Observed Recent Changesin the Tropopause

Dian SeidelNOAA Air Resources Laboratory ~ Silver Spring, Maryland USA

Bill RandelNCAR Atmospheric Chemistry Division ~ Boulder, Colorado USA

AGU Chapman Conference on the Role of the Stratosphere in Climate & Climate Change24-28 September 2007 ~ Santorini, Greece

Motivation• Tropical tropopause changes may relate to

– Stratospheric water vapor changes– Changes in stratospheric and/or tropospheric dynamics

• Global tropopause changes may relate to– Anthropogenic (radiatively forced) climate change– Vertical temperature profile changes

• Evolving understanding of – Thermal tropopause– Multiple tropopauses– Tropopause layers

Observations• Daily radiosoundings from 100 stations• NCEP/NCAR reanalysis (and ERA 40)• GPS Met

Radiosonde Stations

Climatological Tropopauses

Inhomogeneity in Tropopause Times Series

Zonal Tropopause Trends 1980-2004

Global Tropopause Trends 1980-2004

• +64 ± 21 m/decade• -1.7 ± 0.6 hPa/decade• -0.41 ± 0.09 K/decade

• 160 m rise• 4.2 hPa drop• 1.0 K cooling

Short-term Co-Variability of Tropopause Height and Atmospheric Temperature

Tropopause Height and Temperature Trends

Bimodal Tropopause Climatology in Subtropics

Bimodal Tropopause in Radiosonde Data

Bimodal Tropopause In Reanalysis Data

Double Tropopauses in GPS Data

How often is the tropopause high?

Tropopause in Subtropics More Frequently Tropical

Tropopause in Subtropics More Frequently Tropical

Tropopause in Subtropics More Frequently Tropical

Increases in High Tropopause Frequency

Measuring the Width of the Tropics

Expansion of the Tropical Belt

Corroboration in Other Observations

Issues and Implications of a Wider Tropical Belt

• Observed rate exceeds model projections. Why?

• Seasonal, regional structure of changes have not been studied.

• Related changes in – Hydrologic cycle, esp. subtropical dry zones– Jet streams and storm tracks– Tropical weather systems– Brewer-Dobson circulation

Summary• The global tropopause has risen and cooled since 1980.

– Data issues pose problems estimating trends further back.– Short time scale changes assoc. with stratospheric and

tropospheric T changes. Trends only assoc. with strat. T trends.• Tropopause heights are bimodally distributed in

subtropics– Region is also where double tropopauses occur– Indicates ‘edge’ of tropical belt– Robust metric for trend detection• Tropical belt has expanded 2-8 deg lat in past 25 yrs– Evidence from tropopause and other, independent observations– Many open questions about processes, implications

References Hu, Y., and Q. Fu: Observed poleward expansion of the Hadley circulation

since 1979. Atmos. Chem. Phys. Disc., submitted. Hudson R. D., M. F. Andrade, M. B. Follette, and A. D. Frolov, 2006: The total

ozone field separated into meteorological regimes – Part II: Northern Hemisphere mid-latitude total ozone trends. Atmos. Chem. Phys. 6, 5183-5191.

Randel, W. J., D. J. Seidel, and L. L. Pan, 2007: Observational characteristics of double tropopauses. J. Geophys. Res., 112, D07309, doi:10.1029/2006JD007904.

Reichler, T., and I. Held: Widening trend of the Hadley cell over the past 40 years. American Meteorological Society, Conference on Climate Variability and Change (Cambridge, MA, 2005).

Seidel, D.J., and W.J. Randel, 2007: Recent widening of the tropical belt: Evidence from tropopause observations. J. Geophys. Res., in press.

Seidel, D.J., and W.J. Randel, 2006: Variability and trends in the global tropopause estimated from radiosonde data, J. Geophys. Res., 111, D21101, doi:10.1029/2006JD007363.

Thank you

Ευχρισώ

Example of Tropopauses in Sounding Data

WMO (1957) Lapse Rate Tropopause Definition

• The first tropopause is defined as the lowest level at which the lapse rate decreases to 2°C/km or less, provided also the average lapse rate between this level and all higher levels within 2 km does not exceed 2°C/km.

• If above the first tropopause the average lapse rate between any level and all higher levels within 1 km exceeds 3°C/km, then a second tropopause is defined by the same criterion as under (a). This tropopause may be either within or above the 1 km layer.