PASSIVE ADVECTION AND REDISTRIBUTION OF HEAT IN WARMING OCEAN BASINS

English Abstract:

An important parameter for the climate response to increased greenhouse gases or other radiative forcing is the speed at which heat anomalies propagate downward in the ocean. Ocean heat uptake occurs through passive advection/diffusion of surface heat anomalies and through the redistribution of existing temperature gradients due to circulation changes. Atlantic meridional overturning circulation (AMOC) weakens in a warming climate and this should slow the downward heat advection (compared to a case in which the circulation is unchanged). However, weakening AMOC also causes a deep warming through the redistributive effect, thus increasing the downward rate of heat propagation compared to unchanging circulation. Total heat uptake depends on the combined effect of these two mechanisms.

Passive tracers in a control and perturbed CO2-quadrupling experiments are used to investigate the relative effect of passive advection and redistribution. The spatial pattern and mechanisms of warming are examined. One feature of global warming appears to be the deeper heat penetration in the Atlantic than in the Indo-Pacific oceans. Even though our experiments display deep injection of a passive tracer in the deep-water formation region of the North Atlantic, most of the difference in speed of heat penetration is due to the redistribution term rather than this North Atlantic sinking. With further experiments we isolate the effect of weakening overturning on the heat uptake associated with passive advection. The separate effects of wind and salinity changes in changing circulation and the resulting effect on redistribution in the individual basins are also investigated.