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.

1. Introduction

Northern Hemisphere ozone underwent a monotonic decline during the 1980s and early 1990s, before rebounding in later years (Stolarski et al. 1992; WMO 1999; Staehelin et al. 2001). Measuring 5%–10% decade−1, the decline involves systematic changes that operate coherently on time scales longer than a couple of years. The downward trend amplifies during winter, when the stratosphere is disturbed by planetary waves (Hadjinicolaou et al. 1997; Fusco and Salby 1999). It culminates in a trend of spring ozone. During subsequent months, the trend in Northern Hemisphere ozone is gradually erased, leaving little memory by the time the next winter season begins.

The decline of wintertime ozone maximizes at midlatitudes of the Northern Hemisphere. The midlatitude trend has been widely ascribed to increasing halogen levels that support chemical depletion mechanisms, like those operating over the Antarctic (see WMO 1999 and references therein). This interpretation is supported by chemical models, which reproduce some of the features of the observed trend. However, there is growing recognition of the involvement of dynamics, which also introduces changes on decadal time scales (WMO 2003). In fact, the observed trend is separated by a wide gap from many chemical models that account for ozone depletion mechanisms (WMO 1995). Chemical transport models (CTMs), on the other hand, account for changes of the circulation, in addition to increasing halogens. They imply that a significant portion of the ozone trend follows from dynamical changes, especially at subpolar latitudes that are observed continuously by satellite (Chipperfield et al. 1999; Hadjinicolaou et al. 2002).

The interpretation of ozone changes rests, in part, on stratospheric temperature. It too underwent a monotonic decline during the 1980s and 1990s—one analogous to ozone. Observed during winter, the temperature trend has likewise been ascribed to chemical depletion, along with accompanying changes of radiative heating (WMO 1999; Ramaswamy et al. 2001). This interpretation, however, depends critically upon the structure of temperature changes.

A recent study of microwave sounding unit (MSU) temperature shows that systematic changes at high latitude, associated with the temperature trend, are accompanied at low latitude by systematic changes of opposite sign (Salby and Callaghan 2003). The out-of-phase structure of temperature changes suggests a compensation between high and low latitudes.

The structure of temperature changes is consistent with a systematic weakening of the residual mean circulation of the stratosphere. Driven by planetary waves, the residual circulation regulates wintertime temperature (see, e.g., Andrews et al. 1987). Mean downwelling over the winter hemisphere (w* < 0) is attended by adiabatic warming. It offsets radiative cooling, especially inside polar darkness. Through the intensity of downwelling and adiabatic warming, the residual circulation controls how much temperature decreases during individual winters.

The residual circulation also regulates the large wintertime increase of ozone, which culminates each year in its spring maximum. Fueling downwelling over the winter hemisphere is a mean poleward drift (υ* > 0). It carries ozone-rich air poleward from its chemical source in the Tropics. Through such transport, total ozone increases during winter by almost 100%. Supporting poleward transport is chemical production, as air moves to latitudes of longer photochemical lifetime. Jointly, these mechanisms control how much ozone increases during individual winters and, hence, its spring maximum.

Systematic changes, associated with the trend, operate coherently on time scales of a decade and longer. They are but one of two components of interannual variability. The other involves random interannual changes, which operate coherently on time scales of only a couple of years. The random component of interannual variability is thus widely separated from the systematic component associated with the trend.

Ozone and temperature each undergo large swings between years. Those changes are described by the “interannual anomaly”: the deviation from the climatological mean. Interannual anomalies of temperature and ozone are mostly random. However, recent studies have linked those changes to interannual anomalies of the residual circulation (Fusco and Salby 1999; Newman et al. 2001; Hadjinicolaou et al. 2002; Hu and Tung 2002). Interannual changes of Northern Hemisphere temperature track anomalous forcing of the residual circulation and so do interannual changes of Northern Hemisphere ozone.

A companion study (Salby and Callaghan 2002, hereafter SC02) showed that, in fact, much of the interannual variance of Northern Hemisphere ozone is accounted for by anomalous forcing of the residual circulation. Furthermore, the interannual anomaly of ozone has a robust structure, wherein changes at high latitude are compensated at low latitude by changes of opposite sign. In both regions, anomalous ozone varies coherently with anomalous forcing of the residual circulation. Associated chiefly with random changes, this structure characterizes the “climate sensitivity” of ozone with respect to a change of the residual circulation: how ozone changes in response to a perturbation of the residual circulation. Analogous considerations apply to temperature, which exhibits similar behavior.

The climate sensitivity of ozone has been evaluated from random changes over a large population of winters. It is applied here, along with other features of the observed record, to interpret systematic changes of ozone that are associated with its trend over the 1980s and 1990s. Systematic changes of ozone are shown to have a close relationship to random changes, which, in turn, vary coherently with anomalous forcing of the residual circulation. Systematic changes of temperature obey a similar relationship.

