The atmospheric response to an Oyashio SST front shift in an atmospheric GCM
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The atmospheric response to an Oyashio SST front shift in an atmospheric GCM
Dima Smirnov, Matt Newman, Mike Alexander, Young-Oh Kwon & Claude
Frankignoul
August 6, 2013Workshop on SST Fronts
Boulder, Colorado
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Impact of SST fronts on mean state Significant impact has now been shown
Minobe et al., 2008
Nakamura et al., 2008
Front (solid)No front (dash)
SST anomalies in front/no-front experiments approach 10°C
75%300%
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Impact on variability
Is the response mainly in the boundary layer? Locally confined? Is the atmosphere sensitive enough to respond to
realistic SST front variability?
30% ~12% w/ obs SST (solid)smoothed (dash)
ΔSST
Taguchi et al., 2009
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Experimental design
SST anomaly based on the Oyashio Extension Index (1982-2008)
Outside of the frontal region (dSST/dy < 1.5 °C 100 km-1), SST anomalies are masked
dSST/dy (°C 100 km-1)
WARM
COLD
OEI from Frankignoul et al., 2011
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Model information
NCAR’s Community Atmosphere Model (CAM), version 5 25 warm/cold ensembles with different atmospheric initial
states from control run (taken a year apart) Two simulations:
1. High-resolution (HR): Uses 0.25° CAM5.2. Low-resolution (LR): Uses 1° CAM5.
Identical initial land, sea-ice and atmospheric initial conditions
Compare the Ensemble mean difference (WARM – COLD) between the HR and LR model responses
Model Experiments
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Horizontal circulation
Turbulent heat flux is 10-20% stronger in LR
LR response is seasonally dependent
Both models imply a ~6-month persistence time for a 150-m mixed layer
HR LR
L L
Mean Nov-Mar difference: SLP (contour), turbulent heat flux (color), 2-m wind (arrow)
NCEP
L
SST (thin contour), SLP (thick contour)
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Vertical circulationω (contour, 1.5x10-3 Pa s-1) div (color, s-1)
latitude
ERA-Int
HR
LR
What is the cause of the stronger circulation in the HR model?
+50%
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Vertical circulation: forcingDecompose ω using the generalized ω equation:
thermal advection vorticity advection diabatic heatingHR: Model
OutputHR: All forcing
Re-constructed (left) not perfect, but still useful to compare contribution of individual terms.
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Vertical circulation: forcingDiabatic heating:
Vorticity advection:
Δω (contour)ΔQDIAB (color)
HR LR
Δω (contour)Δ(HR-LR) (color)
HR LR
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Role of eddies : high-pass v’T’
NCEP
HR
LR
Eddies in HR show a much greater sensitivity to the SST frontal shift
850mb v’T’ (mean: contour, diff: color)
Cross-section across the front
2
-2K m s-1
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Thermodynamic budget: 950mb
HR LR°C day-1
<5%
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Thermodynamic budget: 700mbHR
°C day-1
LR
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Conclusions A high resolution model (<1°) is required to capture the
atmospheric response to the Oyashio SST front shift For CAM5, movement of heat from the warm side of the
SST front is strongly resolution dependent: In HR, a strong upward heat flux maintains a vertical
circulation through the depth of the troposphere In LR, heat is removed largely by horizontal eddy
fluxes, causing a shallower vertical circulation Unlike the LR, the HR develops a robust shift in the
storm track Collectively, what does this mean for the large scale
response?
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Remote responseHR LRNDJ NDJ
HR JFM
Sea-level pressure
LR JFM
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Looking ahead Can the difference in the HR and LR responses be
explained with a simpler model? Is the difference related to differences in the mean state? Employ a simplified GCM forced by diabatic heating.
How much of the difference in the HR and LR responses is actually due to a better resolved SST front, versus a higher-resolution atmosphere. A “smooth” HR simulation (1° SST with a 0.25° GCM)
appears to suggest that atmospheric resolution plays a larger role than SST front strength.
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Additional Slides
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Precipitation
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Role of eddies
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EHFC – low pass
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Smoothed HRExperiment (0.25° CAM5)