Michelle S. Reboita , Rosmeri P. da Rocha and Tércio Ambrizzi

1. Topography and Surface Heat Fluxes Influence intheExtratropical Cyclones Simulated byRegCM3 on the South Atlantic Michelle S. Reboita, Rosmeri P. da Rocha and Tércio Ambrizzi Department of Atmospheric Sciences University of São Paulo – Brazil

2. Motivation and Objectives DJFMA JJASON • South Atlantic Ocean near the South America east coast South America east is a cyclogenetic region. • The dynamics of the generation of extratropical cyclones in this region is not well understood yet. There is a lack of observational data and numerical experiments. • It will be discussed here the importance of the topography and surface heat turbulent fluxes on the extratropical cyclogenesis during 1990 over the South Atlantic Ocean through regional climate simulations. A climatologically study is under way and it will be presented elsewhere. Source: Sinclair, 1996 Cyclogenetic Density 1990-1999 Density= (systems number/km2) 104 In the color scale 8 ~ 200 systems Source: Reboita et al., 2007

3. Simulation Domain Methodology Four simulations were done with the Regional Climate Model version 3 (RegCM3) • Area: 60º S to 5º S and 84º W to 14 º E • Period: September 1989 to January 1991 • Vertical levels: 18 • Horizontal resolution: 60 km • Integration time: 120 s • Initial and lateral boundary conditions: NCEP reanalyzes dt = 6 hs • Bottom boundary condition in the ocean: Monthly SST (Reynolds et al., 2002) • Convective Precipitation Scheme: Grell with the Fritsch-Chappell closure (Grell, 1993) • Ocean Flux Parameterization: Zeng (Zeng et al., 1998)

4. Simulations 1. Control Experiment (ExpCTRL) everything turned on 2. Without topography (ExpTopo) topography in the model = 0 3. Without sensible and latent heat turbulent fluxes over the sea (ExpFlux) the turbulent fluxes were turn off 4. Without topography in the South and Southeast of Brazil and Argentina east coast (ExpTopo2)

5. Cyclones Tracking • Algorithm: similar to Sinclair (1994, 1995 and 1997) • Cyclones identified through of the relative vorticity minima in the 10 m height wind • Area: 70oW to 10oE and 55oS to 15oS • Tracking only over ocean • Identification Criteria : • Threshold   -1.5 x 10-5 s-1 • Lifetime  24 hs • The cyclones mean density was calculated as the rate of the systems number in 5o x 5o regions by the area using the same procedure as in Murray and Simmonds (1991). To improve the results presentation the output was multiplied by 104. Tracking Area

6. Results b) a) Increase 8% Decrease 11.8% 34.0% Annual (a) and monthly (b) totals of the cyclogeneses in the NCEP and in the experiments. Differences between NCEP and ExpCTRL are due the limited area

7. decrease decrease decrease similar decrease decrease Anual Density – ExpCTRL Anual Density – NCEP a) b) Maximum toward west Annual Density - ExpFlux Annual Density - ExpTopo Affect South American east coast d) c) Affect south/southeast of Brazil, south of Argentina and the central region of the South Atlantic. Cyclogenetic density in 1990-year in: a) NCEP; b) ExpCTRL; c) ExpTopo and d) ExpFlux. Density is equal to (systems number/km2)104. In the color scale 0.8 ~ 20 sytesms.

8. Results ExpCTRL and ExpFlux showed similar results in the cyclogenetic density fields over Uruguay suggesting that other dynamic mechanisms such as the baroclinicity and topography effects are more important in this area. ExpTopo showed a large reduction of cyclogenesis in the Uruguay coast confirming the importance of the Andean mountains to these systems development. Conceptual model of the SALLJ physical environment during the wet season (Marengo et al. 2004 - JC)

9. Less intense LLJ disappear a) b) c) d) Intensity and direction of the wind at 850 hPa in the Summer (DJF)

10. Div(qv) 850 hPa Composites RG1 and RG2 All cyclones that occurred in 1990-year over each region Simulation Domain Analysis Domain

11. Div(qv) 850 hPa Composites – RG1 LLJ position All cyclones that occurred in 1990-year over south/southeast of Brazil LLJ is not well configured Anticyclone intensified NCEP ExpCTRL ExpFlux ExpTopo day (-1) day (0) day (+1)

12. Div(qv) 850 hPa Composites – RG2 LLJ position All cyclones that occurred in 1990-year over Uruguay NCEP ExpCTRL ExpFlux ExpTopo day (-1) day (0) day (+1)

13. Mean Sea Level Pressure Composites – RG1 NCEP ExpCTRL

14. Mean Sea Level Pressure Composites – RG2 NCEP ExpCTRL

15. Annual Density ExpCTRL ExpTopo ExpTopo2 Cyclogenetic density in 1990-year in: ExpCTRL; ExpTopo and ExpTopo2. Density is equal to (systems number/km2)104. In the color scale 0.8 ~ 20 sytesms.

16. RG1 LLJ is not configured Anticyclone intensified ExpTopo2 ExpTopo ExpCTRL day (-1) day (0) day (+1)

17. RG2 ExpTopo2 ExpCTRL ExpTopo day (-1) day (0) day (+1)

18. Conclusions • The removal of the topography affected the cyclogeneses in the south/southeast of Brazil and Uruguay because it does not favor the presence of the LLJ which is responsible for the transport of humidity from the Amazon to these regions. • The cyclogeneses in the south/southeast of Brazil and Uruguay depends on the LLJ position. • ExpTopo2 mostra um anticiclone mais intenso que transporta mais umidade para o sul/sudeste do Brasil favorecendo a ciclogenese. Porém, no Uruguai mostra uma pequena redução da densidade ciclogenética em relação ao ExpCTRL. Isto ainda necessita maior investigação. • The latent and sensible heat fluxes removal affected the cyclogeneses in the central region of the South Atlantic, the south/southeast of Brazil and south Argentine but not the Uruguay coast. It shows that the topography plays a more important role in this case. • A ciclogenêse na costa Argentina não foi discutida porque não está associada com a divergência do fluxo de umidade.

19. Acknowledgments • FAPESP 04/02446-7 and CNPq 475281/03-9 for the financial support. • NCEP for providing the dataset. • International Centre for Theoretical Physics (ICTP) for the RegCM3 availability. THANKS!!!