Subtropical Lower Stratosphere Research Articles

Summertime Transport Pathways From Different Northern Hemisphere Regions Into the Arctic

AbstractTrace gases and aerosols play an important role in Arctic chemistry and climate. As most Arctic tracers and aerosols are transported from midlatitude source regions, long‐range transport into the Arctic is one of the key factors to understand the current and future states of Arctic climate. While previous studies have investigated the airmass fraction and transit time distribution in the Arctic, the actual transport pathways and their underlying dynamics and efficiencies are yet to be understood. In this study, we implement a large ensemble of idealized tagged pulse passive tracers in the Whole Atmosphere Community Climate Model version 5 to identify and analyze summertime transport pathways from different Northern Hemisphere surface regions into the Arctic. Three different transport pathways are identified as those associated with fast, intermediate and slow time scales. Midlatitude tracers can be transported into the Arctic in the troposphere via the fast transport pathway (∼8 days), which moves tracers northward from the source region mainly through transient eddies. For the intermediate transport pathway, which happens on 1–3 weeks’ time scales, midlatitude tracers are first zonally transported by the jet stream, and then advected northward into the Arctic over Alaska and northern North Atlantic. Tropical and subtropical tracers are transported into the Arctic lower stratosphere via the slow transport pathway (1–3 months), as the tracers are lifted upward into the tropical and subtropical lower stratosphere, and then transported into the Arctic following the isentropic surfaces.

Response of stratospheric water vapour to CO2 doubling in WACCM

Stratospheric water vapour (SWV), as a greenhouse gas, modulates the radiative energy budget of the climate system. It is sensitive to, and plays a significant role in the climate change. In this study, we investigate the SWV response to CO2 increase with the Whole Atmosphere Community Climate Model (WACCM). In addition, we study its possible feedback on stratospheric temperature and relevant mechanisms. In our model experiments, the CO2 concentration and sea surface temperature (SSTs) are changed at the same time, as well as separately, to enable separating the radiative-photochemical and dynamical response to CO2 doubling scenarios. The model results show that the response of SWV to CO2 doubling is dominated by the changes in the SSTs, with an increase of the SWV concentration by ~ 6 to 10% in most of the stratosphere and more than 10% in the lower stratosphere, except for winter pole in the lower stratosphere, where the CO2 doubling decreases water vapour. The increase of SWV is mostly due to a dynamical response to the warm SSTs. Doubled CO2 induces warm SSTs globally and further leads to moist troposphere and a warmer tropical and subtropical tropopause, resulting in more water vapour entering stratosphere from below. As a greenhouse gas, large increase of SWV in the lower stratosphere, in turn, affects the stratospheric temperature, resulting in a warming of the tropical and subtropical lower stratosphere, offsetting the cooling caused by CO2 doubling.

The cause of the spring strengthening of the Antarctic polar vortex

The stratospheric polar vortex strengthening from late winter to spring plays a crucial role in polar ozone depletion. The Arctic polar vortex reaches its peak intensity in mid-winter, whereas the Antarctic vortex usually strengthens in early spring. As a result, the strong ozone depletion is observed every year over the Antarctic, while over the Arctic short-term ozone loss occasionally occurs in late winter or early spring. However, the cause of such a difference in the life cycles of the Arctic and Antarctic polar vortices is still not completely clear. Based on the ERA-Interim reanalysis data, we show a high agreement between the seasonal variations of temperature in the subtropical lower stratosphere and zonal wind in the subpolar and polar lower stratosphere in the Southern Hemisphere. Thus, the spring strengthening of the Antarctic polar vortex can occur due to the seasonal temperature increase in the subtropical lower stratosphere in this period.

The cause of the strengthening of the Antarctic polar vortex during October–November periods

The stratospheric polar vortex strength in spring determines to a great extent the duration and intensity of ozone depletion in the polar regions. The size of the Antarctic ozone hole usually reaches its maximum in September, and then drops off during October and November. However, in 1987, 1998, 1999, 2001, 2006, 2011 and especially in 2015, a significant increase in the ozone hole area relative to climatological mean values was observed in October and November under strong polar vortex conditions. Furthermore, in these years it occurred simultaneously with a temperature increase in the subtropical lower stratosphere in the Southern Hemisphere. Based on the ERA-Interim reanalysis temperature and zonal wind data, we reveal a high correlation between interannual variations of the subtropical temperature and the zonal wind at 60° S in October and November (the Pearson correlation coefficients r equal 0.71 and 0.82, respectively). Thus, a temperature increase in the subtropical lower stratosphere from October to November can strengthen the Antarctic polar vortex in this period.

