Alice M. Grimm Marcia T. Zilli Federal University of Paraná, Curitiba, Paraná, Brazil

1. CONNECTION BETWEEN EARLY AND PEAK SUMMER MONSOON PRECIPITATION IN SOUTH AMERICA Alice M. Grimm Marcia T. Zilli Federal Universityof Paraná, Curitiba, Paraná, Brazil grimm@fisica.ufpr.br Jeremy Pal and Filippo Giorgi The Abdus Salam International Centre for Theoretical Physics, Trieste - Italy

2. MOTIVATION • Summer is the rainy season in most of the SA continent. The quality of the monsoon is important for agriculture, hydroelectric power generation, and water management. Very densely populated areas are severely affected by oscillations in the South Atlantic Convergence Zone (SACZ), whose enhancement frequently causes urban floods and landslides. It is therefore important to understand the mechanisms responsible for the variability of the South American Monsoon (SAM) in order to improve the ability to predict it. • The ENSO impact shows strong regional differences and strong changes within the monsoon season, from spring to peak summer, suggesting a relationship between late spring and peak summer, with an important role of regional processes during the peak monsoon. A surface-atmosphere interaction hypothesis involving soil moisture in spring has been proposed to explain this relationship, with a role for the highlands in Southeast Brazil (Grimm 2003, 2004).

3. MOTIVATION Figura 1. Áreas de Característica Operativa (ROC) para as previsões de precipitação de DJF baixo do normal com um mês de antecedência, para o período 1981-2001. A previsão de multi-modelos se baseia em ponderar igualmente as previsões probabilísticas de 3 categorias de 8 CGCMs (painéis superiores) y 3 AGCMs forçados por TSMs prognosticadas (painel inferior). A curva ROC demarca o equilibrio entre eventos “exitosos” e “falsos alarmes” para prognósticos com níveis de probabilidade variável. A falta de discriminação (taxa de éxitos=taxa de falsos alarmes) indica falta de desempenho do modelo, gerando áreas ROC de 0.5. Valores de área ROC menores que 0.5 indicam desempenho negativo e os valores maiores, desempenho positivo, com valor máximo 1.0. A região Centro-leste do Brasil está marcada no último painel inferior direito (Cavalcanti et al. 2006).

4. OBJECTIVES • To verify whether antecedent anomalous conditions in spring influence the summer monsoon not only in ENSO but also in non-ENSO years. • To observationally verify possible links between rainfall in spring and temperature in late spring, as well as between temperature in late spring and rainfall in the peak monsoon season. • To verify the impact of the relationships between spring and summer rainfall on river streamflows. • To investigate the possible role of soil moisture, SST and topography through sensitivity experiments with a regional model.

5. ENSO-related variations La Niña El Niño - moisture flux El Niño Nov Jan Similar intraseasonal changes are present in other years as well… (Grimm, 2003; 2004)

6. SPRING EOFs EOF1 - 18.5 % EOF2 - 16.7 % Grimm and Zilli 2007

7. SUMMER EOFs EOF1 - 22.0 % EOF2 - 10.1 % Grimm and Zilli 2007

8. RESULTS – NOVEMBER AND JANUARY REOFs November REOF1 - 14.6 % January REOF1 - 23.4 % Grimm and Zilli 2007

9. CORRELATION BETWEEN SPRING AND SUMMER PCs November Spring EOF1 EOF1 EOF2 REOF1 r=0.24 r=0.29 r=-0.31 r=-0.32 January Summer EOF1 EOF1 EOF2 REOF1 Grimm and Zilli 2007

10. Spring dry conditions in central-east Brazil More net surface solar radiation and higher SST off southeast Brazil coast in late spring Less soil moisture / Higher surface temperature in late spring Lower surface pressure / more convergence / Enhanced convection over the highlands in southeast Brazil Cyclonic circulation anomaly over southeast Brazil Moisture flux / convergence into central-east Brazil Peak summer wet conditions in central-east Brazil HYPOTHESIS Diagram of the pathway through which spring anomalous dry conditions may lead to subsequent peak summer wet conditions in central-east Brazil. The above diagram is also valid for opposite anomalies, starting from spring wet conditions in central-east Brazil.

