Variability of the Atlantic ITCZ Associated with Amazon

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

Introduction TAV: Tropical Atlantic Variability -- Rainfall and SST ITCZ AMI AMMA SAMS ? 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 These projects operate in the same region of interest, but serve different goals.

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 mm/day October Data: TRMM 2000– 03

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

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

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 data April 2000 -03 (4 years) Satellite Data TRMM Rainfall, 1 o × 1 o Quik. SCAT Ocean surface winds, 1 o × 1 o Reanalysis Data NCEP/NCAR Reanalysis 2. 5 o × 2. 5 o

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

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

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

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

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 Contour: wind Shading: rainfall 3 rly e t es 1 Day W s Ea Day 0 es te W Ea rly ly er t s -1 -3 -5 ly r te S. America Africa mm/day Data: TRMM & QSCAT

Waves can reach Africa Individual Events Precipitation and Zonal Wind Anomalies at the Equator High pass filter: f > 1 / (20 day) Lat. = 0 1 -30 April 2002 Contour: wind Shading: rainfall rly e t es W S. America ly r te s Ea Atlantic Africa mm/day Data: TRMM & QSCAT

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

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

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 Zero absolute vorticity contour at 925 h. Pa Small cross-equatorial pressure gradient Large cross-equatorial pressure gradient Day 0 Stronger cross-equatorial pressure gradient near zero absolute vorticity in the W. Atlantic can enhance inertial instability, and may contribute to strong nearequatorial convection in the western Atlantic.

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 03/01/02 – 01/06/02 Indian Ocean Western Pacific 30 -90 day band-pass filter S. Amer Atlantic Kelvin wave MJO mm/day

Conclusions 1. Using satellite data (TRMM & Quik. SCAT), 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.