Distinguishing Tropical and Extratropical Dynamical Regimes Based on Rossby Number Statistics

Large-scale tropical dynamics tend to be nearly zonally symmetric with large Rossby numbers. Strong zonal asymmetries and small Rossby numbers characterize extratropical regimes. Gaining an understanding of the structure of the Rossby number could be useful in determining the meridional extent of the tropical belt, which determines the locations of subtropical dry zones and their changing positions with climate change. Using Rossby number statistics may be a robust and simple basis for developing diagnostics of the tropical regime and its associated length and time scales for use in a climate model. This study shows how distinguishing tropical and extratropical dynamical regimes can be done based on Rossby number statistics. The study aims to find if meridional changes in the Rossby number could be used to determine what time and length scale approximations are most appropriate to describe the dynamics of a given latitude. In dynamic meteorology courses the length and time scales of atmospheric dynamics and their associated Rossby number are usually approximated with no regard to latitude and consider only a single horizontal scale. In this study we analyze scale dependencies of the Rossby number as a function of latitude using global coverage reanalysis data. We test the assumption that large-scale mid latitude dynamics have a Rossby number of ~0.1 and that the tropics are characterized by large Rossby numbers ~1. Different time and spatial resolutions are studied in order to better quantify the transition between tropical and extratropical dynamical regimes.