The Three Dimensional Stratospheric Circulation in Isentropic Coordinates

Abstract:

The meridional overturning circulation of the stratosphere, often called the Brewer-Dobson Circulation, is traditionally described as a zonally symmetric, 2-dimensional cell driven by gyroscopic pumping. This simplification implicitly treats air as rising equally at all longitudes across the tropics and, therefore, that the composition of the air filling the stratosphere can be described by the tropical zonal mean. This is, of course, not a complete picture of the circulation; the monsoon circulation is important for longitudinal variance, for example. The chemical composition of air throughout the stratosphere is determined by the composition of the tropospheric air at the regions of exchange and the path of parcels to their observed locations. The three-dimensional (3D) structure of stratospheric circulation may therefore be fundamental for determining regional ozone concentrations and other quantities of human interest. Additionally, this 3D circulation may change in a warming climate, so knowing its structure is essential to monitoring any changes and evaluating their impacts.

The 3D stratospheric flow has been a focus of some recent theory and observational papers, including new developments in the residual mean circulation and in the mass-weighted isentropic transport circulation. Using the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) reanalysis data and Lagrangian modeling, we explore the 3D circulation in isentropic coordinates using a Helmholtz decomposition into the rotational and divergent parts of the flow (c.f. Raiter et al. 2020 in troposphere). We then compare this circulation with the traditional residual mean flow and the 3D quasi-residual mean flow to determine if new insights can be gained using our framework. This study aims to answer fundamental questions about stratospheric circulation with implications for the stratosphere’s response to climate change.