On the structural changes in the Brewer-Dobson circulation after 2000

Abstract

Abstract. In this paper we present evidence that the observed increase in tropical upwelling after the year 2000 may be attributed to a change in the Brewer-Dobson circulation pattern. For this purpose, we use the concept of transit times derived from residual circulation trajectories and different in-situ measurements of ozone and nitrous dioxide. Observations from the Canadian midlatitude ozone profile record, probability density functions of in-situ N 2 O observations and a shift of the N 2 O-O 3 correlation slopes, taken together, indicate that the increased upwelling in the tropics after the year 2000 appears to have triggered an intensification of tracer transport from the tropics into the extratropics in the lower stratosphere below about 500 K. This finding is corroborated by the fact that transit times along the shallow branch of the residual circulation into the LMS have decreased for the same time period (1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003). On a longer time scale , the transit time of the shallow residual circulation branch show a steady decrease of about −1 month/decade over the last 30 yr, while the transit times of the deep branch remain unchanged. This highlights that changes in the upwelling across the tropical tropopause are not sufficient as an indicator for changes in the entire BrewerDobson circulation.

Shift of subtropical transport barriers explains observed hemispheric asymmetry of decadal trends of age of air

“…For both data sets, the AoA changes from 2002 to 2012 in the lowermost stratosphere below 380 K were found to be negative equatorwards of 40 • in both hemispheres, but slightly positive polewards of 40 • and between 380 and 420 K for the lower latitudes (Ploeger et al, 2015b). This difference between trends in low and high latitudes suggests a decoupling of the deep and the shallow branch of the BDC, consistent with the general picture presented in Birner and and Bönisch et al (2011). Alternative explanations point to the importance of mixing processes (Ray et al, 2010;Garny et al, 2014;Ploeger et al, 2015b).…”

Airborne limb-imaging measurements of temperature, HNO<sub>3</sub>, O<sub>3</sub>, ClONO<sub>2</sub>, H<sub>2</sub>O and CFC-12 during the Arctic winter 2015/2016: characterization, in situ validation and comparison to Aura/MLS

“…posphere by isentropic mixing, by convective overshooting and by mixing with the troposphere (Holton et al, 1995;Bönisch et al, 2011). The UTLS is challenging to observe.…”

A Lagrangian description on the troposphere-to-stratosphere transport changes associated with the stratospheric water drop around the year 2000

“…The cause and mechanism of this stepwise change have been extensively discussed (e.g., Randel et al, 2006;Rosenlof and Reid, 2008;Bönisch et al, 2011;Fueglistaler, 2012;Fueglistaler et al, 2014;Dessler et al, 2014). While constructing a reliable long-term SWV record is still a challenge (Hegglin et al, 2014), the understanding of a possible stepwise change in SWV is required in assessing possible modulation of the Brewer-Dobson circulation under global warming.…”