11. SCHEMATIC DIAGRAM OF SPRING-SUMMER INVERSE RELATIONSHIP Schematic evolution from (a) spring dry conditions to (b) peak summer wet conditions in Central-east Brazil, through decreasing low-level pressure, convergence and cyclonic anomaly over southeast Brazil. (Grimm, Pal, and Giorgi 2007)

12. TEMPERATURE-PRECIPITATION RELATIONSHIPS Correlation between precipitation in periods within spring and following summer. Correlation between temperature in periods within spring and precipitation averaged over region P2 in following summer. Grimm, Pal and Giorgi (2007)

13. TEMPERATURE-PRECIPITATION RELATIONSHIPS Positive significant correlation of surface air temperature in November vs precipitation averaged in the bold rectangle in January (Grimm, Pal and Giorgi 2007). Negative significant correlation of October-November precipitation vs November surface air temperature averaged in 2º X 2º areas (Grimm, Pal and Giorgi 2007).

14. 15-DAY SLIDING MEAN PRECIPITATION Spring EOF1 Summer EOF1 Composite evolution of the 30-day running mean precipitation (mm day-1), averaged over Central-East Brazil for all years (black line) and (left) for years in which spring (summer) PC1 is above 0.5 standard deviation (green (blue) line) or (right) for years in which spring (summer) PC1 is below -0.5 standard deviation (green (blue) line).

15. RELATIONSHIPS BETWEEN JANUARY AND NOVEMBER STREAMFLOW Example: Três Marias Composite evolution of the 30-day running mean streamflow in Três Marias (left) for years in which spring (summer) PC1 is above 0.5 standard deviation (green (blue) line) or (right) for years in which spring (summer) PC1 is below -0.5 standard deviation (green (blue) line). Spring EOF1 Summer EOF1

16. MODELING STUDIES(Grimm, Pal and Giorgi 2007) Response to 0.5 soil moisture in Central-East Brazil Response to 1.5 soil moisture in Central-East Brazil

17. MODELING STUDIES(Grimm, Pal and Giorgi 2007) Response to reduced soil moisture + warmer SST Response to reduced topography in eastern S. America Realistic topography in eastern South America

18. SCHEMATIC DIAGRAM OF SPRING-SUMMER INVERSE RELATIONSHIP Schematic evolution from (a) spring dry conditions to (b) peak summer wet conditions in Central-east Brazil, through decreasing low-level pressure, convergence and cyclonic anomaly over southeast Brazil. (Grimm, Pal, and Giorgi 2007)

19. CONCLUSIONS • There is an inverse relationship between precipitation in spring and summer in Central-East Brazil, provided that the precipitation anomalies during spring are strong enough. This relationship manifests itself in the connection between the first modes of precipitation variability in spring and summer, as well as in the correlation analysis of precipitation in the two seasons. • Remote influences from SST anomalies are stronger in spring than in summer. This lends support to the hypothesis that the reversal of anomalies in summer might be associated with regional processes of surface-atmosphere interaction that are important during the monsoon season. • The links in that hipothesis are supported by sensitivity experiments of a regional model to soil moisture anomalies, to SST anomalies off the southeast coast of Brazil and to topography in eastern South America. CRN 055

20. References Grimm, A.M., J. Pal, and F. Giorgi, 2007: Connection between Spring Conditions and Peak Summer Monsoon Rainfall in South America: Role of Soil Moisture, Surface Temperature, and Topography in Eastern Brazil. Journal of Climate. In Press. Grimm, A.M., 2003: The El Niño impact on the summer monsoon in Brazil: regional processes versus remote influences. J. Climate,16, 263–280. Grimm, A.M., 2004: How do La Niña events disturb the summer monsoon system in Brazil? Climate Dynamics, 22, n. 2-3, 123-138. Grimm, A.M., and M.T. Zilli, 2007: Interannual variability and seasonal evolution of summer monsoon rainfall in South America. In preparation.