Abstract
AbstractEarth 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.
Highlights
Like the physical version of the model, we find that changes in external forcing, and aerosol forcing, are an important driver of multidecadal change in UKESM1, especially for Atlantic Multidecadal Variability and the Atlantic Meridional Overturning Circulation
Note that the empirical orthogonal functions (EOF) patterns computed from each member are very similar, especially for DJF
To what extent do these sea surface temperatures (SSTs) biases affect the errors in jet latitude and speed in UKESM1, especially in summer? What is the impact of ozone overestimation on the distribution of sulfate aerosols and, aerosol optical depths (AODs) over the North Atlantic; in particular, does the overestimation in ozone in the tropics contribute to the relatively low AOD there, or is this bias related to dust? Why are Arctic‐Atlantic ice and freshwater transports through Fram Strait compensating, and how are they related to the changes in the Atlantic Ocean further south? What controls the bias in the intraseasonal distribution of SSWs, and is this related in any way to the underestimation of variability in the winter (i.e., DJF) North Atlantic Oscillation (NAO)?
Summary
In the current climate the North Atlantic Ocean takes up considerable amounts of heat and carbon from the atmosphere. The subpolar North Atlantic ocean has a crucial role for the formation and maintenance of the Atlantic meridional overturning circulation (AMOC, Buckley & Marshall, 2016; Kuhlbrodt et al, 2007) and its subsequent large northward heat transport (Johns et al, 2011). This large heat transport results in the Atlantic being the only ocean basin with substantial cross‐equator ocean transports (Marshall et al, 2014). Changes in the North Atlantic climate system can have substantial societal impacts on a wide range of sectors and locations (Monerie et al, 2019; Scaife et al, 2008; Smith et al, 2010; Sutton & Hodson, 2005)
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