2. Data and analysis

Ozone column abundance 〈O3〉 is measured by the Total Ozone Mapping Spectrometer (TOMS). The TOMS instrument operated continuously on board Nimbus-7 during 1979–93 (McPeters 1996). The satellite record of ozone has been continued after 1993 in broken records from TOMS instruments on board the Russian satellite, Meteor-3 (1994), and Earth Probe (1997–98). Collectively, these instruments provide a record of ozone over two decades, representing nearly two complete solar cycles.

While affording global coverage, the TOMS record is not without its limitations. The most serious stem from the second of the platforms, Meteor-3. Beyond the gap during 1994–96, before Earth Probe came online, data from Meteor-3 suffer from a drift in equatorial crossing time. They also contain a long gap during the winter of 1994. These limitations handicap TOMS measurements from Meteor-3 relative to those from the other satellites.

The residual mean circulation involves a gradual drift across isentropic surfaces. It is too small to be measured directly. Instead, European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses are used to evaluate anomalous forcing of the residual circulation— dynamical factors that force changes of residual motion from one year to the next (see SC02 for a detailed discussion).

Two major components of anomalous forcing are accounted for. 1) Interannual changes of upward Eliassen– Palm (EP) flux Fz transmitted from the troposphere by planetary waves. Anomalous Fz is evaluated at 100 mb and averaged over 55°–90°N, where upward EP flux is concentrated. Averaging over a season then yields the interannual anomaly Fz (where the overbar denotes the horizontal average, and deviation from the climatological mean is implicit). Absorbed in the middle atmosphere, Fz measures overall wave driving of the residual circulation. Because it is evaluated at the tropopause, Fz also measures how the residual circulation is coupled to the troposphere through planetary waves. 2) The second component is interannual changes of equatorial wind uEQ at 10 mb, averaged over the same season. Associated with the QBO, uEQ controls wind at low latitude. It therefore determines the critical line (u = 0), where planetary waves suffer strong absorption that forces residual motion.

In tandem, Fz and uEQ represent anomalous forcing of the residual circulation. By modulating the strength and distribution of wave absorption (∇ · F), they modify υ* and w* over the winter hemisphere. The latter, in turn, modify how much temperature decreases during individual winters and, likewise, how much ozone increases. Unlike the residual circulation directly, anomalous forcing by Fz and uEQ are well represented in the ECMWF record.

During some years anomalous Fz and uEQ reinforce, but during others they interfere. Their combined influence is accounted for through a multivariate regression. In concert with a variational analysis, it maximizes the projection of interannual variance onto anomalous Fz and uEQ, that is, the interannual variance explained by those major components of anomalous forcing (see SC02 for details). Similar to EOF analysis, the procedure determines those months for which anomalous forcing of residual motion is strongly coherent with anomalous temperature and ozone. Evaluated is how much of the interannual variance is accounted for by anomalous forcing of the residual circulation, as well as the structures of anomalous temperature and ozone that vary coherently with it. The ECMWF record then provides a long history of interannual changes in key dynamical properties that influence temperature and ozone.

The ozone trend amplifies during winter, when the polar vortex is disturbed by planetary waves and total ozone averaged over the Northern Hemisphere 〈O3〉 increases toward its spring maximum (Hadjini-colaou et al. 1997; Fusco and Salby 1999). Then 〈O3〉 develops an anomalous tendency, the rate at which Northern Hemisphere ozone increases. Note that it is the tendency, not ozone directly, that is coupled to the residual circulation (e.g., through transport and chemical production in the ozone continuity equation).

The anomalous tendency accumulates over winter months, yielding an anomaly in spring ozone. This is also when the ozone trend reaches a maximum (Stolarski et al. 1992). Integrating over the disturbed season yields the anomalous net change,

 
formula

which represents the anomalous wintertime-mean tendency. As shown in SC02, Δ〈O3Mar–Nov accounts for most of the interannual anomaly in spring ozone, represented on the rhs of (1). Anomalous spring ozone thus follows chiefly from the anomalous tendency during the preceding months of winter. Little memory is passed forward from earlier winters, also represented on the rhs of (1), a feature of the ozone trend that has long been recognized (Hadjinicolaou et al. 1997; Fusco and Salby 1999; Fioletov and Shepherd 2003).

The Δ〈O3Mar–Nov is forced by anomalous ozone transport, as well as anomalous chemical production (e.g., in the time-averaged continuity equation). Analogous considerations apply to the anomalous wintertime tendency of temperature. It is forced by anomalous adiabatic warming and radiative cooling (e.g., in the time-averaged thermodynamic equation).

