[show abstract][hide abstract] ABSTRACT: We investigate the relative role of volcanic eruptions, El-Niño
Southern-Oscillation (ENSO) and the Quasi-Biennal-Oscillation (QBO) in
the quasi-decadal signal in the tropical stratosphere in temperature and
ozone commonly attributed to the 11 yr solar cycle. For this purpose, we
perform transient simulations with the Whole Atmosphere Community
Climate Model forced from 1960 to 2004 with an 11 yr solar cycle in
irradiance and different combinations of other forcings. An improved
multiple regression technique is used to diagnose the 11 yr solar signal
in the simulations. One set of simulations includes all observed
forcings, and is thereby aimed at closely reproducing observations.
Three idealized sets exclude ENSO variability, volcanic aerosol forcing,
and QBO in tropical stratospheric winds, respectively. Differences in
the derived solar response in the tropical stratosphere in the four sets
quantify the impact of ENSO, volcanic events and the QBO in attributing
quasi-decadal changes to the solar cycle in the model simulations. It is
shown that most of the apparent solar-induced lower stratospheric
temperature and ozone increase diagnosed in the simulations with all
observed forcings is due to two major volcanic eruptions (i.e., El
Chichón in 1982 and Mt. Pinatubo in 1991), that are concurrent
with periods of high solar activity. While in the middle and upper
tropical stratosphere, it is feasible to detect a robust solar signal,
this is not the case in the tropical lower stratosphere, at least in a
45 yr record. The present results suggest that in the tropical lower
stratosphere, the portion of decadal variability that can be
unambigously linked to the solar cycle may be smaller than previously
Atmospheric Chemistry and Physics 11/2013; 13(11):30097-30142. · 4.88 Impact Factor
[show abstract][hide abstract] ABSTRACT:  We investigate the influence of major sudden stratospheric warming (SSW) and elevated stratopause (ES) events in the Northern Hemisphere winter on the transport of NOx produced by energetic particle precipitation (EPP) from the mesosphere–lower thermosphere to the stratosphere using the Whole Atmosphere Community Climate Model (WACCM). Increases in NOx following a major SSW and/or ES event are in excess of 100% compared to winters when no major SSW or ES event occurred. The increase in NOx is attributed to an increase in the descending branch of the residual circulation () following the event. The timing of the event strongly affects the amount of NOx that descends to the stratosphere: the earlier the event occurs, the more NOx descends to the stratosphere. We also quantify the amount of NOx produced by EPP descending to the stratosphere in each winter and find that the largest increases in NOx are in years that have a major SSW followed by an ES event early in the season (December or early January). The strength of following an event shows a very strong seasonal dependence and explains why the timing of the event affects the transport of NOx.
Journal of Geophysical Research: Atmospheres. 10/2013; 118(20).
[show abstract][hide abstract] ABSTRACT:  A global model of sodium in the mesosphere and lower thermosphere has been developed within the framework of the National Center for Atmospheric Research's Whole Atmosphere Community Climate Model (WACCM). The standard fully interactive WACCM chemistry module has been augmented with a chemistry scheme that includes nine neutral and ionized sodium species. Meteoric ablation provides the source of sodium in the model and is represented as a combination of a meteoroid input function (MIF) and a parameterized ablation model. The MIF provides the seasonally and latitudinally varying meteoric flux which is modeled taking into consideration the astronomical origins of sporadic meteors and considers variations in particle entry angle, velocity, mass, and the differential ablation of the chemical constituents. WACCM simulations show large variations in the sodium constituents over time scales from days to months. Seasonality of sodium constituents is strongly affected by variations in the MIF and transport via the mean meridional wind. In particular, the summer to winter hemisphere flow leads to the highest sodium species concentrations and loss rates occurring over the winter pole. In the Northern Hemisphere, this winter maximum can be dramatically affected by stratospheric sudden warmings. Simulations of the January 2009 major warming event show that it caused a short-term decrease in the sodium column over the polar cap that was followed by a factor of 3 increase in the following weeks. Overall, the modeled distribution of atomic sodium in WACCM agrees well with both ground-based and satellite observations. Given the strong sensitivity of the sodium layer to dynamical motions, reproducing its variability provides a stringent test of global models and should help to constrain key atmospheric variables in this poorly sampled region of the atmosphere.
