Sensitivity of a Global Ocean General Circulation Model to Tracer Advection Schemes

Abstract

Vertical diffusivity at the thermocline depths is now believed to be as small as 1 × 10⁻⁵ m² s⁻¹. In order to accomplish a reliable simulation of the World Ocean for the vertical diffusivity of 1 × 10⁻⁵ m² s⁻¹, two advective tracer transport schemes, the Uniformly Third-Order Polynominal Interpolation Algorithm (UTOPIA) of Leonard et al and the Multidimensional Positive Definite Advection Transport Algorithm (MPDATA) of Smolarkiewicz, are incorporated into an ocean general circulation model. Intercomparison is made among simulations using UTOPIA, MPDATA, and the centered differencing scheme. When UTOPIA or MPDATA is adopted, features at the thermocline depths are realistically simulated. Increase in computational cost is moderate. Circulations associated with Antarctic Bottom Water (AABW) in the Atlantic and Circumpolar Deep Water (CDW) in the Pacific are not reproduced at all for such small vertical diffusivity, although the circulation associated with North Atlantic Deep Water (NADW) has reasonable intensity. Another experiment with UTOPIA for the vertical diffusivity of 5 × 10⁻⁵ m² s⁻¹ shows that the circulation associated with NADW is relatively insensitive to vertical diffusivity, compared with the circulation associated with AABW and CDW.