Simulating Rainfall Extremes in South America:

Sensitivity to remote and local forcing

Anji Seth and Maisa Rojas

International Research Institute for Climate Prediction

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

South America: Population Density

• coastal populations• ecosystem diversity• arid Nordeste: 60% rural• Southeast: population And economic center, agricultural breadbasket

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

CLIVAR/VAMOS, Kousky/Halpert

Features of South American Warm Season Rains

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

• Predictability exists, particularly in Nordeste

Motivation

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Rainfall anomalies in Nordeste, 1850-1995

Rainfall, SLP, Surface wind anomalies

SST and Surface wind anomalies

Wallace et al., 1998

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

• Predictability exists, particularly in Nordeste

• GCMs have good anomaly correlation skill in tropical South America

Motivation

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

• Predictability exists, particularly in Nordeste

• GCMs have good anomaly correlation skill in tropical South America

• RegCMs have potential to improve spatial and temporal characteristics of seasonal rainfall characteristics

Motivation

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Outline

• Simulation: FMAM 1983, 1985

• Sensitivity: Remote Forcing – Pacific/Atlantic SST

• Sensitivity: Local Forcing – Amazon moisture

• Sensitivity: Domain Effects – Boundary issues

• Conclusions

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

FMAM SST Anomalies

1983 1985

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

RegCM2: Giorgi et al., 1993

• 110x145x14• 60 km resolution• Remote:

Reanalyses 83,85 CCM3(x3) 83,85

• Local: deforestation

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Sensitivity to (Remote) Large Scale Forcing

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Sensitivity to Local Forcing

The interior Amazon rainfall does not appear to be sensitive to SST forcing.

Is the region more sensitive to local forcing?

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Sensitivity to Local Forcing

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Sensitivity to Local Forcing

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Sensitivity to Domain

•Is the local forcing sensitivity dependent on domain?

•Can a large domain improve the nested model simulation of the ITCZ?

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Sensitivity to Domain

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Conclusions/Lessons• Simulation: Large Scale Forcing is of 1st order importance in determining the seasonal rainfall anomalies. Test the RegCM with more than one set of BCs

• Sensitivity: Remote Forcing. Good anomaly correlation skill is necessary but not sufficient for GCMs. Low level winds and moisture are important. Know your GCM biases!

• Sensitivity: Local Forcing appears to affect the regions upstream and downstream from the Amazon basin. Evidence suggests that Amazon deforestation can affect scales larger than were included in the CONTROL domain. Do deforestation studies require global high-res models?

• Sensitivity: A large domain can potentially improve upon the GCM circulation, and showed the best seasonal evolution of rainfall. Place lateral boundaries where GCM has skill.

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003

Workshop on Theory and Use of Regional Climate Models ICTP, Trieste June 2003