Changes in Stratospheric Climate and Age-Of-Air in Recent GEOS Systems Since MERRA-2

Accurately modeling the large-scale transport of trace gases and aerosols is critical for interpreting past (and projecting future) changes in atmospheric composition. Simulations of the stratospheric mean age-of-air continue to show persistent biases in chemistry climate models, although the drivers of these biases are not well understood. Here we identify one driver of simulated stratospheric transport differences among various NASA Global Earth Observing System (GEOS) candidate model versions under consideration for the upcoming GEOS Retrospective analysis for the $21\text{st}$ Century (GEOS-R21C). In particular, we show that the simulated age-of-air values are sensitive to the so-called “remapping” algorithm used within the finite-volume dynamical core, which controls how individual material surfaces are vertically interpolated back to standard pressure levels after each horizontal advection time step. Differences in the age-of-air resulting from changes within the remapping algorithm approach $\sim$1 year over the high latitude middle stratosphere—or about 30% climatological mean values—and imprint on several trace gases, including methane (${\text{CH}}_4$) and nitrous oxide ($N_2$O). These transport sensitivities reflect, to first order, changes in the strength of tropical upwelling in the lower stratosphere (70–100 hPa) which are driven by changes in resolved wave convergence over northern midlatitudes as (critical lines of) wave propagation shift in latitude. Our results strongly support continued examination of the role of numerics in contributing to transport biases in composition modeling.