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Observed Tropical Expansion: Impact on the Hydrological and Energy Cycles
New Investigator Research Summary
Joel Norris (Lead PI, UC San Diego)Robert Allen (Co-PI, UC Riverside)
Mahesh Kovilakam (postdoc, UC Riverside)
NEWS Science Team MeetingGoddard Space Flight Center
May 1, 2013
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Motivation/Background
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CMIP3 Ensemble Mean Projections
FromLu et al. (2007)
Climate models project expansion of subtropical dry zone
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CMIP3 vs. Observed Tropical Expansion
from Johanson and Fu (2009)
Observed tropical expansion greatly exceeds that for prescribed SST simulations (up to 1999) and CO2 warming projections
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Results So Far
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CMIP5 vs. Observed Tropical Expansion
Observed tropical expansion greatly exceeds that for CMIP5 coupled simulations of 1979-2009Prescribed SST simulations (up to 2009) projections come closest
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CMIP5 vs. Observed SST Trends
Observed SST trend pattern for 1979-2009 resembles the negative phase of the Pacific Decadal Oscillation (PDO)Ensemble mean CMIP5 coupled SST trend pattern for 1979-2009 is much more spatially uniform
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Tropical Expansion and PDO
Jet latitude variations from prescribed SST runs resemble observed −PDO−PDO is a good predictor of variability in JET latitudeJet latitude trends are correlated with PDO trends in 20th century coupled simulations
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1950-1979 vs. 1979-2009 SST Trends
1950-1979 and 1979-2009 SST trends have opposite sign in many regions
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1950-1979 Tropical Contraction and PDO
Jet latitude variations from prescribed SST runs resemble observed −PDO−PDO is a good predictor of variability in JET latitudeJet latitude trends are correlated with PDO trends in 20th century coupled simulations
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1950-1979 vs. 1979-2009 PDO Index
PDO trend has opposite sign in 1950-1979 from 1979-2009 in 20th century coupled simulations with all forcingsTrend sign reversal also occurs in 20th century simulations with only aerosol forcings
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SST Response to Aerosol Radiative Forcing
Simulated 1979-2005 surface temperature trends in response to historical aerosol radiative heating
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Summary
• Subtropical dry zone expanded during 1979-2009• Greatly underestimated by coupled model ensemble mean• Strong association between expansion and −PDO• Could have substantial natural variability component• Could have substantial contribution from aerosol radiative forcing
Implications for NEWS• Does regional aerosol radiative forcing drive circulation, SST, and
precipitation patterns on decadal time scales?• Can we project aerosol radiative forcing changes in coming
decades and likely circulation, SST, and precipitation response?