Evaluation of the N$_2$O Rate of Change to Understand the Stratospheric Brewer‐Dobson Circulation in a Chemistry‐Climate Model

The Brewer-Dobson Circulation (BDC) determines the distribution of long-lived tracers in the stratosphere; therefore, their changes can be used to diagnose changes in the BDC. We evaluate decadal (2005–2018) trends of nitrous oxide (N$_2$O) in two versions of the Whole Atmosphere Chemistry-Climate Model (WACCM) by comparing them with measurements from four Fourier transform infrared (FTIR) ground-based instruments, the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS), and with a chemistry-transport model (CTM) driven by four different reanalyses. The limited sensitivity of the FTIR instruments can hide negative N$_2$O trends in the mid-stratosphere because of the large increase in the lowermost stratosphere. When applying ACE-FTS measurement sampling on model datasets, the reanalyses from the European Center for Medium Range Weather Forecast (ECMWF) compare best with ACE-FTS, but the N$_2$O trends are consistently exaggerated. The N$_2$O trends obtained with WACCM disagree with those obtained from ACE-FTS, but the new WACCM version performs better than the previous above the Southern Hemisphere in the stratosphere. ... mehrModel sensitivity tests show that the decadal N$_2$O trends reflect changes in the stratospheric transport. We further investigate the N$_2$O Transformed Eulerian Mean (TEM) budget in WACCM and in the CTM simulation driven by the latest ECMWF reanalysis. The TEM analysis shows that enhanced advection affects the stratospheric N$_2$O trends in the Tropics. While no ideal observational dataset currently exists, this model study of N$_2$O trends still provides new insights about the BDC and its changes because of the contribution from relevant sensitivity tests and the TEM analysis.