Wave Activity and Its Changes in the Troposphere and Stratosphere of the Northern Hemisphere in Winters of 1979–2016

An analysis of spectra of wave disturbances with zonal wave numbers 1 ≤ k ≤ 10 is carried out using winter (November to March) ERA-Interim reanalysis geopotential data in the troposphere and stratosphere for 1979–2016. Contributions of eastward-traveling (E), westward-traveling (W), and stationary (S) waves are estimated. The intensification of wave activity is observed in the tropical troposphere and stratosphere and in the upper stratosphere of the entire Northern Hemisphere. The intensification of wave activity in the tropics and subtropics is noted for waves of all types (E, W, and S), while in the middle and higher latitudes it is related mainly to stationary and eastward waves. Near the subtropical tropopause, the energy of stationary waves has increased in recent decades. In addition, in the tropical and subtropical troposphere and in the subtropical lower stratosphere, the energy of the eastward-traveling waves in El Nino years may be one and a half times or twice the energy in La Nina years. The spectrally weighted zonal wave numbers for waves of all types (E, W, and S) are the largest in the upper subtropical troposphere. The spectrally weighted zonal wave number for W and S waves is correlated with the Atlantic Multidecadal Oscillation index and varies by 15% in 1979–2016 (on an interdecadal time scale). The spectrally weighted wave period is larger in the stratosphere than in the troposphere. It is maximal in the middle extratropical stratosphere. The spectrally weighted wave periods correlate with the activity of sudden stratospheric warmings. The sign of this correlation depends on the latitude, atmospheric layer, and zonal wave number.

Inter-comparison of stratospheric mean-meridional circulation and eddy mixing among six reanalysis data sets

Abstract. The stratospheric mean-meridional circulation (MMC) and eddy mixing are compared among six meteorological reanalysis data sets: NCEP-NCAR, NCEP-CFSR, ERA-40, ERA-Interim, JRA-25, and JRA-55 for the period 1979–2012. The reanalysis data sets produced using advanced systems (i.e., NCEP-CFSR, ERA-Interim, and JRA-55) generally reveal a weaker MMC in the Northern Hemisphere (NH) compared with those produced using older systems (i.e., NCEP/NCAR, ERA-40, and JRA-25). The mean mixing strength differs largely among the data products. In the NH lower stratosphere, the contribution of planetary-scale mixing is larger in the new data sets than in the old data sets, whereas that of small-scale mixing is weaker in the new data sets. Conventional data assimilation techniques introduce analysis increments without maintaining physical balance, which may have caused an overly strong MMC and spurious small-scale eddies in the old data sets. At the NH mid-latitudes, only ERA-Interim reveals a weakening MMC trend in the deep branch of the Brewer–Dobson circulation (BDC). The relative importance of the eddy mixing compared with the mean-meridional transport in the subtropical lower stratosphere shows increasing trends in ERA-Interim and JRA-55; this together with the weakened MMC in the deep branch may imply an increasing age-of-air (AoA) in the NH middle stratosphere in ERA-Interim. Overall, discrepancies between the different variables and trends therein as derived from the different reanalyses are still relatively large, suggesting that more investments in these products are needed in order to obtain a consolidated picture of observed changes in the BDC and the mechanisms that drive them.

What controls stratospheric water vapor in the NH summer monsoon regions?

AbstractWater vapor in the lower stratosphere exhibits localized maximum values during Northern Hemisphere summer in the Asian and North American (NA) monsoon regions. The processes maintaining these maxima are not well understood. We analyze the variability of water vapor in the monsoon regions based on Aura Microwave Limb Sounder satellite observations during 2005–2013 and quantify links to deep convection, large‐scale temperatures, and monsoonal circulations. Strong subseasonal variations in stratospheric water vapor are closely linked with deep convection in the monsoon regions, with the surprising result that stronger convection leads to a relatively dry stratosphere and weaker convection to a wetter stratosphere. This result is observed for both the Asian and NA monsoon regions. This behavior is explained by temperature changes in the stratosphere tied to deep convection: stronger convection leads to relatively cold temperatures in the subtropical lower stratosphere, which is the key region controlling large‐scale dehydration within the anticyclonic monsoonal circulations. Likewise, weaker convection leads to warmer subtropical stratospheric temperatures, relatively less dehydration, and enhanced water vapor. The observed water vapor changes are in approximate agreement with those expected from the subtropical temperature variations, taking into account dilution of the dehydrated air into the larger monsoon region. These results demonstrate that stratospheric water vapor in the Northern Hemisphere monsoon regions is mainly controlled by large‐scale circulation and temperatures, and overshooting deep convection plays a relatively minor role.