3. Changes of ozone

Figure 1 plots, as a function of year, the anomalous wintertime tendency of Northern Hemisphere ozone, averaged over 10°–65°N (solid). It is nearly equal to the record of anomalous spring ozone 〈O3Mar (SC02). The Δ〈O3Mar–Nov undergoes sizable changes from one year to the next. Largely random, they are accompanied by a systematic decline during the 1980s and early 1990s. Afterward, the ozone tendency rebounds. Following a similar pattern are systematic changes of spring ozone (WMO 1999).

Fig. 1.

Anomalous wintertime tendency of total ozone Δ〈O3Mar–Nov, averaged over 10°–65°N (solid) and compared against the anomalous wintertime tendency that varies coherently with anomalous forcing of the residual circulation (dashed). The latter accounts for 1) changes of upward EP flux transmitted to the middle atmosphere by planetary waves, defined from Fz at 100 hPa averaged over 55°–90°N and 2) changes of equatorial wind at 10 hPa with a lead of 1 month, associated with the quasi-biennial oscillation. Each is averaged over the same months as Δ〈O3Mar–Nov (see SC02 for details). Correlation to the observed record is 0.89 (99.999% significant). Also plotted is the anomalous wintertime tendency that varies coherently with anomalous forcing of the residual circulation and anomalous photochemical environment (dotted). The latter accounts for changes of aerosol and chlorine. Correlation to the observed record is 0.95 (99.999% significant). The data void between 1994 and 1996 coincides with winters of little or no TOMS data. Shaded circles mark winters when ozone was unusually low and temperature was unusually cold

Fig. 1.

Anomalous wintertime tendency of total ozone Δ〈O3Mar–Nov, averaged over 10°–65°N (solid) and compared against the anomalous wintertime tendency that varies coherently with anomalous forcing of the residual circulation (dashed). The latter accounts for 1) changes of upward EP flux transmitted to the middle atmosphere by planetary waves, defined from Fz at 100 hPa averaged over 55°–90°N and 2) changes of equatorial wind at 10 hPa with a lead of 1 month, associated with the quasi-biennial oscillation. Each is averaged over the same months as Δ〈O3Mar–Nov (see SC02 for details). Correlation to the observed record is 0.89 (99.999% significant). Also plotted is the anomalous wintertime tendency that varies coherently with anomalous forcing of the residual circulation and anomalous photochemical environment (dotted). The latter accounts for changes of aerosol and chlorine. Correlation to the observed record is 0.95 (99.999% significant). The data void between 1994 and 1996 coincides with winters of little or no TOMS data. Shaded circles mark winters when ozone was unusually low and temperature was unusually cold

Superposed in Fig. 1 is the ozone tendency that varies coherently with anomalous forcing of the residual circulation (dashed). Reflecting changes of Fz and uEQ, it tracks observed ozone changes closely. The solid and dashed records have a correlation of 0.89, which is 99.999% significant. Anomalous forcing of the residual circulation thus accounts for much, but not all, of the interannual variance of Northern Hemisphere ozone.1

Variance that is not accounted for by changes of the residual circulation is largely accounted for by changes of photochemical environment. Reflecting changes of photochemistry, those changes are represented in an ozone depletion factor (ODF) that characterizes the sensitivity of chemical production and destruction to changes of chlorine and aerosol loading (see SC02; Fusco and Salby 1999 for details). ODF varies with total chlorine and varies logarithmically with aerosol surface area, mirroring the dependence of ozone in chemical models. Observed levels of aerosol from the Stratospheric Aerosol Gas Experiment (SAGE) and levels of chlorine then yield a record of anomalous photochemical environment, which in turn determines ODF during the two decades. With the latter incorporated, the analysis described in section 2 maximizes the projection of interannual variance onto both sources of anomalous ozone: 1) the anomalous residual circulation and 2) anomalous photochemistry.

Superposed in Fig. 1 is the anomalous ozone tendency that varies coherently with both influences (dotted). It tracks observed ozone changes even more closely. Included are years when Northern Hemisphere ozone was unusually low and temperature was unusually cold (shaded circles). The solid and dotted records in Fig. 1 have a correlation of 0.95, which is 99.999% significant. Jointly, anomalous forcing of the residual circulation and anomalous photochemical environment account for nearly all of the observed variation of Northern Hemisphere ozone during the two decades. (The minor residual that remains may reflect contributions from the solar cycle, which has not been accounted for.) Accompanying random changes of Northern Hemisphere ozone is much of its systematic decline.

a. Systematic changes

The wintertime tendency of ozone evidences a monotonic decline over the 1980s and early 1990s, before rebounding in the closing years of the record. Undergoing a similar evolution is anomalous forcing of the residual circulation; it has the same form as the dashed curve in Fig. 1. After 1994, Northern Hemisphere ozone actually increased, despite a continued increase of stratospheric chlorine as well as ozone depletion implied by it (WMO 1999, 2003). The increase of ozone in the closing years of the record is also visible in ground-based measurements. Explanations that have been advanced for the rebound during 1995–98 include 1) a slowing of the chlorine increase and 2) a recovery following Pinatubo. It is noteworthy, however, that during those years, Northern Hemisphere ozone returned almost to levels seen at the opening of the satellite era. Note that the rebound of ozone is mirrored in anomalous forcing of the residual circulation (dashed), which rebounded simultaneously.

