Approach

1. Equilibrium climate sensitivity in light of observations over the warming hiatus. Objective Analyze changes in our understanding of Equilibrium Climate Sensitivity as observations of global average temperature accumulate over the historical period, particularly assessing the impact on the probabilistic characterization of ECS of the warming hiatus period. • Approach • We use a model of intermediate complexity and a fully Bayesian statistical model to update over time the PDF of ECS as we incorporate increasingly longer observational records of global average temperature, ocean heat content, forcings. • We use an ENSO index to account for the possible role of internal variability in the unfolding of the warming hiatus. Changing shape and range of the PDF of Equilibrium Climate Sensitivity as longer observational records are included in the analysis. The black solid line indicates the PDF with information up and including 2011. Larger values of ECS are discounted and the mode of the PDF shifts towards smaller values. Impact This learning exercise quantifies the value of information accumulating over time in shaping our uncertainty over ECS. We find that the inclusion of observations over the hiatus period contributes to a more constrained estimate of ECS, the degree to which this was due to the hiatus per se, as opposed to the accumulation of more data in general, is unclear. Our quantification of the role that unforced internal variability plays in the updating of the PDF leads us to conclude that it is still too early to stipulate that the hiatus has had any particular impact on estimates of ECS. Johansson, D.J.A, B.C. O’Neill, C. Tebaldi and O. Haggstrom, 2015: Equilibrium climate sensitivity in light of observations over the warming hiatus. Nature Climate Change, DOI: 10.1038/nclimate2573.

2. Observational constraints on mixed-phase clouds imply higher climate sensitivity Objective: Given the known tendency for models to simulate clouds with too little liquid below freezing, the authors explored the implications of changing the supercooled liquid fraction (SLF) in a single model for its cloud feedback and climate sensitivity. Research: The authors constrained cloud phase in CAM5.1 by using 79 months of observations obtained by CALIOP. To constrain CAM5.1-simulated SLFs to agree with observations, they adopted a quasi–Monte Carlo sampling approach to select 256 combinations of six cloud microphysical parameters, including the Wegener-Bergeron-Findeisen process time scale for the growth of ice crystals and the fraction of atmospheric aerosols active as ice nuclei. Simulations were run until the global net radiation budget at the top of the atmosphere was balanced with both present-day and doubled CO2 concentrations. (A) Initial-state extratropical SLFs at the –10°C isotherm. (B) Climate sensitivity estimates. (C) Changes in global mean liquid water path. (D) Mean extratropical cloud optical depth feedback. Impact: The cloud phase feedback weakened and the climate sensitivity increased in model versions that maintain liquid down to colder temperatures. In the two model versions in which cloud phase was in closer agreement with observations, climate sensitivity was 1-1.3˚C larger than in the default model. Given that most climate models too readily convert liquid to ice below freezing, the results imply that they may also exaggerate the increase in cloud reflectivity with warming and hence underestimate climate sensitivity. Independent confirmation with other models will be important for establishing robustness. Reference: Tan, I., T. Storelvmo, and M. D. Zelinka, 2016: Observational constraints on mixed-phase clouds imply higher climate sensitivity, Science, 352, 6282, 224-227, doi:10.1126/science.aad5300.

3. Improved Quantification of Jet Stream Variability and Trends • Objective • Develop an objective diagnostic for high-impact midlatitude weather events (e.g., blocking, heat waves, or jet stream meandering) based on theory of local finite-amplitude wave activity (LWA) • Approach • The LWA formalism is used to characterize midtropospheric geopotential height in the reanalysis • The LWA climatology and the pattern associated with the Arctic Oscillation(AO) are compared with blocking • The spatial patterns of LWA linear trends are analyzed • Impact • A local anticorrelation is found, as expected from theory, between the zonal wind anomaly and LWA amplitude for the patterns associated with the AO as well as for the linear trends • no statistically significant evidence either in winter or in summer for a hemispheric-scale increase in wave amplitude associated with recent Arctic warming Zonal wind at 250 hPa (contour) and LWA amplitude (shading) in DJF TOP: the regression patterns with respect to the inverted Arctic Oscillation (AO) index BOTTOM: the linear trends in 1979–2014 Chen, G., J. Lu, D. A. Burrows, and L. R. Leung (2015), Local finite-amplitude wave activity as an objective diagnostic of midlatitude extreme weather, Geophys. Res. Lett., 42, 10,952–10,960, doi:10.1002/2015GL066959.

4. Southern Ocean heat uptake and transport • Objective • To understand the processes controlling anthropogenic heat uptake and transport in the Southern Ocean. • Approach • Analyze heat budgets in a high resolution global climate simulation. • Investigate the role of eddies and changing circulation on the heat transport. • Impact • Heat uptake is large in the Southern Ocean due to the unique upwelling circulation. • Wind-driven increases in upwelling enhance heat uptake. • Heat export northwards out of the Southern Ocean is largely (80%) driven by eddy changes. Spatial distribution of the simulated ocean heat content anomaly under CO2 doubling. The Southern Ocean (30-60°S) dominates ocean heat uptake under climate change. Morrison, A. K., S. M. Griffies, M. Winton, W. G. Anderson, J. L. Sarmiento, (2016). Mechanisms of Southern Ocean heat uptake and transport in a global eddying climate model, Journal of Climate,29, 2059-2075.