The effect of low ancient greenhouse climate temperature gradients on the ocean's overturning circulation

Abstract

Abstract. We examine whether the reduced meridional temperature gradients of past greenhouse climates might have reduced oceanic overturning, leading to a more quiescent subsurface ocean. A substantial reduction of the pole-toEquator temperature difference is achieved in a coupled climate model via an altered radiative balance in the atmosphere. Contrary to expectations, we find that the meridional overturning circulation and deep ocean kinetic energy remain relatively unaffected. Reducing the wind strength also has remarkably little effect on the overturning. Instead, overturning strength depends on deep ocean density gradients, which remain relatively unaffected by the surface changes, despite an overall decrease in ocean density. Ocean poleward heat transport is significantly reduced only in the Northern Hemisphere, as now the circulation operates across a reduced temperature gradient, suggesting a sensitivity of Northern Hemisphere heat transport in greenhouse climates to the overturning circulation. These results indicate that climate models of the greenhouse climate during the Cretaceous and early Paleogene may yield a reasonable overturning circulation, despite failing to fully reproduce the extremely reduced temperature gradients of those time periods.

Major intensification of Atlantic overturning circulation at the onset of Paleogene greenhouse warmth

“…Both the deepening of the Rio Grande Rise and enhanced mixing associated with global warming would have increased the capacity of the overturning circulation in the Atlantic Ocean to transport heat. These interpretations of our new Nd-isotope data are consistent with observed changes in Late Cretaceous to early Paleogene Nd-isotope records from the Pacific Ocean 42 and Earth system modelling results, which indicate that vigorous ocean circulation and strong vertical mixing resulted in increased oceanic heat transport and reduced equator–pole temperature gradients 42 , 43 . Higher oceanic heat transport efficiency likely also set the stage for the occurrence of brief hyperthermals which were frequently superimposed on the overall temperature rise of the Eocene hothouse 41 .…”

Antarctic climate, Southern Ocean circulation patterns, and deep water formation during the Eocene

“…However, modeling studies suggest that Cretaceous to Eocene deep water formation and downwelling may have occurred due to density contrasts created by seasonal changes in surface water temperatures and salinity (with or without the presence of sea ice) [e.g., Lunt et al , ; Huber and Sloan , ; Otto‐Bliesner et al , ]. More recently, Sijp and England [] demonstrated that reduced pole‐to‐equator sea surface temperature gradients, such as those reconstructed from the Tasman Gateway region during the early Eocene [e.g., Bijl et al , ], had little effect on modeled ocean circulation patterns or deep water formation in the southern high latitudes.…”

“…Although deep oceans cooled during the Eocene [e.g., Zachos et al , ], much less is known about the associated changes in circulation patterns and dominant source regions of deep water formation. Modeling and geochemical data‐based studies have proposed both the Southern Ocean and the low latitudes as regions of deep water formation during the warmest periods of the Cretaceous and early Cenozoic [e.g., Brass et al , ; Kennett and Stott , ; Bice et al , ; Cramer et al , ; Hague et al , ; Pak and Miller , ; Scher and Martin , ; Sijp and England , ; Thomas , ; Thomas et al , ], leaving large uncertainty about the link between ocean circulation and climate during the early Cenozoic greenhouse (e.g., the early Eocene “equable climate paradox”) [ Huber and Caballero , ]. This is in part due to a lack of detailed records documenting Eocene climate and ocean circulation in key geographical areas.…”