Analogous changes appear in upper-tropospheric wave structure. How much EP flux is transmitted upward by planetary waves is determined by the circulation near the tropopause, specifically, by the poleward heat flux (e.g., Andrews et al. 1987). Planetary waves in the upper troposphere weakened during the 1980s (Fusco and Salby 1999). Implied is a weakening of the residual circulation, attended by a weakening of adiabatic warming over the winter pole and a commensurate cooling and strengthening of the polar-night vortex (see also Newman and Nash 2000). Similar changes are implied by the Arctic Oscillation (AO), which is likewise coupled to tropospheric wave structure (Baldwin and Dunkerton 1999; Thompson et al. 2000; Hartmann et al. 2000).

A linear trend analysis has been performed on the satellite record of ozone, with years when TOMS data are unavailable masked. It yields the latitudinal structure of Δ〈O3Mar–Nov in Fig. 2. The trend in Δ〈O3Mar–Nov is negative at middle and high latitudes. Amplifying poleward, it approaches a maximum decline of −35 Dobson units (DU) per season per decade. This translates into a decline of spring ozone of about −10% decade−1, consistent with trends that have been deduced elsewhere (WMO 1999). The trend at high latitude is significant at the 99.5% level.

Fig. 2.

Linear trend in the wintertime tendency of total ozone, Δ〈O3Mar–Nov, as a function of latitude. Here, Δ〈O3Mar–Nov has been extended beyond the polar-night terminator by extrapolating 〈O3〉 during Nov, when interannual variance is small (see Salby and Callaghan 2002)

Fig. 2.

Linear trend in the wintertime tendency of total ozone, Δ〈O3Mar–Nov, as a function of latitude. Here, Δ〈O3Mar–Nov has been extended beyond the polar-night terminator by extrapolating 〈O3〉 during Nov, when interannual variance is small (see Salby and Callaghan 2002)

At lower latitude, the trend reverses sign: Δ〈O3Mar–Nov becomes positive across the Tropics and subtropics of the summer hemisphere. However, this compensating trend is an order of magnitude weaker than the negative trend at high latitude. The disparate magnitude at high and low latitudes mirrors the structure of mean vertical motion. Upwelling at low latitude is an order of magnitude weaker than downwelling at high latitude, reflecting the comparatively wide area over which upwelling occurs.2 Despite its smaller magnitude, the trend at low latitude is also significant, at the 96% level. The systematic decrease of ozone tendency at high latitude is thus accompanied at low latitude by a systematic increase. Each accumulates over winter months, yielding a maximum trend in spring ozone.

b. Random changes

The satellite record of ozone spans two decades. It provides a large population of winters, during which ozone changes randomly. In concert with the analysis described in section 2, this population determines the structure of anomalous ozone that varies coherently with anomalous forcing of the residual circulation. Projecting the interannual variance of Δ〈O3Mar–Nov onto records of Fz and uEQ gives the anomalous ozone introduced by a perturbation of the residual circulation.

Plotted in Fig. 3 is the anomalous wintertime tendency of ozone introduced by a 1 standard deviation increase in anomalous forcing of the residual circulation. Derived from random changes, it represents the climate sensitivity of Δ〈O3Mar–Nov with respect to a change of the residual circulation.3

Fig. 3.

Random anomaly in the wintertime tendency of total ozone Δ〈O3Mar–Nov, as a function of latitude: anomalous tendency introduced by a 1 standard deviation increase in anomalous forcing of the residual circulation. Evaluated from random changes of Northern Hemisphere ozone over two decades. Structure has been extended poleward of 65°N by extrapolating ozone during Nov, when interannual variance is small. Values are 95% significant throughout

Fig. 3.

