The Evaluation of the North Atlantic Climate System in UKESM1 Historical Simulations for CMIP6

Robson, Jon and Aksenov, Yevgeny and Bracegirdle, Thomas J. and Dimdore-Miles, Oscar and Griffiths, Paul T. and Grosvenor, Daniel P. and Hodson, Daniel L.R. and Keeble, James and MacIntosh, Claire and Megann, Alex and Osprey, Scott and Povey, Adam C. and Schröder, David and Yang, Mingxi and Archibald, Alexander T. and Carslaw, Ken S. and Gray, Lesley and Jones, Colin and Kerridge, Brian and Knappett, Diane and Kuhlbrodt, Till and Russo, Maria and Sellar, Alistair and Siddans, Richard and Sinha, Bablu and Sutton, Rowan and Walton, Jeremy and Wilcox, Laura J. (2020) The Evaluation of the North Atlantic Climate System in UKESM1 Historical Simulations for CMIP6. Journal of Advances in Modeling Earth Systems, 12 (9): e2020MS002. ISSN 1942-2466

Abstract

Earth system models enable a broad range of climate interactions that physical climate models are unable to simulate. However, the extent to which adding Earth system components changes or improves the simulation of the physical climate is not well understood. Here we present a broad multivariate evaluation of the North Atlantic climate system in historical simulations of the UK Earth System Model (UKESM1) performed for CMIP6. In particular, we focus on the mean state and the decadal time scale evolution of important variables that span the North Atlantic climate system. In general, UKESM1 performs well and realistically simulates many aspects of the North Atlantic climate system. Like the physical version of the model, we find that changes in external forcing, and particularly aerosol forcing, are an important driver of multidecadal change in UKESM1, especially for Atlantic Multidecadal Variability and the Atlantic Meridional Overturning Circulation. However, many of the shortcomings identified are similar to common biases found in physical climate models, including the physical climate model that underpins UKESM1. For example, the summer jet is too weak and too far poleward; decadal variability in the winter jet is underestimated; intraseasonal stratospheric polar vortex variability is poorly represented; and Arctic sea ice is too thick. Forced shortwave changes may be also too strong in UKESM1, which, given the important role of historical aerosol forcing in shaping the evolution of the North Atlantic in UKESM1, motivates further investigation. Therefore, physical model development, alongside Earth system development, remains crucial in order to improve climate simulations.