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Murry Salby and Patrick Callaghan

Abstract

NCEP reanalyses are used to isolate systematic variations in the stratosphere that operated coherently over the last four decades with the 11-yr variation of UV irradiance. Only a small systematic variation is visible at low frequency, which would reflect a simple linear response that drifts with solar flux, F s. However, a systematic variation manifests itself prominently at high frequency, which involves changes between neighboring years. Corresponding to interannual variability, the systematic variation at high frequency reflects a more complex, nonlinear response to the 11-yr variation of UV irradiance. It is analogous to a similar variation found earlier in the quasi-biennial oscillation (QBO) of equatorial wind, u EQ.

Interannual variability undergoes a frequency modulation that systematically alters its phase during winter, when planetary waves couple the polar and equatorial stratosphere. The polar-night vortex is then sensitive to equatorial wind, which itself varies systematically with F s. Monte Carlo simulations indicate that the systematic variation of wintertime phase is highly significant.

The systematic variation appears prominently in the wintertime tendency of temperature, which is coupled directly to the residual mean circulation. In fact, the anomalous wintertime tendency operating coherently with F s has the same basic structure as that operating coherently with anomalous forcing of the residual circulation. Each reflects anomalous downwelling over the Arctic that is compensated at lower latitude by anomalous upwelling. The resemblance of these anomalous structures suggests that the systematic variation at high frequency enters through changes of the residual circulation.

Accompanying the variation of zonal-mean structure is a systematic amplification and decay of wavenumber 1 at high latitude. It represents a poleward advance and retreat of the critical region, or surf zone, where planetary waves experience strong absorption that forces residual motion. This variation of wave structure, along with the anomalous residual motion it forces, parallels the systematic variation of equatorial wind. Wintertime-mean u EQ suggests a reversal of anomalous downwelling between solar min and solar max, one broadly consistent with the observed reversal of anomalous temperature.

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Murry Salby and Patrick Callaghan

Abstract

Evidence of the solar cycle in stratospheric polar temperature rests on a connection to the quasi-biennial oscillation (QBO) of equatorial wind. New evidence reported here establishes a mechanism for how the solar signature in polar temperature follows from the QBO, which itself is shown to vary with the solar cycle. Equatorial westerlies below 30 mb vary systematically with solar activity, as do equatorial easterlies above 30 mb. Changes in their duration introduce a systematic drift into the QBO's phase relative to winter months, when the polar vortex is sensitive to equatorial wind. Corresponding changes in the polar-night vortex are consistent with the solar signature observed in wintertime records of polar temperature that have been stratified according to the QBO.

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Murry Salby and Patrick Callaghan

Abstract

A 3D primitive equation model is used to investigate how the tropical tropopause is influenced by cumulus convection in the troposphere and mean upwelling in the stratosphere. The model simulates the residual mean circulation explicitly, whereas it represents the influence of convection on large-scale structure through statistical properties of cloud. Within this global framework, a change of extratropical planetary waves induces a large change of downwelling over the winter hemisphere, compensated at lower latitudes by a change of upwelling. This exerts a major influence on thermal structure in the extratropics, but only a minor one in the Tropics. On seasonal time scales, however, the influence in the Tropics is significant. During northern winter, extratropical planetary waves are sharply amplified. The accompanying intensification of tropical upwelling, while small compared to the intensification of extratropical downwelling, accounts for about half of the observed seasonal change of the tropical tropopause. Remarkably, anomalous thermal structure extends even into the summer hemisphere where the tropopause is anomalously high and cold. Just the reverse is found in the winter hemisphere.

Contrasting with this is the dependence on convection, which is large in the Tropics. An intensification or deepening of convection elevates and cools the tropical tropopause. Accompanying those changes overhead is anomalous downwelling in the lowermost stratosphere. It is forced by convective cooling, at and above the level of neutral buoyancy (LNB), where overshooting cumulus are colder than their environment. Anomalous temperature is out of phase above and below the LNB, consistent with cumulus detrainment and observed changes that accompany the outbreak of cold cloud. Conversely, an elevation of the LNB, as would accompany an increase of moist static energy (e.g., SST), elevates but warms the tropical tropopause. This dependence may explain geographical variations of the tropopause. A change of tropical convection also influences the extratropical circulation, secondarily through the absorption of planetary waves, which then modulates downwelling and temperature over the winter hemisphere.

Vertical transport into the stratosphere depends on both mechanisms, which interact. Above the LNB, convective cooling drives environmental downwelling that transports stratospheric air into the troposphere at sites of deep convection. There, air of high θ mixes with air of low θ that has been convected above the LNB inside overshooting cumulus. The mixture, having been cooled mechanically, then experiences enhanced radiative warming that carries it upward at sites removed from convection.