Journal of Geophysical Research: Atmospheres. 10/2013; 118(19).
[show abstract][hide abstract] ABSTRACT:  The hydrological impact of enhancing Earth's albedo by solar radiation management is investigated using simulations from 12 Earth System models contributing to the Geoengineering Model Intercomparison Project (GeoMIP). We contrast an idealized experiment, G1, where the global mean radiative forcing is kept at preindustrial conditions by reducing insolation while the CO2 concentration is quadrupled to a 4×CO2 experiment. The reduction of evapotranspiration over land with instantaneously increasing CO2 concentrations in both experiments largely contributes to an initial reduction in evaporation. A warming surface associated with the transient adjustment in 4×CO2 generates an increase of global precipitation by around 6.9% with large zonal and regional changes in both directions, including a precipitation increase of 10% over Asia and a reduction of 7% for the North American summer monsoon. Reduced global evaporation persists in G1 with temperatures close to preindustrial conditions. Global precipitation is reduced by around 4.5%, and significant reductions occur over monsoonal land regions: East Asia (6%), South Africa (5%), North America (7%), and South America (6%). The general precipitation performance in models is discussed in comparison to observations. In contrast to the 4×CO2 experiment, where the frequency of months with heavy precipitation intensity is increased by over 50% in comparison to the control, a reduction of up to 20% is simulated in G1. These changes in precipitation in both total amount and frequency of extremes point to a considerable weakening of the hydrological cycle in a geoengineered world.
Journal of Geophysical Research: Atmospheres. 10/2013; 118(19).
[show abstract][hide abstract] ABSTRACT:  Ozone changes and associated climate impacts in the Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations are analyzed over the historical (1960–2005) and future (2006–2100) period under four Representative Concentration Pathways (RCP). In contrast to CMIP3, where half of the models prescribed constant stratospheric ozone, CMIP5 models all consider past ozone depletion and future ozone recovery. Multimodel mean climatologies and long-term changes in total and tropospheric column ozone calculated from CMIP5 models with either interactive or prescribed ozone are in reasonable agreement with observations. However, some large deviations from observations exist for individual models with interactive chemistry, and these models are excluded in the projections. Stratospheric ozone projections forced with a single halogen, but four greenhouse gas (GHG) scenarios show largest differences in the northern midlatitudes and in the Arctic in spring (~20 and 40 Dobson units (DU) by 2100, respectively). By 2050, these differences are much smaller and negligible over Antarctica in austral spring. Differences in future tropospheric column ozone are mainly caused by differences in methane concentrations and stratospheric input, leading to ~10 DU increases compared to 2000 in RCP 8.5. Large variations in stratospheric ozone particularly in CMIP5 models with interactive chemistry drive correspondingly large variations in lower stratospheric temperature trends. The results also illustrate that future Southern Hemisphere summertime circulation changes are controlled by both the ozone recovery rate and the rate of GHG increases, emphasizing the importance of simulating and taking into account ozone forcings when examining future climate projections.
Journal of Geophysical Research: Atmospheres. 05/2013; 118(10).
[show abstract][hide abstract] ABSTRACT:  A future Maunder Minimum type grand solar minimum, with total solar irradiance reduced by 0.25% over a 50 year period from 2020 to 2070, is imposed in a future climate change scenario experiment (RCP4.5) using, for the first time, a global coupled climate model that includes ozone chemistry and resolved stratospheric dynamics (Whole Atmosphere Community Climate Model). This model has been shown to simulate two amplifying mechanisms that produce regional signals of decadal climate variability comparable to observations, and thus is considered a credible tool to simulate the Sun's effects on Earth's climate. After the initial decrease of solar radiation in 2020, globally averaged surface air temperature cools relative to the reference simulation by up to several tenths of a degree Centigrade. By the end of the grand solar minimum in 2070, the warming nearly catches up to the reference simulation. Thus, a future grand solar minimum could slow down but not stop global warming.
