We aim to reconcile the recently published, apparently contrasting resultsregarding the relative importance of tropical upwelling versus horizontaltransport for the seasonality of ozone above the tropical tropopause.Different analysis methods in the literature (Lagrangian versus Eulerian,and isentropic versus pressure vertical coordinates) yield differentperspectives of ozone transport, and the results must be carefully comparedin equivalent terms to avoid misinterpretation. By examining the Lagrangiancalculations in the Eulerian formulation, we show here that the results arein fact consistent with each other and with a common understanding of theozone transport processes near and above the tropical tropopause.We further emphasize that the complementary approaches are suited foranswering two different scientific questions: (1) what drives the observedseasonal cycle in ozone at a particular level above the tropical tropopause?and (2) how important is horizontal transport from mid-latitudes for ozoneconcentrations in the tropical lower stratosphere? Regarding the firstquestion, the analysis of the transformed Eulerian mean (TEM) ozone budgetshows that the annual cycle in tropical upwelling is the main forcing of theozone seasonality at altitudes with large vertical gradients in the tropicallower stratosphere. To answer the second question a Lagrangian framework mustbe used, and the results show that a large fraction (~50%) of theozone molecules ascending through the tropical lower stratosphere is ofextra-tropical origin and has been in-mixed from mid-latitudes.
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