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Mechanisms and Climatological Structure of the Easterly Jet over Africa and the Atlantic

Mechanisms and Climatological Structure of the Easterly Jet over Africa and the Atlantic. NASA/GSFC/GMAO: Man-Li C. Wu Siegfried D. Schubert, Max Suarez Randy Koster, Winston Chao GSFC/GLA/GEST: Oreste Reale University at Albany: Chris Thorncroft

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Mechanisms and Climatological Structure of the Easterly Jet over Africa and the Atlantic

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  1. Mechanisms and Climatological Structure of the Easterly Jet over Africa and the Atlantic NASA/GSFC/GMAO: Man-Li C. Wu Siegfried D. Schubert, Max Suarez Randy Koster, Winston Chao GSFC/GLA/GEST: Oreste Reale University at Albany: Chris Thorncroft October 5, 2010 : NCU, Chungli, Taiwan

  2. Outline I Introduction II Basic Structure and Variability of the Jet in MERRA and Other Reanalyses III Mechanisms Wave and Jet Interaction Mid-Latitude Influence ENSO MJO III Validation/Case study of Hurricane Helene IV Conclusions

  3. Schematic diagram Our goal is to yield an integrative understanding of Atlantic storm and hurricane development. The schematic diagram here shows the interrelationship among the phenomena of interest to our studies.

  4. AEJ Climatology (1989-2001) u600 Reanalysis minus MERRA MERRA ERA- Interim ERA-40 NCEP-R2 JRA-25 Over all, MERRA, ERA-40 (ERA-Interim) are closer to each other than the others

  5. Resolution MERRA: 0.5x0.625 degree ERA-40: T159L60 (~125km or 1.1 degree) NCEP-R2: T62L28 (~300Km or 2.5degree) JRA-25 (T106L40 (~190km or 1.5degree) ERA-Interim T255L60 (~80Km or 0.75 degree)

  6. Climatology of AEJ (shaded), and K(y), and d[u]dy (contour) – (1980-2001) K(y)=d(f(y)-d[u]/dy)/dy d[u]/dy MERRA ERA-40 NCEP-R2 JRA-25 Note collocation of the region that favors barotropic instability with main development region of Atlantic cyclogenesis

  7. Cross Section of AEJ (shaded), and K(y) and du/dy (contour) at 20W K(y)=d(f(y)-d[u]/dy)/dy d[u]/dy MERRA ERA-40 NCEP-R2 JRA-25

  8. Focus on Disturbances/Waves

  9. La Nina minus El Nino v700 hPa variance based on MERRA 6-hrly data for JAS 1980-2001 02_10day v-variance Spatial distribution suggests there might be two different sources of disturbances.

  10. La Nina minus El Nino Variance based on MERRA 6-hrly data for JAS 1980-2001 v700-variance 02_10day v-variance 06_09day 2.5_6day

  11. La Nina minus El Nino 700 hPa Variance for JAS 1980-2001 2.5_6day 06_09day MERRA ERA-40 v-variance NCEP-R2 JRA-25

  12. Climatology of v700 Variance (1980-2001) 2.5_6 day Percentage (%) MERRA ERA-40 NCEP-R2 JRA-25

  13. La Nina minus El Nino Climatology of Precipitation Variance (1980-2001) Percentage (%) 2.5_6 day MERRA ERA-40 NCEP-R2 JRA-25

  14. II Mechanisms Wave and Jet Interaction

  15. 2.5_6day Using v700 (2.5_6day filter) at a selected location as index to composite various fields

  16. Time evolution from -3day to 1day based on MERRA data for JAS u700 (shaded), streamfunction (contour) 55W and 22N 25W and 12N day -3 -2 -1 0lag 1 2 Jet serves as a wave guide/steering flow for the easterly waves

  17. Time evolution from -3day to 2day based on MERRA data for JAS u700 (shaded), -u’v’d[u]/dy (contour) 25W and 12N day 55W and 22N -3 -2 -1 0lag 1 2 Barotropic energy conversion dominant over the southern side of the Jet and also over the eastern side of the MDR

  18. Time evolution from -3day to 1day based on MERRA data for JAS u700 (shaded), -w’t’ (contour) 25W and 12N day 55W and 22N -3 -2 -1 0lag 1 2 Baroclinic energy conversion dominants over northern Africa

  19. Composites: [u] (shaded) and Energy Conversion (contour) 5W and 26N Land 25W and 12N Ocean -u’v’d[u]/dy -w’t’ -v’t’d[t]/dy

  20. II Mechanisms Mid-Latitude Influence (6-9 days)

  21. Revisit variance distribution and time scales -possible mid-latitude influence

  22. La Nina minus El Nino Variance based on MERRA 6-hrly data for JAS 1980-2001 2.5_09day v-variance 06_09day 2.5_6day

  23. La Nina minus El Nino Marginal Hilbert Spectra: v700 based onMERRA 6-hrly data for JAS in Neutral Years 20W and 26N 6-8day ~24day v-variance Spectral Density Frequency: cycle/JAS

  24. Nature of 6-9 day variance

  25. Lag Correlation Coefficient at 700 hPa Location of index v700 is marked on panel (0lag) 2.5_6day 20W and 12N 06_09day 20W and 26N day -3 -2 -1 0 1 2 3

  26. Lag Correlation Coefficient at 700 hPa; All data in 06_09 Index: v700 at 20W and 26N Streamline of u and v, v (shaded) day -6 -2 -5 -1 0 -4 -3 1

  27. Lag Correlation Coefficient at 700 hPa; All data in 06_09 Index: v700 at 20W and 26N Streamline of u and v, precitation(shaded) day -6 -2 -1 -5 0 -4 -3 1

