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 The Pennsylvania State University October 24-28, 2005

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

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

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

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

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?

Data Daily mean dataApril (4 years) Satellite Data TRMM Rainfall, 1 o × 1 o QuikSCAT Ocean surface winds, 1 o × 1 o Reanalysis Data NCEP/NCAR Reanalysis2.5 o × 2.5 o

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

mm/day 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. Data: TRMM

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

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

Day mm/day Westerly Easterly Westerly S. AmericaAfrica Data: TRMM & QSCAT 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 Day 0 Contour: wind Shading: rainfall

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

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

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

What may cause the intensification of rainfall in W. Atlantic? Small cross-equatorial pressure gradient Large cross-equatorial pressure gradient Composite of SLP and Precipitation Anomaly Zero absolute vorticity contour at 925 hPa Tomas & Webster (1997): Cross-equatorial pressure gradient --- regions between the equator and zero absolute vorticity contour are inertially unstable. 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. Day -3 Day 0

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

Conclusions 1.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. 2.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.