Time-Mean Response over the Tropical Pacific to Increased C02in a Coupled Ocean-Atmosphere Model

The time-mean response over the tropical Pacific region to a quadrupling Of CO2 is investigated using a global coupled ocean-atmosphere general circulation model. Tropical Pacific sea surface temperatures (SSTs) rise by about 4°–5°C. The zonal SST gradient along the equator decreases by about 20%, although it takes about one century (with C02 increasing at 1% per year compounded) for this change to become clearly evident in the model. Over the central equatorial Pacific, the decreased SST gradient is accompanied by similar decreases in the easterly wind stress and westward ocean surface currents and by a local maximum in precipitation increase. Over the entire rising branch region of the Walker circulation, precipitation is enhanced by 15%, but the time-mean upward motion decreases slightly in intensity. The failure of the zonal overturning atmospheric circulation to intensify with a quadrupling of CO2 is surprising in light of the increased time-mean condensation heating over the “warm pool” region. Three aspects of the model response are important for interpreting this result. 1) The time-mean radiative cooling of the upper troposphere is enhanced, due to both the pronounced upper-tropospheric warming and to the large fractional increase of upper-tropospheric water vapor. 2) The dynamical cooling term, −ω̄∂θ̄/∂p, is enhanced due to increased time-mean static stability (−∂θ̄/∂p). This is an effect of moist convection, which keeps the lapse rate close to the moist adiabatic rate, thereby making −∂θ̄/∂p larger in a warmer climate. The enhanced radiative cooling and increased static stability allow for the enhanced time-mean heating by moist convection and condensation to be balanced without stronger time-mean upward motions. 3) The weaker surface zonal winds and wind stress in the equatorial Pacific are consistent with the reduced zonal SST gradient. The SST gradient is damped by the west-east differential in evaporative surface cooling (with greater evaporative cooling in the west than in the east). This evaporative damping increases with increasing temperature, owing to the temperature dependence of saturation mixing ratios, which leads to a reduction in the SST gradient in the warmer climate.