Random anomaly in the wintertime tendency of total ozone Δ〈O3Mar–Nov, as a function of latitude: anomalous tendency introduced by a 1 standard deviation increase in anomalous forcing of the residual circulation. Evaluated from random changes of Northern Hemisphere ozone over two decades. Structure has been extended poleward of 65°N by extrapolating ozone during Nov, when interannual variance is small. Values are 95% significant throughout

Anomalous Δ〈O3Mar–Nov is positive in the extratropics, approaching 30 DU season−1 at high latitude. Hence, changes of the residual circulation between neighboring winters (e.g., between ±1–2 standard deviation) induce ozone changes as large as 50–100 DU. Positive Δ〈O3Mar–Nov reflects intensified downwelling, which displaces (ozone rich) mixing ratio surfaces downward. This increases ozone number density in the lowermost stratosphere, where total ozone is concentrated. Anomalous Δ〈O3Mar–Nov at middle and high latitudes is significant at the 99% level, making it strongly coherent with anomalous forcing of the residual circulation.

At lower latitude, the climate sensitivity reverses sign: anomalous Δ〈O3Mar–Nov becomes negative across the Tropics and subtropics of the summer hemisphere. There, anomalous Δ〈O3Mar–Nov reflects intensified upwelling, which displaces (ozone lean) mixing ratio surfaces upward. However, Δ〈O3Mar–Nov at low latitude is an order of magnitude weaker than Δ〈O3Mar–Nov at high latitude. The disparate magnitude at high and low latitudes reflects comparatively weak upwelling that prevails over the Tropics and subtropics of the summer hemisphere. Despite its smaller magnitude, anomalous Δ〈O3Mar–Nov at low latitude is also strongly significant, at the 99% level. Therefore, it too varies coherently with anomalous forcing of the residual circulation. Random changes of ozone at high latitude are thus accompanied at low latitude by changes of opposite sign.

Figure 3 represents the climate sensitivity of ozone with respect to a change of the residual circulation. (The time scale of this change is arbitrary; it could be isolated to an individual year or it could extend coherently over several years.) Derived from random changes, the climate sensitivity has the same form as systematic changes, which vary coherently over a decade and comprise the ozone trend (Fig. 2). In fact, the two anomalous structures are nearly mirror images of one another. Both amplify poleward, maximizing at high latitude. Likewise, both are compensated at low latitude by changes that are of opposite sign but are an order of magnitude weaker. Minor differences that distinguish the curves are most noticeable where Δ〈O3Mar–Nov is weak. They may reflect anomalous photochemistry (minor at low latitudes). Alternatively, they may simply reflect sampling error, which should become even smaller as the population of winters expands.

Except for a negative scale factor, systematic changes of ozone associated with its trend possess the same basic structure as random changes. They, in turn, vary coherently with anomalous forcing of the residual circulation (Fig. 1).

4. Changes of temperature

A similar analysis has been performed on the record of temperature. To obtain a counterpart of total ozone, temperature in ECMWF analyses has been integrated over pressure from 100 to 10 hPa. The column-averaged temperature 〈T〉 then serves as an analog of the column abundance of ozone 〈O3〉. From it is constructed the anomalous wintertime tendency of temperature, which, like Δ〈O3Mar–Nov, is coupled directly to the residual circulation.

During late winter, total ozone over the Northern Hemisphere increases toward its spring maximum. Simultaneously, its tendency develops a negative trend. The tendency of column-averaged temperature then likewise develops a negative trend. This is especially true over the Arctic, where downwelling is strong and the interannual variance of temperature is large (SC02).

Averaging anomalous temperature over the Arctic yields a record of its anomalous wintertime tendency (not shown), analogous to the one for ozone in Fig. 1. Like ozone, Δ〈TMar–Dec tracks anomalous forcing of the residual circulation. The two records have a correlation of 0.89, which is 99.999% significant. This happens to be the same correlation to anomalous forcing of the residual circulation as for Northern Hemisphere ozone (section 3). Hence, changes of the residual circulation also account for much of the interannual variance of Northern Hemisphere temperature. Accompanying random changes of temperature is much of its systematic decline.

a. Systematic changes

A linear trend analysis has been performed on the record of temperature. It yields the latitudinal structure of Δ〈TMar–Dec in Fig. 4. The trend in the wintertime tendency is negative at high latitude. Amplifying poleward, it approaches a maximum decline of −8 K season−1 decade−1. At these latitudes, the trend is significant at the 99% level. Its magnitude is consistent with the temperature trend observed by MSU (Ramaswamy et al. 2001; Salby and Callaghan 2003).

Fig. 4.

Linear trend in the wintertime tendency of column-averaged temperature between 100 and 10 hPa, Δ〈TMar–Dec, as a function of latitude

Fig. 4.

Linear trend in the wintertime tendency of column-averaged temperature between 100 and 10 hPa, Δ〈TMar–Dec, as a function of latitude

At lower latitude, the trend reverses sign: Δ〈TMar–Dec becomes positive across the Tropics and subtropics. However, like the trend in ozone, this compensating trend is an order of magnitude weaker than the trend at high latitude. Despite its smaller magnitude, the trend at low latitude is also significant, at the 98% level. A compensating trend also appears in the tendency of MSU temperature. The systematic decrease of Δ〈TMar–Dec at high latitude is thus accompanied at low latitude by a systematic increase.

b. Random changes

The temperature record likewise spans two decades. Hence, it too provides a large population of winters during which temperature changes randomly. In concert with the analysis described in section 2, this population determines the structure of anomalous temperature that varies coherently with anomalous forcing of the residual circulation, namely, the change of temperature introduced by a perturbation of the residual circulation.

