Seasonal Climatic Changes Induced by Doubled CO2 as Simulated by the OSU Atmospheric GCM/Mixed-Layer Ocean Model

Abstract

The OSU global atmospheric general circulation model (AGCM) has been coupled to a 60-m deep mixed-layer ocean model to simulate the equilibrium seasonal climatic changes induced by a doubling of the CO2 concentration. Simulations with CO2 concentrations of 326 ppmv (1 ? CO2) and 652 ppmv (2 ? CO2) were performed using an accelerated integration procedure for 45 solar cycles followed by the normal unaccelerated integration procedure for 24 and 16 solar cycles (years), respectively. Averages were then obtained over the last 10 yr of each simulation and were analysed in terms of the annual-mean climate and the annual cycle of climate, the latter defined as the departure of the monthly or seasonal mean from the corresponding annual mean. The 1 ? CO2/observed comparison shows that although the model is capable of simulating many features of the observed climate, it does not do so without error. Annual-mean errors are found in the geographical distributions of sea surface temperature, sea ice area, surface air temperature and precipitation rate, and annual-cycle errors are found in the geographical distribution of sea ice area and precipitation rate. Several of the simulation errors have occurred in previous simulations by the atmospheric GCM with the sea surface temperature and sea ice prescribed from observations. The remainder of the simulation errors occurred as a result of having made the sea surface temperature and sea ice prognostic variables of the model. The 2 ? CO2/1 ? CO2 comparison shows that not all quantities undergo a CO2-induced change in both their annual mean and annual cycle. For the sea surface temperature there is a statistically significant increase everywhere in the annual mean, but the change in the annual cycle is negligible. There is a corresponding statistically significant increase in the annual-mean surface air temperature everywhere, but there are changes in the annual cycle only in the polar regions where the amplitude of the annual cycle is reduced. On the other hand, both the precipitation rate and soil water display increases and decreases in their annual means and annual cycles, but these changes are not statistically significant everywhere. In comparison With CO2-doubling simulations by the Geophysical Fluid Dynamics laboratory (GFDL), Goddard Institute for Space Studies (GISS), National Center for Atmospheric Research (NCAR), and United Kingdom Meteorological Office (UKMO) AGCM/mixed-layer ocean models, the OSU model simulates an annual-mean global-mean surface air temperature warming of 2.8°C compared to 3.5°?5.2°C, and an increase in the global-mean precipitation rate of 7.8% compared to 7.1°?11.0%. The OSU model also simulates a desiccation of the Northern Hemisphere continents almost everywhere in summer. This is in agreement with the GFDL and UKMO models, and to a lesser extent with the GISS model, but is in contrast to the results of the NCAR model.