Tropical Pacific Interannual Variability and CO 2 Climate Change

Abstract

In this paper, an attempt is made to estimate possible sensitivities ofEl Nino-Southern Oscillation (ENSO)-related effects in a climate withincreased carbon dioxide (CO2). To illustrate this sensitivity, resultsare shown from two different interactive ocean-atmosphere modelconfigurations and an atmospheric model with prescribed heatinganomalies. 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 simplenondynamic slab-ocean mixed-layer model (mixed-layer model). In thethird, an atmospheric model is run in perpetual January mode withobserved sea surface temperatures (SSTs) and prescribed tropicaltropospheric heating anomalies (prescribed-heating model). Results fromthe coupled model show that interannual SST variability (with warm andcold events relative to the mean SST) continues to occur in the tropicswith a doubling of CO2. 1 his variability is superimposed on mean SSTsin the tropical eastern Pacific that are higher by about 1-degrees. Thepattern of precipitation and soil-moisture anomalies in the tropics issimilar in model warm events with present amounts of CO2 (1 X CO2) andin warm events with instantaneously doubled CO2 (2 X CO2). When awarm-event SST anomaly is superimposed, the rise in mean SST in thetropical eastern Pacific from the doubling of CO2 leads to increasedevaporation and low-level moisture convergence. greater precipitationover the SST anomaly, and an intensification of atmospheric anomalies inthe tropics involved with the anomalous large-scale east-west (Walker)circulation. Consequently, differences of precipitation and soilmoisture between 1 x CO2 and 2 x CO2 warm events show that mostanomalously dry areas become drier (implying risk of increased droughtin those regions in 2 X CO2 warm events) and anomalously wet areaswetter in the coupled model. In the extratropics, the increased CO2causes a large change in the midlatitude atmospheric circulation. Thisis associated with an alteration of extratropical teleconnections in 2 XCO2 warm events compared to 1 X CO2 warm events in a relative sense.with more zonally symmetric anomalies in sea level pressure and 200-mbheight. Similar results in the tropics and extratropics are obtained forthe mixed-layer model with warm-event SST anomalies in the tropicalPacific prescribed for 1 X CO2 and 2 X CO2 mean climates, and from theprescribed-heating model with anomalous heat sources in the tropicaltroposphere analogous to those in 1 X CO2 and 2 x CO2 warm events. Thisstudy is a precursor to future higher-resolution model studies thatcould also address possible changes in ENSO but with betterrepresentation of coupled mechanisms thought to contribute to ENSO.