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Dynamics of intraseasonal oscillations: the role of surface energy fluxes

Dynamics of intraseasonal oscillations: the role of surface energy fluxes. Adam Sobel. Collaborators. Hezi Gildor, Weizmann Institute Eric Maloney, Oregon State U. Gilles Bellon, Columbia U. (CAOS Alumnus). Intraseasonal OLR variance. Northern Summer. Southern Summer.

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Dynamics of intraseasonal oscillations: the role of surface energy fluxes

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  1. Dynamics of intraseasonal oscillations: the role of surface energy fluxes Adam Sobel

  2. Collaborators • Hezi Gildor, Weizmann Institute • Eric Maloney, Oregon State U. • Gilles Bellon, Columbia U. (CAOS Alumnus)

  3. Intraseasonal OLR variance Northern Summer Southern Summer Lawrence and Webster 2002

  4. Northern summer intraseasonal OLR variance tends to avoid land

  5. Southern summer intraseasonal OLR variance tends to avoid land

  6. Emanuel (87) and Neelin et al (87) proposed that the MJO is driven by wind-induced surface flux perturbations Enhanced sfc flux Mean flow Perturbation flow Wave propagation

  7. This idea has been somewhat abandoned because the real MJO does not look quite like the original WISHE theory Observed cloudiness and wind from TOGA COARE Chen, Houze and Mapes 1996

  8. ocean Shinoda et al. 1998 But the real MJO does have significant net surface heat flux variations, roughly in phase with convection

  9. Net = 0 W/m^2 land ocean Shinoda et al. 1998 Over land, there can be no significant net flux variations on intraseasonal time scales - so if net flux were important to ISO, the observed variance maps should look as they do!

  10. The simplest intraseasonal variability is seen in a local analysis (Waliser 1996) Month -1 SST Month 0 Month +1 Time-varying composites of “hot spots” - SST>29.5C for a period > 1 month Highly reflective cloudiness

  11. This has the appearance of a local recharge-discharge oscillation • Clear skies, no deep convection, • light winds, weak surface fluxes, high • insolation -> SST warms, lower atmos. • humidity increases, troposphere becomes • more unstable to deep convection • Deep convection breaks out, clouds • block sun, high winds & strong sfc fluxes, • SST decreases, lower troposphere dries, • troposphere becomes more stable • Deep convection stops, cycle starts • over • Can be driven by ISO, but doesn’t • necessarily have to be Stephens et al.2004

  12. We can make a very simple model that has such a recharge-discharge oscillation (Sobel and Gildor 2003) with Simple Betts-Miller convection Linear cloud-radiative feedback, SW and LW cancel at TOA Sfc wind constant for starters (will relax this)

  13. The growth rate in this model is sensitive to parameters, period isn’t - it is robustly intraseasonal frequency growth rate 0.1 d^{-1} ~= 60 d period r (cloud-rad/sfc flux feedback) τc (convective time scale) mixed layer depth

  14. Nonlinear solution in unstable regime looks much like observed hot spot evolution precipitation evaporation (perturbed only due to sfc. humidity difference) atmospheric rad. cooling surface shortwave (+ “wind induced” evaporation) SST

  15. A more realistic system may be one that is stable (in a single column) but forced by a traveling ISO disturbance. Amplitude in this system depends non-monotonically on mixed layer depth. Amplitude as function of mld Linear calculation forced in atmospheric temperature equation

  16. At least one GCM behaves similarly (Maloney and Sobel 2004) - MJO amplitude vs. mixed layer depth SST Precip Simple model (amplitude is max-min) GCM (amplitude is std. dev.of filtered data) Mixed layer depth -> (these simple model experiments are nonlinear, and forced by oscillations in surface wind speed)

  17. Wet land is like a mixed layer of zero depth (swamp). Thus if MJO is dependent on surface energy fluxes (turbulent, radiative, or both) it should weaken over land… as observed.

  18. However, the GCM result is very dependent on convective scheme. sfc LH flux (MS04) Precip (Maloney 2002) Precip (Maloney & Sobel 2004)

  19. What about the Northern Summer ISO - monsoon active and break periods? latitude Time Sikka and Gadgil 1980

  20. There is both northward and eastward propagation Wang et al. 2006

  21. The northward part appears to be somewhat independent, justifying axisymmetric models Nanjundiah et al. 1992

  22. A recent model focuses on the “vertical shear mechanism” - essentially dynamical, rather than thermodynamic Jiang et al. 2004

  23. v(t,y,z) = v0 (t,y)V0(z) + v1(t,y)V1(z) + vb(t,y)Vb(z) T(t,y,z) = Tref(z) + T1(t,y)a1(z) + sb (t,y)ab(z) q(t,y,z) = qref(z) + q1(t,y)b1(z) + qb (t,y)bb(z) pt pt V1(z) a1(z) b1(z) V0(z) pb pb ab(z) bb(z) Vb(z) 0 0 q T v 0 0 0 We (Bellon and Sobel 2007) use the “QTCM2” (Sobel and Neelin 2006, building on Neelin and Zeng 2000) Vertical structure: Mass conservation: (pt-pb) ∂yv0(t,y) =- pb∂yvb(t,y)

  24. SST 29 -9 -7 9 y (thousands of km) -2 y0 The model is similar in some but not all ways to those of Jiang et al. (2004), Drbohlav and Wang (2005) Parameterizations : Convection: Betts-Miller (a quasi-equilibrium scheme); Radiation: newtonian cooling towards a uniform temperature.Aquaplanet, axisymmetric, on the β-plane;Forcing :

  25. This model produces a nice intraseasonal northward-propagating oscillation, robustly to parameters Precipitation (mm/d) time Latitude (1000’s km)

  26. Wind-induced sfc fluxes are crucial to the model instability, hence dependence on mixed layer depth is as before Period & growth rate from linear model 1/growth rate period Mixed layer depth

  27. If this model were relevant to reality, it would imply damping of ISO over land… as observed

  28. Summary • Simple models of several types have intraseasonal oscillations that depend on surface fluxes; these oscillations are damped without ocean heat storage • At least one GCM works similarly (though at least one other doesn’t) • Observed ISO (at least in SH summer) has substantial net surface energy flux anomalies in more or less correct phase to drive the oscillation • Observed variance of ISO is maximum over ocean, minimum over land, in both seasons and hemispheres

  29. Concluding remarks • We suggest it is likely that surface fluxes (turbulent and radiative) are important to the energetics of the ISO. • This is testable in models. To what extent is it true in different models, and does the answer correlate with goodness (or other properties) of ISO simulation? • Even if true, it would neither mean we deeply understand the ISO, nor that we could simulate or predict it. • Still, if we could decide conclusively on this it might be a step forward…

  30. Results: two limit cycles Mean states Limit Cycle 1 Limit Cycle 2 CMAP July, 80E-90E

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