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Clio Michel and Gwendal Rivière Météo-France/CNRS, CNRM-GAME, Toulouse, France

Sensitivity of the position and variability of the eddy-driven jet and storm-track to different SST profiles in an aquaplanet general circulation model (Arpège). Clio Michel and Gwendal Rivière Météo-France/CNRS, CNRM-GAME, Toulouse, France. EGU Vienna - April 25, 2012.

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Clio Michel and Gwendal Rivière Météo-France/CNRS, CNRM-GAME, Toulouse, France

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  1. Sensitivity of the position and variability of the eddy-driven jet and storm-track to different SST profiles in an aquaplanet general circulation model (Arpège) Clio Michel and Gwendal Rivière Météo-France/CNRS, CNRM-GAME, Toulouse, France EGU Vienna - April 25, 2012

  2. Literature and problematic • Eddy-driven jet (EDJ) and storm-track position and intensity controlled by:  Intensity of the subtropical jet (SJ). A strong SJ leads to a single jet, a weak SJ leads to two jets. (Lee and Kim 2003, Son and Lee 2005) Anchoring effect of EDJ and storm track by the SST front. Close but on the poleward side of the SST front. (Nakamura et al. 2004, Brayshaw et al. 2008, Ogawa et al. 2011) • EDJ climatology intensively studied but much less the EDJ variability (linked to teleconnections such as AO, NAO).

  3. Problematic Goal: Study of the eddy-driven jet variability using an aquaplanet AGCM Approach: • Aquaplanet simulations using AGCM Arpège-Climat (T127L31) with prescribed axisymmetric SST at the lower boundary and equinoctial conditions. • Systematic analysis of the storm-track and eddy-driven features (latitudinal position, intensity) relative to the SST gradient features (latitudinal position, intensity and width).

  4. SST profiles Idealized piecewise linear distributions to only modify midlatitudes. ERA40 SST Atl-DJF 20° 40° 3 latitudes, 2 widths, 2 SST gradient intensities  12 experiments

  5. The two jet regimes Mass streamfunction(red and blue contours), zonal wind (shading) SST gradient atj=30° SST gradient atj=50° One single jet Two well-separated jets

  6. Storm-track intensity wide strong front wide weak front narrow strong front narrow weak front Higher the gradient to the south, stronger the storm-track. Higher the gradient intensity (intensity and width), stronger the strom-track. (Brayshaw et al. 2008, Graff and LaCasce 2012)

  7. Baroclinicity (Meridional gradient of q) SST gradient atj=30° SST gradient atj=50° Baroclinicity linked to SST gradient Baroclinicity linked to SJ When the SST gradient is more equatorward, the two zones of baroclinic waves growth merged, the two types of baroclinicity add and the storm-track is stronger.

  8. Eddy-driven jet latitude (Umax at 850 hPa) strong gradient One-to-one line weak gradient EDJ located on the poleward side of the SST gradient because of the predominance of AWB. More poleward the gradient, more poleward the EDJ relative to the SST front.

  9. Wave-breaking asymmetries WB event: reversal of the absolute vorticity gradient. SST gradient atj=30° SST gradient atj=50° SST SST U U anticyclonic WB cyclonic WB AWB = CWB AWB >> CWB

  10. Interpretation in terms of refractive index asymmetries n2>0: waves propagation. n2<0: evanescent waves. AWB CWB AWB (Rivière 2009) Proposed interpretation: SST gradient more poleward  WB asymmetries (much more AWB)  EDJ much more poleward relative to the SST gradient.

  11. Eddy-driven jet variability EOF1: first EOF of the vertically and zonally averaged zonal wind U regressed on PC1 (black contours) and U (shading) SST gradient atj=30° SST gradient atj=60° node node PC1>1 PC1>1 U max U PC1<-1 PC1<-1 U U max Only latitudinal shifting Mixed pulsing and latitudinal shifting

  12. ERA40 reanalysis (JJA – Southern hemisphere) Meridional SST gradient Pacific 55°S 120°E-210°E Indian 45°S 30°E-120°E South Pacific SST gradient poleward the South Indian SST gradient

  13. ERA40 reanalysis (JJA - SH) EOF1: first EOF of the vertically and zonally averaged zonal wind U regressed on PC1 (black contours) and U (shading) SST gradient atj=45°S SST gradient atj=55°S Indian ocean 30°E-120°E Pacific ocean 120°E-210°E nodes U max U max PC1>1 PC1>1 PC1<-1 PC1<-1 U U Latitudinal shifting Mainly pulsing

  14. Conclusions • We find two regimes of EDJ variability: latitudinal fluctuations(middle and low latitudes) and pulsing (highlatitudes), similar to the difference between the observed SH Pacific and Indian oceans. (similar to barotropic simulations of Barnes and Hartmann 2011)

  15. Conclusions • We find two regimes of EDJ variability: latitudinal fluctuations(middle and low latitudes) and pulsing (highlatitudes), similar to the difference between the observed SH Pacific and Indian oceans. (similar to barotropic simulations of Barnes and Hartmann 2011) • Intensification of the storm-track for SST gradient closer to the subtropical jet by addition of the two sources of baroclinicity. • The position of the eddy-driven jet relatively to the SST gradient can be explained by wave-breaking asymmetries. (consistent with more idealized simulations of Rivière 2009)

  16. Conclusions • We find two regimes of EDJ variability: latitudinal fluctuations(middle and low latitudes) and pulsing (highlatitudes), similar to the difference between the observed SH Pacific and Indian oceans. (similar to barotropic simulations of Barnes and Hartmann 2011) • Intensification of the storm-track for SST gradient closer to the subtropical jet by addition of the two sources of baroclinicity. • The position of the eddy-driven jet relatively to the SST gradient can be explained by wave-breaking asymmetries. (consistent with more idealized simulations of Rivière 2009) • Perspectives: Stronger subtropical jet. (by increasing SST in the subtropics) Zonally asymmetric SST forcing.

  17. End of talk Thank you for your attention

  18. SST profiles • Idealized piecewise linear distributions to only modify midlatitudes  12 experiments: Solid lines L=20° Dashed lines L=10° 3 latitudes: 30°,40°,50° Black lines strong SST gradient=-10.79 10-6 K/m Blue lines weak SST gradient=-8.54 10-6 K/m

  19. Rossby wave breakings • RWB detection algorithm (based on geometrical considerations) RWB = large-scale and irreversible overturning of the PV gradient over an isentropic surface. Two types (Thorncroft et al., 1993): anticyclonic (SW-NE) cyclonic (SE-NW)

  20. φ φ perturbation ū ū φ0 φ0 Refractive index asymetries (Rivière 2009) n2decreases withj n2increases withj NW-SE orientation, poleward propagation, u’v’<0 CWB SW-NE orientation, equatorward propagation, u’v’>0 AWB AWB CWB AWB

  21. Eddy-driven jet variability One-to-one line pulsing latitudinal shift Results partially similar to those found with a simple barotropic model. (Barnes and Hartmann 2011)

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