We introduce a transformed isentropic coordinate MÎ¸e, defined as the dry air mass under a given equivalent potential temperature surface (Î¸.sub.e) within a hemisphere. Like Î¸.sub.e, the coordinate MÎ¸e follows the synoptic distortions of the atmosphere but, unlike Î¸.sub.e, has a nearly fixed relationship with latitude and altitude over the seasonal cycle. Calculation of MÎ¸e is 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 CO.sub.2 seasonal cycle as a function of pressure and MÎ¸e, where MÎ¸e is thereby effectively used as an alternative to latitude. We show that the CO.sub.2 seasonal cycles are more constant as a function of pressure using MÎ¸e as the horizontal coordinate compared to latitude. Furthermore, short-term variability in CO.sub.2 relative to the mean seasonal cycle is also smaller when the data are organized by MÎ¸e and pressure than when organized by latitude and pressure. We also present a method using MÎ¸e to compute mass-weighted averages of CO.sub.2 on a hemispheric scale. Using this method with the same airborne data and applying corrections for limited coverage, we resolve the average CO.sub.2 seasonal cycle in the Northern Hemisphere (mass-weighted tropospheric climatological average for 2009-2018), yielding an amplitude of 7.8 Â± 0.14 ppm and a downward zero-crossing on Julian day 173 Â± 6.1 (i.e., late June). MÎ¸e may be similarly useful for mapping the distribution and computing inventories of any long-lived chemical tracer.