  28. V-variance (2.5_6day) in JAS in Neutral Years between 1980-2001 based on MERRA V-variance (2.5_6day) in JAS in Neutral Years between 1980-2001 based on MERRA Wave CoherenceActivity MERRA ERA-40

  29. Hilbert Spectra based on MERRA v700 data at 45W and 22N for JAS in Neutral Years between 1980-2001 60day 2.5-6day 6-9day

  30. Composites of u, v, and z using v700 (2.5_6day) at (45W and 22N) as index for JAS in Neutral Years. Each panel is 1day apart from top down and left right. z(shaded), streamline (contour) of u and v at 700 hPa (2.5_9day) day day -5 1 2 -4 -3 3 -2 4 -1 5 6 0-lag

  31. II Mechanisms ENSO

  32. 1980-2001 Neutral Years: 1980, 81, 83, 84, 85, 89, 95, 96, 00, 01 El Nino Years: 1982, 86, 87, 90, 91, 92, 93, 94, 97 La Nina Years: 1988, 89, 99

  33. Seasonal Genesis Parameters (SGP, Gray, 1975) based on MERRA (1980-2001) El Nino La Nina SGP Wind Shear (u200 – u850) Within 10m/s Weighted RH (%) (700 to 500 hPa) SST > 26C

  34. JAS Climatology (1980_2001) Jet Difference at 600 hPa MERRA ERA-40 NCEP-R2 JRA-25 La Nina minus El Nino During La Nina Years, Jet Moves Northward in all reanalyses El Nino minus Neutral La Nina minus Neutral

  35. JAS Climatology (1980_2001): V-variance (2.5-6day) Difference at 600 hPa MERRA ERA-40 NCEP-R2 JRA-25 La Nina minus El Nino El Nino minus Neutral La Nina minus Neutral v600 variance (2.5-6day) increased during the La Nina in all Reanalyses

  36. II Mechanisms MJO

  37. Seasonal Genesis Parameters (SGP, Gray, 1975) based on MERRA Neutral Years SGP Wind Shear (u200 – u850) Within 10m/s Weighted RH (%) (700 to 500 hPa) SST > 26C

  38. Time Evolution from -10day to 6day MERRA JAS Using v700 (2.5_6day) at 15W and 14N as index to composite vlp200 (20-90days) 1980-2001 Neutral Years day -10 -8 -6 -4 -2 0 2 4 6

  39. JAS Climatology based on MERRA data VLP200 (20_90day) for Neutral Years Negative Phase over Africa/Atlantic and Adjacent Regions Positive Phase over Africa/Atlantic and Adjacent Regions v700 var (2.5-6day) Dif (Negative Phase minus Positive Phase) 1980-2001 Neutral Years

  40. Mechanism for Neutral Years MJO Impact: Negative Phase minus Positive Phase MERRA ERA-40 NCEP-R2 JRA-25 Phase dif Var dif When negative phase of the MJO is over Africa and adjacent regions, v600 variance (2.5_6) is increased over Western Africa and Eastern Atlantic

  41. Mechanism for Neutral Years MJO Impact on Precipitation: Negative Phase minus Positive Phase Neg-Phase Pos-Phase Dif VLP200 Precipitation Precipitation variance (2.5-6day) increased with negative phase of the MJO

  42. Conclusions (1 of 2) • AEJ: • All four reanalyses produce an AEJ, but they differ in details • MERRA is a little poleward and has less vertical tilt compared • to the others and is also stronger than the others. The NCEP AEJ • is shifted to the south. The JRA-25 AEJ is the weakest. MERRA • and ERA-40 (ERA-Interim) are closer to each other than the others • Gradients associated with energy conversions exhibit even • more differences, though again MERRA and ERA-40 (and • ERA-Interim) are closer • Variance: • Spatial distribution of wave activity has some frequency • dependence that is robust across the reanalyses: most regions • characterized by 2.5-6 days, but one region in particular is • dominated by 6-9 day time scales with links to middle latitudes

  43. Conclusions (2 of 2) • Mechanisms: • All reanalyses reveal similar impacts from ENSO. The Jet is more poleward • and variance is larger over the MDR when comparing La Nina to El Nino years. • All reanalyses also show that the MJO enhances wave activity over the eastern • side of the MDR during the negative phase (ascending branch) – this is seem most • clearly when limiting the composites to ENSO neutral years • Wave-Jet interaction • •The Jet serves as a wave guide or provides the steering flow for the wave • disturbances. • •Barotropic energy conversion dominants over the eastern side of MDR, and • baroclinic energy conversion dominants on the northern side of the Jet over the • western Africa • •Some of the waves appear to have a middle latitude influence contributing to • the local maximum of 6-9 day variability off the coast of Northern Africa, as • well as play a role in enhancing 2.5-6 day variability over the central Atlantic • Over all, MERRA products are useful new resource for carrying out studies to understand AEJ/AEWs and their downstream effects.

  44. Validation/A case study to show MERRA’s ability to produce Hurricanes Hurricane Helene, 2006

  45. Radiosonde at Cape Verde Island (23.5W, 14.9N) Meridional Wind (m/s) Zonal Wind (m/s) Obs MERRA ERA- Interim NCEP-R2 JRA-25 Aug 18 Aug 18 Sep 13 Sep 13 Aug 21 Debby Sep 12/13 Helene

  46. Time-Longitude Cross Section (11N-16N) at 600 hPa Debby v Helene rotational vorticity RH wind shear speed

  47. MERRA data at 12Z and Hurricane Helene, 2006, Starting on Sept 11 AEJ Axis (black), Curvature Vorticity (shaded), and Cyclone (black contour)

  48. MERRA data at 12Z and Hurricane Helene, 2006, Starting on Sept 11 AEJ Axis (black), Vorticity Gradient (shaded), and Cyclone (black contour)

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