Frontal scale WBC air-sea interaction issues

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					   Frontal scale WBC air-sea
       interaction issues
         Some influences

Also Hisashi Nakamura, Shoshiro Minobe,
               Masami etc
… and the WBC working group…
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?              Overview slides followed by
                         illustrations– discussion at end?
 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.
      Deep response of atmosphere
• Liu et al (2007) air temperature anomalies up to 300hPa above Agulhas
• 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
   – 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
   – 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
   – But diabatic heating is large (TRMM, JRA25)
                              Storm Track
Free tropospheric storm track and WBC
    – Low level baroclinicity important for storm development (Hisashi   g           
       Nakamura, Hoskins and Hodges ) influenced by ocean front                  f    N
    – 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
                                                                                 g     
    – Stability dependent on ocean SST front - Booth                       
                                                                                 f    N
    – New ‘baroclinicity’ index to replace Cione et al?
Decadal variability – SST anomalies in KE potentially influence storm track(Hisashi)
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
• 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
         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?
• Also Affects heat flux
         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 ?
          Schematic from review paper
                                                       Figure 15. Schematic showing boundary
               Momentum mixing important               layer response to an ocean frontal
                                                       boundary. The flow is from cold to warm
                                                       SST in a situation with (a) strong
cool                           warm
                                                       background winds and (b) weak
                                                       background winds. Below each panel are
                                                       horizontal across-front profiles of SST and
                                                       air temperature Tair. Above is shown the
                                                       sea surface, with wave height increasing
                                                       to the right of the SST front. Background
                                                       winds U at left are incident on the front. A
                                                       mixed internal boundary layer (gray
              Pressure gradient important              shading) develops downstream. Circular
                                                       arrows indicate the presence of turbulent
                                                       eddies. Forces due to mixing (Fmix, term
                                                       IV of equation (4a)) and pressure (Fpres,
cool                          warm                     term III) are indicated with thin arrows.
                                                       Due to these forces the wind profile
                                                       becomes uniform with height (thick black
                                                       arrows). To the right are shown vertical
                                                       profiles of the downstream air
                                                       temperature anomalies (dashed) and
                                                       pressure anomalies (solid).

Note: upward heat flux (red) from ocean over warm SST warms internal boundary layer

  Modification to include coriolis effects, thermal wind (O’Neill PhD Thesis)
Wind speed and SST from satellite data
SST (COL) AND                    SST (COL) AND
10M WIND S                       10M WINDS

OBSERVATIONS                       MODEL


                            BACKGROUND WIND

        Winds accelerate towards
         DUE TO GRADP
        low pressure (red)

    Frontal Scale Air-Sea Interaction in High Resolution Versions of
                the Community Climate System Model
•    Fully coupled climate simulations with eddy resolving ocean components are technically
     feasible but still very computationally challenging and costly.
•    The observed relationships between frontal scale SST structure and low level winds and/or
     surface stress may provide useful metrics with which to test coupled climate models in this
     regime that do not require century length integrations.
      –   Local temporal correlation of u10 and sst; Slope of regression of wind stress on sst, div and curl of
          stress on downwind and crosswind sst gradient; Seasonal cycle of regional differences in these
•    Correlation of high pass sst and low level winds weak with lores ocean model, becomes
     systematically stronger over frontal regions with hires ocean model.
•    Coupling between high-pass sst and stress less than ½ observed estimate, independent of
     model resolution, suggesting intrinsic deficiency of atmos. model PBL physics.

                                               0.5° atm / 1.0° ocn                    0.5° atm / 0.1° ocn
              SST vs. |usrf|

                                                   0.5° atm / 1.0° ocn       0.5° atm / 0.1° ocn       0.25° atm / 0.1° ocn

    Agulhas region
     stress vs SST
Seasonal variation of the updraft response (Kuwano-Yoshida)

                                           Figure.B Vertical cross
                                           section averaged between
                                           290oE and 310oE for
                                           vertical velocity (color,),
                                           moisture convergence
                                           (thick contours), and
                                           saturation equivalent
                                           potential temperature
                                           (thin contours).
TRMM estimates of diabatic heating

