Browsing by Author "Li, Feng"
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Item Assessment of the breakup of the Antarctic polar vortex in two new chemistry-climate models(AGU, 2010-04-14) Hurwitz, M. M.; Newman, P. A.; Li, Feng; Oman, L. D.; Morgenstern, O.; Braesicke, P.; Pyle, J. A.Successful simulation of the breakup of the Antarctic polar vortex depends on the representation of tropospheric stationary waves at Southern Hemisphere middle latitudes. This paper assesses the vortex breakup in two new chemistry-climate models (CCMs). The stratospheric version of the UK Chemistry and Aerosols model is able to reproduce the observed timing of the vortex breakup. Version 2 of the Goddard Earth Observing System (GEOS V2) model is typical of CCMs in that the Antarctic polar vortex breaks up too late; at 10 hPa, the mean transition to easterlies at 60°S is delayed by 12–13 days as compared with the ERA-40 and National Centers for Environmental Prediction reanalyses. The two models' skill in simulating planetary wave driving during the October–November period accounts for differences in their simulation of the vortex breakup, with GEOS V2 unable to simulate the magnitude and tilt of geopotential height anomalies in the troposphere and thus underestimating the wave driving. In the GEOS V2 CCM the delayed breakup of the Antarctic vortex biases polar temperatures and trace gas distributions in the upper stratosphere in November and December.Item Characteristics of instabilities in the mesopause region over Maui, Hawaii(AGU, 2005-02-01) Li, Feng; Liu, Alan Z.; Swenson, Gary R.Characteristics of convective and dynamical instabilities in the mesopause region (between 85 and 100 km) over Maui, Hawaii (20.7°N, 156.3°W) are investigated using 19 nights, ∼133 hours of high-resolution wind and temperature data obtained by the University of Illinois Na wind/temperature lidar during the Maui Mesosphere and Lower Thermosphere (Maui MALT) campaigns. The mean probabilities of convective and dynamical instabilities are observed to be ∼3 and 10%, respectively, but there is considerable night-to-night variation. At any given time the probability that an unstable condition is found at some altitudes in the 85–100 km range is ∼90%. The Maui MALT data exhibit a distinct trend for N² to increase with wind shear and vice versa. This correlation has important implications in the understanding of the development of instabilities. The night of 11 April 2002 is studied in detail in order to investigate the spatial and temporal structures of N², wind shear, and convective and dynamical instabilities. A close linkage between instability and the mesosphere inversion layers (MILs) is identified. Most of the convectively and dynamically unstable regions are located above the MILs, with a tendency for dynamical instability to develop below convective instability. It is found that the vertical variations of N² are often correlated with those of wind shear, but with a phase shift such that the maxima and minima of N² are located ∼0.5–1 km below those of wind shear. Because of this shift, dynamical instability tends to develop in the region above the maximum wind shear, where relatively small N² is observed to be associated with large wind shear. We also found that the wind shear is dominated by the contribution of the meridional wind, especially when the wind shear is strong. Possible mechanisms for the observed features are discussed.Item Early action on HFCs mitigates future atmospheric change(IOP, 2016-11-15) Hurwitz, Margaret M; Fleming, Eric L; Newman, Paul A; Li, Feng; Liang, QingAs countries take action to mitigate global warming, both by ratifying the UNFCCC Paris Agreement and enacting the Kigali Amendment to the Montreal Protocol to manage hydrofluorocarbons (HFCs), it is important to consider the relative importance of the pertinent greenhouse gases and the distinct structure of their atmospheric impacts, and how the timing of potential greenhouse gas regulations would affect future changes in atmospheric temperature and ozone. HFCs should be explicitly considered in upcoming climate and ozone assessments, since chemistry-climate model simulations demonstrate that HFCs could contribute substantially to anthropogenic climate change by the mid-21st century, particularly in the upper troposphere and lower stratosphere i.e., global average warming up to 0.19 K at 80 hPa. The HFC mitigation scenarios described in this study demonstrate the benefits of taking early action in avoiding future atmospheric change: more than 90% of the climate change