PNNL

Abstract

The Earth gains energy through absorption of solar radiation at its surface and loses energy by emission of infrared radiation through the troposphere. This differential heating drives a multiscale atmospheric overturning circulation that has so far been poorly quantified, because the contribution from mesoscale overturning associated with convective activity is not well constrained. Here we compare the atmospheric overturning in the newest-generation (ERA5) and third-generation (ERA-Interim) global reanalyses from the European Centre for Medium-Range Weather Forecasts based on isentropic averaging that identifies the contribution of different scales of motions. The two reanalyses feature large-scale overturning of similar magnitude. However, ERA5 reveals a mesoscale overturning that is 72% of the large-scale overturning over the tropics while the former is much weaker in ERA-Interim. Comparison with observed outgoing longwave radiation suggests that the magnitude and seasonal migrations of the mesoscale overturning in the tropics are much better captured by ERA5 than ERA-Interim. Hence in the tropics, mesoscale overturning may play a comparable role to large-scale overturning in vertical mass and heat transport. This result may have important implications for understanding the tropical rain belt and its variability from seasonal to multidecadal timescales.