[show abstract][hide abstract] ABSTRACT: 1] Elevated stratopause (ES) events occurring during Northern Hemisphere winter are identified in four climate simulations of the period 1953–2005 made with the Whole Atmosphere Community Climate Model (WACCM). We find 68 ES events in 212 winters. These events are found in winters when the middle atmosphere is disturbed and there are zonal wind reversals in the stratosphere at high latitudes. These disturbances can be associated with both major and minor stratospheric sudden warming events (SSWs). The ES events occur under conditions where the stratospheric jet, the gravity wave forcing, and the residual circulation remain reversed longer than in those winters where an SSW occurs without an ES. We compare ES events with the type of SSW (vortex splitting and vortex displacement) and find that 68% of ES events form after vortex splitting events. We also present a climatology of ES events based on NASA's Modern-Era Retrospective Analysis for Research and Applications reanalysis data from 1979 to 2012 and compare it to the model results. WACCM composites of major SSW and ES also show enhanced Eliassen-Palm flux divergences in the upper mesosphere after the stratospheric warming, immediately before the formation of an ES. However, the formation of an ES in WACCM is due primarily to adiabatic heating from gravity wave-driven downwelling, which follows the reestablishment of the eastward jet in the upper stratosphere. We find nine winters where an ES forms in the absence of any significant planetary wave activity in the upper mesosphere and illustrate one such event., and L. de la Torre (2013), A climatology of elevated stratopause events in the whole atmosphere community climate model, J. Geophys. Res. Atmos., 118, 1234-1246, doi:10.1002/jgrd.50123.
[show abstract][hide abstract] ABSTRACT: We present a comparison of temperature trends using different satellite and radiosonde observations and climate (GCM) and chemistry-climate model (CCM) outputs, focusing on the role of photochemical ozone depletion in the Antarctic lower stratosphere during the second half of the twentieth century. Ozone-induced stratospheric cooling peaks during November at an altitude of approximately 100 hPa in radiosonde observations, with 1969 to 1998 trends in the range of -3.8 to -4.7 K/dec. This stratospheric cooling trend is more than 50% greater than the previously estimated value of -2.4 K/dec, which suggested that the CCMs were overestimating the stratospheric cooling, and that the less complex GCMs forced by prescribed ozone were matching observations better. Corresponding ensemble mean model trends are -3.8K/dec for the CCMs, -3.5K/dec for the CMIP5 GCMs, and -2.7K/dec for the CMIP3 GCMs. Accounting for various sources of uncertainty-including sampling uncertainty, measurement error, model spread, and trend confidence intervals-observations and CCM and GCM ensembles are consistent in this new analysis. This consistency does not apply to each individual that makes up the GCM and CCM ensembles, and some do not show significant ozone-induced cooling. Nonetheless, analysis of the joint ozone and temperature trends in the CCMs suggests that the modeled cooling/ozone-depletion relationship is within the range of observations. Overall, this study emphasizes the need to use a wide range of observations for model validation as well as sufficient accounting of uncertainty in both models and measurements.
Journal of Geophysical Research-Atmospheres. 01/2013; 118(2):605-613.
[show abstract][hide abstract] ABSTRACT: We investigate the effect of stratospheric ozone recovery on Antarctic
sea ice in the next half-century, by comparing two ensembles of
integrations of the Whole Atmosphere Community Climate Model, from 2001
to 2065. One ensemble is performed by specifying all forcings as per the
Representative Concentration Pathway 4.5; the second ensemble is
identical in all respects, except for the surface concentrations of
ozone depleting substances, which are held fixed at year 2000 levels,
thus preventing stratospheric ozone recovery. Sea ice extent declines in
both ensembles, as a consequence of increasing greenhouse gas
concentrations. However, we find that sea ice loss is ∼33% greater
for the ensemble in which stratospheric ozone recovery does not take
place, and that this effect is statistically significant. Our results,
which confirm a previous study dealing with ozone depletion, suggest
that ozone recovery will substantially mitigate Antarctic sea ice loss
in the coming decades.
