Observed Recent Changesin the Tropopause Dian Seidel NOAA Air Resources Laboratory ~ Silver Spring, Maryland USA Bill Randel NCAR Atmospheric Chemistry Division ~ Boulder, Colorado USA AGU Chapman Conference on the Role of the Stratosphere in Climate & Climate Change 24-28 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
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
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.
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.