4rsta.royalsocietypublishing.orgP h i l .T r a n s .R .S o c .A376:20160452 ........................................................
Rather than using the original CMIP5 ensemble as in previous studies, the aim is to allow for
an improved representation of atmospheric and land surface processes including extremes by
using higher spatial resolution [11].
HadGEM3 (Hadley Centre Global Environment Model version 3) is a conﬁguration of the UK
Met Ofﬁce Uniﬁed Model (MetUM) which has been developed for use for both climate research
and weather prediction applications. It is the result of converging the development of the Met
Ofﬁce’s weather and climate global atmospheric model components so that, where possible,
atmospheric processes are modelled or parametrized seamlessly across spatial resolutions and
timescales.
The high-resolution simulations were performed using the HadGEM3A Global Atmosphere
(GA) 3.0 model [ 12–14] at a resolution of N216 (0.556° of latitude by 0.833° of longitude with
gridboxes of approx. 60 km length in mid-latitudes). This is the atmospheric component of
the HadGEM3-GC2 coupled climate model [ 15,16], which is part of the HadGEM3 family of
climate models [ 12]. This represents the third generation of HadGEM conﬁgurations, leading
on from the HadGEM2 family of climate model conﬁgurations [ 13]w h i c hw a su s e df o rC M I P 5 .
Key improvements over the previous model, HadGEM2, include increased vertical levels in the
atmosphere (85 compared to 38) and substantial changes to the model dynamics (ENDGame) [17].
This version of the HadGEM3 model lies in the transition from CMIP5 to CMIP6 versions. The Met
Ofﬁce is currently operationally running the coupled HadGEM3-GC2 model at N216 resolution
for seasonal and decadal forecasting and clear beneﬁts are emerging from this use at higher
resolution [18,19].
We ran the model using only its atmosphere and land components, with time-varying sea-
surface temperatures (SSTs) and sea-ice concentrations (SICs) prescribed as input quantities. This
approach was taken for two reasons: (i) to provide a rapid ﬁrst analysis of the implications
of the higher resolution for projections of climate extremes and impacts—an atmosphere-
only simulation requires considerably less computing time than a coupled ocean–atmosphere
general circulation model (GCM); (ii) to allow us to explore, to some degree, uncertainties in
regional climate changes by using SSTs and SICs from different climate models. To explore these
uncertainties in the regional impacts of climate change, we carried out six HadGEM3 atmospheric
simulations driven by time-varying SSTs and SICs from a subset of projections from the CMIP5
with the RCP8.5 scenario. The assumption here is that SSTs and SICs provide a substantial
inﬂuence on regional patterns of climate change over land, so using a range of SST and SIC
patterns in a single atmosphere model goes some way towards representing the range of regional
climate changes that would arise in a set of different coupled ocean–atmosphere GCMs. This
approach will not capture the full range of uncertainty affecting regional climate changes over
land, because it still relies on one atmosphere model and one land surface scheme, so responses
to radiative forcing that depend mainly on atmospheric process or land-atmosphere interactions
will still be constrained by the behaviour of that single model. Nevertheless, we consider that
our experimental design avoids the reliance on one single realization of climate and hence allows
some of the uncertainties in regional climate-change impacts to be illustrated and explored.
The SSTs and SICs were taken from a subset of the CMIP5 transient projections performed with
the RCP8.5 scenario from 1979 to 2100—the CMIP5 members were selected as representative of a
range of outcomes for future climate change, including high and low climate sensitivity, different