Sensitivities of the Lower-Stratospheric Transport and Mixing to Tropical SST Heating

Abstract The sensitivities of the Brewer–Dobson circulation (BDC) to different distributions of tropical SST heating are investigated in an idealized aquaplanet model. It is found that an increase in tropical SSTs generally leads to an acceleration of tropical upwelling and an associated reduction in the age of air (AOA) in the polar stratosphere and that the AOA near the subtropical tropopause is correlated with local isentropic mixing of tropospheric air with stratospheric air. The zonal distribution of SST perturbations has a major impact on the vertical and meridional structure of the BDC as compared with other SST characteristics. Zonally localized SST heatings tend to generate a shallow acceleration of the stratospheric residual circulation, enhanced isentropic mixing associated with a weakened stratospheric jet, and a reduction in AOA mostly within the polar vortex. In contrast, SST heatings with a zonally symmetric structure tend to produce a deep strengthening of the stratospheric residual circulation, suppressed isentropic mixing associated with a stronger stratospheric jet, and a decrease of AOA in the entire stratosphere. The shallow versus deep strengthening of the stratospheric residual circulation change has been linked to wave propagation and dissipation in the subtropical lower stratosphere rather than wave generation in the troposphere, and the former can be strongly affected by the vertical position of the subtropical jet. These results suggest that, while the longitudinally localized SST trends under climate change may contribute to the change in the shallow branch of the BDC, the upward shift of the subtropical jet associated with the zonal SST heating can impact the deep branch of the BDC.

Impact of geomagnetic excursions on atmospheric chemistry and dynamics

Abstract. Geomagnetic excursions, i.e. short periods in time with much weaker geomagnetic fields and substantial changes in the position of the geomagnetic pole, occurred repeatedly in the Earth's history, e.g. the Laschamp event about 41 kyr ago. Although the next such excursion is certain to come, little is known about the timing and possible consequences for the state of the atmosphere and the ecosystems. Here we use the global chemistry climate model SOCOL-MPIOM to simulate the effects of geomagnetic excursions on atmospheric ionization, chemistry and dynamics. Our simulations show significantly increased concentrations of nitrogen oxides (NOx) in the entire stratosphere, especially over Antarctica (+15%), due to enhanced ionization by galactic cosmic rays. Hydrogen oxides (HOx) are also produced in greater amounts (up to +40%) in the tropical and subtropical lower stratosphere, while their destruction by reactions with enhanced NOx prevails over the poles and in high altitudes (by −5%). Stratospheric ozone concentrations decrease globally above 20 km by 1–2% and at the northern hemispheric tropopause by up to 5% owing to the accelerated NOx-induced destruction. A 5% increase is found in the southern lower stratosphere and troposphere. In response to these changes in ozone and the concomitant changes in atmospheric heating rates, the Arctic vortex intensifies in boreal winter, while the Antarctic vortex weakens in austral winter and spring. Surface wind anomalies show significant intensification of the southern westerlies at their poleward edge during austral winter and a pronounced northward shift in spring. Major impacts on the global climate seem unlikely.

Long‐term changes in stratospheric age spectra in the 21st century in the Goddard Earth Observing System Chemistry‐Climate Model (GEOSCCM)

In this study we investigate long‐term variations in the stratospheric age spectra using a 21st century simulation with the Goddard Earth Observing System Chemistry‐Climate Model (GEOSCCM). Our purposes are to characterize the long‐term changes in the age spectra, and identify processes that cause the decrease of the mean age in a changing climate. Changes in the age spectra in the 21st century simulation are characterized by decreases in the modal age, the mean age, the spectral width, and the tail decay timescale throughout the stratosphere. Our analyses show that the decrease in the mean age is caused by two processes: the acceleration of the residual circulation that increases the young air masses in the stratosphere, and the weakening of the recirculation that leads to a decrease of the tail of the age spectra and a decrease of the old air masses. Weakening of the stratospheric recirculation is also strongly correlated with the increase of the residual circulation. One important result of this study is that the decrease of the tail of the age spectra makes an important contribution to the decrease of the mean age. Long‐term changes in the stratospheric isentropic mixing are also investigated. Mixing increases in the subtropical lower stratosphere, but its impact on the age spectra is smaller than the increase of the residual circulation. The impacts of the long‐term changes in the age spectra on long‐lived chemical tracers are also investigated.