                         North Atlantic

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

            Surface sensible heat flux            Probability of low-level storm-track axis

Oceanic front acts to anchor a storm track and PFJ by restoring near-surface
   baroclinicity effectively: “oceanic baroclinic adjustment”

c.f. conventional baroclinic adjustment due to differential radiative heating
   (with time scale of 1~2 weeks)
c.f., Moisture supply from a warm western boundary current (Hoskins, Valdes
Results: Atlantic Basin (Jimmy Booth)
           υ850mb                  A.B.L. Stability

             υ10m            Predicted and actual surface storm

   The surface storm track
Results: Pacific Basin
υ850mb           A.B.L. Stability

 υ10m      Predicted and actual surface storm
Genesis density
Genesis density
Storm tracks: That’s the time
mean, what about variability?
      Confinement of extratropical decadal SST variability
              into major oceanic frontal zones
                            Nakamura, Kazmin (2003; JGR)
importance of oceanic processes  atmospheric bridge (basin-scale)
                                                         DJF climatology of
DJF decadal SST variability (COADS)                  Northward SST gradient
  (std. dev. : 0.3, 0.4, 0.5, 0.6 deg.)          (±0.1, 0.3, 0.5, 0.7, …deg./100km)
              Wintertime basin-scale atmospheric response to
                   quasi-decadal SSTA in the KOE region
       SST (Nov) vs Z925 (Jan)
                                            Zonal section of height (contoured) and
–2 mo.                                      temperature (colored) anomalies for 55N


                                            120E                                  120W
                                        Shallow local response and deep, equivalent
                                             barotropic response downstream
120E                             120W                [c.f., Peng et al. (1997)]

 Feedback forcing by transient
  eddies along the poleward-
   shifted storm track on the
equivalent barotropic anomalies.
SST anomalies in north Pacific (Nonaka et al)
Kushnir et al (2002) – response to low
level heating with eddy fluxes included
Minobe et al (this workshop)
Narrow heating (Minobe et al 2008)

                       Problems with this:
                       Heating pattern is not an
                       Based on annual mean
                       (Smooths seasonal
                       No eddy flux feedback
                       Gulf Stream is blocky
Narrow heating (Minobe et al 2008)

                    DESER ET AL 2007. FIG. 9. (left)
                    Vertically averaged diabatic heating
                    responses for days 2–8 and 45–120
                    for the (top) SST (right)Vertical
                    profiles of the diabatic heating
                    responses for days 2–8 (thin curve)
                    and days 45–120 (thick curve)
Interannual variability of local
     forcing from TRMM

 Analysis is challenging because of patchy nature of
 rainfall and narrow swath width of PR
 Perhaps we can start by analysing changes in
 surface convergence any relationship with SST or
        Eddy Kinetic Energy in Tropical Instability Waves

             With understress                    No understress

 WIND & CURRENT  CD U 10  U o U 10  U o   NO   CURRENT    CD U 10 U 10 

        Figure 1. EKE (kgm-1s-2), depth averaged from the surface to 100m. a):
        Experiment 1. b): Experiment 2.

                                                                  The Ekman pumping anomalies
                                                                  compares well with a rough
                                                                  estimate which assumes that only
                                                                  the understress is modifying the
                                                                  stress. The consequent dissipation
                                                                  has an e-folding timescale of 115
     Seasonal variation of the updraft response (Kuwano-

      Figure A. Vertical velocity (color, -10-2 Pa s-1) at 200 (left   Figure.B Vertical cross section averaged
      column), 500 (center column) and 850 (right column) hPa          between 290oE and 310oE for vertical velocity
      for CNTL. (a) (b) (c) Annual mean, (d) (e) (f) in JJA, and       (color,), moisture convergence (thick
      (g) (h) (i) DJF.                                                 contours), and saturation equivalent potential
                                                                       temperature (thin contours).

• The updraft extends vertically up to the upper troposphere over the
  Gulf Stream in summer, while it is restricted the lower troposphere.
  The vertical thermodynamic profile of troposphere controls the
  updraft response to the Gulf Stream.

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