Widening of tropical belt in a changing climate model simulations versus observations
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Widening of Tropical Belt in a Changing Climate: Model Simulations versus Observations. Qiang Fu Dept. of Atmos. Sciences University of Washington. Enhanced midlatitude warming in satellite measurements. Temperature trends 1979-2005. Fu, Johanson, Wallace and Reichler (2006, Science).

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Widening of Tropical Belt in a Changing Climate: Model Simulations versus Observations

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Widening of tropical belt in a changing climate model simulations versus observations

Widening of Tropical Belt in a Changing Climate: Model Simulations versus Observations

Qiang Fu

Dept. of Atmos. Sciences

University of Washington


Enhanced midlatitude warming in satellite measurements

Enhanced midlatitude warming in satellite measurements

Temperature trends 1979-2005

Fu, Johanson, Wallace and Reichler (2006, Science)


Changing meridional temperature gradients in the troposphere

Changing meridional temperature gradients in the troposphere

Temperature trends 1979-2005

  • Meridional temperature gradients

    • reduced (equatorward), weaker westerlies

    • increased (poleward), stronger westerlies

  • Total widening of Hadley cell ~2°

Fu, Johanson, Wallace and Reichler (2006, Science)


Enhanced midlatitude warming in satellite measurements1

Enhanced midlatitude warming in satellite measurements

Temperature trends 1979-2005 (JJA)

Fu, Johanson, Wallace and Reichler (2006, Science)


Implications of hadley cell widening

Implications of Hadley cell widening

Poleward movement of Hadley cell edges ~ poleward displacement of the subtropical dry zone boundaries

Changes in rainfall patterns at subtropical dry zone margins

May contribute to desertification of marginal lands


Hadley cell

Hadley cell

Meridional overturning circulation driven by differential heating of atmosphere

Transports energy and momentum poleward

Rising, moist air near Equator

Large scale sinking of dry, warm air ~30NS


Locating hadley cell edges

Locating Hadley cell edges

Stream function

Evaporation – Precipitation (E-P)

Hadley cells

OLR contours

Upper tropospheric jets


Locating hadley cell edges1

Locating Hadley cell edges

Days/year with height > 15 km

Total ozone (e.g., TOMS total ozone, 3/11/1990

Upper tropospheric jets


Observed hadley cell widening

Observed Hadley cell widening

Consensus: Hadley cell has widened by ~3° since 1979

Seidel, Fu, Randel, and Reichler (2008, Nature Geoscience)

Also see Hudson et al. (2006), Hu and Fu (2007), Seidel and Randel (2007), Archer and Caldeira (2008)


Mechanisms for hadley cell widening

Mechanisms for Hadley cell widening

Global warming: wider Hadley cell in warmer world

Held (2000):

Aquaplanet GCMs (Frierson et al. 2007)

Simulations of 21st century climate (Lu et al. 2007)

Warming of the Indo-West Pacific ocean (Lau et al. 2008)

Strengthening of polar stratospheric vortex(Polvani and Kushner 2002)

Tropospheric warming/stratospheric cooling

Altered wind shear across tropopause, increasing phase speed of baroclinic eddies(Chen and Held 2007)


Objective comparison of tropical widening between model simulations and observations

Objective: Comparison of tropical widening between model simulations and observations

Tools


Description of models used to investigate hadley cell widening

Description of models used to investigate Hadley cell widening

CMIP3 Coupled Model Intercomparison Project

AMIP2 Atmospheric Model Intercomparison Project

Forcings included in 20C simulations


Description of observations used to investigate hadley cell widening

Description of observations used to investigate Hadley cell widening

Latitude where stream function = 0 at 500 hPa from reanalysis

NCEP/NCAR

NCEP/DOE

ERA-40

Latitude where OLR = 250 W m-2 from satellite

HIRS

ISCCP

GEWEX

Hu and Fu (2007)


Results time series of hadley cell width

Results: time series of Hadley cell width

Interannual timescales: variability similar in observations and individual models

Hadley cell width 1979-2005

Observations: increase

20th century simulations: no change

Stream function

OLR

Johanson and Fu (2008, submitted)

  • Hadley cell width 2000-2099

    • A1B: increase

    • By 2099, Hadley cell much wider than pre-industrial control


Observed widening not explained by natural variability

Observed widening not explained by natural variability

Mean widening

Multi-dataset ensemble mean trends from observations

Ensemble mean trends from samples of 25-yr time periods in PIC

Range of widening

Trend distribution from observational datasets

Trend distribution from samples of 25-yr time periods in PIC

Observed

3° on average

Range 2.3-4.5°

Pre-industrial control (PIC)

No tenancy to shift Hadley cell towards wider or narrower state

Widening occurs as often as narrowing

Hadley cell width variation within ±1.5°

Mean Hadley cell widening

Distributions of Hadley cell widening


20 th century simulations do not reproduce observations

20th century simulations do not reproduce observations

Mean widening

Multi-dataset ensemble mean trends from observations and 20C-A1B from 1979-2005

Range of widening

Trend distributions from observational datasets

Trend distributions from 20C-A1B ensemble members

Widening in 20C-A1B from 1979-2005

Less than 1/5th observed values

At least 97.5% of the 20C-A1B trends from ensemble members show less widening than observed

Mean Hadley cell widening

Distributions of Hadley cell widening


Observed widening not explained by pattern of sst warming

Observed widening not explained by pattern of SST warming

Mean widening

Multi-dataset ensemble mean trends from observations and AMIP2 from 1979-1999

Range of widening

Trend distributions from observational datasets

Trend distributions from AMIP2 ensemble members from 1979-model end year

Widening in AMIP2

less than 1/4 observed values

Only 2 of 57 realizations had widening larger than 3º

Next largest trend less than 2º

Mean Hadley cell widening

Distributions of Hadley cell widening


Observed widening not explained by global warming

Observed widening not explained by global warming

1. Hu and Fu (2007)

2. Johanson and Fu (2008)

3. Lu et al. (2007)

4. Frierson et al. (2007)

Rate of widening per degree of global warming is ~10 times larger in observations than models

Global warming alone can't explain recent widening as simulated by GCMs


Observed widening not explained by stratospheric ozone depletion

Observed widening not explained by stratospheric ozone depletion

Strengthening of polar stratospheric jet → wider Hadley cell(Polvani and Kushner 2002)

Strengthening of S. Hem jet in recent decades largely attributable to ozone depletion(Thompson and Solomon 2002, Science; Gillett and Thompson 2003, Science)

No consensus among model simulations of 20th century


Gcm results with interactive chemistry and good representation of stratospheric dynamics

GCM results with interactive chemistry and good representation of stratospheric dynamics

Lamarque & Solomon (2008)


Conclusions

Conclusions

Observed widening of about 3º since 1979

Not due to natural variability

Cannot be explained by historical changes in forcings as simulated by GCMs

Cannot be explained by pattern of SST warming

Occurs at a faster rate than in projections of future widening

Further investigation required to resolve discrepancy between observations and GCM simulations


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