SIMULATION OF THERMOHALINE CIRCULATION WITH A 20-LAYER OCEANIC GENERAL-CIRCULATION MODEL

This paper presents the basic configuration and preliminary performanc e of a twenty-layer oceanic general circulation model which represents a portion of the recent progress in developing coupled ocean-atmosphe re general circulation models made by the authors. The model uses lati tude/depth-dependent thermohaline-stratification subduction, ''eta''-c oordinate, three-dimensional implicit diffusion, complete convective a djustment, separating and coupling of external and internal modes and Asselin temporal filter, and thermodynamic sea-ice calculation. With s easonally varying climatological forcing at the surface and enhanced s urface salinities in the region adjacent Antarctica, the model has bee n integrated for one thousand years to reach a quasi-equilibrium state . Preliminary verification shows that the model is capable of simulati ng successfully not only many aspects of the upper ocean circulation b ut also an acceptable thermohaline circulation. The modelled overturni ng rate of the North Atlantic Deep Water (NADW) is greater than 15Sv. The simulated overturning rate of the Antarctic Bottom Water (AABW) is about 20Sv. The southward outflow of NADW can be identified from not only the meridional overturning streamfunction but also the current fi elds at four deeper levels from 1455m to 2475m. The AABW northward out flow exists at some bottom levels below 2600m, and mainly flows toward s the Pacific basin. Major problems in the present simulation include the underestimate of the NADW outflow, the failure to simulate the Ant arctic Intermediate Water (AAIW), the too fresh bottom water and the t oo diffuse thermocline of the model. A sensitivity experiment has reve aled that the model diffusion process has an important impact on the s imulation of both the thermocline and the NADW outflow.