TROPICAL PACIFIC INTERANNUAL VARIABILITY AND CO2 CLIMATE CHANGE

In this paper, an attempt is made to estimate possible sensitivities o f El Nino-Southern Oscillation (ENSO)-related effects in a climate wit h increased carbon dioxide (CO2). To illustrate this sensitivity, resu lts are shown from two different interactive ocean-atmosphere model co nfigurations and an atmospheric model with prescribed heating anomalie s. In the first, an atmospheric general circulation model (GCM) is cou pled to a global coarse-grid dynamical ocean GCM (coupled model). In t he second, the same atmospheric model is coupled to a simple nondynami c slab-ocean mixed-layer model (mixed-layer model). In the third, an a tmospheric model is run in perpetual January mode with observed sea su rface temperatures (SSTs) and prescribed tropical tropospheric heating anomalies (prescribed-heating model). Results from the coupled model show that interannual SST variability (with warm and cold events relat ive to the mean SST) continues to occur in the tropics with a doubling of CO2. 1 his variability is superimposed on mean SSTs in the tropica l eastern Pacific that are higher by about 1-degrees. The pattern of p recipitation and soil-moisture anomalies in the tropics is similar in model warm events with present amounts of CO2 (1 X CO2) and in warm ev ents with instantaneously doubled CO2 (2 X CO2). When a warm-event SST anomaly is superimposed, the rise in mean SST in the tropical eastern Pacific from the doubling of CO2 leads to increased evaporation and l ow-level moisture convergence. greater precipitation over the SST anom aly, and an intensification of atmospheric anomalies in the tropics in volved with the anomalous large-scale east-west (Walker) circulation. Consequently, differences of precipitation and soil moisture between 1 x CO2 and 2 x CO2 warm events show that most anomalously dry areas be come drier (implying risk of increased drought in those regions in 2 X CO2 warm events) and anomalously wet areas wetter in the coupled mode l. In the extratropics, the increased CO2 causes a large change in the midlatitude atmospheric circulation. This is associated with an alter ation of extratropical teleconnections in 2 X CO2 warm events compared to 1 X CO2 warm events in a relative sense. with more zonally symmetr ic anomalies in sea level pressure and 200-mb height. Similar results in the tropics and extratropics are obtained for the mixed-layer model with warm-event SST anomalies in the tropical Pacific prescribed for 1 X CO2 and 2 X CO2 mean climates, and from the prescribed-heating mod el with anomalous heat sources in the tropical troposphere analogous t o those in 1 X CO2 and 2 x CO2 warm events. This study is a precursor to future higher-resolution model studies that could also address poss ible changes in ENSO but with better representation of coupled mechani sms thought to contribute to ENSO.