impacts of HFCs can be avoided if emissions stop by 2030.Item Effects of Greenhouse Gas Increase and Stratospheric Ozone Depletion on Stratospheric Mean Age of Air in 1960–2010(AGU, 2018-02-10) Li, Feng; Newman, Paul; Pawson, Steven; Perlwitz, JudithThe relative impacts of greenhouse gas (GHG) increase and stratospheric ozone depletion onstratospheric mean age of air in the 1960–2010 period are quantified using the Goddard Earth ObservingSystem Chemistry-Climate Model. The experiment compares controlled simulations using a coupledatmosphere-ocean version of the Goddard Earth Observing System Chemistry-Climate Model, in which eitherGHGs or ozone depleting substances, or both factors evolve over time. The model results show that GHGsand ozone depleting substances have about equal contributions to the simulated mean age decrease, butGHG increases account for about two thirds of the enhanced strength of the lower stratospheric residualcirculation. It is also found that both the acceleration of the diabatic circulation and the decrease of the meanage difference between downwelling and upwelling regions are mainly caused by GHG forcing. The resultsshow that ozone depletion causes an increase in the mean age of air in the Antarctic summer lowerstratosphere through two processes: (1) a seasonal delay in the Antarctic polar vortex breakup that inhibitsyoung midlatitude air from mixing with the older air inside the vortex, and (2) enhanced Antarcticdownwelling that brings older air from middle and upper stratosphere into the lower stratosphere.Item Empirical Bayes Methods for Proteomics(2008-02-05) Li, Feng; Seillier-Moiseiwitsch, Francoise; Rukhin, Andrew; Mathematics and Statistics; StatisticsProteomics is the science that deals with high-throughput analysis of proteins. The study of proteins relies on efficient protein separation technique. 2D PAGE is a powerful technique for separating complex mixtures of proteins, where thousands of proteins are separated and measured simultaneously. The analysis of 2D PAGE images needs efficient methods able to cope with large-scale dataset. Empirical Bayes methods have been shown to be very efficient at combining information across dimensions of high-dimensional data. In the first part of this dissertation, the construction of empirical Bayes confidence intervals under different model assumptions is studied. Numerical simulations are conducted to demonstrate the satisfactory performance of the proposed methods. In the second part, a new comprehensive procedure for statistical analysis of 2D PAGE images is proposed, including protein quantification, normalization and statistical analysis. It reduces the dimension of the data. It also bypasses the current bottleneck in the analysis of 2D PAGE images in that it does not require spot matching. A strategy for multiple hypothesis testing based on multivariate analysis combined with empirical Bayes methods is formulated and applied to the differential analysis of 2D PAGE images. The new methodologies are implemented in a custom software package.Item Evidence for changes in stratospheric transport and mixing over the past three decades based on multiple data sets and tropical leaky pipe analysis(AGU, 2010-11-06) Ray, Eric A.; Moore, Fred L.; Rosenlof, Karen H.; Davis, Sean M.; Boenisch, Harald; Morgenstern, Olaf; Smale, Dan; Rozanov, Eugene; Hegglin, Michaela; Pitari, Gianni; Mancini, Eva; Braesicke, Peter; Butchart, Neal; Hardiman, Steven; Li, Feng; Shibata, Kiyotaka; Plummer, David A.Variability in the strength of the stratospheric Lagrangian mean meridional or Brewer-Dobson circulation and horizontal mixing into the tropics over the past three decades are examined using observations of stratospheric mean age of air and ozone. We use a simple representation of the stratosphere, the tropical leaky pipe (TLP) model, guided by mean meridional circulation and horizontal mixing changes in several reanalyses data sets and chemistry climate model (CCM) simulations, to help elucidate reasons for the observed changes in stratospheric mean age and ozone. We find that the TLP model is able to accurately simulate multiyear variability in ozone following recent major volcanic eruptions and the early 2000s sea surface temperature changes, as well as the lasting impact on mean age of relatively short-term circulation perturbations. We also find that the best quantitative agreement with the observed mean age and ozone trends over the past three decades is found assuming a small strengthening of the mean circulation in the lower stratosphere, a moderate weakening