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

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.

Opportunistic experiments to constrain aerosol effective radiative forcing

“…The model N d and trend match those from MODIS very well over the 2003–2014 period, giving confidence in the CMIP6 emissions and the ability of this model to accurately translate emissions into changes in cloud properties, which involves several stages. However, Robson et al (2020) and Grosvenor and Carslaw (2020) show that this model does exhibit biases in N d and its trends in other regions.…”

“…It is tempting to relate these changes in N d to observed and simulated trends in cloud fraction, LWP, and shortwave fluxes. For example, Robson et al (2020) suggest that the negative upwelling shortwave top-of-atmosphere flux trend in UKESM1 for the wider North Atlantic region is too strong compared to CERES, with the model also displaying a positive bias in upwelling shortwave top of atmosphere fluxes coincident with a cloud fraction that is too high compared to CALIPSO (see also Grosvenor and Carslaw, 2020 ). The overly strong trend may be interpreted as an overly strong cloud response to aerosol.…”

Envisioning an Integrated Assessment System and Observation Network for the North Atlantic Ocean

“…Model capacity is limited by the lack of simultaneous, high-quality measurements of characterised aerosols necessary for verification and development of model parameterisations [126,157]. There is a need for more robust understanding of aerosol-cloud-radiation feedbacks to allow for decadal-scale regional climate forcing and subsequent feedbacks [158][159][160], which could be facilitated by the integrated monitoring and assessment of atmospheric constituents as proposed here [134].…”