Tropical Atlantic Variability

1. Tropical Atlantic Variability Ching-Yee Chang 張 靜 宜 University of Maryland The Department of Atmospheric Sciences and Institute of Atmospheric Physics National Central University, December 18 2007

2. Outline • Introduction: Seasonal cycle, and Tropical Atlantic Variability • The relationship between the Meridional and Equatorial modes • Spring-time equatorial westerly wind bias of the CAM3 and CCSM3 model output

3. Tropical Atlantic • Atlantic about 1/3 of Pacific in width • Tropical Atlantic more land-locked Similarity to Pacific Ocean The tilting coastline in the eastern boundary

4. Seasonal cycle • Sea Surface Temperature (SST) • Thermocline • Surface winds • InterTropical Convergence Zone (ITCZ)

5. 4°N-4°S,16°W-4°E Depth of thermocline (ave. btw 3S-2N) Scatter diagram of monthly mean SST (Mitchell and Wallace 1992) Sea Surface Temperature (SST) Warmest in MAM; coldest in JJA Thermocline: Flat and “deep” in MAM Steep and “shallow” in JJA

6. Surface winds • Southeastern Trade winds relax in Mar-Aprs • Southeastern trade winds strengthen in Jun-Jul

7. Precipitation • ITCZ in its most Southern position in Apr-May

8. Tropical Atlantic Variability (TAV) modes • Equatorial mode (zonal mode, Atlantic Nino) • Meridional mode (dipole mode, gradient mode) • ENSO influence • NAO influence

9. Empirical Orthogonal Function (EOF) analysis • Decomposition of the climate variability into a linear combination of base functions (EOFs) • EOFs are eigenvectors of a covariance matrix • Each EOF has a corresponding Principal Component (PC), which is the time series of the magnitude of the EOF • EOFs are spatially and temporally orthogonal

10. JJA climatological SST Equatorial (zonal) mode • Oftener and stronger in June, July, August • Warm/cold anomaly in the eastern basin near the equator • Zonal winds anomalies along the equator • Equatorial thermocline movement Ruiz-Barradas et al. 2000. Regression to the PC of a 5-variable rotated EOF analysis

11. MAM climatological SST Meridional (dipole) mode • Oftener and stronger in March, April, May • Meridional gradient of SST anomaly • Cross-equatorial flow • North-south shift of ITCZ • Reduced/increased trade winds in different hemispheres Ruiz-Barradas et al. 2000. Regression to PC of a 5-variable rotated EOF analysis

12. The Meridional and Equatorial Modes of Tropical Atlantic Variability: Related? Ching-Yee Chang James Carton Sumant Nigam

13. The relationship of the two modes • Servain et al., 1999 and 2000 • Related at decadal and short-interannual time scale. • zonal mode leading dipole mode • Murtugudde et al., 2001 • The correlation of the two modes falls apart when a longer time-series data were used • dipole mode leading zonal mode • Okumura and Xie, 2006 • Nov-Dec equatorial SSTA related to following Mar-Apr dipole index

14. EOF analysis on SST anomaly of different seasons • Apply EOF analysis separately for MAM and JJA averaged SST anomalies. Data SST Data for EOF analysis: Hadley SST, 1949-2002 Other datasets: Wind: NCAR-NCEP reanalysis, 1949-2002 Precipitation: CRU TS 2.1, 1949-2002

15. 37.5 % 41.1 % EOF patterns and PCs Correlation coefficient=0.71

16. 41.9 % 43.6 % ENSO influence removed • Remove the regression of monthly SSTA onto DJF Nino3 index The correlation is not resulted from ENSO

17. 1958 1984 Dipole mode leading to zonal mode • Dipole pattern in spring • Later on, along the equator, a corresponding phase of SST anomaly strengthens Cl=0.2ºC

18. 22.4 % 42.0 % GFDL cm2.1 model, flux corrected

19. Spring dipole PC and meridional winds Summer zonal PC and meridional winds Regression of PC and meridional winds Contours: regression coefficients; Shading: significant correlation (above 95%)

20. Spring dipole PC and zonal winds Summer zonal PC and zonal winds Regression of PC and zonal winds Wind anomaly patterns that are related to one mode are also related to the other mode

