Strongly eddying ocean simulations required to resolve Eocene model-data
mismatch
Abstract
Model simulations of past climates are increasingly found to compare
well with proxy data at a global scale, but regional discrepancies
remain. A persistent issue in modeling past greenhouse climates has been
the temperature difference between equatorial and (sub-)polar regions,
which is typically much larger in simulations than proxy data suggest.
Particularly in the Eocene, multiple temperature proxies suggest extreme
warmth in the southwest Pacific Ocean, where model simulations
consistently suggest temperate conditions. Here we present new global
ocean model simulations at 0.1° horizontal resolution for the
middle-late Eocene. The eddies in the high-resolution model affect
poleward heat transport and local time-mean flow in critical regions
compared to the non-eddying flow in the standard low-resolution
simulations. As a result, the high-resolution simulations produce higher
surface temperatures near Antarctica and lower surface temperatures near
the equator compared to the low-resolution simulations, leading to
better correspondence with proxy reconstructions. Crucially, the
high-resolution simulations are also much more consistent with
biogeographic patterns in endemic-Antarctic and low-latitude-derived
plankton, and thus resolve the long-standing discrepancy of warm
subpolar ocean temperatures and isolating polar gyre circulation. The
results imply that strongly eddying model simulations are required to
reconcile discrepancies between regional proxy data and models, and
demonstrate the importance of accurate regional paleobathymetry for
proxy-model comparisons.