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Donal O'Sullivan and Murry L. Salby

Abstract

A significant fraction of the interannual variability of the wintertime stratosphere in the Northern Hemisphere is related to the quasi-biennial oscillation (QBO) in tropical winds. Disturbed conditions of the polar night vortex are favored in years when the winds over the equator are easterly. More quiescent conditions prevail at high latitudes when the phase of the tropical QBO is westerly.

The effects of tropical winds on the extratropical circulation are studied in the equivalent barotropic framework on the sphere. Calculations are performed with the tropical flow representative of easterly and westerly phases of the QBO. With planetary wave forcing representative of conditions in the lower stratosphere, the wintertime polar vortex is highly disturbed during the easterly phase of the QBO, the evolution taking the form of a “Canadian warming.” In contrast, more quiescent conditions prevail with the same forcing but with tropical winds representative of the westerly phase of the QBO, the vortex preserving a high degree of polar symmetry in that case.

The dependence of the extratropical circulation on tropical winds and the QBO in these calculations originates in planetary wave transport. When tropical winds are easterly, the zero wind line is shifted into the winter hemisphere. Under such conditions, disturbance amplitudes at middle and high latitudes are amplified as is eddy transport which acts to displace and degrade the vortex. On the other hand, when tropical winds are westerly the zero wind line is removed into the summer hemisphere, in particular away from the heart of the vortex. Under these conditions disturbance amplitudes at middle and high latitudes are weaker, resulting in reduced eddy transport and a more orderly circumpolar flow. Modulation of planetary wave activity by the QBO is interpreted in terms of a latitudinal shift of the critical region, where eddy displacements are large. Both eddy stirring and displacement of the vortex out of polar symmetry appear to be involved in modifying the extratropical flow, the latter resulting in nonconservative behavior through thermal dissipation. Modulation of planetary wave transport by tropical winds may also explain interannual variability of ozone at middle and high latitudes.

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Patrick F. Callaghan and Murry L. Salby

Abstract

Integrations with the nonlinear primitive equations are used to study 3D diabatic structure underlying the Brewer–Dobson circulation of the middle atmosphere. Such structure reveals zonally asymmetric contributions to mean downwelling over the winter hemisphere. It is used to evaluate contributions to w* from mechanical dissipation of planetary waves, associated with irreversible eddy dispersion, and from thermal dissipation of planetary waves, associated with irreversible heat transfer.

Zonal-mean downwelling follows disproportionately from those longitudes where air is deflected across contours of radiative equilibrium. This zonally asymmetric contribution to w* is pronounced at high latitudes, where the displaced vortex achieves cross-polar flow that drives air across sharply different radiative environments. Air parcels orbiting about the vortex then experience a wide swing in radiative-equilibrium temperature, driving them well out of thermal equilibrium. This renders the heat transfer experienced by them irreversible, resulting in net cooling and descent to lower θ with each orbit about the displaced vortex. By destroying anomalous potential vorticity (PV), irreversible heat transfer also leads to thermal dissipation of planetary waves and acts to resymmetrize the vortex diabatically.

Integrations in which irreversible dispersion is suppressed recover much the same diabatic motion as the full integration. Downwelling is reduced at midlatitudes, where the contribution from irreversible eddy dispersion is concentrated, but it is virtually unchanged at high latitudes, where the contribution from irreversible heat transfer prevails. Lagrangian integrations show that thermal dissipation of wave activity accounts for a major fraction of the downwelling over the winter hemisphere. This is especially true at high latitudes, where cross-polar flow leads to irreversible cooling and a systematic drift of air to lower θ. Were it not for this contribution to w*, the Arctic stratosphere would be several tens of Kelvin colder.

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Murry L. Salby and Patrick F. Callaghan

Abstract

Interannual changes of stratospheric dynamical structure and ozone are explored in observed variations over the Northern Hemisphere during the 1980s and 1990s. Changes of dynamical structure are consistent with a strengthening and weakening of the residual mean circulation of the stratosphere. It varies with the Eliassen–Palm (E–P) flux transmitted upward from the troposphere and, to a lesser degree, with the quasi-biennial oscillation (QBO). These two influences alone account for almost all of the interannual variance of wintertime temperature over the two decades, even during unusually cold winters.

Stratospheric changes operating coherently with anomalous forcing of the residual circulation are coupled to changes of tropospheric wave structure. Those changes of dynamical structure share major features with the Arctic Oscillation. Both involve an amplification of the ridge over the North Pacific and an expansion of the North Atlantic storm track. Changes of tropospheric wave structure lead to a temperature signature of anomalous downwelling in the Arctic stratosphere. Accompanying it at a lower latitude is a temperature signature of anomalous upwelling. That compensating change operates coherently but out of phase with the temperature change over the Arctic. However, it is an order of magnitude smaller, making it difficult to isolate in individual years or in small systematic changes that characterize trends.

