Influence of the Increased SST on Baroclinic Instability Wave Activity under an Aqua Planet Condition Chihiro Kodama and Toshiki Iwasaki Graduate School of Science, Tohoku University, Japan 1. Introduction It is widely accepted that tropospheric temperature rise due to global warming is great in the tropical upper troposphere and NH polar lower troposphere (Fig. 1). Considering midlatitude meridional temperature gradient (MTG), extratropical transient wave activity is expected to enhance at least in the SH. According to the results of Yin (2005), however, future projection of global mean transient wave energy seems to be rather inconsistent among global climate models, though its enhancement is found on multi-model mean. Instead, poleward shift of transient wave activity is evident in almost all the models. In this study, we conduct a series of aqua planet experiments to investigate influence of the global-warming-like idealized SST rise on extratropical baroclinic instability wave activity. Patterns of SST rise strongly affects midlatitude MTG in the lower and upper troposphere. 2. Experimental Design Fig. 2. Schematic diagram of the experiments. Red shadings are the regions where warming occurs and MTG changes Fig. 3. SST distribution [ºC] Control All+3 High+3 Control : SST distribution proposed by Neale and Hoskins (2000) High+3 : The control SST is warmed by 3K only in the high latitudes All+3 : The control SST is uniformly warmed by 3K All the conditions including SST are uniform longitudinally and symmetric against the equator. After a 1-year spin up, each run is integrated for 10 years in perpetual 20 Mar (~equinox) condition. Therefore, all the results are shown as NH & SH average plots. Zonal mean variables such as wave energy (W = K E + A E ) are diagnosed using Mass-weighted Isentropic zonal Mean (MIM; Iwasaki, 1989; Iwasaki, 2001). MRI: Meteorological Research Institute, JMA: Japan Meteorological Agency 3.3. Wave energy generation rate 3.2. Wave Activity 6. Acknowledgements ・ Meteorological Research Institute (MJ98 GCM) ・ The 21st century GCOE program “Global Education and Research Center for Earth and Planetary Dynamics” at Tohoku University 3.1. Zonal Mean Temperature and Zonal Wind 4. Concluding Remarks 5. References Fig. 4: Zonal mean (top) temperature [K] and (bottom) zonal wind [m/s]. High+3 ・ Great polar warming & Decreased midlatitude MTG in the lower troposphere ・ Weaker midlatitude jet in the troposphere (& slight equatorward shift) All+3 ・ Great tropical warming & Increased midlatitude MTG in the upper troposphere ・ Stronger midlatitude jet, especially above the upper troposphere ・ Poleward shift of midlatitude jet in the troposphere High+3 ・ Suppressed wave activity almost everywhere in the troposphere All+3 ・ Rather unchanged global mean wave energy (though slight enhancement is found) ・ Poleward & upward shift of wave activity (similar to realistic global warming) Important question (All+3 run) Despite the increased MTG in the upper troposphere, why is global mean baroclinic instability wave energy rather unchanged? Low-level midlatitude MTG is much more important for global mean baroclinic instability wave activity than upper- level one. Increased upper-level midlatitude MTG shifts the location of wave activity poleward through increase in vertical shear of westerly and static stability. Poleward shift of westerly jet due to uniform SST rise is an open question. Note that, in the actual NH during winter, where the stationary waves are active, suppression of stationary waves due to global warming (Kodama et al. 2007) may enhance transient wave activity (Iwasaki et al., submitted). Fig. 7. Same as Fig. 5 but for wave energy generation rate [W m -2 ]. All+3 ・ Baroclinic process enhances W mainly through increased vertical shear of westerly in the higher latitude. ・ Baroclinic process suppresses W through decreased vertical EP flux in the lower latitude, which is related to the increased static stability. ・ Barotropic process suppresses W in the lower latitude. ・ Diabatic heating secondarily enhances W in the higher latitude. Ocean Troposphere E.Q.N.P.S.P. Control Run upper increase lower decrease High+3 All+3 Fig. 1. Future projection of zonal mean temperature [K]. See IPCC (2007) for details. Fig. 5. (top) K E [m 2 s -2 ]. (bottom) Vertically integrated wave energy (W=K E +A E ) [10 5 J m -2 ]. Thick lines show vertically integrated values multiplied by cosφ. The Control values divided by 10 are shaded in the center and right panels. Global means are shown on the upper-left side of the each panel [10 5 J m -2 ] Atmospheric GCM : MJ98 ・ MRI & JMA (Shibata et al. 1999; Yukimoto et al. 2006) ・ T63 (1.9º×1.9º) in horizon, 45 vertical levels (top: 0.01hPa) ・ Aqua planet condition ・ Rayleigh Friction (middle atmosphere) ・ Prognostic Arakawa-Schubert cumulus convection F Z form Vertical shear Fig. 4. (top) Temperature [K] and (bottom) zonal wind [m s -1 ]. (a)(d) The Control run. (b)(e) The High+3 run change. (c)(f) The All+3 run change. Green contours in (e)(f) show zonal wind in the Control run. Fig. 8. Contributions of (top) vertical shear and (bottom) F Z form to the changes in baroclinic conversion in the All+3 run [W m -2 ]. ~ ~ Diabatic heating Q E Baroclinic conversion C(K Z →A E ) Barotropic conversion C(K Z →K E ) QZQZ QEQE δZδZ δEδE W AZAZ KZKZ KEKE AEAE Fig. 6. MIM energy cycle. Fig. 9. Change in N 2 in the All+3 run [10 -4 s -2 ]. [1] IPCC (2007), Climate Change 2007: The Physical Science Basis, 996pp. [2] Iwasaki (1989), J. Meteor. Soc. Japan, 67, [3] Iwasaki (2001), J. Atmos. Sci., 58, [4] Kodama et al. (2007), J. Geophys. Res., doi: /2006JD [5] Kodama and Iwasaki, J. Atmos. Sci., under revision. [6] Neale and Hoskins (2000), Atmos. Sci. Lett., 1, [7] Shibata et al. (1999), Pap. Meteor. Geophys., 50, [8] Yukimoto et al. (2006), J. Meteor. Soc. Japan, 84, Feel free to tell us if you want our paper (Kodama and Iwasaki, submitted to JAS) W: wave energy K E : eddy kinetic energy A E : eddy available potential energy

Appendix 2. Eliassen-Palm flux Appendix 1. Mass Streamfunction Fig. A1. Mass streamfunction [10 10 kg/s]. Fig. A2. EP flux and its divergence [m/(s day)]. Tohoku Univ. : My Webpage : Appendix 4. Precipitation Fig. A4. (left) Precipitation and (right) its zonal mean [mm/day]. Appendix 3. Maximum Growth Rate Fig. A3. Maximum growth rate of baroclinic instability wave [/day].