21. Regression of SSTA onto May western equatorial zonal winds Correlation of equatorial zonal wind and E. equatorial SSTA (Okumura and Xie, 2006) Dipole patterns change into zonal patterns May zonal wind anomaly related to dipole pattern of SSTA at earlier time

22. Regression of Meridional winds onto JJA Atl3 index • Warm JJA equatorial SSTA related to convergence flow to the equatorial in Jun, July, August • Warm JJA equatorial SSTA related to cross-equatorial flow in March, April, May

23. A schematic plot of the relationship between the two modes

24. Climatological zonal winds May June Red line is the place of zero meridional winds ~ position of ITCZ

25. Cold SSTA Warm SSTA Warm SSTA Cold SSTA

26. Regression of PCs and Precipitation

27. Regression of African Precipitation onto PCs JJA zonal PC Mar.Apr May.Jun Jul.Aug MAM dipole PC Equatorial SSTA highly related to the precipitation anomaly along the Coast of Guinea Precipitation Climatology

28. Mar.Apr May.Jun Jul.Aug MAM-dipole PC JJA-zonal PC Climatological precipitation

29. Corr = 0.71 Corr = 0.51 Corr = 0.76

30. 0.65 0.71 0.70 0.72 Spring EOF1 Winter EOF1 Prediction of Amazonian rainfall using DJF EOF/PC1 Fall EOF1 Summer EOF1

31. Mar.Apr May.Jun Jul.Aug MAM-dipole PC DJF PC1 Climatological precipitation

32. 0.65 0.43 0.37 0.71 0.70 0.58 0.34 0.72

33. Regression of PC and African Precipitation Mar.Apr May.Jun Jul.Aug DJF PC1 and precip JJA zonal PC and precip. Precipitation Climatology

34. Extra-tropics influencing tropics • Czaja et al. 2002: remote forcing to the North Atlantic SSTA DJF NAO corr with MAM SSTA

35. Summary I • Spring-time dipole mode is highly correlated to the summer-time zonal mode. • This relationship is not due to ENSO • Western equatorial zonal wind anomaly is the bridge • Anomalous meridional movement of ITCZ can result in zonal wind anomaly • This relationship can be helpful in predicting West African rainfall

36. The origin of the spring-time westerlies in the equatorial Atlantic in CAM3 and CCSM3 Ching-Yee Chang Sumant Nigam James Carton Semyon Grodsky Chang et al, 2007. Seasonal Climate of the Tropical Atlantic Sector in the NCAR Community Climate System Model 3: Error Structure and Probable Causes of Errors. J. Climate Chang et al, 2008. Origin of the Spring-time Westerly Bias in Equatorial Atlantic Surface Winds in CAM3 Model Simulations. J. Climate. In revision

37. Bias structures of the seasonal climate in the tropical Atlantic in CCSM3 and CAM3 Observational data SST: Hybrid Reynolds and Smith Winds: SSMI GPCP precipitation ERA40 reanalysis SODA Model output data CCSM3: 20C3M, T85_gx1v3 CAM3: AMIP run, T85

38. CCSM3 SST

39. Surface zonal wind bias

40. Precipitation ` ` Extra rainfall in Africa Deficient Rainfall in S. America ITCZ southward shift

41. Depth of thermocline Equatorial surface zonal winds SST standard deviation (Deser et al. 2006) red: CCSM3 blue: observation

42. Depth of thermocline Equatorial surface zonal winds red: CCSM3 blue: observation

43. Some possible origins of the westerly winds • Rainfall deficiency in tropical South America • Extra rainfall in tropical Africa • Southward shift of ITCZ

44. Diagnosis of the origin of the westerly wind bias using a diagnostic model

45. Diagnostic linear model • Linear • Primitive equation • Dry model • Steady state • Forcings: diabatic heating, eddy momentum flux, topography……. • In this study, CAM3 and ERA40 diabatic heatings are the forcing to the model.

46. Assessment of the diagnostic model Zonally asymmetric components of SLP and surface winds

47. Equatorial zonal winds Why diabatic heating?

48. Simulated bias: Effect of S. America heating Contour: SLP Vector: winds • Westerly wind bias due to land heating bias equally important as the oceanic heating

49. Regional-averaged vertical profile of diabatic heating

50. Simulated bias: Effect of African heating Contour: SLP Vector: winds • Effect due to African land heating is small and confined