Interannual changes of dynamical structure are mirrored by changes of total ozone. Like temperature, ozone changes are large at high latitudes. They are accompanied at lower latitudes by coherent changes of opposite sign. Those compensating changes, however, are an order of magnitude smaller—like temperature. Ozone changes operating coherently with anomalous forcing of the residual circulation track observed changes. They account for most of the interannual variance. What remains (about 20%) is largely accounted for by changes of the photochemical environment, associated with volcanic perturbations of aerosol and increasing chlorine. The close relationship between these changes and observed ozone is robust: It is obeyed even during years of unusually low ozone. Total ozone then deviates substantially from climatological-mean levels. However, it remains broadly consistent with the relationship deduced from the overall population of years.

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Harry H. Hendon and Murry L. Salby

Abstract

A composite life cycle of the Madden–Julian oscillation (MJO) is constructed from the cross covariance between outgoing longwave radiation (OLR), wind, and temperature. To focus on the role of convection, the composite is based on episodes when a discrete signal in OLR is present. The composite convective anomaly possesses a predominantly zonal wavenumber 2 structure that is confined to the eastern hemisphere. There, it propagates eastward at about 5 m s−1 and evolves through a systematic cycle of amplification and decay. Unlike the convective anomaly, the circulation anomaly is not confined to the eastern hemisphere.

The circulation anomaly displays characteristics of both a forced response, coupled to the convective anomaly as it propagates across the eastern hemisphere, and a radiating response, which propagates away from the convective anomaly into the western hemisphere at about 10 m s−1. The forced response appears as a coupled Rossby–Kelvin wave while the radiating response displays predominantly Kelvin wave features.

When it is amplifying, the convective anomaly is positively correlated to the temperature perturbation, which implies production of eddy available potential energy (EAPE). A similar correlation between upper-tropospheric divergence and temperature implies conversion of EAPE to eddy kinetic energy during this time. When it is decaying, temperature has shifted nearly into quadrature with convection, so their correlation and production of EAPE are then small. The same correspondence to the amplification and decay of the disturbance is mirrored in the phase relationship between surface convergence and anomalous convection. The correspondence of surface convergence to the amplification and decay of the convective anomaly suggests that frictional wave–CISK plays a key role in generating the MJO.

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Murry L. Salby, Harry H. Hendon, Karen Woodberry, and Ken Tanaka

Synoptic images of the global cloud field have been created from infrared measurements taken aboard four geostationary and two polar-orbiting platforms simultaneously observing the earth. A series of spatial and temporal interpolations together with data reliability criteria are used to composite data from the individual satellites into synoptic images of the global cloud pattern. The composite Global Cloud Imagery (GCI) have a horizontal resolution of about half a degree and a temporal resolution of 3 h, providing an unprecedented view of the earth's cloud field. Each composite image represents a nearly instantaneous snapshot of the global cloud pattern. Collectively, the composite imagery resolve, on a global basis, most of the variability associated with organized convection, including several harmonics of the diurnal cycle.

The dense and 3-dimensional nature of the GCI make them a formidable volume of information to treat in a practical and efficient manner. To facilitate analysis of global cloud behavior, the GCI has been constructed with certain homogeneous properties. In addition to synoptic coverage of the globe, data are spaced uniformly in longitude, latitude, and time, and contain no data voids. An interactive Image Analysis System (IAS) has been developed to investigate the space-time behavior of global cloud activity. In the IAS, data, hardware, and software are integrated into a single system capable of providing a variety of space-time covariance analyses. Because of its customized architecture and the homogeneous properties of the GCI, the IAS can perform such analyses on the 3-dimensional data with interactive speed. Statistical properties of cloud variability are presented along with other preliminary results derived from the GCI.

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Murry L. Salby, Rolando R. Garcia, and Harry H. Hendon

Abstract

Interaction between the large-scale circulation and the convective pattern is investigated in a coupled system governed by the linearized primitive equations. Convection is represented in terms of two components of heating:A “climatological component” is prescribed stochastically to represent convection that is maintained by fixed distributions of land and sea and SST. An “induced comonent” is defined in terms of the column-integrated moisture flux convergence to represent convection that is produced through feedback with the circulation. Each component describes the envelope organizing mesoscale convective activity.