Geophysical Research Letters 10/2012; 39(20):20701-. · 3.98 Impact Factor
[show abstract][hide abstract] ABSTRACT: Over the last three decades, ozone depletion over Antarctica has
affected temperature and winds in the lower stratosphere, and even in
the troposphere and at the surface. The second Chemistry Climate Model
Validation activity (CCMVal2) concluded that chemistry-climate models
simulate stratospheric cooling that is too large compared to
observations, even though the modeled and observed ozone trends are
similar. However, these comparisons were based only on radiosonde data
available for 1969-1998. Here, we investigate trends in the
Southern Hemisphere polar cap in the latest version of the Community
Earth System Model (CESM1) with its high-top atmospheric component,
WACCM4, fully coupled to an ocean model. We compare model trends with
observations for different periods and with other modeling studies to
show much better agreement with more recent data, and conclude that the
discrepancy between observed trends and those calculated by high-top
models may not be as large as previously reported.
Geophysical Research Letters 08/2012; 39(16):16803-. · 3.98 Impact Factor
[show abstract][hide abstract] ABSTRACT: The Whole Atmosphere Community Climate Model (WACCM) simulates the
dynamical and chemical interactions in the lower and middle atmosphere.
Dramatic perturbations in dynamics and chemical composition occur in
both the free-running model and in a version constrained with
observationally based winds and temperature. However, downward transport
of NO and CO into the upper stratosphere during active periods in the
northern hemisphere winter is weaker than observed. We will present
results of an investigation of the model dynamics and chemistry to
understand the processes that affect the downward fluxes. Particular
attention will be paid to the gravity wave momentum forcing that drives
the mesospheric circulation, the photochemical production and loss rates
in the MLT, and the diffusion of trace species.
[show abstract][hide abstract] ABSTRACT: Previous studies using a multiple linear regression model to fit both
observations and reanalysis data pointed to a significant 11 year solar
cycle response in the tropical stratosphere. Temperature and ozone
increase during solar maximum, with the largest relative response near
20 and 45 km altitude. However, continuous satellite data has only been
available since 1978, which means that the analysis period covers at
most three solar cycles. This hinders a clean separation of a solar
signal from other natural sources of variability. In particular, in the
tropical lower stratosphere part of the decadal variability in this
region is due to ENSO and major volcanic events (e.g., El Chichon in
1982 and Pinatubo in 1991 occurred during the peak of solar activity).
We investigate the relative contribution of volcanic eruptions and ENSO
in the quasi-decadal signal commonly attributed to the 11 year solar
cycle. For this purpose, we present results from transient simulations
with the Whole Atmosphere Community Climate Model, which were carried
out with observed forcings, including the 11 year solar cycle in
irradiance. In one simulation, we exclude the ENSO variability by
prescribing climatologically varying sea surface temperatures. In the
second simulation, we exclude volcanic aerosol forcing. We compare the
solar signal in both simulations, with a focus on the tropical lower
stratosphere, to the signal in simulations run with all natural and
anthropogenic forcings. Differences in the derived solar response
quantify the impact ENSO and volcanic events have in correctly
attributing decadal changes to the solar cycle in the relatively short
[show abstract][hide abstract] ABSTRACT: Observations over the last few decades have shown a cooling trend in the
Southern Hemisphere (SH) upper troposphere and lower stratosphere in
conjunction with and acceleration of the SH westerly winds which
directly affects tropospheric climate. General Circulation Models
corroborate this result and reveal that both the increase in greenhouse
gases and polar ozone depletion are responsible for this trend. On the
contrary, the increase in greenhouse gases and the expected ozone
recovery have an opposite impact on future trends in the SH. Thus, the
Chemistry Climate Models (CCMs) from the CCM Validation 2 activity
(CCMVal-2 SPARC report) predict a deceleration of the polar jet on the
poleward side during the SH summer, although at a weaker rate than the
acceleration observed in the past decades. CCMVal2 models are fully
chemistry-coupled models with their top well beyond the stratopause.
However, they typically use observed surface temperatures as boundary
conditions. We make use of 95year AR5 simulations with the Whole
Atmosphere Community Climate Model WACCM4 coupled to an ocean model to
investigate the impact of ozone recovery on Southern Hemisphere climate
under three different climate change scenarios. The results show that
the effect of ozone recovery on temperature and wind trends is the
weakest in the intermediate scenario in relation to the competing
effects of ozone recovery and increase in GHGs.
