Amanda Maycock & Piers Forster Impact of forcing agents on the width of the tropical belt in PDRMIP models Tom Wood pm11tw@leeds.ac.uk Supervised by: Amanda Maycock & Piers Forster

Introduction Tropical Expansion Expansion of the Hadley Cells ARID ZONE Expansion of the Hadley Cells into sub-tropics Intensification & poleward shift of hydrological cycle ‘LUSH’ ZONE

Magnitude and Rate of Expansion Observations: 0.25° – 3.00° decade-1 since 1979 (Davis & Rosenlof 2012) Satellite observations Radiosonde data Reanalysis Total expansion estimates over last 30-40 years range between 2° - 5° Could be up to 8° Reproduction in models tends to be slower than observations Models tend to estimate rate of expansion at around 0.1 – 0.2 ° decade-1 (eg. Hu & Fu, 2007) Closest statistically significant estimates: 1.0 – 1.5 ° decade-1 between 1979 – 2009 (Davis & Rosenlof 2012) Sign of change consistently in same direction i.e. +ve in NH and –ve in SH Hemispherically asymmetric Stronger in SH than NH (Davis et al, 2016) Varies with longitude Highly dependent on metric used to define tropical edge

Metrics used to define tropical edge Mean meridional streamfunction. Latitudes of the subtropical jet streams. Latitudes of the subtropical tropopause breaks / tropopause height Precipitation – evaporation (P - E) (especially poleward zero crossing point) Outgoing longwave radiation (OLR) Extratropical storm tracks Subtropical dry zones Total column ozone Subtropical ridges in sea level pressure Latitude of maximum downwelling in the Hadley cell. Width varies by more than 10° latitude depending on the metric used

Metrics used to define tropical edge Latitude of edge of Hadley Cells (∫Ψdp) (vertically averaged mean meridional streamfunction) Latitude of tropopause breaks (Δθ) Latitude of subtropical jet (Umax) Latitude of maximum downwelling in the Hadley Cell (∂y∫Ψdp) 500-hPa mean meridional streamfunction (Ψ500) (Grey contour indicates Ψ = 0) Latitude of zero zonal-mean surface wind (Usfc) Precipitation – Evaporation (P-E) Figure from Davis & Birner, JOC (2017)

Possible Drivers of Tropical Expansion Tropospheric warming in the Hadley cells Increases in GHG concentrations Stratospheric cooling Ozone stratospheric water vapour Mean SST and meridional temperature gradients Black carbon, aerosols & tropospheric ozone Especially in NH (Allen et al., 2012) Natural cycles e.g.: Pacific decadal oscillation (PDO) El Nino Southern Oscillation (ENSO) Southern annular mode (SAM) Recent studies (eg. Mantsis et al, 2017) have emphasised natural cycles as playing significant role Drivers likely to depend on the hemisphere in question

Current Work Change in poleward location of zonal mean P-E = 0 in Southern Hemisphere Responding to PDRMIP coupled idealised perturbation experiments: abrupt2xCO2 abrupt3xCH4 abrupt5xSul abrupt10xBC abruptSol Why edge of subtropical dry zone (P-E=0) metric? Location could have significant impacts on population living in and around dry zones Important metric in PDRMIP Relatively straightforward calculation Why Southern Hemisphere? Circulation more zonally symmetric than NH Less land than NH makes SH analysis more simple Response tends to be stronger in the SH than NH (Davis et al., 2016)

Current work 2xCO2 3xCH4 5xSul 10xBC 2%Sol P-E=0 MMM P-E=0 MMM 10-year running mean P-E=0 MMM GMST MMM 10-year running mean GMST MMM 5xSul 10xBC 2%Sol

Current Work CCCma tends to respond most strongly to forcings 2xCO2 and 2%Sol result in largest movement of P-E=0

Current Work CCCma has largest response ratio in 4/5 experiments CCCma appears to be highly sensitive to BC Response to CH4 most sensitive Response to BC has large inter-model spread CAM5 produces slight equatorward movement Sulphate perturbation has lowest sensitivity System more sensitive to expansion than contraction? Large inter-model spread result of model differences?

Some Open Questions Why does 5xSul have a lower sensitivity than others Contraction rather than expansion? Some characteristic of the forcing agent? What accounts for the large inter-model spreads? And why are the spreads different for different forcings? Why does BC perturbation result in contraction in CESM1-CAM5? What makes certain models so sensitive to certain perturbations? What is the regional variability?

Future Work Examine further aspects of the zonal-mean Hadley cell circulation Examine other metrics for tropical width including: Mean meridional streamfunction = 0 at 500 hPa (Ψ500) Latitude of mid-latitude eddy driven jet (u850) Examine responses of upper tropospheric metrics vs lower tropospheric metrics to different forcings Examine inter-model error Analyse precipitation and evaporation responses separately Analyse fixed SST experiments Fast adjustments Analyse the vertical atmospheric stability i.e. upper tropospheric temperature changes vs lower tropospheric temperature changes. These will respond differently depending on the forcing used Analyse results alongside maps of surface temperature response Some forcings produce more localised results than others Temperature response will vary by hemisphere Closely examine initial response to forcings i.e. first ~20 years after abrupt perturbation Inter-model differences between rate of response Differences between rate of response to different forcings Analyse signal to noise issues Examine seasonal responses