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Murry L. Salby

Abstract

Monitoring climate variability from space is considered from the standpoint of satellite sampling. Asynoptic sampling leads to well-defined limits in spatial and temporal resolution which are violated by behavior involving sufficiently small scales. Because of aliasing to larger scales, unresolved behavior can influence long-term and spatially averaged behavior important in climate.

From physical processes operating within the climate system, two classes of space-time variability which challenge the information content of asynoptic sampling are identified. Random fluctuations coherent on small space and time scales are characteristic of convective processes in the troposphere and of the dispersion of long-lived constituents such as ozone in the lower stratosphere. Diurnal variations are also an important component of tropospheric convection, as they are of short-lived chemical species such as ozone in the upper stratosphere. Each of these forms of variability can violate the sampling limitations inherent to satellite data from a single orbiting platform.

Implications of undersampling to instantaneous and long-term mean diagnostics are discussed for each of these classes of behavior for orbital and viewing geometries relevant to climate. When the field being monitored has significant variance beyond the Nyquist limits of asynoptic sampling, the complete space-time behavior cannot be recovered faithfully. Diagnostics such as synoptic maps and space-time spectra are prone to contamination from unresolved scales. Aliasing from unresolved random variability cancels in averages over a sufficiently long record, leading to accurate time-mean behavior provided no other forms of unresolved variability are present. A similar cancellation occurs for unresolved diurnal variability if the satellite orbit precesses through local time.

Through careful selection of sampling, long-term mean diagnostics can in principle be retrieved from a single orbiting platform even though the complete behavior may be seriously undersampled. Although such diagnostics represent the primary tool for studying climate, it may be necessary to observe behavior on shorter time scales (e.g., diurnal) to meaningfully interpret these quantities and understand how changes in the climate system occur. To do so will require measurements from multiple orbiting platforms. Sampling strategies and how such measurements can be assimilated so as to recover the full information content of the collective data are suggested.

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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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John W. Bergman and Murry L. Salby

Abstract

The ISCCP-C2 cloud climatology is used to describe the three-dimensional structure of cloud diurnal variations and to investigate their relationship to local climatological conditions. The latter follows from the regression of diurnal components onto climatological state variables.

Four important diurnal cloud categories are identified. The diurnal variation of maritime high-cloud fraction Chi maximizes at 1700 local solar time (LST) and is strongest over maritime convective locations where the mean high-cloud fraction is C hi > 0.1. The diurnal variation of maritime low-cloud fraction maximizes at 0400 LST and is strong over maritime nonconvective locations where C hi < 0.1. Diurnal variations of high-cloud fraction (persistent during the night, minimum at 1100 LST) and low-cloud fraction (1300 LST maximum) are strong over all continental locations in the latitude band 40°S–40°N.

In each cloud category, most of the diurnal amplitude and phase at individual locations is explained by the regression of diurnal amplitude onto only three climatological state variables. For most categories, the diurnal amplitude has its strongest relationship with mean cloud fraction. The relationship between relative diurnal amplitude (amplitude divided by the mean) and other climatological properties is then particularly meaningful. The relative amplitude of maritime high-cloud fraction is related to the mean total-cloud fraction and the noon-time solar zenith angle, which measures the solar diurnal amplitude. The diurnal amplitude of maritime low-cloud fraction does not have its strongest relationship with the mean low-cloud fraction, but has strong relationships to the upper-level cloud fraction, cloud-top height, and the solar diurnal amplitude. The relative amplitude of continental high-cloud fraction is related most strongly to the time-mean surface temperature, the diurnal amplitude of surface temperature, and the solar diurnal amplitude. The relative amplitude of continental low-cloud fraction has strong relationships with atmospheric moisture content and the diurnal amplitude of surface temperature.

In contrast to amplitude, diurnal phase does not exhibit a strong relationship with any climatological variable. Instead, it is uniform within individual categories, which makes cloud diurnal variations independent of geographical location and, therefore, highly spatially coherent. The spatial coherence of cloud diurnal variations makes them an important ingredient of climate, one that affords some predictability in terms of local climatological conditions.

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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

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 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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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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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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