of the mean circulation in the middle and upper stratosphere, and a moderate increase in the horizontal mixing into the tropics. The mean age trends are strongly sensitive to trends in the horizontal mixing into the tropics, and the uncertainty in the mixing trends causes uncertainty in the mean circulation trends. Comparisons of the mean circulation and mixing changes suggested by the measurements with those from a recent suite of CCM runs reveal significant differences that may have important implications on the accurate simulation of future stratospheric climate.Item The frequency and dynamics of stratospheric sudden warmings in the 21st century(AGU, 2008-08-27) Charlton-Perez, A. J.; Polvani, L. M.; Austin, J.; Li, FengChanges to stratospheric sudden warmings (SSWs) over the coming century, as predicted by the Geophysical Fluid Dynamics Laboratory (GFDL) chemistry climate model [Atmospheric Model With Transport and Chemistry (AMTRAC)], are investigated in detail. Two sets of integrations, each a three-member ensemble, are analyzed. The first set is driven with observed climate forcings between 1960 and 2004; the second is driven with climate forcings from a coupled model run, including trace gas concentrations representing a midrange estimate of future anthropogenic emissions between 1990 and 2099. A small positive trend in the frequency of SSWs is found. This trend, amounting to 1 event/decade over a century, is statistically significant at the 90% confidence level and is consistent over the two sets of model integrations. Comparison of the model SSW climatology between the late 20th and 21st centuries shows that the increase is largest toward the end of the winter season. In contrast, the dynamical properties are not significantly altered in the coming century, despite the increase in SSW frequency. Owing to the intrinsic complexity of our model, the direct cause of the predicted trend in SSW frequency remains an open question.Item Impacts of Stratospheric Ozone Recovery on Southern Ocean Temperature and Heat Budget(AGU, 2023-09-14) Li, Feng; Newman, Paul A.; Waugh, Darryn W.The impacts of stratospheric ozone recovery on Southern Ocean surface and interior temperature, heat content, heat uptake, and heat transport are investigated by contrasting two ensemble chemistry-climate model simulations in 2005–2099: one with fixed ozone depleting substances (ODSs) and another with decreasing ODSs. In our simulations ozone recovery significantly affects Southern Ocean temperature, with large latitudinal and vertical variations. Ozone recovery causes a dipole change of the full-depth ocean heat content (OHC) with an increase south of 60°S and a decrease between 45°S and 60°S. Integrated over latitudes south of 40°S, OHC decreases in response to ozone recovery. This ocean heat loss is shown to be driven by weakened poleward ocean heat transport (OHT) across 40°S, which is partly canceled by enhanced heat uptake. The weakening of poleward OHT into the Southern Ocean is caused by the ozone-induced equatorward shift of the meridional overturning circulation.Item Impacts of the Eruption of Mount Pinatubo on Surface Temperatures and Precipitation Forecasts With the NASA GEOS Subseasonal-to-Seasonal System(AGU, 2021-07-26) Aquila, Valentina; Baldwin, Colleen; Mukherjee, Nikita; Hackert, Eric; Li, Feng; Marshak, Jelena; Molod, Andrea; Pawson, StevenA contemporary seasonal forecasting system is used to study the impacts of a volcanic sulfate injection into the stratosphere on the seasonal forecasts for surface temperatures, the El Niño Southern Oscillation (ENSO), and precipitation. The focus is a case study of the June 1991 eruption of Mt. Pinatubo, Philippines and the period from July 1991 to February 1992. Version 2 of the Goddard Earth Observing System (GEOS) subseasonal-to-seasonal (S2S) forecasting system is used in this study. GEOSS2S includes the GOddard Chemistry, Aerosols, Radiation and Transport (GOCART) aerosol module, which allows to prognostically simulate aerosol distributions. GOCART is coupled to the radiation and cloud modules to include the impact of the eruption on forecasted radiation and precipitation. The coupled GEOS-S2S system was initialized in May 1991 with fields based on observations to produce ten-member 9-month forecasts with and without the volcanic sulfur injection. The results of these ensemble experiments demonstrate that including Mt. Pinatubo in seasonal forecasts would improve the forecasts of the 1991–1992 global mean temperature and precipitation but worsen the forecast of ENSO by