, Volume 55, Issue 1-4, pp 65-87

Simulation of thermohaline circulation with a twenty-layer oceanic general circulation model

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This paper presents the basic configuration and preliminary performance of a twenty-layer oceanic general circulation model which represents a portion of the recent progress in developing coupled ocean-atmosphere general circulation models made by the authors. The model uses latitude/depthdependent thermohaline-stratification subduction, “η”-coordinate, three-dimensional implicit diffusion, complete convective adjustment, separating and coupling of external and internal modes and Asselin temporal filter, and thermodynamic sea-ice calculation. With seasonally varying climatological forcing at the surface and enhanced surface salinities in the region adjacent Antarctica, the model has been integrated for one thousand years to reach a quasiequilibrium state. Preliminary verification shows that the model is capable of simulating successfully not only many aspects of the upper ocean circulation but also an acceptable thermohaline circulation. The modelled overturning 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 fields at four deeper levels from 1455m to 2475m. The AABW northward outflow exists at some bottom levels below 2600m, and mainly flows towards the Pacific basin.

Major problems in the present simulation include the underestimate of the NADW outflow, the failure to simulate the Antarctic Intermediate Water (AAIW), the too fresh bottom water and the too diffuse thermocline of the model. A sensitivity experiment has revealed that the model diffusion process has an important impact on the simulation of both the thermocline and the NADW outflow.

With 16 Figures