Abstract:
Attempts to understand early Paleogene climate using models have provided insights, but have consistently had difficulty explaining essential climatic features. A main limitation of those studies has been the lack of interaction between dynamical ocean and atmospheric models. After reviewing previous model studies and quantitatively defining the "low gradient problem," we present results from the first fully coupled general circulation model simulations in equilibrium with reconstructed early Paleogene topography, bathymetry, vegetation, and pCO2 (560 ppm). Although our results should be understood as preliminary, predictions of both Paleogene sea surface temperatures and salinities are, for the first time, made without assuming an arbitrary value of the oceanic or atmospheric heat transport. Model-predicted bottom water temperatures are 7 °C warmer than modern. Deep convection occurs in the North Atlantic and Tethys, forming warm (12-15 °C), salty water masses (35.5 ppt). Through most of the Southern Ocean, vigorous wind-driven upwelling and a stable water column caused by low salinities (∼32 ppt) produce a temperature inversion that may explain proxy interpretations of "warm salty deep water" formation in the tropics. Estimates of seawater oxygen isotopic ratios reveal substantially different patterns than usually predicted. Ocean heat transport is little-changed from modern values and warming of tropical temperatures (by ∼3 °C) is about half that occurring in high latitudes. The mean annual temperature and seasonality patterns produced by the simulation are biased toward colder and more seasonal values than those reconstructed from temperature proxies.