[show abstract][hide abstract] ABSTRACT: 1] The atmospheric response to the 11 year solar cycle (SC) and its combination with the quasi-biennal oscillation (QBO) are analyzed in four simulations of the Whole Atmosphere Community Climate Model version 3.5 (WACCM3.5), which were performed with observed sea surface temperatures, volcanic eruptions, greenhouse gases, and a nudged QBO. The analysis focuses on the annual mean response of the model to the SC and on the evolution of the solar signal during the Northern Hemispheric winter. WACCM3.5 simulates a significantly warmer stratosphere under solar maximum conditions compared to solar minimum. The vertical structure of the signal in temperature and ozone at low latitudes agrees with observations better than previous versions of the model. The temperature and wind response in the extratropics is more uncertain because of its seasonal dependence and the large dynamical variability of the polar vortex. However, all four simulations reproduce the observed downward propagation of zonal wind anomalies from the upper stratosphere to the lower stratosphere during boreal winter resulting from solar-induced modulation of the polar night jet and the Brewer-Dobson circulation. Combined QBO-SC effects in the extratropics are consistent with observations, but they are not robust across the ensemble members. During boreal winter, solar signals are also found in tropospheric circulation and surface temperature. Overall, these results confirm the plausibility of proposed dynamical mechanisms driving the atmospheric response to the SC. The improvement of the model climatology and variability in the polar stratosphere is the basis for the success in simulating the evolution and magnitude of the solar signal.
Journal of Geophysical Research 03/2012; 117. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: The combined effects of El Niño-Southern Oscillation (ENSO) and
the 11 year solar cycle on the Northern Hemisphere polar stratosphere
have been analyzed in the Whole Atmosphere Community Climate Model
version 3 in the absence of the quasi-biennial oscillation. The polar
response to ENSO agrees with previous studies during solar minimum;
composites of warm minus cold ENSO events show a warmer polar
stratosphere and a weaker polar vortex, propagating downward as the
winter evolves. During solar maximum conditions, little downward
propagation of the ENSO signal is simulated, leading to colder
temperatures and stronger winds in the polar lower stratosphere. The
analysis of the Eliassen-Palm flux and wave index of refraction shows
that this is mainly due to a reduction of upward propagating
extratropical planetary wave number 1 component caused by changes in the
background winds in the subtropics related to a warmer tropical upper
stratosphere during solar maximum. The effect of the 11 year solar cycle
variability on the polar stratosphere is not significant during cold
ENSO events until February. During warm ENSO events, a statistically
significant colder polar lower stratosphere and stronger polar vortex
are simulated throughout the winter, and no downward propagation of this
signal occurs. This is mainly due to the combined effects of solar
maximum and warm ENSO conditions on the wave mean flow interaction.
These results show a nonlinear behavior of the extratropical
stratosphere response to the combination of the two forcings and
highlight the need to stratify with respect to ENSO and solar conditions
and analyze the seasonal march throughout the winter.
Journal of Geophysical Research 12/2011; 116(D23):23112-. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: A model study of the atmosphere/ocean response from the surface to the
lower thermosphere to changes in solar and geomagnetic forcing over the
11-year solar cycle is presented. The study utilizes the NCAR Community
Earth System Model with the Whole Atmosphere Community Climate Model as
its atmospheric component (CESM-WACCM). It is a 'high-top' chemistry
climate model with coupled ocean, sea ice and land components. A
three-member ensemble of simulations was conducted with observed
variability in solar radiation, auroral activity and trends in
radiatively active chemical species for the period 1850 to present.
Multiple linear regression is used to separate secular changes in
chemistry and dynamics from those correlated to the periodic solar
forcing. Emphasis will be placed on the responses in the stratosphere of
ozone in the tropics and the dynamics at high latitudes during the
northern hemisphere winter including interactions with the Quasi
Biennial Oscillation. The extent to which 11-year solar cycle
variability leads directly or indirectly to changes in the troposphere
and sea surface temperatures are also investigated. Future simulations
are also addressed.