A Robust Mechanism for Strengthening of the Brewer–Dobson Circulation in Response to Climate Change: Critical-Layer Control of Subtropical Wave Breaking

Abstract Climate models consistently predict a strengthened Brewer–Dobson circulation in response to greenhouse gas (GHG)-induced climate change. Although the predicted circulation changes are clearly the result of changes in stratospheric wave drag, the mechanism behind the wave-drag changes remains unclear. Here, simulations from a chemistry–climate model are analyzed to show that the changes in resolved wave drag are largely explainable in terms of a simple and robust dynamical mechanism, namely changes in the location of critical layers within the subtropical lower stratosphere, which are known from observations to control the spatial distribution of Rossby wave breaking. In particular, the strengthening of the upper flanks of the subtropical jets that is robustly expected from GHG-induced tropospheric warming pushes the critical layers (and the associated regions of wave drag) upward, allowing more wave activity to penetrate into the subtropical lower stratosphere. Because the subtropics represent the critical region for wave driving of the Brewer–Dobson circulation, the circulation is thereby strengthened. Transient planetary-scale waves and synoptic-scale waves generated by baroclinic instability are both found to play a crucial role in this process. Changes in stationary planetary wave drag are not so important because they largely occur away from subtropical latitudes.

Residual circulation trajectories and transit times into the extratropical lowermost stratosphere

Abstract. Transport into the extratropical lowermost stratosphere (LMS) can be divided into a slow part (time-scale of several months to years) associated with the global-scale stratospheric residual circulation and a fast part (time-scale of days to a few months) associated with (mostly quasi-horizontal) mixing (i.e. two-way irreversible transport, including extratropical stratosphere-troposphere exchange). The stratospheric residual circulation may be considered to consist of two branches: a deep branch more strongly associated with planetary waves breaking in the middle to upper stratosphere, and a shallow branch associated with synoptic and planetary scale waves breaking in the subtropical lower stratosphere. In this study the contribution due to the stratospheric residual circulation alone to transport into the LMS is quantified using residual circulation trajectories, i.e. trajectories driven by the (time-dependent) residual mean meridional and vertical velocities. This contribution represents the advective part of the overall transport into the LMS and can be viewed as providing a background onto which the effect of mixing has to be added. Residual mean velocities are obtained from a comprehensive chemistry-climate model as well as from reanalysis data. Transit times of air traveling from the tropical tropopause to the LMS along the residual circulation streamfunction are evaluated and compared to recent mean age of air estimates. A time-scale separation with much smaller transit times into the mid-latitudinal LMS than into polar LMS is found that is indicative of a separation of the shallow from the deep branch of the residual circulation. This separation between the shallow and the deep circulation branch is further manifested in a distinction in the aspect ratio of the vertical to meridional extent of the trajectories, the integrated mass flux along the residual circulation trajectories, as well as the stratospheric entry latitude of the trajectories. The residual transit time distribution reproduces qualitatively the observed seasonal cycle of youngest air in the extratropical LMS in fall and oldest air in spring.

Narrowing of the upwelling branch of the Brewer‐Dobson circulation and Hadley cell in chemistry‐climate model simulations of the 21st century

Changes in the width of the upwelling branch of the Brewer‐Dobson circulation and Hadley cell in the 21st Century are investigated using simulations from a coupled chemistry‐climate model. In these model simulations the tropical upwelling region narrows in the troposphere and lower stratosphere. The narrowing of the Brewer‐Dobson circulation is caused by an equatorward shift of Rossby wave critical latitudes and Eliassen‐Palm flux convergence in the subtropical lower stratosphere. In the troposphere, the model projects an expansion of the Hadley cell's poleward boundary, but a narrowing of the Hadley cell's rising branch. Model results suggest that eddy forcing may also play a part in the narrowing of the rising branch of the Hadley cell.

To What Extent Does High-Latitude Wave Forcing Drive Tropical Upwelling in the Brewer–Dobson Circulation?

