Changes in Global Ocean Bottom Properties and Volume Transports in CMIP5 Models under Climate Change Scenarios

Abstract:

Changes in bottom temperature, salinity and density in the global ocean by 2100 for 24 CMIP5 climate models are investigated for the climate change scenarios RCP4.5 and RCP8.5. The multimodel mean shows a decrease in density in all deep basins except for the North Atlantic which becomes denser. The individual model responses to climate change forcing are more complex: regarding temperature, only one model predicts a cooling of the bottom waters while the 23 others predict a warming; in salinity, there is less agreement regarding the sign of the change, especially in the Southern Ocean. The magnitude and equatorward extent of these changes also vary strongly among models. The changes in properties can be linked with the changes in transport of key water masses. The Atlantic Meridional Overturning Circulation weakens in most models and is directly linked to changes in bottom density in the North Atlantic. These changes are due to the intrusion of modified Antarctic Bottom Water, made possible by the decrease in North Atlantic Deep Water formation. In the Indian, Pacific and South Atlantic basins, changes in bottom density are congruent with the weakening in Antarctic Bottom Water transport through these basins. We argue that the greater the meridional transport, the more the change is propagated towards the equator. Then strong decreases in density over 100 years of climate change cause a weakening of the transports. The speed at which these property changes reach the deep basins is critical for a correct assessment of the heat storage capacity of the oceans as well as for predictions of future sea level rise.