strengthening of El Niño beyond what showed in observations. Most significant changes in the forecasts of temperatures and precipitation are limited to the tropics. The only land area where the inclusion of Pinatubo significantly lowered the forecasted precipitation is tropical Africa.Item Investigation of a “wall” wave event(AGU, 2007-02-20) Li, Feng; Swenson, Gary R.; Liu, Alan Z.; Taylor, Michael; Zhao, YuchengA bright airglow event was observed at Maui, Hawaii, on the night of 11–12 August 2004 with multiple instruments including a Na wind/temperature lidar, an airglow imager, and a mesospheric temperature mapper. The characteristics of this event were investigated with measurements from these instruments. Analysis showed that this event was caused by a large-amplitude, upward-propagating gravity wave with a period of about 4–5 hours and a vertical wavelength of about 20 km, i.e., a “wall” wave. This wall wave induced dramatic changes in temperature (60 K), airglow intensity (doubled in the OH and tripled in the O₂ emissions), and Na abundance (tripled). It experienced strong dissipation and induced large downward heat flux with values about an order of magnitude larger than the annual mean. The wave also carried large momentum flux (∼70 m² s⁻²).Item Long-term changes in stratospheric age spectra in the 21st century in the Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM)(AGU, 2012-10-30) Li, Feng; Waugh, Darryn W.; Douglass, Anne R.; Newman, Paul A.; Strahan, Susan E.; Ma, Jun; Nielsen, J. Eric; Liang, QingIn 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.Item Narrowing of the upwelling branch of the Brewer-Dobson circulation and Hadley cell in chemistry-climate model simulations of the 21st century(AGU, 2010-07-10) Li, Feng; Stolarski, Richard. S.; Pawson, Steven; Newman, Paul A.; Waugh, DarrynChanges 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.Item Observationally derived and general circulation model simulated tropical stratospheric upward mass fluxes(AGU, 2008-10-30) Yang, Qiong; Fu, Qiang; Austin, John; Gettelman, Andrew; Li, Feng; Vomel, HolgerWe quantify the vertical velocity and upward mass flux in the tropical lower stratosphere on the basis of accurate radiative heating rate calculations using 8-year Southern Hemisphere Additional Ozonesondes balloon-borne measurements of temperature and ozone and cryogenic frost-point hygrometer measured water vapor in the tropics (15°S—10°N). The impact of tropospheric clouds on the stratospheric heating rates is considered using cloud distributions from the International Satellite Cloud Climatology Project. We find a nearly constant annual mean upward mass flux in the tropical lower stratosphere above the top of the tropical tropopause layer (i.e., ∼70 hPa), which is 1.13 ± 0.40 kgm⁻²d⁻¹ for the 40- to 30-hPa layer, and 0.89 ± 0.48 kgm⁻²d⁻¹ for the 70- to 50-hPa layer. A strong seasonal cycle exists in the upward mass flux and it is found that the mass flux below ∼70 hPa is decoupled from that above in the Northern Hemisphere summer. Simulations of the tropical lower stratosphere from two stratospheric General Circulation Models (GCMs) are compared with observations. The annual mean upward mass fluxes from both GCMs for the 40- to 30-hPa layer agree well with observations, while the simulated mass fluxes for the 70- to 50-hPa layer are twice as large. Both GCMs also simulate seasonal variation of the mass flux reasonably well but are incapable of simulating the observed interannual variability of the upward mass flux, which is closely correlated with the quasi-biennial oscillations.Item Observations of gravity wave breakdown into ripples associated with dynamical instabilities(AGU, 2005-03-30) Li, Feng; Liu, Alan Z.; Swenson, Gary R.; Hecht, James H.; Robinson, Walter A.The breakdown of a high-frequency quasi-monochromatic gravity wave into small-scale ripples in OH airglow was observed on the night of 28 October 2003 at Maui, Hawaii (20.7°N, 156.3°W). The ripples lasted ∼20 min. The phase fronts of the ripples were parallel to the phase fronts of the breaking wave. The mechanism for the ripple generation is investigated using simultaneous wind and temperature measurements made by a sodium (Na) lidar. The observations suggest that the wave breaking and the subsequent appearance of ripples were related to dynamical (or Kelvin-Helmholtz) instabilities. The characteristics of the ripples, including the alignment of the phase fronts with respect to the wind shear, the motion of the ripples, and the horizontal separation of the ripple fronts were consistent