It is a daunting challenge to conduct initialized hindcasts with enough ensemble members and associated start years to form a drifted climatology from which to compute the anomalies necessary to ...quantify the skill of the hindcasts when compared to observations. This limits the ability to experiment with case studies and other applications where only a few initial years are needed. Here we run a set of hindcasts with CESM1 and E3SMv1 using two different initialization methods for a limited set of start years and use the respective uninitialized free-running historical simulations to form the model climatologies. Since the drifts from the observed initial states in the hindcasts toward the uninitialized model state are large and rapid, after a few years the drifted initialized models approach the uninitialized model climatological errors. Therefore, hindcasts from the limited start years can use the uninitialized climatology to represent the drifted model states after about lead year 3, providing a means to compute forecast anomalies in the absence of a large hindcast sample. There is comparable skill for predicting spatial patterns of multi-year Pacific sea surface temperature anomalies in the domain of the Interdecadal Pacific Oscillation using this method compared to the conventional methodology with a large hindcast data set, though there is a model dependence to the drifts in the two initialization methods.
In initialized seasonal to decadal (S2D) predictions, model hindcasts rapidly drift away from the initial observed state and converge toward a preferred state characterized by systematic error, or ...bias. Bias and drift are among the greatest challenges facing initialized prediction today. Differences in trends between initial states and drifted states, combined with bias and drift, introduce complexities in calculating anomalies to assess skill of initialized predictions. We examine several methods of calculating anomalies using the Decadal Prediction Large Ensemble (DPLE) using the Community Earth System Model (CESM) initialized hindcasts and focus on Pacific and Atlantic SSTs to illustrate issues with anomaly calculations. Three methods of computing anomalies, one as differences from a long term model climatology, another as bias-adjusted differences from the previous 15 year average from observations, and a third as differences from the previous 15 year average from the model, are contrasted and each is shown to have limitations. For the first, trends in bias and drift introduce higher skill estimates earlier and later in the hindcast period due to the trends that contribute to skill. For the second, higher skill can be introduced in situations where low frequency variability in the observations is large compared to the hindcasts on timescales greater than 15 years, while lower skill can result if the predicted signal is small and the bias-correction itself produces a transition of SST anomalies to the opposite sign of those that are observed. The third method has somewhat lower skill compared to each of the others, but has less difficulties with not only the long term trends in the model climatology, but also with the unrealistic situational skill from using observations as a reference. However, the first 15 years of the hindcast period cannot be evaluated due to having to wait to accumulate the previous 15 year model climatology before the method can be applied. The IPO transition in the 2014–2016 time frame from negative to positive (predicted by Meehl et al. in in Nat Commun, 10.1038/NCOMMS11718, 2016) did indeed verify using all three methods, though each provides somewhat different skill values as a result of the respective limitations. There is no clear best method, as all are roughly comparable, and each has its own set of limitations and caveats. However, all three methods show generally higher overall skill in the AMO region compared to the IPO region.
This paper describes the Stratospheric Aerosol Geoengineering Large Ensemble (GLENS) project, which promotes the use of a unique model dataset, performed with the Community Earth System Model, with ...the Whole Atmosphere Community Climate Model as its atmospheric component CESM1(WACCM), to investigate global and regional impacts of geoengineering. The performed simulations were designed to achieve multiple simultaneous climate goals, by strategically placing sulfur injections at four different locations in the stratosphere, unlike many earlier studies that targeted globally averaged surface temperature by placing injections in regions at or around the equator. This advanced approach reduces some of the previously found adverse effects of stratospheric aerosol geoengineering, including uneven cooling between the poles and the equator and shifts in tropical precipitation. The 20-member ensemble increases the ability to distinguish between forced changes and changes due to climate variability in global and regional climate variables in the coupled atmosphere, land, sea ice, and ocean system. We invite the broader community to perform in-depth analyses of climate-related impacts and to identify processes that lead to changes in the climate system as the result of a strategic application of stratospheric aerosol geoengineering.
The January 2022 Hunga Tonga-Hunga Ha'apai (HTHH) volcanic eruption injected a relatively small amount of SO2, but significantly more water into the stratosphere than previously seen in the modern ...satellite record. Here we show that the large amount of water resulted in large perturbations to stratospheric aerosol evolution. Our Community Earth System Model simulation reproduces the enhanced water vapor observed by the Microwave Limb Sounder at pressure levels between 10 and 50 hPa for three months. Compared with a simulation without a water injection, this additional source of water vapor increases OH, which halves the SO2 lifetime. Subsequent coagulation creates larger sulfate particles that double the stratospheric aerosol optical depth. A seasonal forecast of volcanic plume transport in the southern hemisphere indicates this eruption will greatly enhance the aerosol surface area and water vapor near the polar vortex until at least October 2022, suggesting that there will continue to be an impact of the HTHH eruption on the climate system.
