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 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 ( )  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 Mean Nov-Mar difference: SLP (contour), turbulent heat flux (color), 2-m wind (arrow) NCEP SST (thin contour), SLP (thick contour)

Vertical circulation ω (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: forcing Decompose ω using the generalized ω equation: thermal advection vorticity advection diabatic heating HR: Model Output HR: All forcing Re-constructed (left) not perfect, but still useful to compare contribution of individual terms.

Vertical circulation: forcing Diabatic heating: Vorticity advection: Δω (contour) ΔQ DIAB (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 -2 K m s -1

Thermodynamic budget: 950mb HRLR °C day -1 <5%

Thermodynamic budget: 700mb HR °C day -1 LR

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 HRLR NDJ HR JFM Sea-level pressure LR JFM

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.

Additional Slides

Precipitation

Role of eddies

EHFC – low pass

Smoothed HR Experiment (0.25° CAM5)