Tropical Response to Increased CO2 in a GCM with a Simple Mixed Layer Ocean: Similarities to an Observed Pacific Warm Event

Abstract

A version of the NCAR Community Climate Model (CCM), a global spectral atmospheric general circulation model, is coupled to a simple ocean mixed layer model with fixed depth. This mixed layer model is a simple slab of water that crudely accounts for seasonal heat storage but has no dynamics (e.g., advection, diffusion or upwelling). As a result, tropical sea surface temperatures (SSTs) produced by the simple mixed layer are warmer than observed in the equatorial tropics, in particular, in the tropical eastern Pacific. When CO2 is doubled in the model, SSTs increase at all longitudes in the equatorial tropics on the order of 1°?2°C. However, this uniform longitudinal warming is not associated with uniform longitudinal increases of tropical precipitation. Greatest area-averaged increases of precipitation occur in the tropical Pacific, while decreases of area-averaged precipitation are noted in the Asian monsoon regions. This is attributed to a disruption of the east?west circulation between the Pacific and Indian sectors similar to that observed during Pacific Warm Events. Accordingly, a number of the features of the model response to increased CO2 resemble those of a Warm Event. These include, for example, westerly upper-level wind anomalies in the subtropics of the Pacific in both hemispheres, westerly anomalies in the equatorial tropical western Pacific, and easterly wind anomalies to the east. In spite of these similarities, it is emphasized that the real climate system may respond differently to an increase in CO2 due to the importance of tropical ocean dynamics that are not included in the present simple mixed-layer ocean model. The next step in the modeling hierarchy, an ocean GCM coupled to the NCAR CCM, will show how the response in the tropics, documented here for SSTs determined only by surface energy balance and crude ocean heat storage, may be affected by the addition of ocean dynamics.