Frontal scale WBC air-sea interaction issues

1. Frontal scale WBC air-sea interaction issues Also Hisashi Nakamura, Shoshiro Minobe, Masami etc

2. Outline – summaries and questions 1. Atmospheric boundary layer response to WBCs, some findings and questions. 2. Deep atmospheric response, some findings and questions. Importance for mean state 3. Storm track response to WBCs, some findings and questions. 4. Feedback onto ocean 5. Way forward – SST anomalies and climate variability. How to address this – joint model project?

3. Response of marine boundary layer to SST gradients WBCs regions of largest ocean to atmosphere heat flux in midlatitudes (Bunker and Worthington 1976, CLIMODE group, KESS) Wind and stress correlation with SST over ocean eddies, fronts (reviewed by Xie et al 2004, Chelton et al 2004, Small et al 2008) Direct stress effects (Liu, also Ralph Foster) – drag coefficient changes across front. Estimates of importance range from ~30% (O’Neill, Small) to 70% (Liu). Stress convergence but no wind convergence a problem for deeper response? Momentum mixing (Hayes et al 1989) driven by surface instability Pressure gradients and secondary circulations (Lindzen and Nigam 1987, Small et al 2003, Minobe et al 2008, laplacian of SLP, others). SLP anomalies in KE over warm SST cause cloud formation (winter): in summer warm KE inhibits fog formation (Tokinaga) Stress proportional to boundary layer height (Samelson) Refer Spall work, Takatama Actually mixing (sensible heat fluxes ~ Ts-Ta) important for all mechanisms---- Precise mechanism not really important – what is important is the magnitude of convergence and/or buoyancy/latent heat flux which may influence troposphere Climate model requirements: Metrics for atmospheric response: hi-pass SST vs. Usfc.correlation; downwind SST gradient vs. wind stress anomaly (Chelton). Need higher ocean resolution.

4. Deep response of atmosphere Liu et al (2007) air temperature anomalies up to 300hPa above Agulhas meanders Minobe et al (2008) ascent throughout troposphere, upper level divergence and clouds, low level convergence, seasonal modifications Winter has strong low level convergence, shallow heating – baroclinic instability Summer has high SSTs, deep heating (weak low level convergence, more latent heating?) Atmosphere preconditioned by fairly unstable thetae profile – warm SST and surface buoyancy leads to deep convection Also seen in various model experiments (Hand, Kuwano-Yoshida) Xie, Tokinaga et al (2009/2010/2011??), high level clouds over KE in winter How large is this effect? Samelson (pers comm) – low level convergence and consequent ascent over GS equivalent to that for a hill of a few hundred meters – is this important? Same question for Agulhas meanders (Liu). But diabatic heating is large (TRMM, JRA25)

5. Storm Track Free tropospheric storm track and WBC Low level baroclinicity important for storm development (Hisashi Nakamura, Hoskins and Hodges ) influenced by ocean front Hatteras-North Wall temperature gradient - Land-sea contrast modified by WBC? (Cione et al 1993). Upper level vorticity as second independent variable (Jacobs et al 2005). Surface storm track and WBC Free tropospheric storm track modified by near surface stability Sampe and Shang-Ping Xie, Terry Joyce, Jimmy Booth work Ocean baroclinic adjustment (Taguchi, Nonaka, Hisashi) Ocean front helps fix storm track via sensible heating – restores ?T Low level sensible heating counters eddy heat flux Alternative to Hoskins and Valdes idea Eady growth rate ?– dependence on stability and horizontal temp gradient (vertical shear) Hisashi – compute growth rate using near surface data Stability dependent on ocean SST front - Booth New ‘baroclinicity’ index to replace Cione et al? Decadal variability – SST anomalies in KE potentially influence storm track(Hisashi)

6. Response to midlatitude SST anomaly Local linear baroclinic response to diabatic heating (Hoskins and Karoly, Palmer and Sun,) Feedback from eddy vorticity and heat fluxes may be important, can cause equivalent barotropic response (Peng and Whitaker, Peng et al, Yochanan Kushnir et al 2002) Time-evolving response shows linear response first (~ a week) followed by effect of eddy fluxes – equilibrium after 2-4 months (Deser et al 2007, JCLI, Frankignoul et al) Broad, fairly low level heating How is this changed by narrow, concentrated, possibly deeper heating? Limited observational evidence for climate response to ocean extratropical SST anomalies (Frankignoul) Limitations due lack of resolution in Reanalysis- compare with QuikSCAT measurements of surface ‘atmosphere’ and storm track (local mean response is finescale in atmosphere –minobe- ) Model experiments need SST anomalies

7. Response of ocean (1) Ekman pumping Stress gradients at the surface due to SST-wind feedback (Seo et al) and gradients in surface ocean current (Dewar and Flierl, Small et al – top drag, AMS talk) Significant effect – O(1) we (Seo), EKE damping of 30% (Small) Effect of GS currents on fluxes: 10% errors in heat flux, 20% errors in wind stress. How important is this relative to SST effects? (Bigorre) Also Affects heat flux

8. Response of ocean (2) Mode water formation – ’50% at or close to Gulf Stream front ‘ (Joyce, this workshop) Driven by surface buoyancy fluxes but vertical shear important High resolution models necessary ?

9. Schematic from review paper

10. Wind speed and SST from satellite data

18. TRMM estimates of diabatic heating

20. Anchoring of a storm track by a mid-latitude oceanic front Nakamura, Sampe, Goto, Ohfuchi, Xie (2008 GRL)

22. Results: Atlantic Basin (Jimmy Booth)

23. Results: Pacific Basin

25. Genesis density

26. Storm tracks: That’s the time mean, what about variability?

27. Confinement of extratropical decadal SST variability into major oceanic frontal zones Nakamura, Kazmin (2003; JGR)

28. Wintertime basin-scale atmospheric response to quasi-decadal SSTA in the KOE region  (Nakamura)

29. SST anomalies in north Pacific (Nonaka et al)

31. Narrow heating (Minobe et al 2008)

33. Interannual variability of local forcing from TRMM