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Analysis of the 1997 Floods over East and Central Africa

Analysis of the 1997 Floods over East and Central Africa. M. Bessafi 1 , B. Morel 1 , A. Babu 2 , L. Chang’a 3 , H. Kabengel 4 , S. Mwangi 5 1 LE2P, University of Reunion, Saint-Denis, Reunion 2 Met. Agency, Addis Ababa, Ethiopia 3 Met. Agency, Dar es Salam, Tanzania

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Analysis of the 1997 Floods over East and Central Africa

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  1. Analysis of the 1997 Floods over East and Central Africa M. Bessafi1, B. Morel1, A. Babu2, L. Chang’a3, H. Kabengel4, S. Mwangi51LE2P, University of Reunion, Saint-Denis, Reunion 2Met. Agency, Addis Ababa, Ethiopia 3Met. Agency, Dar es Salam, Tanzania 4Met. Agency, Kinshasa, D.R. Congo; 5Met. Agency, Nairobi, Kenya May 8, 2012 Geneva, Switzerland WWRP

  2. Case study For the predictability studies, the workshop identified a number of target flooding events on a sub-regional basis 2008 2009 2009 West Africa 2008Algeria 2008SouthernAfrica 1997 Central Africa (DR Congo) 1997 East Africa (Ethiopia, Kenya, Tanzania) 1997 1997 2008

  3. What’s known?OND 1997 Rainy Season Major global climate anomalies and episodicevents in 1997 (CPC, NOAA) " " WMO Report – No 877

  4. OND 1997 Rainy Season June-Dec 1997 accumulatedprecipitationanomaly (mm) Green shades: positive anomalies Brown shades:negative anomalies Regionalarea-averagedestimates of monthlymeanprecipitationamounts (mm) for the box regionsduring 1997 Blue line:monthlymean Red line:monthlyclimatology BAMS ClimateAssessment, 1997 http://www.ncdc.noaa.gov/bams-state-of-the-climate/ 1997 BAMS Climate Assessment indicates excessive rainfall over equatorial eastern Africa during OND 1997 in association with strong El Niño

  5. 0ctober Cumulative Rainfall (Station Observed) Dot is indicative of station observation Colors distinguish different precipitation amounts’ values In the anomaly figure shown below, the parma dots are indicative of NEGATIVE values 1997 Mean Anomaly

  6. 0ctober Cumulative Rainfall (Station Observed) During October 1997, wetter-than-normal conditions occurred across most East Africa The largest anomalies were recorded over south/southeast Ethiopia, Kenyan coast, and Tanzanian northern coast/northeastern Highland 1997 Mean Anomaly

  7. 0ctober Cumulative Rainfall Violet/blue shading: low precipitation amounts Yellow/orange shading: high precipitation amounts Station Satellite (GPCP) October 1997 Octobermean

  8. 0ctober Cumulative Rainfall Satellite (GPCP) October 1997 GPCP satellite data confirm station observations showing excessive rainfall over most Ethiopia, Kenya and Northern Tanzania Drier-than-normal conditions occurred to the West of East Africa (D.R. Congo) Octobermean

  9. 0ctober Cumulative Rainfall Satellite (GPCP) + October 1997 - GPCP satellite data confirm station observations showing excessive rainfall over most Ethiopia, Kenya and Northern Tanzania Drier-than-normal conditions occurred to the West of East Africa (D.R. Congo) suggesting a dipole structure over Africa Octobermean

  10. October 1997, station rainfall time series

  11. What was observed? • 0ctober Cumulative Rainfall Second row: Rainfall total during October 1997 averaged over selected stations across different areas over East Africa Percentage contributions to the climatological total for each area are shown inside the brackets Excessive rainfall (6 to 8 times the normal) was observed

  12. 0ctober Cumulative Rainfall (Station Observed) Second row:Rainfall total during 17-23/18-28 October 1997 averaged over selected stations acrossdifferent areas over East Africa Percentage contributions to the monthly total for each area are showninside the brackets 60% of the monthly total rainfall were received within 11 days during the period 18-28 October, 1997 in Ethiopia, and within 7 days during the period 17-23 October, 1997 in Kenya and Tanzania

  13. MeanRainfall Anomalies Satellite (CMAP) During October 1997 the tropical West Indian Ocean and the Eastern part of Africa experienced extreme rainfall Anomalies are computed as the deviation of the current monthly mean from the climatological average of the corresponding month for the period 1979 to 2007 Gray shading denotes area where rainfall anomalies exceed 3 times the standard deviation

  14. MeanRainfall Anomalies Satellite (CMAP) Standardizedrainfallanomaly Time series of standardized rainfall anomalies from 1981 to 2007 averaged over the East African domain (red box) 10S  15N 30E  45E Standardized anomalies are computed computed from raw data as these data minus the mean value and divided by the standard deviation October 1997 Time

  15. Evolution of climate indices: ENSO, IOD, NAO Standardizedrainfallanomaly During October 1997, MEI (SOI) was highly positive (negative) indicating a strong El Niño event IOD index was at its highest value indicating a strong positive IOD event Moderate negative NAO event SOI MEI Red shading: positive values Blue shading: negative values Positive MEI and negative SOI are indicative of El Niño conditions Vertical arrow is indicative of October 1997 IOD NAO

  16. Low October monthly mean OLR values associated with deep atmospheric convection are depicted over the equatorial land masses and the warm pool Octobermean state OLR & SST Orange shading: high OLR values Blue shading: low OLR values Low OLR is a proxy for the enhanced convection