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Murry Salby and Patrick Callaghan

Abstract

Evidence of the solar cycle in stratospheric polar temperature rests on a connection to the quasi-biennial oscillation (QBO) of equatorial wind. New evidence reported here establishes a mechanism for how the solar signature in polar temperature follows from the QBO, which itself is shown to vary with the solar cycle. Equatorial westerlies below 30 mb vary systematically with solar activity, as do equatorial easterlies above 30 mb. Changes in their duration introduce a systematic drift into the QBO’s phase relative to winter months, when the polar vortex is sensitive to equatorial wind. Corresponding changes in the polar-night vortex are consistent with the solar signature observed in wintertime records of polar temperature that have been stratified according to the QBO.

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Murry L. Salby and Patrick Callaghan

Abstract

The diurnal cycle present in many climate properties is undersampled in asynoptic data, which, through aliasing, introduces a bias into time-mean behavior derived from satellite measurements. This source of systematic error is investigated in high-resolution Global Cloud Imagery (GCI), which provides a proxy, with realistic space–time variability, for several climate properties to be observed from space. The GCI, which resolves mesoscale and diurnal variability on a global basis, is sampled asynoptically according to orbital and viewing characteristics from one and multiple platforms. Sampling error is then evaluated by comparing the resulting time-mean behavior against the true time-mean behavior in the GCI.

The bias from undersampled diurnal variability is most serious in polar-orbiting measurements from an individual platform. However, it emerges even in precessing measurements, which drift through local time, because diurnal variability is still sampled too slowly to be truly resolved in such observations. A “mean diurnal cycle” can be constructed by averaging precessing measurements, provided that the ensemble of observations at individual local times is large enough (e.g., that observations are averaged over a long enough duration). The pattern of time-mean error closely resembles the pattern of error in the mean diurnal cycle. Time-mean behavior can therefore be determined only about as accurately as can the mean diurnal cycle. Determining accurate time-mean properties often requires averaging measurements from an individual platform over several months, which cannot be performed without contaminating mean behavior with seasonal variations. The sampling limitations from an individual orbiting platform are alleviated by sampling from multiple platforms, which provide observations frequently enough in space and time to determine accurate monthly mean properties.

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Murry L. Salby and Patrick F. Callaghan

Abstract

Recent evidence points to a decadal modulation of the quasi-biennial oscillation (QBO), one that varies with the 11-yr cycle of UV irradiance and ozone heating in the upper stratosphere. Interaction between the QBO and the Hadley circulation is considered here through an analysis that accounts for cyclic variations in their relationship, which may cancel and, hence, be invisible in the long-term average.

The analysis reveals coherent changes in the tropical stratosphere and troposphere. Involving periods shorter than 5 yr, their relationship manifests itself in major properties associated with the QBO and the Hadley circulation. Like the QBO’s relationship to the polar stratosphere, its relationship to the Hadley circulation reverses on the time scale of a decade. The systematic swing in their relationship leads to two important implications: 1) Interannual changes of one circulation operate coherently with changes of the other, reflecting their interaction. 2) At least one is influenced by a decadal variation. The latter is interpreted in light of the cyclic variation of ozone heating in the upper stratosphere, where the phase of the QBO is set.

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Murry L. Salby and Patrick F. Callaghan

Abstract

Interannual changes of the stratospheric circulation are studied in relation to coherent changes of the tropospheric circulation. Emerging over the winter pole is a clear signature of adiabatic warming and anomalous downwelling. Reflecting an intensification of the Brewer–Dobson circulation, the signature of anomalous downwelling extends from stratospheric levels into the troposphere. Compensating for it at subpolar latitudes is a signature of adiabatic cooling and anomalous upwelling. Equally coherent, the signature of anomalous upwelling occupies the same levels as the signature of anomalous downwelling. Inside the tropical troposphere, anomalous cooling is replaced by anomalous warming. It reflects an intensification of organized convection and the Hadley circulation, one that accompanies the intensification of the Brewer–Dobson circulation.

These signatures of anomalous vertical motion represent changes that operate coherently in the stratosphere and troposphere. They share major features with the Arctic Oscillation. Extending across the tropopause, they couple the stratosphere and troposphere through a transfer of mass. By modifying vertical motion inside the Tropics, anomalous upwelling influences organized convection. Support for this interpretation comes from anomalous divergence in the tropical upper troposphere; it is shown to vary coherently with anomalous downwelling in the Arctic stratosphere. Exhibiting analogous behavior are changes of the tropical tropopause. Coupled to stratospheric changes, these variations of the tropical circulation act to organize convection about the equator, favoring a split ITCZ. They reflect as much as 40% of the interannual variance of tropical divergence, representing an important complement to ENSO.

Much of the covariance between the polar stratosphere and the tropical troposphere is concentrated at periods shorter than 5 yr. Included is variability that is associated with the quasi-biennial oscillation (QBO) in the tropical stratosphere. Also included is biennial variability, which accompanies the QBO in the polar stratosphere. These stratospheric variations involve the same time scales as biennial variability in the tropical troposphere, which likewise influences convection.

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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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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

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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