The atmospheric response to an Oyashio SST front shift in an atmospheric GCM

1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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.

9. Vertical circulation: forcing Diabatic heating: HR LR Δω(contour) ΔQDIAB (color) Vorticity advection: HR LR Δω(contour) Δ(HR-LR) (color)

10. 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

11. Thermodynamic budget: 950mb <5% HR LR °C day-1

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

13. 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?

14. Remote response Sea-level pressure NDJ NDJ HR LR JFM JFM HR LR

15. 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.

16. Additional Slides

17. Precipitation

18. Role of eddies

19. EHFC – low pass

20. Smoothed HR Experiment (0.25° CAM5)