A mass-weighted isentropic coordinate for mapping chemical tracers and computing atmospheric inventories

Abstract

We introduce a transformed isentropic coordinate Mθe, defined as the dry air mass under a given equivalent potential temperature surface (θe) within a hemisphere. Like θe, the coordinate Mθefollows the synoptic distortions of the atmosphere but, unlike θe, has a nearly fixed relationship with latitude and altitude over the seasonal cycle. Calculation of Mθeis straightforward from meteorological fields. Using observations from the recent HIAPER Pole-to-Pole Observations (HIPPO) and Atmospheric Tomography Mission (ATom) airborne campaigns, we map the CO2seasonal cycle as a function of pressure and Mθe, where Mθeis thereby effectively used as an alternative to latitude. We show that the CO2seasonal cycles are more constant as a function of pressure using Mθeas the horizontal coordinate compared to latitude. Furthermore, short-term variability in CO2relative to the mean seasonal cycle is also smaller when the data are organized by Mθeand pressure than when organized by latitude and pressure. We also present a method using Mθeto compute mass-weighted averages of CO2on a hemispheric scale. Using this method with the same airborne data and applying corrections for limited coverage, we resolve the average CO2seasonal cycle in the Northern Hemisphere (mass-weighted tropospheric climatological average for 2009-2018), yielding an amplitude of 7.8±0.14 ppm and a downward zerocrossing on Julian day 173±6.1 (i.e., late June).Mθemay be similarly useful for mapping the distribution and computing inventories of any long-lived chemical tracer.