Global long‐lived chemical modes excited in a 3‐D chemistry transport model: Stratospheric N2O, NOy, O3 and CH4 chemistry

The two longest‐lived, major chemical response patterns (eigenmodes) of the atmosphere, coupling N2O and CH4, are identified with the UCI chemistry‐transport model using a linearized (N2O, NOy, O3, CH4, H2O)‐system for stratospheric chemistry and specified tropospheric losses. As in previous 1D and 2D studies, these century‐long 3D simulations show that the e‐folding decay time of a N2O perturbation (mode‐1: 108.4 y) caused by a pulse emission of N2O is 10‐years shorter than the N2O atmospheric lifetime (118.2 y). This mode‐1 can also be excited by CH4 emissions due to CH 4 ‐O 3 stratospheric chemistry: a pulse emission of 100 Tg CH4 creates a +0.1 Tg N2O perturbation in mode‐1 with a 108‐yr e‐folding decay time, thus increasing the CH4 global warming potential by 1.2%. Almost all of the 100 Tg CH4 appears in mode‐2 (10.1 y).