Plotted in Fig. 5 is the anomalous wintertime tendency of temperature introduced by a 1 standard deviation increase in anomalous forcing of the residual circulation. Derived from random changes, it represents the climate sensitivity of Δ〈TMar–Dec with respect to a change of the residual circulation.4 Anomalous Δ〈TMar–Dec is positive at high latitude, where it approaches 8 K season−1. Positive Δ〈TMar–Dec reflects intensified downwelling and adiabatic warming, which offsets radiative cooling inside polar darkness. Anomalous Δ〈TMar–Dec at high latitude is significant at the 99% level, making it strongly coherent with anomalous forcing of the residual circulation.

Fig. 5.

Random anomaly in the wintertime tendency of column-averaged temperature Δ〈TMar–Dec, as a function of latitude: anomalous tendency introduced by a 1 standard deviation increase in anomalous forcing of the residual circulation. Evaluated from random changes over a population of 20 winters. Values are 95% significant throughout

Fig. 5.

Random anomaly in the wintertime tendency of column-averaged temperature Δ〈TMar–Dec, as a function of latitude: anomalous tendency introduced by a 1 standard deviation increase in anomalous forcing of the residual circulation. Evaluated from random changes over a population of 20 winters. Values are 95% significant throughout

At lower latitude, the climate sensitivity reverses sign: anomalous Δ〈TMar–Dec becomes negative across the Tropics and subtropics. There, anomalous Δ〈TMar–Dec reflects intensified upwelling, which intensifies adiabatic cooling. However, like ozone, anomalous temperature at low latitude is an order of magnitude weaker than anomalous temperature at high latitude. Despite its smaller magnitude, anomalous Δ〈TMar–Dec at low latitude is also strongly significant, at the 99% level. Therefore, it too varies coherently with anomalous forcing of the residual circulation. Random interannual changes of temperature at high latitude are thus accompanied at low latitude by changes of opposite sign.

Figure 5 represents the climate sensitivity of temperature. Derived from random changes, it has the same form as systematic changes that comprise the temperature trend (Fig. 4). In fact, the two anomalous structures are nearly mirror images of one another. Both amplify poleward, maximizing over the Arctic. Likewise, both are compensated at low latitude by changes that are of opposite sign but are an order of magnitude weaker. As for ozone, minor differences that distinguish the curves are most noticeable where Δ〈TMar–Dec is weak. They may reflect anomalous radiative heating associated with changes of chemical composition. Alternatively, they may simply reflect sampling error, which should become even smaller as the population of winters expands.

Except for a negative scale factor, systematic changes of temperature possess the same basic structure as random changes. They, in turn, vary coherently with anomalous forcing of the residual circulation.

5. Seasonality of systematic changes

The trends in temperature and ozone at high latitude are each accompanied at low latitude by trends of opposite sign. It is instructive to examine how those trends evolve over the course of the year. We explore this in the seasonal-mean tendency. It is calculated in terms of a sliding 4-month difference:

 
Δ〈O3MjMj−4 = 〈O3Mj − 〈O3Mj−4.
(2)

This is analogous to (1), only now as a function of closing month Mj. Here, Δ〈O3MjMj−4 represents the seasonal mean tendency of Northern Hemisphere ozone as a function of month. It develops a trend, which, for j = 3 (Mj = March), reduces to the trend in the wintertime tendency of ozone (Fig. 2).

Figure 6 plots, as a function of closing month, the trend in Δ〈O3MjMj−4 at high latitude (solid). The trend in ozone tendency becomes negative after fall equinox, amplifying sharply during the disturbed months of winter. This is the same season when the interannual variance of random changes amplifies (SC02). After spring equinox, westerlies collapse, along with planetary wave propagation and the anomalous residual circulation. The anomalous ozone tendency then reverses: the trend swings to positive values, which are likewise 99% significant. The positive trend in Δ〈O3MjMj−4 prevails during summer, gradually erasing a negative ozone trend that accumulated from the negative trend in Δ〈O3MjMj−4 during winter. The reversed tendency and subsequent approach to small values reflect photochemical relaxation of ozone, which prevails under undisturbed conditions and strong illumination. Photochemical relaxation drives 〈O3〉 toward climatological-mean values for summer, after the perturbing influence of winter has been removed.

Fig. 6.

