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1. Sea Level Rise in the CESM Large Ensemble: The Role of Individual Climate Forcings and Consequences for the Coming Decades

   https://doi.org/10.1175/JCLI-D-19-1001.1  

   Fasullo, John T. ; Gent, Peter R. ; Nerem, R. Steven ( August 2020 , Journal of Climate)

   null (Ed.)

   Abstract The emergence of a spatial pattern in the externally forced response (FR) of dynamic sea level (DSL) during the altimeter era has recently been demonstrated using climate models but our understanding of its initial emergence, drivers, and implications for the future is poor. Here the anthropogenic forcings of the DSL pattern are explored using the Community Earth System Model Large Ensemble (CESM-LE) and Single-Forcing Large Ensemble, a newly available set of simulations where values of individual forcing agents remain fixed at 1920 levels, allowing for an estimation of their effects. Statistically significant contributions to the DSL FR are identified for greenhouse gases (GHGs) and industrial aerosols (AERs), with particularly strong contributions resulting from AERs in the mid-twentieth century and GHGs in the late twentieth and twenty-first century. Secondary, but important, contributions are identified for biomass burning aerosols in the equatorial Atlantic Ocean in the mid-twentieth century, and for stratospheric ozone in the Southern Ocean during the late twentieth century. Key to understanding regional DSL patterns are ocean heat content and salinity anomalies, which are driven by surface heat and freshwater fluxes, ocean dynamics, and the spatial structure of seawater thermal expansivity. Potential implications for the interpretation of DSL during the satellite era and the longer records from tide gauges are suggested as a topic for future research. 

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2. Understanding the Uncertainty in the 21st Century Dynamic Sea Level Projections: The Role of the AMOC

   https://doi.org/10.1029/2018GL080676  

   Chen, Changlin ; Liu, Wei ; Wang, Guihua ( January 2019 , Geophysical Research Letters)

   Climate models show that the largest uncertainties in the 21st century dynamic sea level (DSL) projections are in the high latitudes of the North Atlantic and Southern Oceans. We conduct an intermodel singular value decomposition analysis and find that the DSL uncertainties in these two oceans are both intrinsically connected to the uncertainty in the change of the Atlantic meridional overturning circulation (AMOC). We further conduct a freshwater hosing experiment to show that the AMOC decline not only accounts for the dipole pattern in the DSL change in the North Atlantic but also remotely induces a poleward shift in the Southern Hemisphere westerlies that helps build a belted pattern of DSL change in the Southern Ocean. Our results suggest that reducing the intermodel spread in the change of the AMOC can greatly improve the consistency of DSL projection among models not only in individual basins but over the global ocean.

    

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3. Extratropical Cyclone Response to Projected Reductions in Snow Extent over the Great Plains

   https://doi.org/10.3390/atmos14050783  

   Clare, Ryan M. ; Desai, Ankur R. ; Martin, Jonathan E. ; Notaro, Michael ; Vavrus, Stephen J. ( May 2023 , Atmosphere)

   Extratropical cyclones develop in regions of enhanced baroclinicity and progress along climatological storm tracks. Numerous studies have noted an influence of terrestrial snow cover on atmospheric baroclinicity. However, these studies have less typically examined the role that continental snow cover extent and changes anticipated with anthropogenic climate change have on cyclones’ intensities, trajectories, and precipitation characteristics. Here, we examined how projected future poleward shifts in North American snow extent influence extratropical cyclones. We imposed 10th, 50th, and 90th percentile values of snow retreat between the late 20th and 21st centuries as projected by 14 Coupled Model Intercomparison Project Phase Five (CMIP5) models to alter snow extent underlying 15 historical cold-season cyclones that tracked over the North American Great Plains and were faithfully reproduced in control model cases, providing a comprehensive set of model runs to evaluate hypotheses. Simulations by the Advanced Research version of the Weather Research and Forecast Model (WRF-ARW) were initialized at four days prior to cyclogenesis. Cyclone trajectories moved on average poleward (μ = 27 +/− σ = 17 km) in response to reduced snow extent while the maximum sea-level pressure deepened (μ = −0.48 +/− σ = 0.8 hPa) with greater snow removed. A significant linear correlation was observed between the area of snow removed and mean trajectory deviation (r2 = 0.23), especially in mid-winter (r2 = 0.59), as well as a similar relationship for maximum change in sea-level pressure (r2 = 0.17). Across all simulations, 82% of the perturbed simulation cyclones decreased in average central sea-level pressure (SLP) compared to the corresponding control simulation. Near-surface wind speed increased, as did precipitation, in 86% of cases with a preferred phase change from the solid to liquid state due to warming, although the trends did not correlate with the snow retreat magnitude. Our results, consistent with prior studies noting some role for the enhanced baroclinity of the snow line in modulating storm track and intensity, provide a benchmark to evaluate future snow cover retreat impacts on mid-latitude weather systems. 