with their attribution to Kelvin-Helmholtz billows. It is likely that the dynamical instability was initiated by the superposition of the background wind shear and the shear induced by the wave. The wind shear, the mean wind acceleration, and the propagation of the breaking wave were found to be in the same direction, suggesting that wave-mean flow interactions contributed significantly to the generation of the strong (>40 m/s/km) wind shear and instability.Item On the relationship between the strength of the Brewer-Dobson circulation and the age of stratospheric air(AGU, 2006-09-08) Austin, John; Li, FengThe strength of the Brewer-Dobson circulation is computed for multi-decadal simulations of a coupled chemistry-climate model covering the period 1960 to 2100. The circulation strength, as computed from the tropical mass upwelling, generally increases throughout the simulations. The model also includes an age of air tracer which generally decreases during the simulations. The two different transport concepts of mass upwelling and reciprocal of the age of air are investigated empirically from the model simulations. The results indicate that the variables are linearly related in the model but with a change of gradient some time near 2005. Possible reasons for the change of gradient are discussed.Item Ozone depletion by hydrofluorocarbons(AGU, 2015-10-22) Hurwitz, Margaret M.; Fleming, Eric L.; Newman, Paul A.; Li, Feng; Mlawer, Eli; Cady-Pereira, Karen; Bailey, RoshelleAtmospheric concentrations of hydrofluorocarbons (HFCs) are projected to increase considerably in the coming decades. Chemistry climate model simulations forced by current projections show that HFCs will impact the global atmosphere increasingly through 2050. As strong radiative forcers, HFCs increase tropospheric and stratospheric temperatures, thereby enhancing ozone-destroying catalytic cycles and modifying the atmospheric circulation. These changes lead to a weak depletion of stratospheric ozone. Simulations with the NASA Goddard Space Flight Center 2-D model show that HFC-125 is the most important contributor to HFC-related atmospheric change in 2050; its effects are comparable to the combined impacts of HFC-23, HFC-32, HFC-134a, and HFC-143a. Incorporating the interactions between chemistry, radiation, and dynamics, ozone depletion potentials (ODPs) for HFCs range from 0.39 × 10⁻³ to 30.0 × 10⁻³, approximately 100 times larger than previous ODP estimates which were based solely on chemical effects.Item Prescribing stratospheric chemistry overestimates southern hemisphere climate change during austral spring in response to quadrupled CO₂(Springer Nature, 2022-11-23) Li, Feng; Newman, Paul A.The interaction of stratospheric chemistry with a changing climate from an abrupt CO₂ quadrupling is assessed using the coupled atmosphere–ocean Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM). Two abrupt 4 × CO₂ experiments were performed, one with interactive stratospheric chemistry and the other with a prescribed stratospheric chemistry that does not simulate stratospheric ozone response to 4 × CO₂. The interactive and prescribed chemistry experiments simulate similar global mean surface temperature change. Nevertheless, interactive chemistry is critical to capture the Southern Hemisphere tropospheric midlatitude jet response to 4 × CO₂. When stratospheric ozone response to 4 × CO₂ is neglected, GEOSCCM overestimates Southern Hemisphere tropospheric circulation change. This stratospheric chemistry-induced climate impact has large seasonal variability. During the austral spring season September–October–November (SON), prescribed chemistry yields a stronger poleward shift and intensification of the Southern Hemisphere midlatitude tropospheric jet, surface wind stress, and the Southern Ocean meridional overturning circulation than occurs with interactive chemistry. In other seasons interactive and prescribed chemistry have similar effects on the Southern Hemisphere circulation. The seasonality of stratospheric chemistry-induced climate impact is related to the seasonality of Antarctic lower stratospheric ozone response to 4 × CO₂. In contrast to this stratospheric ozone response to 4 × CO₂, stratospheric ozone recovery from decline of the ozone depleting substances has its largest impact on the Southern Hemisphere tropospheric circulation in austral summer (December–January–February), but no effects in SON. It is found that the different seasonality for these two stratospheric ozone layer change scenarios is related to the different seasonality of tropopause