8.3.2.2 Simulation of Circulation Features Important for Climate Response
8.3.2.2.1 Meridional overturning circulation
The MOC is an important component of present-day climate and many models indicate that it will change in the future (Chapter 10). Unfortunately, many aspects of this circulation are not well observed. The MOC transports large amounts of heat and salt into high latitudes of the North Atlantic Ocean, where the relatively warm, salty surface waters are cooled by the atmosphere, making the water dense enough to sink to depth. These waters then flow southward towards the Southern Ocean where they mix with the rest of the World Ocean waters (see Supplementary Material, Figure S8.15).
The models simulate this major aspect of the MOC and also simulate a number of distinct wind-driven surface cells (see Supplementary Material, Figure S8.15). In the tropics and subtropics, these cells are quite shallow, but at the latitude of the Drake Passage (55°S) the wind-driven cell extends to a much greater depth (2 to 3 km). Most models in the multi-model data set have some manifestation of the wind-driven cells. The strength and pattern of the overturning circulation varies greatly from model to model (see Supplementary Material, Figure S8.15). The GISS-AOM exhibits the strongest overturning circulation, almost 40 to 50 Sv (106 m3 s–1). The CGCM (T47 and T63) and FGOALS have the weakest overturning circulations, about 10 Sv. The observed value is about 18 Sv (Ganachaud and Wunsch 2000).
In the Atlantic, the MOC, extending to considerable depth, is responsible for a large fraction of the northward oceanic heat transport in both observations and models (e.g., Hall and Bryden, 1982; Gordon et al., 2000). Figure 10.15 contains an index of the Atlantic MOC at 30°N for the suite of AOGCM 20th-century simulations. While the majority of models show an MOC strength that is within observational uncertainty, some show higher and lower values and a few show substantial drifts which could make interpretation of MOC projections using those models very difficult.
Overall, some aspects of the simulation of the MOC have improved since the TAR. This is due in part to improvements in mixing schemes, the use of higher resolution ocean models (see Section 8.2) and better simulation of the surface fluxes. This improvement can be seen in the individual model MOC sections (see Supplementary Material, Figure S8.15) by the fact that (1) the location of the deep-water formation is more realistic, with more sinking occurring in the Greenland-Iceland-Norwegian and Labrador Seas as evidenced by the larger stream function values north of the sill located at 60°N (e.g., Wood et al., 1999) and (2) deep waters are subjected to less spurious mixing, resulting in better water mass properties (Thorpe et al., 2004) and a larger fraction of the water that sinks in the northern part of the North Atlantic Ocean exiting the Atlantic Ocean near 30°S (Danabasoglu et al., 1995). There is still room for improvement in the models’ simulation of these processes, but there is clear evidence of improvement in many of the models analysed here.
8.3.2.2.2 Southern Ocean circulation
The Southern Ocean wind stress error has a particularly large detrimental impact on the Southern Ocean simulation by the models. Partly due to the wind stress error identified above, the simulated location of the Antarctic Circumpolar Current (ACC) is also too far north in most models (Russell et al., 2006). Since the AAIW is formed on the north side of the ACC, the water mass properties of the AAIW are distorted (typically too warm and salty: Russell et al., 2006). The relatively poor AAIW simulation contributes to the multi-model mean error identified above where the thermocline is too diffuse, because the waters near the base of thermocline are too warm and salty.
It is likely that the relatively poor Southern Ocean simulation will influence the transient climate response to increasing greenhouse gases by affecting the oceanic heat uptake. When forced by increases in radiative forcing, models with too little Southern Ocean mixing will probably underestimate the ocean heat uptake; models with too much mixing will likely exaggerate it. These errors in oceanic heat uptake will also have a large impact on the reliability of the sea level rise projections. See Chapter 10 for more discussion of this subject.