Enhanced “wind-evaporation effect” drove the “deep-tropical contraction” in the early Eocene

The early Eocene is the warmest epoch in the last 65 million years, with a global mean temperature 9 to 23°C higher than the modern era. According to state-of-the-art climate models, the tropical rainfall contracted towards the equator during this extremely warm period. However, the physical mechanism causing this phenomenon remains unclear. In this study, we examined the hemispheric energy balance in the early Eocene that causes the equatorward contraction of tropical precipitation. A novel mechanism underlying this phenomenon is revealed. Based on the climate modeling of CESM1.2, we show that the GHG-induced warmth enhances the sensitivity of evaporation to surface wind speed changes in the early Eocene. Thus, the stronger tropical trade wind in the winter hemisphere will drive out stronger latent heat flux than in the summer hemisphere. This interhemispheric asymmetric response reduces the interhemispheric heating contrast in the solstice seasons. As a result, the ascending motion in the tropical atmosphere migrates towards the equator, finally decreases the width of tropical precipitation in the early Eocene.