Future trends in stratosphere-to-troposphere transport in CCMI models

Abstract

Abstract. One of the key questions in the air quality and climate sciences is how tropospheric ozone concentrations will change in the future. This will depend on two factors: changes in stratosphere-to-troposphere transport (STT) and changes in tropospheric chemistry. Here we aim to identify robust changes in STT using simulations from the Chemistry Climate Model Initiative (CCMI) under a common climate change scenario (RCP6.0). We use two idealized stratospheric tracers to isolate changes in transport: stratospheric ozone (O3S), which is exactly like ozone but has no chemical sources in the troposphere, and st80, a passive tracer with fixed volume mixing ratio in the stratosphere. We find a robust increase in the tropospheric columns of these two tracers across the models. In particular, stratospheric ozone in the troposphere is projected to increase 10 %–16 % by the end of the 21st century in the RCP6.0 scenario. Future STT is enhanced in the subtropics due to the strengthening of the shallow branch of the Brewer–Dobson circulation (BDC) in the lower stratosphere and of the upper part of the Hadley cell in the upper troposphere. The acceleration of the deep branch of the BDC in the Northern Hemisphere (NH) and changes in eddy transport contribute to increased STT at high latitudes. These STT trends are caused by greenhouse gas (GHG) increases, while phasing out of ozone-depleting substances (ODS) does not lead to robust transport changes. Nevertheless, the decline of ODS increases the reservoir of ozone in the lower stratosphere, which results in enhanced STT of O3S at middle and high latitudes. A higher emission scenario (RCP8.5) produces stronger STT trends, with increases in tropospheric column O3S more than 3 times larger than those in the RCP6.0 scenario by the end of the 21st century.

GISS Model E2.2: A Climate Model Optimized for the Middle Atmosphere—2. Validation of Large‐Scale Transport and Evaluation of Climate Response

“…This weaker response at high latitudes reflects the fact that the extra‐tropical transport properties are not solely controlled by changes in tropical advection but also by mixing between the tropics and extra‐tropics, with enhanced mixing resulting in older mean ages over high latitudes (Neu & Plumb, 1999). In addition, as shown in Abalos et al (2019) the high‐latitude changes in lower stratospheric e90 are to a large extent tethered to changes in tropopause height, which rises in response to increased CO 2 , as discussed further in the next section. Therefore, both changes in extra‐tropical mixing and tropopause height likely modulate the amplitude of the high‐latitude e90 response to increased CO 2 .…”

“…The residual mean circulation (Ψ ∗ ), here used to describe the advective component of the Brewer‐Dobson circulation, accelerates throughout the stratosphere as CO 2 is increased (Figure 8, top), a change that is among the more robust dynamical responses in models (Butchart & Scaife, 2001; Butchart et al, 2010; Garcia & Randel, 2008; Hardiman et al, 2014; Li et al, 2008; Oberländer et al, 2013; Rind et al, 1998, 2002; Sigmond et al, 2004). While in more realistic climate change scenarios changes in ODS may significantly weaken this projected acceleration over the 21st century (Abalos et al, 2019; Polvani et al, 2018), it is nonetheless important that the underlying CO 2 ‐induced response of the Brewer‐Dobson circulation (BDC) be rigorously understood.…”

“…Our discussion of the climate change response in the stratospheric and tropospheric transport circulations is in the context of the abrupt CO 2 forcing simulations which, despite their simplicity, afford a mechanistic look into the response characteristics of models that can be unambiguously attributed to an increase in CO 2 concentrations. This simple forcing lens is especially important for understanding the large‐scale transport response to climate change, which has been relatively unexplored in previous studies that have either focused almost exclusively on “dynamical sensitivity” (Chemke & Polvani, 2019; Grise & Polvani, 2016; Menzel et al, 2019) or on the large‐scale transport circulation response to changes in both CO 2 (and other greenhouse gases [GHGs]) and ozone‐depleting substances (ODS) (Abalos et al, 2019; Doherty et al, 2017). The CO 2 ‐induced response of the large‐scale transport circulation therefore remains poorly understood.…”

Twenty‐First Century Trends in Mixing Barriers and Eddy Transport in the Lower Stratosphere

“…In particular, the effect of strengthened transport barriers does not translate into reduced two‐way mixing because it is compensated by the concomitant strengthening of PV gradients. Consistently, annual mean isentropic eddy transport in the lower stratosphere increases for tracers of tropospheric and stratospheric origin (Abalos et al, 2017, 2020). The trends in eddy tracer and PV fluxes show the same strong seasonality, not found in the κ eff trends (cf.…”

On the Changing Role of the Stratosphere on the Tropospheric Ozone Budget: 1979–2010

“…Horizontal wind components (u and v) and potential temperature (θ) are nudged (following (Telford et al, )) to ERA‐Interim Reanalysis data (Dee et al, ). The meteorological reanalysis includes dynamical effects of stratospheric ozone depletion and increased greenhouse gas concentration, both of which enhance the speed of the Brewer‐Dobson circulation (Abalos, Orbe, et al, ).…”