Upload
letitia-day
View
214
Download
0
Tags:
Embed Size (px)
Citation preview
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
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%
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
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
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
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)
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%
Vertical circulation: forcingDecompose ω using the generalized ω equation:
thermal advection vorticity advectiondiabatic heatingHR: Model
OutputHR: All forcing
Re-constructed (left) not perfect, but still useful to compare contribution of individual terms.
Vertical circulation: forcingDiabatic heating:
Vorticity advection:
Δω (contour)
ΔQDIAB (color)
HR LR
Δω (contour)Δ(HR-LR) (color)
HR LR
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
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?
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.