On the hiatus in the acceleration of tropical upwelling since the beginning of the 21st century

Abstract

Abstract. Chemistry-climate models predict an acceleration of the upwelling branch of the Brewer-Dobson circulation as a consequence of increasing global surface temperatures, resulting from elevated levels of atmospheric greenhouse gases. The observed decrease of ozone in the tropical lower stratosphere during the last decades of the 20th century is consistent with the anticipated acceleration of upwelling. However, more recent satellite observations of ozone reveal that this decrease has unexpectedly stopped in the first decade of the 21st century, challenging the implicit assumption of a continuous acceleration of tropical upwelling. In this study we use three decades of chemistry-transport-model simulations to investigate this phenomenon and resolve this apparent contradiction. Aside from a high-bias between 1985-1990, our model is able to reproduce the observed tropical lower stratosphere ozone record. A regression analysis identifies a significant decrease in the early period followed by a statistically robust trend-change after 2002, in qualitative agreement with the observations. We demonstrate that this trend-change is correlated with structural changes in the vertical transport, represented in the model by diabatic heating rates taken from the reanalysis product Era-Interim. These changes lead to a hiatus in the acceleration of tropical upwelling between 70-30 hPa and a southward shift of the tropical pipe at 30 and 100 hPa during the past decade, which appear to be the primary causes for the observed trendchange in ozone.

Quantifying the effects of mixing and residual circulation on trends of stratospheric mean age of air

“…As shown in Figure , the strong hemispheric asymmetry with decreasing age in the SH and increasing age in the NH between 2002 and 2012 is clearly reflected in the integrated effect of mixing. Only below about 20 km the net mean age decrease coincides with decreasing RCTT, indicating an acceleration of the residual circulation in the lowest part of the stratosphere during 2002–2012, in agreement with Aschmann et al []. Remarkably, for the 1990–2013 period the residual circulation shows acceleration (negative RCTT trend) extending to higher altitudes (up to ∼26 km, Figure d), compared to the 2002–2012 period.…”

Inter-comparison of stratospheric mean-meridional circulation and eddy mixing among six reanalysis data sets

“…The trend is negative only in ERA-Interim at both altitudes (−3.9 % decade −1 at 440 K and −6.7 % decade −1 at 560 K). The estimated trends are statistically significant for the new data sets both at 440 and 560 K. Several recent studies have pointed out that AoA and BDC changes occur over timescales of several years to decades (Aschmann et al, 2014;Mahieu et al, 2014;Ray et al, 2014;Abalos et al, 2015;Poelger et al, 2015b). As summarized in Table 5, the upward mass flux at 440 K shows a strengthening trend during the first 22 years (1979-2000, +1.3 to +9.6 % decade −1 ), but a weakening trend (or slowing strengthening trend) during the last 12 years (2001-2012, −6.3 to +0.9 % decade −1 ) at 440 K in all data sets except for ERA-Interim.…”

The upward branch of the Brewer‐Dobson circulation quantified by tropical stratospheric water vapor and carbon monoxide measurements from the Aura Microwave Limb Sounder

“…On the other hand, Aschmann et al . [] analyzed chemistry‐transport model simulations for the period between 1980 and 2013 and found that strengthening in tropical upwelling between 70 and 30 hPa ceased around 2002 and actually weakened between 2002 and 2013. Their results could partly explain the apparent inconsistency between our implied decreasing trend for the period of 2004–2014 and any increasing trends derived from longer‐term reanalysis data sets.…”