We present new insights into the evolution and interactions of stratospheric aerosol using an updated version of the Whole Atmosphere Community Climate Model (WACCM). Improved horizontal resolution, ...dynamics, and chemistry now produce an internally generated quasi‐biennial oscillation and significant improvements to stratospheric temperatures and ozone compared to observations. We present a validation of WACCM column ozone and climate calculations against observations. The prognostic treatment of stratospheric sulfate aerosols accurately represents the evolution of stratospheric aerosol optical depth and perturbations to solar and longwave radiation following the June 1991 eruption of Mount Pinatubo. We confirm the inclusion of interactive OH chemistry as an important factor in the formation and initial distribution of aerosol following large inputs of sulfur dioxide (SO2) to the stratosphere. We calculate that depletion of OH levels within the dense SO2 cloud in the first weeks following the Pinatubo eruption significantly prolonged the average initial e‐folding decay time for SO2 oxidation to 47 days. Previous observational and model studies showing a 30 day decay time have not accounted for the large (30–55%) losses of SO2 on ash and ice within 7–9 days posteruption and have not correctly accounted for OH depletion. We examine the variability of aerosol evolution in free‐running climate simulations due to meteorology, with comparison to simulations nudged with specified dynamics. We assess calculated impacts of volcanic aerosols on ozone loss with comparisons to observations. The completeness of the chemistry, dynamics, and aerosol microphysics in WACCM qualify it for studies of stratospheric sulfate aerosol geoengineering.
Plain Language Summary
Stratospheric aerosols form after volcanoes inject SO2 into the stratosphere, and can cool global surface temperatures. A new capability for simulating stratospheric aerosols from SO2 injections in the Whole Atmosphere Community Climate Model is shown to reproduce well observed climate and chemistry responses. The ability of the model to calculate accurately the reductions in sunlight and losses of ozone that have been observed following historical eruptions in the satellite era gives strong confidence in the model's ability to simulate such responses to potential future deliberate injections of SO2 to offset global warming. Such responses to geoengineering are presented in a series of companion papers.
Key Points
WACCM accurately calculates radiative and chemical responses to stratospheric sulfate, validating its use for geoengineering studies
Interactive OH chemistry is key to the study of aerosol formation from large stratospheric SO2 perturbations
OH depletion extended the calculated average initial e‐folding time for oxidation of SO2 from the 1991 Pinatubo eruption by >50%
Abstract
Prediction systems to enable Earth system predictability research on the subseasonal time scale have been developed with the Community Earth System Model, version 2 (CESM2) using two ...configurations that differ in their atmospheric components. One system uses the Community Atmosphere Model, version 6 (CAM6) with its top near 40 km, referred to as CESM2(CAM6). The other employs the Whole Atmosphere Community Climate Model, version 6 (WACCM6) whose top extends to ∼140 km, and it includes fully interactive tropospheric and stratospheric chemistry CESM2(WACCM6). Both systems are utilized to carry out subseasonal reforecasts for the 1999–2020 period following the Subseasonal Experiment’s (SubX) protocol. Subseasonal prediction skill from both systems is compared to those of the National Oceanic and Atmospheric Administration CFSv2 and European Centre for Medium-Range Weather Forecasts (ECMWF) operational models. CESM2(CAM6) and CESM2(WACCM6) show very similar subseasonal prediction skill of 2-m temperature, precipitation, the Madden–Julian oscillation, and North Atlantic Oscillation to its previous version and to the NOAA CFSv2 model. Overall, skill of CESM2(CAM6) and CESM2(WACCM6) is a little lower than that of the ECMWF system. In addition to typical output provided by subseasonal prediction systems, CESM2 reforecasts provide comprehensive datasets for predictability research of multiple Earth system components, including three-dimensional output for many variables, and output specific to the mesosphere and lower-thermosphere (MLT) region from CESM2(WACCM6). It is shown that sudden stratosphere warming events, and the associated variability in the MLT, can be predicted ∼10 days in advance. Weekly real-time forecasts and reforecasts with CESM2(CAM6) and CESM2(WACCM6) are freely available.
Significance Statement
We describe here the design and prediction skill of two subseasonal prediction systems based on two configurations of the Community Earth System Model, version 2 (CESM2): CESM2 with the Community Atmosphere Model, version 6 CESM2(CAM6) and CESM 2 with Whole Atmosphere Community Climate Model, version 6 CESM2(WACCM6) as its atmospheric component. These two systems provide a foundation for community-model based subseasonal prediction research. The CESM2(WACCM6) system provides a novel capability to explore the predictability of the stratosphere, mesosphere, and lower thermosphere. Both CESM2(CAM6) and CESM2(WACCM6) demonstrate subseasonal surface prediction skill comparable to that of the NOAA CFSv2 model, and a little lower than that of the ECMWF forecasting system. CESM2 reforecasts provide a comprehensive dataset for predictability research of multiple aspects of the Earth system, including the whole atmosphere up to 140 km, land, and sea ice. Weekly real-time forecasts, reforecasts, and models are publicly available.
The potential for multiyear prediction of impactful Earth
system change remains relatively underexplored compared to shorter
(subseasonal to seasonal) and longer (decadal) timescales. In this study, ...we
introduce a new initialized prediction system using the Community Earth
System Model version 2 (CESM2) that is specifically designed to probe
potential and actual prediction skill at lead times ranging from 1 month out
to 2 years. The Seasonal-to-Multiyear Large Ensemble (SMYLE) consists of a
collection of 2-year-long hindcast simulations, with four initializations per
year from 1970 to 2019 and an ensemble size of 20. A full suite of output is
available for exploring near-term predictability of all Earth system
components represented in CESM2. We show that SMYLE skill for El
Niño–Southern Oscillation is competitive with other prominent seasonal
prediction systems, with correlations exceeding 0.5 beyond a lead time of 12
months. A broad overview of prediction skill reveals varying degrees of
potential for useful multiyear predictions of seasonal anomalies in the
atmosphere, ocean, land, and sea ice. The SMYLE dataset, experimental
design, model, initial conditions, and associated analysis tools are all
publicly available, providing a foundation for research on multiyear
prediction of environmental change by the wider community.
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