Structural changes in the shallow and transition branch of the Brewer–Dobson circulation induced by El Niño

Abstract

Abstract. The stratospheric Brewer–Dobson circulation (BDC) determines the transport and atmospheric lifetime of key radiatively active trace gases and further impacts surface climate through downward coupling. Here, we quantify the variability in the lower stratospheric BDC induced by the El Niño–Southern Oscillation (ENSO), using satellite trace gas measurements and simulations with the Lagrangian chemistry transport model, CLaMS, driven by ERA-Interim and JRA-55 reanalyses. We show that despite discrepancies in the deseasonalized ozone (O3) mixing ratios between CLaMS simulations and satellite observations, the patterns of changes in the lower stratospheric O3 anomalies induced by ENSO agree remarkably well over the 2005–2016 period. Particularly during the most recent El Niño in 2015–2016, both satellite observations and CLaMS simulations show the largest negative tropical O3 anomaly in the record. Regression analysis of different metrics of the BDC strength, including mean age of air, vertical velocity, residual circulation, and age spectrum, shows clear evidence of structural changes in the BDC in the lower stratosphere induced by El Niño, consistent with observed O3 anomalies. These structural changes during El Niño include a weakening of the transition branch of the BDC between about 370 and 420 K (∼100–70 hPa) and equatorward of about 60∘ and a strengthening of the shallow branch at the same latitudes and between about 420 and 500 K (∼70–30 hPa). The slowdown of the transition branch is due to an upward shift in the dissipation height of the large-scale and gravity waves, while the strengthening of the shallow branch results mainly from enhanced gravity wave breaking in the tropics–subtropics combined with enhanced planetary wave breaking at high latitudes. The strengthening of the shallow branch induces negative tropical O3 anomalies due to enhanced tropical upwelling, while the weakening of the transition branch combined with enhanced downwelling due to the strengthening shallow branch leads to positive O3 anomalies in the extratropical upper troposphere–lower stratosphere (UTLS). Our results suggest that a shift in the ENSO basic state toward more frequent El Niño-like conditions in a warmer future climate will substantially alter UTLS trace gas distributions due to these changes in the vertical structure of the stratospheric circulation.

Large Anomalies in the Tropical Upper Troposphere Lower Stratosphere (UTLS) Trace Gases Observed during the Extreme 2015–16 El Niño Event by Using Satellite Measurements

“…However, the observed high decrease of O 3 in the present study is in July and August that belong to boreal summer. These results matched well with those reported by Tweedy et al [17] and Diallo et al [11]. They clearly demonstrated that the changes in the O 3 anomalies in NH are due to the meridional advection in northern subtropics altered by boreal summer ENSO events and in SH due to the tropical upwelling [17].…”

“…Recent 2015-16 El Niño was one of the strongest and long lasted events in the 21st century. Lots of unusual changes happened in the atmosphere in 2015-16 El Niño event and were well reported by several studies [7][8][9]11,25,26].…”

“…They clearly demonstrated that the changes in the O 3 anomalies in NH are due to the meridional advection in northern subtropics altered by boreal summer ENSO events and in SH due to the tropical upwelling [17]. The structural changes in the BDC due to 2015-16 El Niño event is also one of the reasons for the O 3 anomalies observed in the tropical LS during this event [11]. Overall, instead of chemical reactions, the transport processes (due to BDC) are the major possible reason for the presently observed O 3 anomalies.…”

“…This decrease is very high from September to December 2015 (peaking in November) with large negative anomalies and continued in 2016 with less magnitude. This might be due to the strengthening of the Brewer Dobson circulation (BDC), that is, strong enhancement of tropical upwelling during El Niño period [9][10][11]. Previous studies clearly demonstrated the impact of the ENSO on the inter-annual variability of O 3 at the tropopause level [32].…”

“…The negative zonal mean O 3 and temperature anomalies in the LS are also evident during the El Niño period. These anomalies are attributed to the strengthening tropical upwelling of the BDC in El Niño period [10,11]. Due to the warm, high troposphere temperatures in El Niño period, drought conditions generally observed over South East Asia (SEA) region with lot of biomass burning and forest fires [12][13][14].…”