Transient and long-term behaviour of the world ocean under global warming

The thermohaline circulation (THC) of the oceans plays a crucial role in adjusting the global thermal and hydrological budget in the climate system. Knowledge about its stability and change is very important for understanding the evolution of past climates and assessing possible climate changes in the future. In this study, we investigate the transient and long-term behaviour of the THC, particularly the Southern Ocean overturning in the CSIRO climate model, under increasing atmospheric greenhouse gases (as equivalent CO2 , referred to simply as CO2) following the IPCC/IS92a scenario to stabilisation at three times preindustrial CO2 (3x CO2) then continuing at that level of stabilisation. Firstly the CSIRO ocean model is further developed by modifying the surface boundary forcing, for the purpose of ensuring a stable and realistic ocean climate to be used as the initial condition of the ocean for coupled model climate change studies. The new formulation leads to a significantly improved spinup solution and coupled control climate of the ocean. The world ocean water mass properties, in particular the Southern Ocean stratification, the THC, and the Antarctic Circumpolar Current (ACC) are all in broad agreement with observations. In the following global warming experiment, pronounced changes of the global THC occur during the CO2 increase period: the North Atlantic Deep Water Formation (NADWF) weakens rapidly and the Antarctic Bottom Water Formation (AABWF) collapses completely before CO2 triples. During the subsequent period of 1100 years with stabilised 3x CO2, the NADWF intensity shows a tendency to recover gradually in the upper part of the ocean but the AABWF shows no sign of returning and the residual deep overturning dies away. Also evident is the change of ACC transport under CO2 forcing: it increases along with the CO2 increase and keeps increasing steadily for a few centuries after CO2 tripling. While both the surface freshening and heating from above are responsible for the weakening of NADWF, the surface freshening around Antarctica, including the enhancement of precipitation over evaporation (P - E), runoff from continents and reduction in the sea ice formation and outflux, suppresses the deep convection off Antarctica and causes the shutdown of AABWF. The strengthening of the ACC transport is attributable to the enhanced meridional density contrast across the ACC due to the uneven warming in the Southern Ocean, both at the surface and in the interior. This change in density structure leads to an acceleration in the upper layer currents which outweighs the deceleration in the mid-depth layer caused by the weakening and shutoff of the AABWF. Using the Bryan (1984) technique to accelerate the convergence of the deep ocean towards equilibrium under the 3x CO2 condition, it is found that the global THC eventually reaches a near-stable state in which the NADWF is fully recovered, the AABWF is also partly re-established and deep ocean ventilation is activated again. The recovery of the THC is attributed to the slow but persistent warming in the deep ocean which gradually destabilizes the water column. After thousands of years, a stratification structure close to the initial state becomes re-built in the high latitude Southern Ocean, which allows deep convection and hence overturning off Antarctica to occur, bringing the system into a new regime. However, this regime needs some more time to further adjust and settle down to a more stable and slightly different normal mode solution. This is verified by an extension to the accelerated run for 500 years with the acceleration switched off. This result shows the development of a possible new quasi-equilibrium for the ocean under long-term global warming induced by the anthropogenic CO2 increase. Comparison has been made with the results from an earlier version of the coupled model which has a clearly different initial climate for the ocean. We conclude that, for the CSIRO coupled model (mark2), the oceanic response to global warming is not strongly dependent on the basic state of the ocean. However, some differences in the oceanic behaviour under CO2 forcing between the two versions are of scientific interest and have been discussed in detail.