Seasonal-mean tendency of total ozone, expressed as a sliding 4-month difference (see text), as a function of ending month and latitude at 55°N (solid) and 5°S (dashed)

Fig. 6.

Seasonal-mean tendency of total ozone, expressed as a sliding 4-month difference (see text), as a function of ending month and latitude at 55°N (solid) and 5°S (dashed)

Superposed in Fig. 6 is the trend in Δ〈O3MjMj−4 at low latitude (dashed). Although smaller, it too varies with season—just out of phase with the trend at high latitude. When the negative trend at high latitude amplifies, a positive trend at low latitude amplifies simultaneously. Likewise, when the negative trend at high latitude reverses, after spring equinox, so too does the positive trend at low latitude. The negative trend at low latitude that prevails during summer is 95% significant. It likewise reflects photochemical relaxation, gradually erasing a positive ozone trend that accumulated there from the positive trend in Δ〈O3MjMj−4 during winter.

By summer's end, the trend in Δ〈O3MjMj−4 in both regions has been reduced to small values. Little memory from the preceding year remains after fall equinox, at the onset of the next disturbed season (Hadjinicolaou et al. 1997; Fusco and Salby 1999; Fioletov and Shepherd 2003). Anomalous ozone, out of phase between high and low latitudes, then amplifies again.

If this behavior reflects a systematic change of the residual circulation, then similar seasonality should govern the trend in temperature. Figure 7 plots the same information for the seasonal-mean tendency of temperature Δ〈TMjMj−3. The trend at high latitude (solid) is negative and sharply amplified during the disturbed months of winter. After spring equinox, when planetary wave propagation and the residual circulation collapse, the anomalous temperature tendency reverses: the trend then swings to positive values, which are likewise 99% significant. The positive trend in Δ〈TMjMj−3 prevails during summer, gradually erasing a negative temperature trend that accumulated from the negative trend in Δ〈TMjMj−3 during winter. The reversed tendency and approach to small values reflect radiative relaxation of temperature, which prevails under undisturbed conditions and strong illumination. Radiative relaxation drives thermal structure toward climatological-mean 〈T〉 for summer, after the perturbing influence of winter has been removed.

Fig. 7.

As in Fig. 6, but for the seasonal-mean tendency of column-averaged temperature above 100 hPa at 65° (solid) and 10°N (dashed)

Fig. 7.

As in Fig. 6, but for the seasonal-mean tendency of column-averaged temperature above 100 hPa at 65° (solid) and 10°N (dashed)

Superposed is the trend in Δ〈TMjMj−3 at low latitude (dashed). It too varies with season—again, just out of phase with the trend at high latitude. When the negative trend at high latitude amplifies, a positive trend at low latitude amplifies simultaneously. Likewise, when the negative trend at high latitude reverses, so too does the positive trend at low latitude. The negative trend at low latitude that prevails during summer is 99% significant. It likewise reflects radiative relaxation, gradually erasing a positive temperature trend that accumulated there from the positive trend in Δ〈TMjMj−3 during winter.

By summer's end, the trend in Δ〈TMjMj−3 in both regions has been reduced to small values. Little memory of the preceding year remains at the onset of the next disturbed season. Anomalous temperature, out of phase between high and low latitudes, then amplifies again.

6. Interpretation

Except for a negative scale factor, the ozone trend has the same basic structure (Fig. 2) as random changes of ozone (Fig. 3). The negative scale factor separating those anomalous structures is not arbitrary. It reflects a physical link between these two components of interannual variability. Since random changes of ozone vary coherently with anomalous forcing of the residual circulation, that scale factor should reflect a systematic variation in such forcing (implied by the dashed curve in Fig. 1).

The climate sensitivity in Fig. 3 describes the change of ozone that is introduced by a perturbation of the residual circulation. Derived from random changes over a large population of years, the climate sensitivity can be used to anticipate the ozone trend that would result from a trend in anomalous forcing of the residual circulation, like the one evident in Fig. 1 (dashed). Scaling the ozone sensitivity by that negative trend yields an ozone trend with structure identical to that in Fig. 3, only reversed in sign. It then reproduces the basic structure of the observed ozone trend (Fig. 2), amplifying poleward to about −24 DU decade−1. Thus, random changes of ozone (represented in its climate sensitivity) predict an ozone trend with the same structure as the observed trend and with 70% of its magnitude, which approaches −35 DU decade−1 at high latitude. This result is broadly consistent with CTM calculations, which imply a comparable contribution from dynamics (Chipperfield and Jones 1999; Hadjinicolaou et al. 2002).

