Numerical evidence against reversed thermohaline circulation in the warm Paleocene/Eocene ocean

The question of whether deep water formation might have occurred in subtropical latitudes in the early Cenozoic is examined through use of a global ocean model forced by mixed boundary conditions. Zonal mean surface temperatures and wind stresses are derived from an atmospheric general circulation model (AGCM) simulation of the warm Paleocene/Eocene boundary interval (∼55 Ma) and are held constant for a series of sensitivity tests. The control case for moisture flux (evaporation minus precipitation, E-P), also derived from the AGCM, is perturbed so that the subtropical evaporation increases and high-latitude precipitation increases. A dramatic response is seen in the temperature and salinity structure of the model ocean, but the perturbation does not result in deep convection in subtropical latitudes. In all cases, bottom water is formed in the southern high latitudes, and the global meridional overturning is characterized by a strongly asymmetric circulation. No multiple equilibria have been found for any particular E-P configuration. In the most extreme case (5 times the control E-P) the model oscillates between meridional overturning circulation "on" and "off". Shorter-lived thermohaline slowing and reinvigoration are observed as a transient response under less extreme E-P perturbations. Despite the high evaporation implied in the perturbation experiments, mean mixed layer salinities in the subtropics do not rise much above the control case because of efficient removal of salt (and heat) through deepened subduction beneath the subtropical gyres. The sensitivity of the results to the parameterization of continental runoff and the specified diapycnal mixing coefficient (K_ν) are also examined. Distributing runoff purely zonally, rather than globally, has approximately the same effect as a 50% increase in the strength of the hydrologic cycle. Decreasing K_ν to 0.3 cm² s^-1 from the standard value of 1.0 cm² s^-1 increases the sensitivity to an increased hydrologic cycle considerably, but in no case does low-latitude deep water formation occur, indicating that subtropical bottom water formation is implausible in a model with some degree of realism. These experiments support changes in moisture flux as a mechanism for ocean warming (largely in the thermocline through intermediate water depths), but the process involved is deepened subtropical subduction and not subtropical deep water formation.