Higher CO2 concentrations increase extreme event risk in a 1.5C world
Nature Climate Change Nature Publishing Group 8 (2018) 604-608
Abstract:
The Paris Agreement1 aims to ‘pursue efforts to limit the temperature increase to 1.5°C above pre-industrial levels.’ However, it has been suggested that temperature targets alone are unable to limit the risks associated with anthropogenic emissions2, 3. Here, using an ensemble of model simulations, we show that atmospheric CO2 increase - a more predictable consequence of emissions compared to global temperature increase - has a significant impact on Northern Hemisphere summer temperature, heat stress, and tropical precipitation extremes. Hence in an iterative climate mitigation regime aiming solely for a specific temperature goal, an unexpectedly low climate response may have corresponding ‘dangerous’ changes in extreme events. The direct impact of higher CO2 concentrations on climate extremes therefore substantially reduces the upper bound of the carbon budget, and highlights the need to explicitly limit atmospheric CO2 concentration when formulating allowable emissions. Thus, complementing global mean temperature goals with explicit limits on atmospheric CO2 concentrations in future climate policy would reduce the adverse effects of high-impact weather extremes.The impact of tropical precipitation on summertime Euro-Atlantic circulation via a circumglobal wave-train
Journal of Climate American Meteorological Society 31:16 (2018) 6481-6504
Abstract:
The influence of tropical precipitation variability on summertime seasonal circulation anomalies in the Euro-Atlantic sector is investigated. The dominant mode of the maximum covariance analysis (MCA) between the Euro-Atlantic circulation and tropical precipitation reveals a cyclonic anomaly over the extratropical North Atlantic, contributing to anomalously wet conditions over western Europe and dry conditions over eastern Europe and Scandinavia (in the positive phase). The related mode of tropical precipitation variability is associated with tropical Pacific SST anomalies and is closely linked to the El Niño/Southern Oscillation (ENSO). The second MCA mode consists of weaker tropical precipitation anomalies but a stronger extratropical signal which reflects internal atmospheric variability. The teleconnection mechanism is tested in barotropic model simulations, which indicate that the observed link between the dominant mode of tropical precipitation and the Euro-Atlantic circulation anomalies is largely consistent with linear Rossby wave dynamics. The barotropic model response consists of a circumglobal wave-train in the extratropics that is primarily forced by divergence anomalies in the eastern tropical Pacific. Both the eastward and westward group propagation of the Rossby waves are found to be important in determining the circulation response over the Euro-Atlantic sector. The mechanism was also analysed in an operational seasonal forecasting system, ECMWF’s System 4. Whilst System 4 is well able to reproduce and skillfully forecast the tropical precipitation, the extratropical circulation response is absent over the Euro-Atlantic region, which is likely related to biases in the Asian jetstream.The roles of static stability and tropical-extratropical interactions in the summer interannual variability of the North Atlantic sector
Climate Dynamics Springer Verlag 52:3-4 (2018) 1299-1315
Abstract:
Summer seasonal forecast skill in the North Atlantic sector is lower than winter skill. To identify potential controls on predictability, the sensitivity of North Atlantic baroclinicity to atmospheric drivers is quantified. Using ERA-INTERIM reanalysis data, North Atlantic storm-track baroclinicity is shown to be less sensitive to meridional temperature-gradient variability in summer. Static stability shapes the sector’s interannual variability by modulating the sensitivity of baroclinicity to variations in meridional temperature gradients and tropopause height and by modifying the baroclinicity itself. High static stability anomalies at upper levels result in more zonal extratropical cyclone tracks and higher eddy kinetic energy over the British Isles in the summertime. These static stability anomalies are not strongly related to the summer NAO; but they are correlated with the suppression of convection over the tropical Atlantic and with a poleward-shifted subtropical jet. These results suggest a non-local driver of North Atlantic variability. Furthermore, they imply that improved representations of convection over the south-eastern part of North America and the tropical Atlantic might improve summer seasonal forecast skill.Seasonal sensitivity of the Hadley cell and cross-hemispheric responses to diabatic heating in an idealized GCM
Geophysical Research Letters American Geophysical Union 45:5 (2018) 2533-2541
Abstract:
The seasonal sensitivity of the Hadley cell to localized diabatic forcing is studied using a dry idealized atmospheric general circulation model. Sensitivities are broadly consistent with Hadley cell responses in observations and climate models to ENSO and global warming-like forcings. However, the exact seasonal sensitivity patterns highlight the importance of reducing the uncertainty in the size and position of expected anthropogenic forcings to understand how the atmospheric circulation will respond. The sensitivities reveal cross-hemispheric Hadley cell responses which project onto the eddy-driven jets and storm tracks. For summer hemisphere heating, the winter Hadley cell extent and jet latitude responses are highly correlated. For winter hemisphere heating, the summer Hadley cell extent and jet speed responses are highly correlated. These seasonal differences arise due to the contrast between the dominant winter Hadley cell and weaker summer Hadley cell.Skilful seasonal predictions of Summer European rainfall
Geophysical Research Letters American Geophysical Union 45:7 (2018) 3246-3254