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, 2013 Workshop on SST Fronts Boulder, Colorado. Impact of SST fronts on mean state. Significant impact has now been shown.

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The atmospheric response to an oyashio sst front shift in an atmospheric gcm

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, 2013

Workshop on SST Fronts

Boulder, Colorado


Impact of SST fronts on mean state

  • Significant impact has now been shown

Minobe et al., 2008

Front (solid)

No front (dash)

SST anomalies in front/no-front experiments approach 10°C

75%

300%

Nakamura et al., 2008


Impact on variability

ΔSST

w/ obs SST (solid)

smoothed (dash)

30%

~12%

  • Is the response mainly in the boundary layer?

  • Locally confined?

  • Is the atmosphere sensitive enough to respond to realisticSST front variability?

Taguchi et al., 2009


Experimental design
Experimental design

dSST/dy (°C 100 km-1)

WARM

COLD

  • 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

OEI from Frankignoul et al., 2011


Model information
Model information

Model Experiments

  • 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:

    • High-resolution (HR): Uses 0.25° CAM5.

    • Low-resolution (LR): Uses 1° CAM5.

  • Identical initial land, sea-ice and atmospheric initial conditions

  • Compare the Ensemblemeandifference (WARM –COLD)between the HR and LR model responses


Horizontal circulation
Horizontal circulation

Mean Nov-Mar difference: SLP (contour), turbulent heat flux (color), 2-m wind (arrow)

LR

HR

L

L

  • 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

SST (thin contour), SLP (thick contour)

NCEP

L


Vertical circulation
Vertical circulation

ERA-Int

ω (contour, 1.5x10-3 Pa s-1)

div (color, s-1)

HR

+50%

What is the cause of the stronger circulation in the HR model?

LR

latitude


Vertical circulation forcing
Vertical circulation: forcing

Decomposeω using the generalized ωequation:

diabatic heating

vorticity advection

thermal advection

HR: All forcing

HR: Model Output

Re-constructed (left) not perfect, but still useful to compare contribution of individual terms.


Vertical circulation forcing1
Vertical circulation: forcing

Diabatic heating:

HR

LR

Δω(contour)

ΔQDIAB (color)

Vorticity advection:

HR

LR

Δω(contour)

Δ(HR-LR) (color)


Role of eddies high pass v t
Role of eddies : high-pass v’T’

850mb v’T’ (mean: contour, diff: color)

2

Eddies in HR show a much greater sensitivity to the SST frontal shift

NCEP

-2

Cross-section across the front

K m s-1

HR

LR


Thermodynamic budget 950mb
Thermodynamic budget: 950mb

<5%

HR

LR

°C day-1



Conclusions
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?


Remote response
Remote response

Sea-level pressure

NDJ

NDJ

HR

LR

JFM

JFM

HR

LR


Looking ahead
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.






Smoothed HR

Experiment (0.25° CAM5)


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