Changes of photochemistry, associated with increasing chlorine and perturbations in aerosol loading, can be accounted for in similar fashion. The ozone sensitivity can be scaled by the collective trend in the residual circulation and in photochemistry, also represented in Fig. 1 (dotted). As in the preceding analysis, this recovers a trend with the same structure as in Fig. 3, only reversed in sign. Thus, it too reproduces the basic structure of the observed trend (Fig. 2). However, the anticipated trend now amplifies poleward to about −37 DU decade−1, just over the observed value.

7. Conclusions

Systematic changes in the tendency of Northern Hemisphere ozone accumulate during winter, yielding a trend in spring ozone. Those systematic changes have the same basic structure as random interannual changes. The latter, in turn, vary coherently with anomalous forcing of the residual circulation. For each of these two components of interannual variability, ozone changes at high latitude are compensated at low latitude by changes of opposite sign. The same compensation is evident in the wintertime tendency of temperature.

Compensating trends of ozone at high and low latitude also possess the same seasonality. They amplify simultaneously—during winter, when the polar-night vortex is disturbed by planetary waves that force residual motion. This is also when the interannual variance of random changes amplifies. Analogous relationships are obeyed by temperature.

The ozone trend is broadly consistent with its climate sensitivity with respect to a perturbation of the residual circulation. Derived from random changes over a large population of years, 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.

The strong resemblance between systematic and random changes of Northern Hemisphere ozone indicates that a major portion of its decline during the 1980s and 1990s involved a weakening of the residual circulation. This interpretation is consistent with an observed cooling and intensification of the Arctic vortex, for example, as represented in the AO. Implications of this analysis are also consistent with CTM calculations, which account for those observed changes of dynamical structure. Such changes must be attended by reduced ozone transport and chemical production, associated with air drifting more slowly into the winter hemisphere.

While underscoring the importance of the residual circulation, these results do not exclude the role of photochemistry in systematic changes. About a third of the trend in the Northern Hemisphere average is, in fact, accounted for directly by changes of chlorine and aerosol loading. Furthermore, anomalous forcing of the residual circulation is not entirely extraneous to the stratosphere. Although Fz is dictated chiefly by tropospheric structure, uEQ is not. Systematic changes of equatorial wind during winter (only then is the polar vortex sensitive to uEQ) could follow from anomalous radiative heating and cooling in the stratosphere, for example, introduced by systematic changes of ozone or CO2. Understanding systematic changes associated with the residual circulation will require such considerations to be nailed down.

Ozone can also change through feedback between the anomalous circulation and photochemistry. Colder temperature produced by a weakening of the residual circulation can reinforce chemical destruction, notably, through heterogeneous processes that enhance chlorine activation at high latitude (WMO 1999 and references therein). Changes of ozone that are induced directly by anomalous transport and chemical production may then be augmented by anomalous chemical destruction. Depending nonlinearly on temperature, such feedback can modify the response of ozone to anomalous transport. The climate sensitivity of ozone may thus include an indirect contribution from photochemistry, one that is stimulated by anomalous transport and, therefore, also varies coherently with anomalous forcing of the residual circulation. Feedback onto photochemistry is most likely at high latitude, where cold temperature favors heterogeneous processes that support chlorine activation. Anomalous ozone destruction could then amplify the climate sensitivity of ozone there, over that from anomalous transport alone. On the other hand, it is unlikely that photochemical considerations can explain the out-of-phase relationship between high and low latitudes, which is inherent to the trend in ozone, to the trend in temperature, and to random variations of each.

Isolating individual contributions from transport and photochemistry will be essential to meaningfully interpret systematic changes of ozone. The origin of those contributions, however, is strongly suggested by the present analysis. Much of the systematic decline of Northern Hemisphere ozone during the preceding two decades involved the same mechanisms that figure in random interannual changes.

Acknowledgments

The authors are grateful for constructive comments provided during the review. This work was performed while the authors were supported by extraneous funding under NSF Grant ATM-0121853.

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Footnotes

Corresponding author address: Dr. Murry L. Salby, University of Colorado, Campus Box 311, Boulder, CO 80309

1

Ozone changes operating coherently with the residual circulation reflect anomalous transport. However, they also reflect anomalous (net) chemical production that is stimulated by anomalous transport. The latter enter through changes in temperature that are induced by anomalous downwelling, as well as through changes in the rate at which air is driven to latitudes of longer photochemical lifetime.

2

Mean vertical motion controls the elevation of ozone mixing ratio surfaces, which in turn controls total ozone.

3

The structure in Fig. 3 is robust; it is nearly identical to the structure of Δ〈O3Mar–Nov that varies coherently with anomalous ozone at high latitude.

4

As for ozone, the structure in Fig. 5 is robust; it is nearly identical to the structure of Δ〈TMar–Dec that varies coherently with anomalous temperature over the North Pole. It is also nearly identical to the structure of anomalous temperature earlier during winter, as temperature approaches its minimum around solstice (cf. SC02; Fig. 5).