Weak Miocene meridional SST gradients imply wetter subtropical conditions

Surface temperature reconstructions from the Mid and Late Miocene reveal polar amplified warmth and a reduced equator to pole temperature gradient. Vegetation reconstructions provide little evidence of arid desert conditions within the subtropics but rather shrublands, grasslands, savannas and woodlands suggesting generally wetter conditions. This is interesting because following a simple thermodynamic scaling implies enhanced subtropical moisture divergence and subtropical drying with warming - as seen in climate model projections for the future. What is distinct in the case of the Miocene is the presences of weak meridional SST gradients, supporting reduced atmospheric meridional circulation and hence subtropical moisture divergence. We demonstrate this using available surface temperature reconstructions to provide a best estimate of Miocene zonal-mean surface temperature gradients and a zonal-mean Energy Balance Model (EBM), modified to represent the Hadley Cell, the vertically integrated moisture flux, and P-E.