Development of Efficient, Dynamical Ocean-Atmosphere Models for Climatic Studies

Abstract

Three ocean-atmosphere models are constructed and evaluated for further studies of climatic variability and climatic sensitivity. All the models are based on the two-layer, highly truncated spectral atmospheric model of Held and Suarez modified to have a seasonal cycle of solar forcing and to interact with simple geographical distributions of land and sea. For simplicity, a 120° sector configuration is adopted. The first coupled model has as its oceanic component two motionless layers, connected by vertical diffusion and convective adjustment. This is chosen to represent the seasonal cycle in the upper ocean more realistically than a constant-thickness mixed layer. The effect of wind stirring on mixed-layer thickness is added in the second coupled model. The third coupled model is one in which three-dimensional ocean circulation is included, using a relatively fine oceanic grid (1.5°). Long integrations of the three coupled models are carried out in order to understand their intrinsic dynamics. The first coupled model exhibits many aspects of the seasonally varying atmospheric circulation, such as strong unstable wintertime westerlies, a summertime intertropical convergence zone at latitude 12°, and a low-latitude monsoonal circulation. Eddy transports of heat and momentum are realistic, except for a reduced contribution by stationary waves which is apparently due to the lack of mountains in the model. Eddy statistics are relatively insensitive to changes in the model parameters governing dissipation and precipitation. Inclusion of a prognostic mixed layer in the second coupled model produces a realistic dependence of layer depth on season and on synoptic variations in wind forcing. The model's annual cycle of upper ocean heat storage and surface temperature agrees well with weather ship data. Statistically significant modifications in the summertime structure of zonal winds and atmospheric temperatures result from a reduced upper ocean heat capacity. When active ocean circulation is included in the third coupled model, major current systems analogous to the Gulf stream, the oceanic interior flow, and the zonal equatorial currents, are produced on a seasonally averaged basis. Large barotropic oscillations are excited by the atmospheric forcing on synoptic time scales. Reduced ocean temperatures in the tropics result from equatorial upwelling. The annual-average oceanic heat transport is dominated by meridional overturning at low latitudes and is shared with gyre and diffusive components at midlatitudes. The time variation of net heat transport shows a semiannual oscillation in the tropics and an annual cycle at higher latitudes. Further improvements in the coupled models are suggested. A program of future experiments on climatic variability and sensitivity is outlined.