A design of an oceanic GCM without the rigid lid approximation and its application to the numerical simulation of the circulation of the Pacific Ocean

Abstract

Since the approximation of rigid lid of the ocean excludes the “available surface energy” and consequently, introduces errors in the computation of surface currents, propagation of very long waves and the variation of gigantic gyres, it is desirable to remove this approximation. Such an oceanic general circulation model has been designed in the Laboratory of Numerical Modelling for Atmospheric Sciences and Geophysical Fluid Dynamics, Chinese Academy of Sciences. In addition, the model differs from other oceanic GCM's (General Circulation Models) in the world by the following features: 1. (1) subtraction of the standard stratification and introduction of departures of temperature, T′, salinity, S′, density,ϱ′ and pressure, p′, from their standard values; 2. (2) suitable transformation of coordinates and variables, which makes the energy equation more compact and the total “available energy” conserved, if the forcing and dissipation are neglected and some small reasonable modifications are introduced; 3. (3) perfect conservation of total available energy (under the conditions mentioned in point 2) and no computational mode in the finite-difference scheme; 4. (4) introduction of “flexible coefficients” and a modified splitting method for flexibility of computation and acceleration of convergence in time-integration. A four-level version of such oceanic GCM has been applied to simulations of annual mean circulations in the World Ocean and the annual cycle in the Pacific Ocean under the forcings of the climatological surface wind stress, surface heat flux and air pressure. More than 60 years integration has been made and the results show successful simulations of most observed large-scale features of the annual mean and annual cycle of currents, temperature, sea surface elevation and other features in the Pacific Ocean.