbation to the net hemispheric overburden of ozone.

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Rolando R. Garcia and Murry L. Salby

Abstract

In Part I of this investigation, we described the stochastic, near-field behavior of disturbances excited by randomly evolving tropical heating. In the present paper, we examine how these disturbances are modified as they propagate through the far field in the presence of spatially-varying background states. Although the behavior can no longer be broken down into individual Hough modes, it can still be understood in terms of projection and barotropic components of the response.

Responses to fast heating, as may be produced by daily fluctuations in convection, and to slow heating, evolving over seasonal time scales, are studied separately. For fast heating the projection response consists mainly of a spectrum of Kelvin waves which, in the lower stratosphere, is centered at frequencies corresponding to twice the effective depth of the heating. The spectrum shifts to higher frequency with increasing altitude due to differential damping. As a result, the slow, fast and ultrafast Kelvin waves identified in observations all appear in our calculations as manifestations of the same response modified by dissipation. The barotropic response to fast forcing is dominated by the (1,1) Rossby normal mode throughout the tropics and in the stratosphere. In the extratropical troposphere, a transient barotropic wavetrain composed of low frequency Rossby waves of zonal wavenumber 1–3 is also present.

For slowly evolving heating projection and barotropic components from various modes overlap in the spectrum, coalescing into a continuum near zero frequency. Nevertheless, it is still possible to distinguish projection from barotropic responses because the former are dominant in the tropics while the latter are responsible for the extratropical behavior. The projection response to slow heating does not propagate effectively in the vertical and is largely confined to the troposphere, where its behavior is dictated by the particular part of the solution and assumes the form of a slowly evolving Walker circulation. The barotropic response is dominated by the same transient wavetrain found in the fast forcing case, but its amplitude is larger as a result of the greater amount of power available at low frequencies. Radiation of the barotropic response to higher latitudes is strongly dependent on the presence of westerly shear near the source region. Thus, maximum radiation takes place in the winter hemisphere, where the subtropical jet is closest to the source. The evolution of the wavetrain is also sensitive to the wind within the source region. Given the variability of winds in the tropical troposphere, the extratropical wavetrain can be expected to be a highly variable feature of the response to tropical heating. By contrast, the tropical Walker cell, which is essentially a forced response, is the most robust feature found in our slow heating calculations.

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John W. Bergman and Murry L. Salby

Abstract

The contribution to time-mean energetics from cloud diurnal variations is investigated. Cloud diurnal contributions to radiative fluxes follow as the differences between time-mean radiative fluxes based on diurnally varying cloud properties and those based on fixed cloud properties. Time-mean energetics under both conditions are derived from an observationally driven radiative transfer calculation in which cloud cover, temperature, and moisture are prescribed from satellite observations.

Cloud diurnal contributions to time-mean energetics arise from the nonlinear dependence of radiative fluxes on diurnally varying properties. Diurnal variations of cloud fractional coverage and solar flux are the main factors of the cloud diurnal contributions to shortwave (SW) flux, although the diurnal variation of cloud type is also important. The cloud diurnal contribution to longwave (LW) flux at the top of the atmosphere (TOA) is produced by diurnal variations of cloud fractional coverage, cloud-top height, and surface temperature. The cloud diurnal contribution to LW flux at the surface is produced by diurnal variations of cloud fractional coverage and cloud-base height. Cloud diurnal contributions to SW fluxes at the surface and TOA are much larger than the contribution to SW atmospheric absorption. The contribution to radiative heating in the atmosphere is concentrated inside the cloud layer. Its vertical profile changes sign, so the cloud diurnal contribution to atmospheric energetics is significantly larger than is implied by the column average.

Cloud diurnal contributions to SW flux at the surface and TOA are 5–15 W m−2 over continental and maritime subsidence regions, where the diurnal variation of cloud fractional coverage is large. The contributions to LW fluxes are 1–5 W m−2 over continental regions, where diurnal variations of cloud fractional coverage and surface temperature are large. A cancellation between contributions of opposite sign makes the cloud diurnal contributions to globally averaged energetics much smaller than regional contributions. However, a shift in regional climate from one dominated by high clouds to one dominated by low clouds can alter time-mean surface energetics by as much as 20 W m−2.

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Andrew C. Fusco and Murry L. Salby

Abstract

Interannual variations of total ozone at midlatitudes of the Northern Hemisphere are shown to operate coherently with variations of upwelling planetary wave activity from the troposphere. Variations of upwelling wave activity, which modulate ozone transport and chemical production by the diabatic mean circulation of the stratosphere, account for much of the interannual variance of total ozone, including its systematic decline during the 1980s. Chemical depletion, enhanced by increasing halocarbon levels, accounts for the remainder of the midlatitude trend, consistent with values widely reported by chemical models that do not account for observed changes in upwelling planetary wave activity. Much of the chemical contribution comes from sharply enhanced depletion following the eruption of Mt. Pinatubo, during the final years of the satellite record. Incomplete representation of the 3–5-yr recovery toward normal aerosol and ozone after Pinatubo appears to distort the trend inferred from the overall satellite record to values that are unrepresentative of the rest of the record. The impact on ozone of interannual changes of upwelling planetary wave activity is evaluated in calculations with a three-dimensional model of stratospheric dynamics and photochemistry, which reproduce the magnitude and structure of observed interannual variations.

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