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peter j. webster: georgia institute of technology

Variability of the Indian Ocean monsoon system on timescales of weeks to years. peter j. webster: georgia institute of technology. Outline:. Brief description of coupled phenomena Weather Intraseasonal varaibility Interannual variability Regulation Predictability. resource.

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peter j. webster: georgia institute of technology

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  1. Variability of the Indian Ocean monsoon system on timescales of weeks to years peter j. webster: georgia institute of technology IOGOOS: November 4-9, 2002

  2. Outline: • Brief description of coupled phenomena • Weather • Intraseasonal varaibility • Interannual variability • Regulation • Predictability IOGOOS: November 4-9, 2002

  3. IOGOOS: November 4-9, 2002

  4. resource IOGOOS: November 4-9, 2002

  5. and hazard……… IOGOOS: November 4-9, 2002

  6. description IOGOOS: November 4-9, 2002

  7. mean august and january precipitation IOGOOS: November 4-9, 2002

  8. IOGOOS: November 4-9, 2002

  9. Annual cycle of convection (OLR) in the Indian Ocean-South Asia region IOGOOS: November 4-9, 2002

  10. annual cycle of surface winds over Indian Ocean and South Asia IOGOOS: November 4-9, 2002

  11. IOGOOS: November 4-9, 2002

  12. IOGOOS: November 4-9, 2002

  13. Monsoon regions are the largest region where precipitation exceeds evaporation. This excess water must come from somewhere … IOGOOS: November 4-9, 2002

  14. …….which defines the scale of the monsoon. The monsoon is intrinsically inter-hemispheric with the winter hemisphere being the source of moisture IOGOOS: November 4-9, 2002

  15. Monsoon tied to both zonal & meridional land-sea contrasts IOGOOS: November 4-9, 2002

  16. To a large degree the system seems “well behaved” and appears to act as a large scale sea-breeze system. But there are surprises and issues that are difficult to understand IOGOOS: November 4-9, 2002

  17. Time Scales of Monsoon Variability • Interannual variability: Variations on the annual cycle of the monsoon generated by SST/land surface variations in PO and IO. • Monsoon Weather: Monsoon lows and depressions, tropical cyclones (hurricanes) and etc. Produce short-lived local flooding (or drought), erosion, high winds and etc. • Intraseasonal Variability: “Envelopes”: or clusters of weather events producing 20-40 day droughts or flood periods. Arguably most important but also most difficult to forecast IOGOOS: November 4-9, 2002

  18. IOGOOS: November 4-9, 2002

  19. IOGOOS: November 4-9, 2002

  20. Persistence of SST over 6-month period North Indian Ocean shows strong persistence (I.e., if you know the SST now, you know it 6 months from now). Much more persistent than in Pacific Ocean which contains springtime “persistence barrier: Inference:predictability exists in Indian Ocean. IOGOOS: November 4-9, 2002

  21. El Nino influences Indian Ocean Changes SST, sea surface slope and regions of maximum precipitation But we are uncertain how this influence is manifested and large amounts of variance are unexplained IOGOOS: November 4-9, 2002

  22. Relationship Between Indian Monsoon Rainfall and ENSO La Nina El Nino IOGOOS: November 4-9, 2002

  23. Decadal Changes in Monsoon-ENSO Relationships Correlations over 125 years average about -0.6 but there are long periods where the correlation drops to very small numbers. IOGOOS: November 4-9, 2002

  24. also……….. Four near normal years show very different precipitation patterns. ENSO offers only “broad-brush” prediction of rainfall over India. Does rainfall over India define a strong or weak monsoon? Are fine “fine brush” forecasts possible? IOGOOS: November 4-9, 2002

  25. Other modes of variability? • are there other modes of variability besides El Nino? • Is there predictability in the these modes? • Are these phenmena independent of El Nino? • Is there a basic building block of variability in the Indian Ocean/monsoon regime? • Can understanding these phenomena help design n observing array in the Indian Ocean? IOGOOS: November 4-9, 2002

  26. Indian Ocean SST and All-India Rainfall IOGOOS: November 4-9, 2002

  27. Indian Ocean Zonal Mode or Dipole • “Recently” discovered mode of variability following 1997-1998 period (earlier evidence exists, though) • Exhibits strong coupled characteristics between ocean and atmosphere • Appears linked to ENSO and to vigor of monsoon • Strongly tied to annual cycle as is El Nino in Pacific Ocean • Strongly tied to climate variability especially equinoctial “short rains” in East Africa • Vigorous for decades and occasionally takes a statistical vacation IOGOOS: November 4-9, 2002

  28. most unusual events in 1997 IOGOOS: November 4-9, 2002

  29. IOGOOS: November 4-9, 2002

  30. IOGOOS: November 4-9, 2002

  31. Coupled mechanism for the Indian Ocean Zonal Mode IOGOOS: November 4-9, 2002 Webster et al. (1999)

  32. Composite growth of the positive phase of IOZM (i) IOGOOS: November 4-9, 2002

  33. Composite growth of the positive phase of IOZM (ii) IOGOOS: November 4-9, 2002

  34. Composite growth of the negative phase of IOZM (i) IOGOOS: November 4-9, 2002

  35. Composite growth of the negative phase of IOZM (ii) IOGOOS: November 4-9, 2002

  36. Indian Ocean Zonal Mode: Inherent mode of Indian Ocean invoked by outside perturbations including ENSO Time series of E-W SST gradient frequency spectra IOGOOS: November 4-9, 2002

  37. IOGOOS: November 4-9, 2002

  38. IOGOOS: November 4-9, 2002

  39. IOGOOS: November 4-9, 2002

  40. Sea-level variations associated with IOZM: Positive phase: warm SST WIO, low SSH EIO Negative phase: warm SST EIO, high SSH EIO IOGOOS: November 4-9, 2002

  41. Intraseasonal variability in Indian Ocean • Predominant feature of IO system • Arguably the most important time scale for prediction (variability on 20-40 day scales far larger than interannual • Possess features very similar to interannual variability • Shows great robustness in behavior, effect and duration • A chaotic instability? Predictability? • Is intraseasonal variability a coupled ocean-atmosphere phenomenon? IOGOOS: November 4-9, 2002

  42. Intraseasonal Modes: Impact on Rainfall • Intraseasonal variability imposes a distinct form to precipitation. • Histograms of precipitation, shown for 9 years show distinct wet periods with lulls in between. • Lower diagram shows the distinct spatial character Precipitation histograms in central India IOGOOS: November 4-9, 2002

  43. Pattern of Intraseasonal Modes IOGOOS: November 4-9, 2002

  44. IOGOOS: November 4-9, 2002

  45. Temporal evolution of MISO along 90E: 1995 Active phases of the monsoon commence near the equator and propagate northward (and southward) across South Asia. Active phase northward propagation commence active phase IOGOOS: November 4-9, 2002

  46. Latitude-time section of OLR along 90E: 1988 IOGOOS: November 4-9, 2002

  47. Sequence of Monsoon Intraseasonal Variability Sequence shows the evolution of the “average” MISO over a 30-day period IOGOOS: November 4-9, 2002

  48. Composite wind stress during intraseasonal oscillation IOGOOS: November 4-9, 2002

  49. IOGOOS: November 4-9, 2002

  50. Composite zonally & vertically heat flux during intraseasonal oscillation 20ºN -0.7PW +0.5PW 10ºN latitude 0° 10ºS 20ºS -5 +10 +15 -10 +5 0 -15 day IOGOOS: November 4-9, 2002

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