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4. Common Era sea-level budgets along the U.S. Atlantic coast

   https://doi.org/10.1038/s41467-021-22079-2  

   Walker, Jennifer S. ; Kopp, Robert E. ; Shaw, Timothy A. ; Cahill, Niamh ; Khan, Nicole S. ; Barber, Donald C. ; Ashe, Erica L. ; Brain, Matthew J. ; Clear, Jennifer L. ; Corbett, D. Reide ; et al ( December 2021 , Nature Communications)

   null (Ed.)

   Abstract Sea-level budgets account for the contributions of processes driving sea-level change, but are predominantly focused on global-mean sea level and limited to the 20th and 21st centuries. Here we estimate site-specific sea-level budgets along the U.S. Atlantic coast during the Common Era (0–2000 CE) by separating relative sea-level (RSL) records into process-related signals on different spatial scales. Regional-scale, temporally linear processes driven by glacial isostatic adjustment dominate RSL change and exhibit a spatial gradient, with fastest rates of rise in southern New Jersey (1.6 ± 0.02 mm yr −1 ). Regional and local, temporally non-linear processes, such as ocean/atmosphere dynamics and groundwater withdrawal, contributed between −0.3 and 0.4 mm yr −1 over centennial timescales. The most significant change in the budgets is the increasing influence of the common global signal due to ice melt and thermal expansion since 1800 CE, which became a dominant contributor to RSL with a 20th century rate of 1.3 ± 0.1 mm yr −1 . 

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5. Ocean-forcing of cool season precipitation drives ongoing and future decadal drought in southwestern North America

   https://doi.org/10.1038/s41612-023-00461-9  

   Seager, Richard ; Ting, Mingfang ; Alexander, Patrick ; Liu, Haibo ; Nakamura, Jennifer ; Li, Cuihua ; Newman, Matthew ( September 2023 , npj Climate and Atmospheric Science)

   The US Southwest is in a drought crisis that has been developing over the past two decades, contributing to marked increases in burned forest areas and unprecedented efforts to reduce water consumption. Climate change has contributed to this ongoing decadal drought via warming that has increased evaporative demand and reduced snowpack and streamflows. However, on the supply side, precipitation has been low during the 21st century. Here, using simulations with an atmosphere model forced by imposed sea surface temperatures, we show that the 21st century shift to cooler tropical Pacific sea surface temperatures forced a decline in cool season precipitation that in turn drove a decline in spring to summer soil moisture in the southwest. We then project the near-term future out to 2040, accounting for plausible and realistic natural decadal variability of the Pacific and Atlantic Oceans and radiatively-forced change. The future evolution of decadal variability in the Pacific and Atlantic will strongly influence how wet or dry the southwest is in coming decades as a result of the influence on cool season precipitation. The worst-case scenario involves a continued cold state of the tropical Pacific and the development of a warm state of the Atlantic while the best case scenario would be a transition to a warm state of the tropical Pacific and the development of a cold state of the Atlantic. Radiatively-forced cool season precipitation reduction is strongest if future forced SST change continues the observed pattern of no warming in the equatorial Pacific cold tongue. Although this is a weaker influence on summer soil moisture than natural decadal variability, no combination of natural decadal variability and forced change ensures a return to winter precipitation or summer soil moisture levels as high as those in the final two decades of the 20th century.

    

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