As SST on the equator is increased, induced heating amplifies in the gravest zonal wavenumbers at eastward frequencies, where positive feedback offsets dissipation. Under barotropic stratification, a critical SST of 29.5°C results in positive feedback exactly cancelling dissipation in wavenumber 1 for an eastward phase speed of 6 m s−1. The neutral disturbance is dominated by Kelvin structure along the equator and Rossby gyres in the subtropics of each hemisphere. Heating induced by the neutral disturbance is magnified in a neighborhood of the equator, where nearly geostrophic balance in the boundary layer gives way to frictional balance. Moisture convergence induced by the Kelvin and Rossby structures fuels heating that is positively correlated with the temperature anomaly. Induced heating then generates eddy available potential energy, which offsets dissipation in the neutral disturbance. This sympathetic interaction between the circulation and the induced heating is the basis for “frictional waveCISK,” which is distinguished from classical wave-CISK by rendering the gravest zonal dimensions most unstable. Under baroclinic stratification, the coupled system exhibits similar behavior. The critical SST is only 26.5°C for conditions representative of equinox, but in excess of 30°C for conditions representative of solstice. However, the neutral disturbance is then no longer confined to the tropical troposphere. Forced by the induced heating, wave activity radiates poleward into extratropical westerlies and vertically into the stratosphere.

Having the form of an unsteady Walker circulation, the disturbance produced by frictional wave-CISK compares favorably with the observed life cycle of the Madden–Julian oscillation (MJO). SST above the critical value produces an amplifying disturbance in which enhanced convection coincides with upper-tropospheric westerlies and is positively correlated with temperature and surface convergence. Conversely, SST below the critical value produces a decaying disturbance in which enhanced convection coincides with upper-tropospheric easterlies and is nearly in quadrature with temperature and surface convergence. The observed convective anomaly, which propagates across the Eastern Hemisphere at some 5 m s−1, undergoes a similar shift between amplifying and decaying stages of the MJO. During the same transition, enhanced convection remains phase-locked to inviscid convergence above the boundary layer, as does induced heating in the calculations. Frictional wave-CISK also predicts seasonality in accord with that observed. The coupled system is most unstable under equinoctial conditions, for which climatological convection and maximum SST neighbor the equator. While sharing essential features with the MJO in the Eastern Hemisphere, frictional wave-CISK does not explain observed behavior in the Western Hemisphere, where the convective signal is largely absent. Comprised of Kelvin structure with the same frequency, observed behavior in the Western Hemisphere can be understood as a propagating response that is excited in and radiates away from the fluctuation of convection in the Eastern Hemisphere.

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Murry L. Salby, Patrick J. McBride, and Patrick F. Callaghan

Abstract

Global cloud imagery (GCI) is constructed from multiple satellite platforms that simultaneously monitor the earth. The GCI overcomes sampling limitations that are inherent to measurements from an individual platform, and provides a continuous and high-resolution description of the global convective pattern. However, it must reconcile inconsistencies in the measurements from different platforms. Escaping operational stages of error detection is a spurious brightening (cooling of brightness temperature), which appears sporadically in the composited imagery and must be removed a posteriori. The spurious brightening is shown to follow from a bias between measurements from polar-orbiting platforms and those from geostationary platforms. The bias is related to the zenith angle dependence of geostationary measurements, which enables its efficient removal. GCI is then composited from satellites in which the zenith angle–dependent bias has been removed a priori. The corrected imagery is shown to be virtually free of the systematic error, leaving a more accurate representation of the global convective pattern.

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Murry L. Salby, Dennis L. Hartmann, Paul L. Bailey, and John C. Gille

Abstract

Eastward propagating disturbances over the equator are diagnosed in two independent Nimbus-7 LIMS (Limb Infrared Monitor of the Stratosphere) data sets. They are evident consistently at several pressure levels throughout the stratosphere and account for much of the temperature variance in the tropics. The disturbances, which can be seen in wavenumbers 1–3, are in phase and symmetric about the equator, latitudinally evanescent, and have short-moderate vertical phase structure, 10–40 km, which progresses downward.

Wavenumber 1 has spectral components which propagate eastward at periods of 6.7–8.6 days (54–69 m s−1) and 3.5–4.0 days (115–135 m s−1). Wavenumber 2 exhibits eastward propagating variance at periods of 6.0–7.5 days (31–39 m s−1) and 3.8–4.3 days (55–62 m s−1). The faster waves appear principally in the upper stratosphere. These features are in reasonable agreement with the structure and dispersion characteristics of simple, quasi-separable Kelvin modes. With the exception of the slower wavenumber 1 feature, reported earlier by Hirota, these components are newly documented for the middle and upper stratosphere.

Interpretations of wave structure in terms of refractive properties of the basic flow are supported by the zonal-mean winds for the period. Power structures exhibit several maxima and minima in height, with phase variations across the maxima slower than across the minima. This behavior, supported by the longer vertical wavelengths, suggests that some reflection may be occurring.

