Variability in the speed of the Brewer–Dobson circulation as observed by Aura/MLS

Abstract

Abstract. We use Aura/MLS stratospheric water vapour (H2O) measurements as tracer for dynamics and infer interannual variations in the speed of the Brewer–Dobson circulation (BDC) from 2004 to 2011. We correlate one-year time series of H2O in the lower stratosphere at two subsequent pressure levels (68 hPa, ~18.8 km and 56 hPa, ~19.9 km at the Equator) and determine the time lag for best correlation. The same calculation is made on the horizontal on the 100 hPa (~16.6 km) level by correlating the H2O time series at the Equator with the ones at 40° N and 40° S. From these lag coefficients we derive the vertical and horizontal speeds of the BDC in the tropics and extra-tropics, respectively. We observe a clear interannual variability of the vertical and horizontal branch. The variability reflects signatures of the Quasi Biennial Oscillation (QBO). Our measurements confirm the QBO meridional circulation anomalies and show that the speed variations in the two branches of the BDC are out of phase and fairly well anti-correlated. Maximum ascent rates are found during the QBO easterly phase. We also find that transport of H2O towards the Northern Hemisphere (NH) is on the average two times faster than to the Southern Hemisphere (SH) with a mean speed of 1.15 m s−1 at 100 hPa. Furthermore, the speed towards the NH shows much more interannual variability with an amplitude of about 21% whilst the speed towards the SH varies by only 10%. An amplitude of 21% is also observed in the variability of the ascent rate at the Equator which is on the average 0.2 mm s−1.

Quantifying contributions to the recent temperature variability in the tropical tropopause layer

“…The QBO is closely related to the tropical upwelling Flury et al (2013). A regression of temperature differences onto the differences in the vertical component of BDC between the Natural and NOQBO run shows a very similar result than the regression of temperature differences onto the QBO time series (not shown).…”

“…The temperature in the TTL is determined by the combined influences of latent heat release, thermally as well as dynamically driven vertical motion and radiative cooling (Gettelman and Forster, 2002;Fueglistaler et al, 2009;Grise and Thompson, 2013). The thermal structure, static stability and zonal winds in the TTL affect the two-way interaction between the troposphere and the stratosphere (Flury et al, 2013;Simpson et al, 2009) as well as the surface climate, since the relative minimum temperature (usually known as the cold point tropopause) subsequently influences the radiation and water vapour budget (Andrews, 2010). The TTL reacts particularly sensitively to anthropogenically induced radiative, chemical and dynamical forc-…”

Vertical Profiles and Temporal Variations of Greenhouse Gases in the Stratosphere over Syowa Station, Antarctica

“…However, their results showed that the difference in CO 2 mole fraction between the middle and lower stratosphere was about 7−8 ppm, which is twice as large as our finding obtained in the Antarctic stratosphere. Such a latitude-dependent vertical CO 2 difference could be mainly related with the effects (1) that the mole fraction of upper tropospheric CO 2 is higher in the northern hemisphere than in the southern hemisphere on average (Matsueda et al 2015) and the tropospheric and stratospheric air exchange occurs through various processes, such as blockings, cut-off cyclones, and tropopause folds (e.g., Holton et al 1995), and (2) that the tropospheric air with high CO 2 mole fractions, intruded into the stratosphere in the tropics, has a stronger influence on lower stratospheric CO 2 in northern midlatitudes than in Antarctica, since its poleward transport through the lower stratosphere is faster in the northern hemisphere than in the southern hemisphere (Flury et al 2013). …”

Enhanced stratospheric water vapor over the summertime continental United States and the role of overshooting convection

“…Poleward of the subtropical jet, water may be transported into the lowermost stratosphere through isentropic troposphere-stratosphere exchange (Holton et al, 1995) or through convective overshoot of the local tropopause Hanisco et al, 2007;Liu at el., 2008;Liu and Liu, 2016). Isentropic transport from the tropics is the dominant pathway for water into the lowermost stratosphere, with evidence from the seasonal cycle of lower stratospheric water (e.g., Flury et al, 2013). How important the sublimation of ice from convective overshoot is for hydrating the stratosphere is a topic of ongoing debate (e.g., Randel et al, 2015;Wang, 2003).…”