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Variability of the Atlantic ITCZ Associated with

Variability of the Atlantic ITCZ Associated with Amazon Rainfall and Convectively Coupled Kelvin Waves Hui Wang and Rong Fu School of Earth and Atmospheric Scoences Georgia Institute of Technology Atlanta, Georgia The 30 th Annual Climate Diagnostics and Prediction Workshop

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Variability of the Atlantic ITCZ Associated with

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  1. Variability of the Atlantic ITCZ Associated with Amazon Rainfall and Convectively Coupled Kelvin Waves Hui Wang and Rong Fu School of Earth and Atmospheric Scoences Georgia Institute of Technology Atlanta, Georgia The 30th Annual Climate Diagnostics and Prediction Workshop The Pennsylvania State University October 24-28, 2005

  2. Introduction TAV: Tropical Atlantic Variability -- Rainfall and SST TACE: Tropical Atlantic Climate Experiment 2006 – 2011 AMMA: African Monsoon Multidisciplinary Analysis 2006 – 2008 AMI: Study of Processes in the Atlantic Marine ITCZ 2007 ITCZ AMI AMMA ? SAMS SST TACE These projects operate in the same region of interest, but serve different goals.

  3. Seasonal Migration Rainfall Climatology In boreal spring, both the ITCZ and Amazon convection merge into one convective zone centered at the Equator. January April July October mm/day Data: TRMM 2000–03

  4. Boreal Spring AMI: rapid intensification SST: maximum variability Correlation: SST vs. NINO 3 SSTA: dominated by air-sea interaction within the Atlantic. Largest uncertainty in prediction of springtime SST. Saravanan and Chang, J. Climate, 2000

  5. Zonal Variation • West-East oscillation • Stronger in west phase Two Consecutive 4-day Mean Precipitation April 13–16, 2000 April 17–20, 2000 West Phase East Phase mm/day Data: TRMM

  6. Questions to be addressed: • What causes the zonal variation of the Atlantic ITCZ? • What causes the intensification of the ITCZ in the • west phase? • How does the zonal variation of the ITCZ relate to • precipitation in South America and Africa?

  7. Data Daily mean dataApril 2000-03 (4 years) Satellite Data TRMMRainfall, 1o × 1o QuikSCATOcean surface winds, 1o × 1o Reanalysis Data NCEP/NCAR Reanalysis2.5o × 2.5o

  8. Amazon vs. Atlantic ITCZ Daily rainfall April 2000–03 Period: 6-7 days Zonal oscillation of the Atlantic ITCZ appears to be coupled to the changes of Amazon rainfall. Amazon Atlantic ITCZ used for composite analysis 2000 2001 2002 2003 Time Series Days Zonal structure SVD Mode 1 Amazon Atlantic ITCZ Spatial Patterns Data: TRMM

  9. West and East Phases Linear Regression April 2000–03 West Phase ITCZ Index = 1 East Phase ITCZ Index = – 1 The first SVD mode well captures the zonal oscillation of the ITCZ. mm/day Data: TRMM

  10. Signal in QuikSCAT Wind Composite of Rainfall and Ocean Surface Wind Anomalies April 2000-2003 Eastward propagation Kelvin waves Phase speed: 10-12 m/s Day 0 Day 1 Day 2 Day 3 Day -3 Day -2 Day -1 Data: TRMM & QSCAT

  11. Equatorial Waves Wavenumber - Frequency Kelvin Wave Eastward propagating Life time: 6 – 7 days Phase speed: 10–12 m/s Zonal wavenumber 6 Wheeler and Kiladis (JAS, 1999)

  12. Kelvin Wave: Zonal Wind Structure Westerly to the west of convection Easterly to the east of convection Composite: Longitude-Time Diagram of Rainfall and Surface Zonal Wind Anomalies at the Equator 5 3 1 -1 -3 -5 Contour: wind Shading: rainfall Westerly Easterly Day 0 Day Easterly Westerly S. America Africa mm/day Data: TRMM & QSCAT

  13. Individual Events Waves can reach Africa Precipitation and Zonal Wind Anomalies at the Equator High pass filter: f > 1 / (20 day) Lat. = 0 Contour: wind Shading: rainfall 1-30 April 2002 Westerly Easterly S. America Atlantic Africa mm/day Data: TRMM & QSCAT

  14. 200-hPa Circulation Composites of 200-hPa Height (contour), Wind (vector) and Precipitation (shading) Anomalies Kelvin wave (Matsuno 1966) Equator Convection Day 0 L H Day -3 Day 1 Day -2 Day 2 Day -1 Day 3 Data: NCEP/NCAR Reanalysis -5 5 mm/day

  15. Vertical Cross-Section Composite: Vertical Velocity and x-z Wind Anomalies at the Equator Day 0 Day -3 Day 1 Day -2 Day 2 Day -1 Day 3 Data: NCEP/NCAR Reanalysis

  16. What may cause the intensification of rainfall in W. Atlantic? Composite of SLP and Precipitation Anomaly Tomas & Webster (1997): Cross-equatorial pressure gradient --- regions between the equator and zero absolute vorticity contour are inertially unstable. Day -3 Day 0 Zero absolute vorticity contour at 925 hPa Small cross-equatorial pressure gradient Large cross-equatorial pressure gradient Stronger cross-equatorial pressure gradient near zero absolute vorticity in the W. Atlantic can enhance inertial instability, and may contribute to strong near-equatorial convection in the western Atlantic.

  17. Relation to MJO No significant correlation between high frequency (6-7 days) variability of the Atlantic ITCZ and precipitation (either unfiltered or 30-90 day band-pass filtered data) in other equatorial regions. Precipitation at the Equator 30-90 day band-pass filter Indian Ocean Western Pacific S. Amer Atlantic Kelvin wave MJO 03/01/02 – 01/06/02 mm/day

  18. Conclusions • Using satellite data (TRMM & QuikSCAT), we have found that convectively coupled Kelvin waves appear to dominate the changes of zonal structure of the Atlantic ITCZ in boreal spring. • These waves originate from deep convection in the equatorial South America, then propagate eastward across the Atlantic, and result in a zonal oscillation of the ITCZ. 3. A strengthened cross-equatorial pressure gradient and associated inertial instability probably intensify the ITCZ in the W. Eq. Atlantic, leading to a stronger ITCZ in the west phase.

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