Chemistry–climate models predict an acceleration of the upwelling branch ofthe Brewer–Dobson circulation as a consequence of increasing global surfacetemperatures, resulting from elevated levels of atmospheric greenhouse gases.The observed decrease of ozone in the tropical lower stratosphere during thelast decades of the 20th century is consistent with the anticipatedacceleration of upwelling. However, more recent satellite observations ofozone reveal that this decrease has unexpectedly stopped in the first decadeof the 21st century, challenging the implicit assumption of a continuousacceleration of tropical upwelling. In this study we use three decades ofchemistry-transport-model simulations (1980–2013) to investigate thisphenomenon and resolve this apparent contradiction. Aside from a high-biasbetween 1985–1990, our model is able to reproduce the observed tropicallower stratosphere ozone record. A regression analysis identifies asignificant decrease in the early period followed by a statistically robusttrend-change after 2002, in qualitative agreement with the observations. Wedemonstrate that this trend-change is correlated with structural changes inthe vertical transport, represented in the model by diabatic heating ratestaken from the reanalysis product Era-Interim. These changes lead to a hiatusin the acceleration of tropical upwelling between 70–30 hPa and a southwardshift of the tropical pipe at 30 and 100 hPa during the past decade, whichappear to be the primary causes for the observed trend-change in ozone.
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