Fasullo
John T.
Fasullo
John T.
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ArticleClimate variability, volcanic forcing, and last millennium hydroclimate extremes(American Meteorological Society, 2018-05-03) Stevenson, Samantha ; Overpeck, Jonathan T. ; Fasullo, John T. ; Coats, Sloan ; Parsons, Luke A. ; Otto-Bliesner, Bette ; Ault, Toby ; Loope, Garrison ; Cole, JuliaMultidecadal hydroclimate variability has been expressed as “megadroughts” (dry periods more severe and prolonged than observed over the twentieth century) and corresponding “megapluvial” wet periods in many regions around the world. The risk of such events is strongly affected by modes of coupled atmosphere–ocean variability and by external impacts on climate. Accurately assessing the mechanisms for these interactions is difficult, since it requires large ensembles of millennial simulations as well as long proxy time series. Here, the Community Earth System Model (CESM) Last Millennium Ensemble is used to examine statistical associations among megaevents, coupled climate modes, and forcing from major volcanic eruptions. El Niño–Southern Oscillation (ENSO) strongly affects hydroclimate extremes: larger ENSO amplitude reduces megadrought risk and persistence in the southwestern United States, the Sahel, monsoon Asia, and Australia, with corresponding increases in Mexico and the Amazon. The Atlantic multidecadal oscillation (AMO) also alters megadrought risk, primarily in the Caribbean and the Amazon. Volcanic influences are felt primarily through enhancing AMO amplitude, as well as alterations in the structure of both ENSO and AMO teleconnections, which lead to differing manifestations of megadrought. These results indicate that characterizing hydroclimate variability requires an improved understanding of both volcanic climate impacts and variations in ENSO/AMO teleconnections.
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ArticleOrigin of interannual variability in global mean sea level(National Academy of Sciences, 2020-06-08) Hamlington, Benjamin D. ; Piecuch, Christopher G. ; Reager, John T. ; Chandanpurkar, Hrishikesh A. ; Frederikse, Thomas ; Nerem, R. Steven ; Fasullo, John T. ; Cheon, Se-HyeonThe two dominant drivers of the global mean sea level (GMSL) variability at interannual timescales are steric changes due to changes in ocean heat content and barystatic changes due to the exchange of water mass between land and ocean. With Gravity Recovery and Climate Experiment (GRACE) satellites and Argo profiling floats, it has been possible to measure the relative steric and barystatic contributions to GMSL since 2004. While efforts to “close the GMSL budget” with satellite altimetry and other observing systems have been largely successful with regards to trends, the short time period covered by these records prohibits a full understanding of the drivers of interannual to decadal variability in GMSL. One particular area of focus is the link between variations in the El Niño−Southern Oscillation (ENSO) and GMSL. Recent literature disagrees on the relative importance of steric and barystatic contributions to interannual to decadal variability in GMSL. Here, we use a multivariate data analysis technique to estimate variability in barystatic and steric contributions to GMSL back to 1982. These independent estimates explain most of the observed interannual variability in satellite altimeter-measured GMSL. Both processes, which are highly correlated with ENSO variations, contribute about equally to observed interannual GMSL variability. A theoretical scaling analysis corroborates the observational results. The improved understanding of the origins of interannual variability in GMSL has important implications for our understanding of long-term trends in sea level, the hydrological cycle, and the planet’s radiation imbalance.
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ArticlePaleoclimate constraints on the spatiotemporal character of past and future droughts(American Meteorological Society, 2020-10-15) Coats, Sloan ; Smerdon, Jason E. ; Stevenson, Samantha ; Fasullo, John T. ; Otto-Bliesner, Bette ; Ault, TobyMachine-learning-based methods that identify drought in three-dimensional space–time are applied to climate model simulations and tree-ring-based reconstructions of hydroclimate over the Northern Hemisphere extratropics for the past 1000 years, as well as twenty-first-century projections. Analyzing reconstructed and simulated drought in this context provides a paleoclimate constraint on the spatiotemporal characteristics of simulated droughts. Climate models project that there will be large increases in the persistence and severity of droughts over the coming century, but with little change in their spatial extent. Nevertheless, climate models exhibit biases in the spatiotemporal characteristics of persistent and severe droughts over parts of the Northern Hemisphere. We use the paleoclimate record and results from a linear inverse modeling-based framework to conclude that climate models underestimate the range of potential future hydroclimate states. Complicating this picture, however, are divergent changes in the characteristics of persistent and severe droughts when quantified using different hydroclimate metrics. Collectively our results imply that these divergent responses and the aforementioned biases must be better understood if we are to increase confidence in future hydroclimate projections. Importantly, the novel framework presented herein can be applied to other climate features to robustly describe their spatiotemporal characteristics and provide constraints on future changes to those characteristics.
