Abstract
We investigate the opposing effects of direct radiative forcing and sea surface warming on the atmospheric circulation using a hierarchy of models. In large ensembles of three general circulation models, direct hbox {CO}_2 forcing produces a wavenumber 5 stationary wave over the Northern Hemisphere in summer. Sea surface warming produces a similar wave, but with the opposite sign. The waves are also present in the Coupled Model Intercomparison Project phase 5 ensemble with opposite signs due to direct hbox {CO}_2 and sea surface warming. Analyses of tropical precipitation changes and equivalent potential temperature changes and the results from a simple barotropic model show that the wave is forced from the tropics. Key forcing locations are the Western Atlantic, Eastern Atlantic and in the Indian Ocean just off the east coast of Africa. The stationary wave has a significant impact on regional temperature anomalies in the Northern Hemisphere summer, explaining some of the direct effect that hbox {CO}_2 concentration has on temperature extremes. Ultimately, the climate sensitivity and future changes in the land–sea temperature contrast will dictate the balance between the opposing effects on regional changes in mean and extreme temperature and precipitation under climate change.
Highlights
IntroductionIt is well established that there are differing, and sometimes opposing, effects of radiative forcing versus sea surface temperature (SST) change on the atmospheric circulation
It is well established that there are differing, and sometimes opposing, effects of radiative forcing versus sea surface temperature (SST) change on the atmospheric circulationElectronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.(e.g. Mitchell 1983; He and Soden 2015; Shaw and Voigt 2015; Chadwick et al 2017)
Using results from three general circulation models (GCMs) and Coupled Model Intercomparison Project phase 5 (CMIP5), we have shown here that there are opposing effects of direct CO2 forcing and sea surface warming on tropical precipitation and forced summer Northern Hemisphere (NH) stationary waves
Summary
It is well established that there are differing, and sometimes opposing, effects of radiative forcing versus sea surface temperature (SST) change on the atmospheric circulation. Shaw and Voigt (2015) investigate the large scale circulation responses to direct CO2 radiative forcing versus SST warming in the Coupled Model Intercomparison Project phase 5 (CMIP5) ensemble, finding that CO2 forcing leads to positive land–sea equivalent potential temperature ( e ) contrasts, and SST forcing leads to negative land–sea e contrasts. Shaw and Voigt (2015) do not look at changes in midlatitude stationary waves in response to the two types of forcing, which may have an important impact on future changes in regional Northern Hemisphere (NH) summer temperatures and heatwave hazard due to the direct effect of CO2 shown by Baker et al (2018a)
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