What is the extratropical tropopause and how might it change in the future? Peter Haynes, University of Cambridge. Introduction Simple models for the extratropical tropopause Transport structure of the extratropical tropopause Future changes in the tropopause The Tropopause Inversion Layer

UTLS Ozone schematic Tropopause is boundary between different dynamical regimes and boundary between air masses -- alternative definitions (but also compatible since PV is a dynamical tracer) -- dynamics and transport are closely coupled. Persistence of air mass contrast implies the tropopause is a (semi- permeable) transport barrier. Communication with tropics is important.

What is the tropopause and what sets its position? Radiative-equilibrium temperature profile is unstable near the ground, hence dynamical adjustment to temperature at low levels. Upper limit to this adjustment is the tropopause. Mechanisms for dynamical adjustment? Tropics: moist convection Extratropics: baroclinic eddies (Held 1982), perhaps significantly modified by moist dynamics (Juckes 2000), or moist slantwise convection (Emanuel 2002)? Radiative constraint + dynamical constraint determine tropospheric lapse rate + tropopause height (Held 1982)

troposphere/stratosphere transport schematic Tropics: vertical transport/mixing associated with convection Extratropics: quasi-isentropic transport/mixing associated with baroclinic eddies

Multi-level quasi-geostrophic flow Continuously stratified quasi-geostrophic beta-plane flow Thermal relaxation (K_T) towards temperature profile with consistent with broad jet- like flow with vertical shear, hence baroclinically unstable Rigid upper and lower boundaries, Ekman friction (K_M) at lower boundary only Greenslade and H 2008

Results from two-layer simulation Upper layer (central transport barrier) Lower layer (mixing across broad central region)

Results from multi-layer simulation

Transport and mixing structure in multi- level flow Transition between upper level and lower level transport regimes PV transport barrier: sharp contrast in PV could be consequence of central barrier + neighbouring mixing regions or cause of central barrier by shaping coherent jet

Transition height is determined by internal dynamics, given external parameters Variation of transition height

PV on isentropic surfaces A simple dynamical model of the extratropical tropopause (H, Scinocca and Greenslade 2001) Can a realistic tropopause structure be realised by large-scale dynamics of baroclinic eddies alone? Height-latitude structure of PV

Effective diffusivity A simple dynamical model of the extratropical tropopause (H, Scinocca and Greenslade 2001) Height-latitude structure of transport and mixing

PE ‘real-atmosphere’ structure QG structure Key quantities: transition height + latitude of central barrier

Schneider (2004): tropopause defined in terms of meridonal mass circulation (closely related to eddy fluxes of PV) (Held and Schneider 1999)

HSG2001, GH2008: baroclinic eddies determine latitudinal barrier and transition height. (Not a theory.) Schneider (2004): predictive theory based on baroclinic eddy PV and surface temperature fluxes. Emanuel (2002): troposphere is neutral to moist slantwise convection. Juckes (2000): moist neutrality in mature cyclones. Frierson (2008): important role for moist dynamics (but is this equivalent to Juckes?) Current state of theory for extratropical tropopause Schneider (2004)

The extratropical tropopause visualised by effective diffusivity (H and Shuckburgh 2000) (but motion in extratropical troposphere is not exactly isentropic)

NH winter NH summer Proportion of back trajectories that visit boundary layer in 30 days (Berthet et al 2007) PV=2? Note implied transition layer

Proportion of back trajectories that visit 10N in 30 days NH winter NH summer (Berthet et al 2007)

Liu PhD thesis (2009) Proportion of back trajectories that visit boundary layer in 30 days

Berthet et al 2007 Surface origin of air that enters stratosphere Entry to 305K surfaceEntry to 335K surfaceEntry to 365K surface

(lapse-rate) tropopause as fingerprint of climate change? (Santer el 2003)

(Santer el 2003)

(Seidel et al 2007) Increase in width of tropics?

Changing width of Hadley circulation in 2 GCMs Frierson et al (2007)

Tropopause Inversion Layer How does it form? (What exactly is it?) What does it imply about transport and mixing?

Son and Polvani (2007) Model containing only large- scale dynamics can capture a layer of enhanced static stability above the tropopause

PV structure of TIL (Birner 2006) Standard averaging TP averaging Enhanced PV contrast along isentropic surfaces would enhance tropopause transport barrier

Questions How realistic could extratropical tropopause structure be in ‘large-scale dynamics only’ world? Will PV transition across tropopause become sharper and sharper? Does large-scale dynamics provide overall structure with important modifications from small-scale processes (e.g. convection, GWs)? Prediction of future change in tropopause structure requires confidence about mechanism Does transition layer in transport time from BL correspond to observed transition layer in chemical species What is mechanism for formation of TIL -- is ‘large-scale’ formation convincing and how does it work. What are implications of TIL for transport -- if corresponds to genuine increase in PV jump then could affect transport by baroclinic eddies, if artifact of averaging with respect to thermal tropopause then could affect short-time transport, e.g. convective penetration or GWs What is active radiative role of chemical distributions near tropopause -- in formation of TIL and in climate more generally?

Summary Transport criteria for extratropical tropopause New criterion based on ‘time from boundary layer’ Identification of changes in extratropical tropopause depend on criterion used Changes in extratropical tropopause manifested both in height (at midlatitudes) and in subtropical transition (links to many other aspects of tropospheric general circulation)

WMO 2006 Chapter 2 ExTL: extratropical tropopause layer

(Berthet et al 2007) Proportion of back trajectories that visit boundary layer in 30 days