Intermodel CMIP5 relationships in the baseline Southern Ocean climate
system and with future projections
Abstract
Climate models exhibit a broad range in the simulated properties of the
global climate. In the early historical period, the absolute global mean
surface air temperature of models contributing to the fifth phase of the
Coupled Model Intercomparison Project (CMIP5) spans a range of
~12-15 °C. Other climate parameters are linked to the
global mean temperature, such as sea ice area, atmospheric circulation
patterns, and by extension cloudiness, precipitation and albedo.
Accurate representation of the baseline climate state is crucial for
meaningful future climate projections, since the baseline conditions may
dictate the capacity for change. For example, a model with initially
smaller sea ice area has less potential to lose sea ice as the planet
warms. Amongst the CMIP5 models, it is found that in the baseline
climate state there are coherences between Southern Ocean temperature,
outgoing shortwave radiation, cloudiness, the position of the
mid-latitude eddy-driven jet, and Antarctic sea ice area. The baseline
temperature relationship extends to projected future changes in the same
set of variables. The tendency for models with initially cooler Southern
Ocean surface temperature to exhibit more global warming, and vice versa
for initially warmer models, can therefore be linked to baseline
Southern Ocean climate system biases. A first look at emerging data from
CMIP6 reveals a shift of the tendency towards the Antarctic region,
potentially linked to a reduction in biases over the Southern Ocean,
which prompts an examination of biases in the Antarctic region as more
CMIP6 model data becomes available.