Tropical Pacific Interannual Variability and CO2Climate Change

Abstract

In this paper, an attempt is made to estimate possible sensitivities of El Niño-Southern Oscillation (ENSO)-related effects in a climate with increased carbon dioxide (CO2). To illustrate this sensitivity, results are shown from two different interactive ocean-atmosphere model configurations and an atmospheric model with prescribed heating anomalies. In the first, an atmospheric general circulation model (GCM) is coupled to a global coarse-grid dynamical ocean GCM (coupled model). In the second, the same atmospheric model is coupled to a simple nondynamic slab-ocean mixed-layer model (mixed-layer model). In the third, an atmospheric model is run in perpetual January mode with observed sea surface 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 relative to the mean SST) continues to occur in the tropics with a doubling of CO2. This variability is superimposed on mean SSTs in the tropical eastern Pacific that are higher by about 1°. The pattern of precipitation and soil-moisture anomalies in the tropics is similar in model warm events with present amounts of CO2 (1 × CO2) and in warm events with instantaneously doubled CO2 (2 × 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 low-level moisture convergence, greater precipitation over the SST anomaly, and an intensification of atmospheric anomalies in the tropics involved with the anomalous large-scale east-west (Walker) circulation. Consequently, differences of precipitation and soil moisture between 1 × CO2 and 2 × CO2 warm events show that most anomalously dry areas become drier (implying risk of increased drought in those regions in 2 × CO2 Warm events) and anomalously wet areas wetter in the coupled model. In the extratropics, the increased CO2 causes a large change in the midlatitude atmospheric circulation. This is associated with an alteration of extratropical teleconnections in 2 × CO warm events compared to 1 × CO2 warm events in a relative sense, with more zonally symmetric 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 × CO2 and 2 × CO2 mean climates, and from the prescribed-heating model with anomalous heat sources in the tropical troposphere analogous to those in 1 × CO2 and 2 × CO2 warm events. This study is a precursor to future higher-resolution model studies that could also address possible changes in ENSO but with better representation of coupled mechanisms thought to contribute to ENSO.