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ArticleUnderstanding of contemporary regional sea-level change and the implications for the future(American Geophysical Union, 2020-04-17) Hamlington, Benjamin D. ; Gardner, Alex S. ; Ivins, Erik ; Lenaerts, Jan T. M. ; Reager, John T. ; Trossman, David S. ; Zaron, Edward D. ; Adhikari, Surendra ; Arendt, Anthony ; Aschwanden, Andy ; Beckley, Brian D. ; Bekaert, David P. S. ; Blewitt, Geoffrey ; Caron, Lambert ; Chambers, Don P. ; Chandanpurkar, Hrishikesh A. ; Christianson, Knut ; Csatho, Beata ; Cullather, Richard I. ; DeConto, Robert M. ; Fasullo, John T. ; Frederikse, Thomas ; Freymueller, Jeffrey T. ; Gilford, Daniel M. ; Girotto, Manuela ; Hammond, William C. ; Hock, Regine ; Holschuh, Nicholas ; Kopp, Robert E. ; Landerer, Felix ; Larour, Eric ; Menemenlis, Dimitris ; Merrifield, Mark ; Mitrovica, Jerry X. ; Nerem, R. Steven ; Nias, Isabel J. ; Nieves, Veronica ; Nowicki, Sophie ; Pangaluru, Kishore ; Piecuch, Christopher G. ; Ray, Richard D. ; Rounce, David R. ; Schlegel, Nicole‐Jeanne ; Seroussi, Helene ; Shirzaei, Manoochehr ; Sweet, William V. ; Velicogna, Isabella ; Vinogradova, Nadya ; Wahl, Thomas ; Wiese, David N. ; Willis, Michael J.Global sea level provides an important indicator of the state of the warming climate, but changes in regional sea level are most relevant for coastal communities around the world. With improvements to the sea‐level observing system, the knowledge of regional sea‐level change has advanced dramatically in recent years. Satellite measurements coupled with in situ observations have allowed for comprehensive study and improved understanding of the diverse set of drivers that lead to variations in sea level in space and time. Despite the advances, gaps in the understanding of contemporary sea‐level change remain and inhibit the ability to predict how the relevant processes may lead to future change. These gaps arise in part due to the complexity of the linkages between the drivers of sea‐level change. Here we review the individual processes which lead to sea‐level change and then describe how they combine and vary regionally. The intent of the paper is to provide an overview of the current state of understanding of the processes that cause regional sea‐level change and to identify and discuss limitations and uncertainty in our understanding of these processes. Areas where the lack of understanding or gaps in knowledge inhibit the ability to provide the needed information for comprehensive planning efforts are of particular focus. Finally, a goal of this paper is to highlight the role of the expanded sea‐level observation network—particularly as related to satellite observations—in the improved scientific understanding of the contributors to regional sea‐level change.
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ArticleInternal variability and forcing influence model-satellite differences in the rate of tropical tropospheric warming(National Academy of Sciences, 2022-11-22) Po-Chedley, Stephen ; Fasullo, John T. ; Siler, Nicholas ; Labe, Zachary M. ; Barnes, Elizabeth A. ; Bonfils, Céline J. W. ; Santer, Benjamin D.Climate-model simulations exhibit approximately two times more tropical tropospheric warming than satellite observations since 1979. The causes of this difference are not fully understood and are poorly quantified. Here, we apply machine learning to relate the patterns of surface-temperature change to the forced and unforced components of tropical tropospheric warming. This approach allows us to disentangle the forced and unforced change in the model-simulated temperature of the midtroposphere (TMT). In applying the climate-model-trained machine-learning framework to observations, we estimate that external forcing has produced a tropical TMT trend of 0.25 ± 0.08 K⋅decade between 1979 and 2014, but internal variability has offset this warming by 0.07 ± 0.07 K⋅decade. Using the Community Earth System Model version 2 (CESM2) large ensemble, we also find that a discontinuity in the variability of prescribed biomass-burning aerosol emissions artificially enhances simulated tropical TMT change by 0.04 K⋅decade . The magnitude of this aerosol-forcing bias will vary across climate models, but since the latest generation of climate models all use the same emissions dataset, the bias may systematically enhance climate-model trends over the satellite era. Our results indicate that internal variability and forcing uncertainties largely explain differences in satellite-versus-model warming and are important considerations when evaluating climate models.