meridional temperature gradient response.Item Relationships between the Brewer-Dobson circulation and the southern annular mode during austral summer in coupled chemistry-climate model simulations(AGU, 2010-08-06) Li, Feng; Newman, Paul A.; Stolarski, Richard S.The Brewer-Dobson circulation (BDC) is the mean meridional mass circulation in the stratosphere and the southern annular mode (SAM) is the prime variability pattern of the Southern Hemisphere extratropical troposphere. Motivated by previous studies showing that both the strengths of the BDC and the SAM have the largest trends in the austral summer in the recent past, this paper investigates the relationships between the BDC and the SAM using coupled chemistry-climate model simulations. The model results show that the strengthening of the BDC in the Southern Hemisphere during November–February (NDJF) is strongly projected onto the high index of the SAM. The trends in the BDC and the SAM are driven by Antarctic ozone depletion, which increases stratosphere-troposphere interactions through a delayed Antarctic vortex breakup. The prolonged persistence of stratospheric westerlies enhances upward propagation of tropospheric wave activity into the stratosphere and strengthens the BDC. The wave flux and westerly anomalies in the stratosphere in turn drive a SAM trend toward its high index. Model results also show that the BDC-SAM relationship is robust on the interannual time scale.Item Response of the Upper-Level Monsoon Anticyclones and Ozone to Abrupt CO₂ Changes(AGU, 2021-10-05) Tweedy, Olga V.; Oman, Luke D.; Waugh, Darryn W.; Schoeberl, Mark R.; Douglass, Anne R.; Li, FengThe summer monsoon anticyclones are the dominant climatological features of the Northern Hemispheric (NH) summertime circulation in the upper troposphere and lower stratosphere (UTLS). However, the response of these anticyclones to the increased levels of E CO₂ remains highly uncertain, as does the impact on the distribution of UTLS ozone and other tracers. This study examines the response of the NH summertime monsoon anticyclones and UTLS ozone to the abrupt increase in E CO₂ forcing using output from a suite of coupled ocean–atmosphere general circulation model simulations. These models show an equatorward shift of the Asian summer monsoon anticyclone, a weakening of the North American summer monsoon anticyclone, and a stronger westerly flow penetrating deep into the tropics above the Pacific Ocean and North America. We use additional idealized experiments from atmosphere-only general circulation models with prescribed SSTs and sea ice concentration to isolate the direct atmospheric radiative effects from the indirect effect of SST warming on the UTLS monsoon anticyclones. Comparison between atmosphere-only and coupled ocean–atmosphere experiments shows that SST warming is the principal mechanism producing UTLS monsoonal circulation changes. The 4*CO₂ experiments result in a significant reduction up to 40%–50% of the UTLS ozone in the northern tropics, which could have an impact on radiative balance near the surface.Item Seasonal variations of stratospheric age spectra in the Goddard Earth Observing System Chemistry Climate Model (GEOSCCM)(AGU, 2012-03-15) Li, Feng; Waugh, Darryn W.; Douglass, Anne R.; Newman, Paul A.; Pawson, Steven; Stolarski, Richard S.; Strahan, Susan E.; Nielsen, J. EricThe stratospheric age spectrum is the probability distribution function of the transit times since a stratospheric air parcel had last contact with a tropospheric boundary region. Previous age spectrum studies have focused on its annual mean properties. Knowledge of the age spectrum's seasonal variability is very limited. In this study, we investigate the seasonal variations of the stratospheric age spectra using the pulse tracer method in the Goddard Earth Observing System Chemistry Climate Model (GEOSCCM). The relationships between the age spectrum and the boundary impulse response (BIR) are reviewed, and a simplified method to reconstruct seasonally varying age spectra is introduced. The age spectra in GEOSCCM have strong seasonal cycles, especially in the lowermost and lower stratosphere and in the subtropical overworld. These changes reflect the seasonal evolution of the Brewer-Dobson circulation, isentropic mixing, and transport barriers. We also investigate the seasonal and interannual variations of the BIRs. Our results clearly show that computing an ensemble of seasonally dependent BIRs is necessary in order to capture the seasonal and annual mean properties of the stratospheric age spectrum.