Abstract The causes of the annual cycle and nonseasonal variability in the globally averaged, equator-to-pole Brewer–Dobson circulation (BDC; defined here as the equatorially symmetric component of the Lagrangian-mean meridional circulation) are investigated based on zonally averaged, lower-stratospheric temperature data from satellite-borne Microwave Sounding Unit (MSU) and Advanced Microwave Sounding Unit (AMSU). Time-varying vertical velocities in the BDC are inferred from departures of the meridional temperature profiles from the respective radiative equilibrium temperature profiles. Equatorward of ∼45°N/S, the annual-mean profile of lower-stratospheric temperature and the seasonal and nonseasonal variations about it project almost exclusively onto the equatorially symmetric component. The climatological-mean annual cycle accounts for nearly 90% of the month-to-month variance of the equatorially symmetric component of the temperature field; January/February is colder than July/August equatorward of ∼45°N/S and warmer than July/August poleward of that latitude. The equator-to-subpolar temperature contrast roughly doubles from July/August to January/February, implying an approximate doubling of the strength of the BDC. The nonseasonal variability is dominated by a similar pattern. Tropical upwelling in the BDC, as inferred from of the temperature field, varies in response to variations in eddy heat fluxes at high latitudes with comparable strength on the intraseasonal and interannual time scales; it does not appear to be correlated with equatorial tropospheric planetary wave activity or with variations in wave forcing in subtropical lower stratosphere. It is concluded that high-latitude wave forcing plays an important role in modulating tropical upwelling in the BDC across a wide range of frequencies.

Acceleration of the Brewer–Dobson Circulation due to Increases in Greenhouse Gases

Abstract The acceleration of the Brewer–Dobson circulation under rising concentrations of greenhouse gases is investigated using the Whole Atmosphere Community Climate Model. The circulation strengthens as a result of increased wave driving in the subtropical lower stratosphere, which in turn occurs because of enhanced propagation and dissipation of waves in this region. Enhanced wave propagation is due to changes in tropospheric and lower-stratospheric zonal-mean winds, which become more westerly. Ultimately, these trends follow from changes in the zonal-mean temperature distribution caused by the greenhouse effect. The circulation in the middle and upper stratosphere also accelerates as a result of filtering of parameterized gravity waves by stronger subtropical westerly winds.

Characteristics of inertia gravity waves over the South Pacific as revealed by radiosonde observations

Using horizontal wind and temperature data with fine vertical resolution obtained by radiosondes, we investigate the characteristics of parameters and propagation properties of inertia gravity waves in the upper troposphere and lower stratosphere in the subtropics and extratropics over the South Pacific where very few observations have so far been done. Components extracted by a band‐pass filter having cutoff vertical wavelengths of 0.5 and 5 km are analyzed as gravity waves. Gravity wave parameters such as intrinsic frequency , vertical and horizontal wavelengths, and phase and group velocities are estimated by hodograph analysis. The statistics on such parameters are presented for both the stratosphere and troposphere for the three latitudinal ranges 17°–24°S, 25°–33°S, and 34°–48°S. The horizontal wavelengths decrease and increases with increasing latitude south. The dominant values for the Coriolis parameter f divided by are ∼0.5 in the lower stratosphere and ∼0.3 in the troposphere, with little dependence on latitude. The dominant vertical wavelengths for upward propagating waves are longer in the troposphere than in the lower stratosphere. The most dominant direction of horizontal propagation is southward, with a little bias toward southeastward. A case study using ray tracing suggests that the major source of such southeastward waves is located in the upper troposphere. Notable northward propagation of gravity waves is also found in the subtropical lower stratosphere. The source of the gravity waves is considered to be a cutoff cyclone developing in the upper troposphere.

Characterization of the 11-Year Solar Signal Using a Multiple Regression Analysis of the ERA-40 Dataset

Abstract A multiple linear regression analysis of the ERA-40 dataset for the period 1979–2001 has been used to study the influence of the 11-yr solar cycle on atmospheric temperature and zonal winds. Volcanic, North Atlantic Oscillation (NAO), ENSO, and quasi-biennial oscillation (QBO) signatures are also presented. The solar signal is shown to be readily distinguishable from the volcanic signal. The main solar signal is a statistically significant positive response (i.e., warmer in solar maximum) of 1.75 K over the equator with peak values at 43 km and a reversed signal of similar magnitude at high latitudes that is seasonally dependent. Consistent with this is a statistically significant zonal wind response of up to 6 m s−1 in the subtropical upper stratosphere/lower mesosphere that is also seasonally dependent. The wind anomalies are westerly/easterly in solar maximum/minimum. In addition, there is a statistically significant temperature response in the subtropical lower stratosphere that shows similarity in spatial structure to the QBO response, suggesting a possible interaction between the solar and QBO signals in this region. The solar response in tropospheric zonal winds is small but significant, confirming previous studies that indicate a possible modulation of the Hadley circulation.

