Fundamental framework and experiments of the third generation of IAP/LASG world ocean general circulation model

A new generation of the IAP / LASG world ocean general circulation model is designed and presented based on the previous 20-layer model, with enhanced spatial resolutions and improved parameterizations. The model uses a trian gular-truncated spectral horizontal grid system with its zonal wave number of 63 (T63) to match its atmospheric counterpart of a T63 spectral atmosphe re general circulation model in a planned coupled ocean-atmosphere system. There are 30 layers in vertical direction, of which 20 layers are located a bove 1000 m for better depicting the permanent thermocline. As previous oce an models developed in IAP / LASG, a free surface (rather than "rigid-lid" approximation) is included in this model. Compared with the 20-layer model, some more detailed physical parameterizations are considered, including th e along / cross isopycnal mixing scheme adapted from the Gent-MacWilliams s cheme. The model is spun up from a motionless state. Initial conditions for temper ature and salinity are taken from the three-dimensional distributions of Le vitus' annual mean observation. A preliminary analysis of the first 1000-ye ar integration of a control experiment shows some encouraging improvements compared with the twenty-layer model, particularly in the simulations of pe rmanent thermocline, thermohaline circulation, meridional heat transport, e tc. resulted mainly from using the isopycnal mixing scheme. However, the us e of isopycnal mixing scheme does not significantly improve the simulated e quatorial thermocline. A series of numerical experiments show that the most important contribution to the improvement of equatorial thermocline and th e associated equatorial under current comes from reducing horizontal viscos ity in the equatorial regions. It is found that reducing the horizontal vis cosity in the equatorial Atlantic Ocean may slightly weaken the overturning rate of North Atlantic Deep Water.