A rapid phase variation is evident in both wavenumbers 1 and 2 near the stratopause, overlying a region of magnified amplitude. The latitudinal structure at this level, can be seen to contract as well. Such behavior is suggestive of disturbance focusing, due to Doppler shifting to small intrinsic frequencies, and attendant wave absorption. Coincident with this region of enhanced power and steep phase tilt, is a layer of sharp westerly shear, which, as reported by Leovy and others, descends over a period of weeks. The concurrent observation of the two phenomena supports earlier suggestions that Kelvin waves are instrumental in the westerly acceleration of the semiannual oscillation.

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Murry L. Salby, Rolando R. Garcia, Donal O'sullivan, and Patrick Callaghan

Abstract

The interaction of horizontal eddy motions and thermal drive in the stratosphere is investigated in equivalent barotropic calculations on the sphere. Eddy advection tends to homogenize the distribution of potential vorticity Q while thermal dissipation drives the motion towards a state of radiative equilibrium. The average circulation is shaped by a balance between these competing influences. Realistic mechanical and thermal forcing lead to time-mean behavior in reasonable agreement with observed monthly mean fields.

Mechanical forcing representative of disturbances near 10 mb produces behavior resembling a wide critical layer, for which strong wave-mean flow interaction occurs over much of the winter hemisphere. Under adiabatic conditions, eddy stirring across the hemisphere destroys the polar vortex in just a couple of weeks. Thermal dissipation modifies that behavior by confining irreversible dispersion to low latitudes, where diabatic effects are inefficient at damping the motion field. Outside the tropics, the circumpolar flow is maintained efficiently by destruction of potential vorticity anomalies and absorption of enstrophy at large scales, which prevent secondary eddies from amplifying and rearranging air irreversibly. Although eddy dispersion is small at middle and high latitudes, a sizable flux of potential vorticity results there from diabatic effects acting on large scales. This nondispersive source of eddy Q flux is analogous to thermal dissipation of planetary waves and may represent an important source of error in meridional diffusivities calculated from eddy potential vorticity flux.

Eddy stirring weakens the meridional gradient of Q at low latitudes. However, thermal drive restores the gradient after episodes of mixing. Variations in the tropical Q gradient are accompanied by fluctuations in the equatorward flux of wave activity. While reflection plays a role in thew fluctuations, variations of the flow at high latitudes, brought about through wave-mean flow interaction and concomitant variations in wave generation are equally responsible for the changes in equatorward propagation. Although some reflection occurs (e.g., when the Q gradient at low latitudes is weakened intermittently), a steady state is not reached before seasonal variations in mechanical and thermal forcing would have altered the circulation externally. Sustained phase tilt in the extratropical wave field and equatorward propagation of wave activity indicate that the primary role of the dispersive region at low latitudes is absorption.

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Murry L. Salby, Donal O'Sullivan, Rolando R. Garcia, and Patrick Callaghan

Abstract

Horizontal air motions accompanying the development of a planetary wave critical layer are investigated on the sphere in the equivalent barotropic framework. For small wave amplitude or strong shear in the neighborhood of the zero wind line, the critical region is confined laterally to a narrow zone adjacent to the critical latitude. Increasing the wave amplitude or reducing the zonal shear near the zero wind line expands the critical region poleward. Stirring within the critical region is controlled by a competition between wave advection, which promotes instability by rearranging potential vorticity, and dissipation, which limits the steepening of gradients and damps instability. Eddy advection strains bodies of air inside the critical region down to small dimensions, until eventually dissipation becomes efficient and anomalies in potential vorticity and other tracers “dissolve.”

For a narrow critical region, eddy stirring tends to homogenize potential vorticity near the zero wind line. The reduced gradient of potential vorticity resulting under weakly damped conditions lead to reflection of wave activity that suppresses wave generation and allows the flow to approach equilibrium. For a wide critical region, the flow can adjust nonlinearly directly at the forcing, making eddy stirring at low latitudes less important. The picture in that case is one of “global-scale entrainment,” where material lines fold across much of the hemisphere initially in westerlies. Air injected into the polar cap spins up anticyclonically, sweeping easterlies across mid-latitudes that shield the critical line from wave activity. Once this occurs, the eddy field at low latitudes collapses and easterlies advance over the forcing to suppress wave generation and restore equilibrium.

The preliminary development of the critical region resembles the familiar “wave breaking” signature in stratospheric potential vorticity maps and is reproduced quite well in linear and quasi-linear restrictions of the complete system. Although the nonlinear planetary wave critical layer captures a number of observed features, this paradigm of stratospheric air motions breaks down quickly with realistic wave amplitudes. Without a restoring mechanism to counteract the disruptive influence of eddy advection, the vortex is seriously compromised after only a month.

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