188.8.131.52 Horizontal and Vertical Resolution
There has been a general increase in resolution since the TAR, with a horizontal resolution of order one to two degrees now commonly used in the ocean component of most climate models. To better resolve the equatorial waveguide, several models use enhanced meridional resolution in the tropics. Resolution high enough to allow oceanic eddies, eddy permitting, has not been used in a full suite of climate scenario integrations due to computational cost, but since the TAR it has been used in some idealised and scenario-based climate experiments as discussed below. A limited set of integrations using the eddy-permitting MIROC3.2 (hires) model is used here and in Chapter 10. Some modelling centres have also increased vertical resolution since the TAR.
A few coupled climate models with eddy-permitting ocean resolution (1/6° to 1/3°) have been developed (Roberts et al., 2004; Suzuki et al., 2005), and large-scale climatic features induced by local air-sea coupling have been successfully simulated (e.g., Sakamoto et al., 2004).
Roberts et al. (2004) found that increasing the ocean resolution of the HadCM3 model from about 1° to 0.33° by 0.33° by 40 levels (while leaving the atmospheric component unchanged) resulted in many improvements in the simulation of ocean circulation features. However, the impact on the atmospheric simulation was relatively small and localised. The climate change response was similar to the standard resolution model, with a slightly faster rate of warming in the Northern Europe-Atlantic region due to differences in the Atlantic Meridional Overturning Circulation (MOC) response. The adjustment time scale of the Atlantic Basin freshwater budget decreased from being of order 400 years to being of order 150 years with the higher resolution ocean, suggesting possible differences in transient MOC response on those time scales, but the mechanisms and the relative roles of horizontal and vertical resolution are not clear.
The Atlantic MOC is influenced by freshwater as well as thermal forcing. Besides atmospheric freshwater forcing, freshwater transport by the ocean itself is also important. For the Atlantic MOC, the fresh Pacific water coming through the Bering Strait could be poorly simulated on its transit to the Canadian Archipelago and the Labrador Sea (Komuro and Hasumi, 2005). These aspects have been improved since the TAR in many of the models evaluated here.
Changes around continental margins are very important for regional climate change. Over these areas, climate is influenced by the atmosphere and open ocean circulation. High-resolution climate models contribute to the improvement of regional climate simulation. For example, the location of the Kuroshio separation from the Japan islands is well simulated in the MIROC3.2 (hires) model (see Figure 8.1), which makes it possible to study a change in the Kuroshio axis in the future climate (Sakamoto et al., 2005).
Guilyardi et al. (2004) suggested that ocean resolution may play only a secondary role in setting the time scale of model El Niño-Southern Oscillation (ENSO) variability, with the dominant time scales being set by the atmospheric model provided the basic speeds of the equatorial ocean wave modes are adequately represented.
Figure 8.1. Long-term mean ocean current velocities at 100 m depth (vectors, unit: m s–1) and sea surface temperature (colours, °C) around the Kuroshio and the Kuroshio Extension obtained from a control experiment forced by pre-industrial conditions (CO2 concentration 295.9 ppm) using MIROC3.2 (hires).