  17. Low October monthly mean OLR values associated with deep atmospheric convection are depicted over the equatorial land masses and the warm pool Octobermean state OLR & SST Rainfall & SST Orange shading: high precipitation values Green shading: low precipitation values High mean precipitation is associated with warm SSTs and low mean OLR over the warm pool and with low mean OLR over the equatorial land masses

  18. Drier-than-normal conditions, positive OLR anomalies (redshading) Wetter-than-normal conditions, negative OLR anomalies (blueshading) OLR and SST anomalies duringOctober 1997 During October 1997, suppressed convection prevailed over the Eastern Indian Ocean, the Maritime Continent and the Western Pacific…

  19. Drier-than-normal conditions, positive OLR anomalies (redshading) Wetter-than-normal conditions, negative OLR anomalies (blueshading) OLR and SST anomalies duringOctober 1997 … while enhanced convection occurred across the Western Indian Ocean and the Central and equatorial Eastern Pacific associated with ENSO and IOD events The SST pattern shows positive (negative) anomalies over the regions of enhanced (suppressed) convection

  20. October 1997 Moisture Flux Anomaly Arrows: 850-hPa moisture flux anomaly Shades: magnitude of the 850-hPa moisture flux anomaly Consistent with the El Niño and positive IOD events during October 1997, a strong easterly moisture flux anomaly is evident over equatorial Western and Central Indian Ocean http://www.esrl.noaa.gov/psd/data/composites/nssl/day/index.html

  21. October 1997 Moisture Flux Anomaly Arrows: 850-hPa moisture flux anomaly Shades: magnitude of the 850-hPa moisture flux anomaly This flux converges over East Africa bringing large proportion of moisture from the Indian Ocean to this region which results in enhanced convection and increased rainfall http://www.esrl.noaa.gov/psd/data/composites/nssl/day/index.html

  22. What was the status of the Madden-Julian Oscillation and the convectively coupled equatorial waves?

  23. Raw power spectra of OLR in 15S-15N band for years 1979-2000 Separately for anti-symmetric and symmetric parts about the equator Normalized power spectra Convectively-coupled equatorial waves (CCEWs) Courtesy of NCAR, adapted from Wheeler and Kiladis (1999) MJO

  24. Normalized Principal Component time series of wave-filtered OLR 1st Jan. 96 – 31stDec. 97 Madden-Julian Oscillation (MJO) Bessafi and Wheeler, 2006 During October 1997, the MJO was almost inexistent The MJO in October 1997 had the weakest activity in the period 1989-2005 in comparison with the other October months Line: October’s mean of MJO variance ≡ amplitude (PC1²+PC2²) of the 91-day running mean of daily MJO’s PC1 and PC2 over 1989-2005

  25. Normalized Principal Component time series of wave-filtered OLR Dashed horizontal line:one σ Solid horizontal line:twoσ During October 1997, MRG activity was quite normal (amplitude of PC1 and PC2 within 1 σ) ER activity was strong PC1 (PC2) had the lowest value of the year 1997 around October 10 (15) with this lowest value exceeding around 3 σ and indicating strong negative OLR anomalies and thus enhanced convection Equatorial Rossby (ER) Mixed Rossby-Gravity (MRG)

  26. ER Index – Evolution over October 1997 • The axes represent daily values of the principal components from the two leading modes (PC1, PC2) • The triangular areas indicate the location of the enhanced phase of the ER • Anticounter-clockwise motion is indicative of westward propagation The dots and numbers are indicative of the daily observations • Distance from the origin is proportional to ER strength

  27. ER Index – Evolution over October 1997 ER wave spanned the entire Indian Ocean during 1-27 October 1997 Significant westward propagation started on October 6 over the Maritime continent From there ER wave rapidly strengthened attaining maximum intensity on October 9, and reaching rapidly Western Indian Ocean on October 16 with the highest amplitude of the month Following this period ER-wave activity decreased drastically over Eastern African from October 18 and became a weak quasi-stationary wave until October 25 staying a dozen days over East Africa

  28. Spatial map of ER-filtered OLR anomalies in October 1997 Dashed contours/gray shading: negative OLR values indicative of enhanced convection Solid contours: positive OLR values 5/10 At the beginning of October, the western Indian ocean was concerned with negative OLR anomalies which were located around 15° of latitude At the same time, a second negative ER wave train appeared over the eastern part of Indian Ocean propagating then westward and equatorward during the next days 10/10 15/10

  29. Spatial map of ER-filtered OLR anomalies in October 1997 17/10 The ER wave train converged towards East Africa in the second half of October At the time of the heavy rainfalls, eastern Africa was progressively concerned by the north and south symmetric components negative OLR anomalies especially of the ER wave train 18/10 19/10 20/10

  30. Conclusions and Future Work • Excess (deficit) rainfall was observed over East (Central) Africa during October 1997 • Record rainfall was depicted across the south/southeast Ethiopia, Kenyan coast, and Tanzanian northern coast/northeastern Highland in the second half of October 1997 • Assessment of the state of global to regional phenomena indicates strong El Niño and positive IOD events as well as quasi-inexistent MJO and significant ER wave activity occurring at this time period • Next steps to achieve Phase #1 of work plan: synoptic description of the flooding episode; conceptual model of the event

  31. Thanks for your attention

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