The Walker circulation (WC) responds to CO2 forcing at both short and long timescales. The fast response accounts for a substantial portion of the total responses, but its mechanisms, particularly those pertaining to air-sea interactions remain unclear. We found the initial land-sea warming contrast drives the anomalous easterlies (westerlies) in the Pacific warm pool (cold tongue) region. When the anomalous easterlies dominate, a Bjerknes feedback is triggered, leading to fast Equatorial Pacific cooling and WC strengthening. Otherwise, the Central and Eastern Pacific warms via the wind-SST-evaporation feedback and a weakened upwelling, leading to WC weakening. While the WC weakens and the equatorial Pacific warms eventually, the transition into the slow response differs among climate models. If initial cooling happens, the Equatorial warming emerges as the Bjerknes feedback wanes and the subsurface warm water accumulated in the warm pool migrates eastward via downwelling oceanic Kelvin waves.

Aerosols interact with radiation and clouds. Substantial progress made over the past 40 years in observing, understanding, and modeling these processes helped quantify the imbalance in the Earth’s radiation budget caused by anthropogenic aerosols, called aerosol radiative forcing, but uncertainties remain large. This poster presents the outcome of an international workshop and subsequent review paper, which quantify the likely range of aerosol radiative forcing over the industrial era based on multiple lines of evidence, including modelling approaches, theoretical considerations, and observations. Improved understanding of aerosol absorption and the causes of trends in surface radiative fluxes narrow the range of the forcing from aerosol-radiation interactions compared to the latest assessment by the Intergovernmental Panel on Climate Change (IPCC). A robust theoretical foundation and convincing evidence constrain the forcing caused by aerosol-driven increases in liquid cloud droplet number concentration. However, the influence of anthropogenic aerosols on cloud liquid water content and cloud fraction and on mixed-phase and ice clouds remains poorly constrained. Observed changes in surface temperature and radiative fluxes provide additional constraints. These multiple lines of evidence lead to total aerosol radiative forcing ranges that are of similar width to the last IPCC assessment but more clearly based on physical arguments.