Maris Sandstad Senior Advisor, CICERO, Norway

Title: METEOR: A novel framework for emulating multi-timescale regional climate responses

Abstract: Introducing METEOR (Multivariate Emulation of Time-Evolving and Overlapping Responses) a spatially resolved impacts emulator. METEOR uses Earth system model output to emulate impact response patterns of varying decay timescales to forcing changes. In-built scaled emissions to forcing mapping enables a full chain emulation of impact variables from emissions scenarios to spatially resolved impacts. METEOR can emulate multiple independent forcer responses, relaying on at least one abrupt-CO2-change experiment as training data, and using either more abrupt forcer change experiments or a residual technique to emulate additional responses. This presentation will describe the model and its design philosophy and show results for CMIP6 models trained on abrupt-4xCO2 and with a residual based emulation of sulphate aerosol responses applied to yearly mean temperature and precipitation. The flexibility of the framework allows application to a wide range of other more impact specific variables, and in addition the emulation patterns and timescales in themselves may reveal interesting patterns in the emulated data.

Stephanie Hay Postdoctoral Research Fellow, University of Exeter, UK

Title: The impact of Arctic sea-ice loss on winter weather in the British Isles

Abstract: We use simulations from the Polar Amplification Model Intercomparison Project to understand the impact of Arctic sea-ice loss in isolation, as well as within the context of general greenhouse warming, on projections of wintertime weather for the British Isles. At large scales, sea-ice loss results in a more negative North Atlantic Oscillation across all participating models, as well as more positive East Atlantic and Scandinavian Patterns, alongside an equatorward shift in the North Atlantic eddy-driven jet. Strong extratropical cyclones become weaker and less frequent, and they propagate more slowly. At the regional scale of the British Isles, there is often little inter-model agreement in the responses of mean and extreme precipitation, surface winds, and temperature, but the spread is generally correlated with model spread in larger-scale circulation and storminess. Whether the effect of sea-ice loss can be expected to be a dominant contributor to projections of weather in the region is examined by comparing with experiments that isolate ocean surface warming, as well as by considering the range of plausible storylines of sea-ice loss and warming. Simulated dynamical changes due to ocean warming are often of the opposite sign to those due to sea-ice loss, creating a tug-of-war on the weather of the region. While ocean warming is the dominant driver of projected thermodynamical changes over the British Isles, projected dynamical changes depend strongly on the relative magnitudes of future sea-ice loss and ocean warming.

Michael Lai Scientist (Climate), Met Office, UK

Title: 1950-control vs 1850-control: How do HighResMIP simulations relate to CMIP simulations?

Abstract: A common set of simulation is important for inter-comparison between different models. The ‘entry-card’ to participate in CMIP is to perform the baseline DECK simulations (1850-control, 1pctCO2, abrupt-4xCO2, historical-amip). However, performing the control and historical simulations from an 1850 baseline is prohibitively expensive for high-resolution fully-coupled models. Therefore, HighResMIP chose to use a shorter experimental protocol based on 1950 conditions alongside a shorter spin-up length and simplified aerosols. Because of this difference in protocol, it is not clear exactly how the HighResMIP simulations relate to the other CMIP simulations. In this study we analyse the control and historical simulations of the HadGEM3-GC3.1 model, which performed control and historical simulations based on both 1950 and 1850 baselines. Our results show that the absolute temperature is sensitive to the different experimental protocol, but the anomalies are much more comparable. This opens an interesting discussion on whether climate change should be discussed in terms of absolute values or anomalies. The difference in the absolute value (and mean state) is largely due to the different aerosol scheme used in CMIP and HighResMIP for this particular model. The second phase of HighResMIP no longer require models to use EasyAerosol, so modelling centres should use the same aerosol scheme if they would like their HighResMIP simulations to be comparable to CMIP simulations.