Global long‐lived chemical modes excited in a 3‐D chemistry transport model: Stratospheric N 2 O, NO y , O 3 and CH 4 chemistry

The two longest‐lived, major chemical response patterns (eigenmodes) of the atmosphere, coupling N 2 O and CH 4 , are identified with the UCI chemistry‐transport model using a linearized (N 2 O, NO y , O 3 , CH 4 , H 2 O)‐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 N 2 O perturbation (mode‐1: 108.4 y) caused by a pulse emission of N 2 O is 10‐years shorter than the N 2 O atmospheric lifetime (118.2 y). This mode‐1 can also be excited by CH 4 emissions due to CH 4 ‐O 3 stratospheric chemistry: a pulse emission of 100 Tg CH 4 creates a +0.1 Tg N 2 O perturbation in mode‐1 with a 108‐yr e‐folding decay time, thus increasing the CH 4 global warming potential by 1.2%. Almost all of the 100 Tg CH 4 appears in mode‐2 (10.1 y).