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

Abstract

<p><strong>Abstract.</strong> The stratospheric Brewer–Dobson circulation (BD-circulation) determines the transport and lifetime of key radiatively active trace gases and further impacts surface climate through downward coupling. Here, we quantify the variability in the lower stratospheric BD-circulation induced by the El Nino 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 deseasonalised ozone (O<sub>3</sub>) mixing ratios between CLaMS simulations and satellite observations, the patterns of changes in the lower stratospheric O<sub>3</sub> 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 O<sub>3</sub> anomaly in the record. Regression analysis of different metrics of the BD-circulation strength, including mean age of air, vertical velocity, residual circulation and age spectrum, shows clear evidence for structural changes of the BD-circulation in the lower stratosphere induced by El Niño, consistent with observed O<sub>3</sub> anomalies. These structural changes during El Niño include a weakening of the transition branch of the BD-circulation between about 370–420 K (∼ 100–70 hPa) and equatorward of about 60° and, a strengthening of the shallow branch at the same latitudes and between about 420–500 K (∼ 70–30 hPa). The strengthening of the shallow branch induces negative tropical O<sub>3</sub> 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 O<sub>3</sub> anomalies in the extratropical upper troposphere-lower stratosphere (UTLS). Our results suggest that a shift of the ENSO basic state toward more frequent El Niño-like conditions in a warming future climate will substantially alter UTLS trace gas distributions due to these changes in the vertical structure of the stratospheric circulation.</p>

Stratospheric Fluorine as a Tracer of Circulation Changes: Comparison Between Infrared Remote‐Sensing Observations and Simulations With Five Modern Reanalyses

“…The QBO itself is inherently unpredictable as it has, on the one hand, a variable period of ∼22 to ∼36 months (e.g., Osprey et al, 2016) and, on the other hand, an intrinsic variability as illustrated by the unprecedented QBO disruption in 2015-2016 (Diallo et al, 2018;Osprey et al, 2016;Tweedy et al, 2017). In addition to the interaction between the BDC and the QBO, the El Niño-Southern Oscillation (ENSO) is also at play and its alignment with QBO phases can lead to large stratospheric water vapor and O 3 anomalies (Diallo et al, 2018(Diallo et al, , 2019. Here follows a hypothesis based on the results of previous studies to explain the interhemispheric asymmetry building up in long-lived tracers and AoA from 2006-2007 to 2011-2012. As in earlier studies (see e.g., Figure 13 in Ploeger & Birner, 2016, Figure 4 in Diallo et al, 2018, or Figure 3 in Strahan et al, 2020, the 2010-2011 period stands out in Figure 8 as a period of particularly strong interhemispheric difference, whether in long-lived tracer or in AoA spectrum time series.…”

“…The QBO itself is inherently unpredictable as it has, on the one hand, a variable period of ∼22 to ∼36 months (e.g., Osprey et al, 2016) and, on the other hand, an intrinsic variability as illustrated by the unprecedented QBO disruption in 2015-2016 (Diallo et al, 2018;Osprey et al, 2016;Tweedy et al, 2017). In addition to the interaction between the BDC and the QBO, the El Niño-Southern Oscillation (ENSO) is also at play and its alignment with QBO phases can lead to large stratospheric water vapor and O 3 anomalies (Diallo et al, 2018(Diallo et al, , 2019. (Nedoluha et al, 2015).…”

Tropical Stratospheric Circulation and Ozone Coupled to Pacific Multi‐Decadal Variability

“…This low‐frequency signal due to changes in SSTs in the middle stratosphere in CESM1(WACCM) may arise from longer and sustained conditions in the background climate state associated with the position of the subtropical tropospheric jets (Palmeiro et al., 2014) and decadal variability in the QBO (see Figure ). The IPO signal in the tropical stratosphere is analogous to that of ENSO at shorter time scales (Diallo et al., 2019; Marsh & Garcia, 2007).…”

“…Inter‐annual variability in the BDC is coupled to modes of natural variability, including El Niño/Southern Oscillation (ENSO) (Calvo et al., 2010; Marsh & Garcia, 2007), the quasi‐biennial oscillation (QBO), and natural forcings (e.g., volcanic eruptions) (Abalos et al., 2015; Garfinkel et al., 2017). Indeed, sub‐decadal SST variability in the tropical Pacific Ocean associated with ENSO has been linked to interannual changes in lower stratospheric tropical upwelling (Diallo et al., 2019), with a small (0.8%) but significant influence extending to the tropical middle stratosphere (Marsh & Garcia, 2007). On multi‐decadal time scales, low frequency variability in the troposphere has been shown to impact the lower stratosphere and its circulation (Hu et al., 2018; Jadin et al., 2010), but the coupling to higher altitudes is less well understood, including any impacts on stratospheric composition.…”

Multi-timescale variations of modelled stratospheric water vapor derived from three modern reanalysis products

“…Among the additional sources of climate variability, ENSO and stratospheric volcanic aerosols have been shown to modulate both the tropical ascending branch of the BDC (e.g. Diallo et al, 2017Diallo et al, , 2018a and tropical tropopause temperatures (e.g.…”