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Steven Feldstein
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Transcript of Steven Feldstein
The link between tropical convection and the Northern Hemisphere Arctic surface air temperature change on intraseaonal
and interdecadal time scales
Steven Feldstein
The Pennsylvania State University
September 11, 2012
Collaborators: Sukyoung Lee, Tingting Gong, Nat Johnson,, Changhyun Yoo, David Pollard, and Tim White
NOAA Global Historical Climatology Network Gridded surface air temperature
Source: JISAO, University of Washington
Warming trend is greater toward higher latitudes(1) Present-day observations
Source: Hoffert & Covey (1992, Nature)
Barron & Washington reconstruction data (1985, AGU)
Warmer climate hasa smaller equator-to-pole temperature gradient
(2) Past climates (from reconstruction)
Present day
Multi-model mean changes in surface air temperature (°C, left) and precipitation (mm/day) A1B Scenario, 2080-2090 relative to 1980-1999
IPCC (2007)
(3) Future projections (from models)
“Advances in climate modeling and reconstruction of paleoclimates from proxy data have resolved some controversies but have underscored areas
where understanding of greenhouse climates remains imperfect. Latitudinal temperature gradients are particularly problematic.”
from Warm Climates in Earth History (Huber et al. 2002)
surplus solarterrestrial
deficit
deficit Poleward heat flux
Poleward heat flux
Baroclinic eddy heat flux = -Diffusivity x mean gradient
What other mechanisms?
• CO2 warming causes tropical convective heating to be more localized • Convective heating excites Rossby waves• Rossby waves propagate poleward, transporting westerly momentum
equatorward: eastward acceleration in tropics• Atmosphere adjusts toward a balanced state• Poleward heat transport by the wavesEastward acceleration
Proposed mechanism for polar amplification:Tropical Rossby wave source, poleward wave propagation, &
high-latitude adiabatic warming
pole
heig
ht
equator
X
westward acceleration
Adiabatic warming
Poleward Rossby propagation
Inspiration of the hypothesis: Rossby waves excited by a tropical heat source cause equatorial superrotation [Saravanan 1993]
Upper level zonal wind of normal state
Upper-level zonal wind of a superrotating state
longitude
latit
ude
Wave source
Inspiration of the hypothesis: Rossby waves excited by a tropical heat source cause equatorial superrotation [Saravanan 1993]
Eddy heat flux of a superrotating state
Eddy heat flux of normal state
Upper level zonal wind of normal state
Upper-level zonal wind of a superrotating state
In a superrotating state, there must be a process other than baroclinic eddy heat flux that maintains the weak temperature gradient
Test of the hypothesis for the Cretaceous climates
• Coupled atmosphere-mixed layer ocean GCM (GENESIS)• 50-m oceanic mixed layer with diffusive heat flux• Atmospheric GCM: T31 (3.75o), 18 vertical levels• CO2: 4 X pre-industrial value 1120 ppmv• CTL run: no prescribed tropical heating• EXP run: local tropical heating – compensating cooling• Heating/cooling profile: maximum in the mid-troposhere
longitude
equator
Warm pool heating (150 W/m2)
Compensating cooling (-50 W/m2)
tropical perturbation run - control run
Surface temperature 250-hPa height and winds
To test the hypothesis, consider thermodynamic energy equation
heat fluxconvergence
adiabatic warming
radiative cooling
diabatic heating
heat flux convergence stationary eddies
temperature change
heat flux convergence transient eddies
adiabatic warming
Change in stationary eddy heat flux
Change in stationary eddy heat flux convergence
0.3 K/day;
for g = 20 day, dT ~ 6 K
Change in transient eddy heat flux
Change in transient eddy heat flux convergence
0.3 K/day;
for g = 20 day, dT ~ 6 K
change in vertical velocity (dP/dt) change in adiabatic warming
0.4 K/day;
for g = 20 day, dT ~ 8 K
change in vertical velocity (dP/dt) change in stationary eddy momentum flux convergence
EE W W
what drives high-latitude adiabatic warming?
January zonal mean surface air temperature
Tropical heating contrast (W/m2)
Main findings of the Cretaceous modeling study
1. Localized tropical heating (net zero heating) can warm the Arctic.
2. Winter Arctic warming is driven by dynamic warming (adiabatic warming driven by stationary eddy momentum flux + stationary eddy heat flux), and possibly by downward IR (cloud cover increases).
3. Within the range of 0-150 W/m2, zonal mean Arctic temperature increases 0.8oC per 10 W/m2 for CO2 level of 4 X PAL (Cretaceous);
increases 0.3oC per 10 W/m2 for CO2 level of 1 X PAL (present)
4. Above 150 W/m2, the Arctic warming saturates
5. Tropical upper tropospheric zonal wind becomes more strongly super-rotating.
Does localized tropical convective heating increase as the climate warms?
mm/day/century
Climatology
Linear trend (1979-2002)
(1982-2001) minus (1958-1977)
Testing the hypothesis with ECMWF reanalysis
Surface temperature 250-hPa streamfunction
250-hPa eddy streamfunction Convective precipitation
To test the hypothesis, consider the thermodynamic energy equation
heat fluxconvergence
adiabatic warming
radiative cooling
diabatic heating
heat flux convergence
temperature change
adiabatic warming
diabatic heating
Surface heat flux
(1982-2001) minus (1958-1977)
Testing the hypothesis with ECMWF reanalysis
Surface temperature Downward IR flux
Horizontal temperature advection + Adiabatic warmingDirect consequence of Rossby wave dynamics
The findings so far:
Dynamical processes & downward IR radiative flux warm the Arctic
Questions:
1. Are these two processes linked?
2. How does the inter-decadal time-scale Arctic warming occur through Rossby wave dynamics which takes place on intraseasonal time scales?
Daily evolution associated with the 250-hPa streamfunction trend pattern
250-hPa streamfunction Downward IR flux Surface temperature
Spatial correlations between trend patterns & daily patterns
(day)
Tropical precip => atm circulation (Rossby waves) => IR warming, surface warming
Surface Air Temperature Response to Madden-Julian Oscillation
MJO Phase 1 MJO Phase 5
Interdecadal changes in MJO phase frequency
Interdecadal changes in surface air temperature due to MJO
Main findings: from (1958-1979) to (1980-2001)
1. Tropical convective precipitation (thus heating) became more confined into the Indo-Pacific warm pool region.
2. Winter Arctic warming is driven by dynamic warming and downward IR.
3. This decadal time-scale Arctic warming occurs through changes in the frequency of occurrence of a small number of intraseasonal-scale teleconnection patterns.
4. The warm pool convection leads the teleconnection patterns by 3-4 days; the teleconnection patterns lead the IR flux by 2-3 days;
5. Interdecadal changes in the Madden-Julian Oscillation contribute toward polar amplification.
MJO phase 1 MJO phase 5
Passive tracer evolution