A large SST variability in the tropical Pacific and its relationship to the CO2-induced warming anomaly pattern are investigated using an MRI coupled atmosphere/mixed-layer (slab) ocean model (SGCM). The model was integrated for 60 years with constant (345ppm) CO2, and for 20 years with doubled CO2 starting from four different initial conditions derived from the 60-year control run (Monte Carlo method). The leading mode of the variability in the control run has a strong similarity to that simulated with an MRI coupled atmosphere-ocean model (AOGCM). This mode bears an ENSO-like spatial pattern and the Walker circulation/SST anomaly relation although no ocean dynamical processes are involved in the SGCM.
The ensemble doubling CO2 runs show that the SST spatial anomaly pattern for the CO2-induced warming resembles that for the leading mode of the natural variability. The response pattern is robust and has an El Nino-like polarity with referring to the observed Walker circulation/SST anomaly relation. Moreover, the amplitude of the ENSO-like SST variability is substantially reduced in the CO2-induced warm climate.
The positive anomaly in the tropical SST is attributable to the decrease (increase) in stratus clouds (solar radiation) in the eastern Pacific, while it is caused by the increase in longwave radiation due to the CO2 doubling and enhanced convections reduce the solar heating in the western Pacific. The stratus clouds play a very similar role in the heat budget for the eastern Pacific SST during the warming phase of the natural variability. However, their role is reduced in warmer climate due to the decrease in relative humidity because the cloudiness strongly depends on the relative humidity in the present cloud parameterization. This fact may be attributable to the selection of the El Nino-like polarity and the reduction in the ENSO-like variability in the CO2-induced warmer climate simulated with the MRI SGCM.
Differences in CO2-induced responses between the AOGCM and the SGCM experiments will be discussed in a separate paper.