Large‐scale equatorward transport of ozone in the subtropical lower stratosphere

Anomalous vertical profiles of ozone were observed in the subtropical lower stratosphere (near south Florida) in July 2002 during the NASA‐sponsored Cirrus Regional Study of Tropical Anvils and Cirrus Layers‐Florida Area Cirrus Experiment measurement campaign. It is shown that there is an enhancement of ozone (initially >150%) above the tropopause extending up to ∼410 K potential temperature. This ozone increase is the result of recent transport of middle‐and high‐latitude lower stratospheric air into the subtropics. This meridional transport was a consequence of a geostrophic flow pattern established by a quasi‐stationary anticyclone centered over the south central United States that persisted for much of July 2002. We show the spatial and temporal extent of meridional isentropic transport into the subtropics by examining the ozone vertical profiles in combination with the NOy:O3 correlations as well as isentropic back trajectory calculations. The anomalous ozone profiles are also reproduced in a global chemical transport model.

Static Stability in the Extratropical Tropopause Region

Idealized axisymmetric anomalies of potential vorticity (PV) on a midlatitude f plane and their related response in terms of balanced wind and temperature are investigated with special focus on the static stability in the tropopause region. The PV anomalies are specified such that they can be interpreted as the result of conservative advection in the tropopause region across the gradients of a prescribed background atmosphere with piecewise constant buoyancy frequency squared N2. Related cyclones and anticyclones are treated identically except for the sign of the tropopause potential temperature anomaly. Composite profiles of N2 are computed, for which the thermal tropopause is used as a common reference level and where the number of cyclones in the composite equals the number of anticyclones. One obtains a pronounced peak of N2 just above the tropopause and slightly enhanced values below the tropopause in comparison with the background profile. Within the framework of PV inversion various mechanisms are identified. Important contributions to the peak in N2 are due to the pronounced cyclone–anticyclone asymmetry in the vertical structure of the tangential wind, and to the poleward advection of high values of PV from the subtropical lower stratosphere. Qualitatively, the key features of the composite profiles are in good agreement with observations above southern Germany that have recently been compiled.

2 * CO2 and Solar Variability Influences on the Troposphere Through Wave-Mean Flow Interactions.

A variety of climate forcings are now thought to be able to influence planetary wave dynamics in the troposphere by affecting the propagation of planetary waves out of the troposphere. However, this propagation pattern is sensitive to the details of the corresponding zonal wind changes. Here we discuss two forcing mechanisms that alter zonal winds and subsequent tropospheric responses: changes in atmospheric CO 2 concentrations, and solar forcing in conjunction with the QBO. Increased atmospheric CO 2 concentrations can be shown to influence planetary wave refraction so as to produce an intensified residual circulation in the subtropical lower stratosphere (which increases transport of tropospheric species into the stratosphere). In our GCM experiments, the low latitude response appears qualitatively robust over a wide range of tropical warming magnitudes, although the quantitative circulation change depends upon the degree of tropical warming as influenced by convection and cloud cover changes; it varies by a factor of three with a factor of three change in tropical warming. At higher latitudes, this equatorward planetary wave refraction has been associated with an increase in the high phase of the Arctic Oscillation. In the model experiments, the extratropical response depends upon the magnitude of both low and high latitude warming in the troposphere; with SST and sea ice changes that result in a weaker Hadley Cell and greater high latitude warming, the Arctic Oscillation phase change may be negative. The QBO alters the latitudinal gradient of the zonal wind in the stratosphere, and solar heating, in association with ozone response, alters the vertical gradient of the zonal wind. Both gradients affect the refractive properties of planetary waves uniquely for each individual combination of tropical east/west winds and solar maximum/minimum activity. In the model, when we consider solar maximum compared to solar minimum conditions, the east (west) phase of the QBO results in a relative high (low) phase of the Arctic Oscillation with corresponding temperature changes. Observed and modeled surface air temperature variations calculated between the solar cycle extremes in the different QBO phases are similar in magnitude to those derived from regression of monthly data on the AO, both being on the order of observed interannual variations.