The Strength of El Ni ñ o and the Spatial Extent of Tropical Drought

1. The Strength of El Niño and theSpatial Extent of Tropical Drought Bradfield Lyon International Research Institute for Climate Prediction, The Earth Institute at Columbia University Climate Diagnostics and Prediction Workshop Madison, WI October 18-22, 2004

2. Lyon, B. 2004, The strength of El Niño and the spatial extent of tropical drought, GRL (in press). Lyon, B. 2004, ENSO and the Spatial Extent of Interannual Precipitation Extremes in Tropical Land Areas, J. Climate (in review). Thanks to... Mark Cane, Tony Barnston, Adam Sobel, Alessandra Giannini, Simon Mason, Chet Ropelewski

3. El Niño and tropical droughts • General tendency for tropical land areas to dry during ENSO (+) events • Want to examine / quantify the spatial extent of droughts 30S-30N

4. Questions... • How large a role does El Niño play in forcing drought • (particularly) when viewed from a tropics-wide perspective? • Does the relationship between drought and El Niño vary • depending on the severity of drought considered? • How does this behavior vary between individual El Niño • events? • Briefly: How symmetric is the behavior of excessively • wet conditions across the tropics during La Niña? • Any ideas on possible forcing mechanisms?

5. Data • Monthly rainfall - UEA CRU and CMAP • Monthly SST - Smith and Reynolds ERSST • Atmospheric data - NCEP/NCAR Reanalysis • Domain – 30S to 30N, land areas only • Period of analysis – 1950 to 2003 • Base period for climatology – 1961 to 1990 • (1980 to 2000 for CDAS)

6. Defining drought (and excessively wet conditions) • Weighting factor accounts for seasonality; damps extremely high • monthly values which can occur at start/end of rainy seasons • Climatologically dry areas (i.e. deserts) masked • Drought inherently an accumulated moisture deficit – here we’ll look • at (1 and) 12 month accumulations

7. Much of the variance in soil moisture is associated with the variability in precipitation • Need to compute values for 4 parameters based on a training period • The PDSI is highly correlated with other, simpler indices

8. WASP12 is well correlated with PDSI... Lyon 2004, J. Climate(in review)

9. Drought is an integrated moisture deficit WASP1 < - 0.5 WASP12 < - 1.5 % Tropical Land Area • El Nino: 5-month running average of Nino 3.4 SST anomaly > + 0.4 C • La Nina: “ “ “ “ “ “ “ “ < - 0.4 C

10. Percent of tropical land area in drought of varying severity (1950-2003) WASP12 Note: 5 month lag between max. Nino 3.4 SSTA and extent peaks Lyon, 2004 GRL (in press)

11. Drought extent vs. Nino 3.4 SST anomaly Lyon, 2004 GRL (in press)

12. Drought occurrence increases with drought severity during El Niño WASP12 < -1 (moderate) WASP12 < -2 (severe) Lyon 2004, GRL (in press)

13. Is there a lag between the onset of moderate vs. severe conditions? Lyon 2004, J. Climate (in review)

14. What about positive rainfall anomalies? L- x x % tropical land area L- Lyon, 2004 J. Climate (in review)

15. Asymmetric behavior relative to El Niño and drought… ENSO is asymmetric… Lyon, 2004 J. Climate (in review) Nino 3.4 SST Anomaly

16. Forcing? • Why should drought extent be linearly related to • Nino 3.4 SST anomalies? • There are different “flavors” of El Niño, so why • should drought extent be captured by such a • simple index? • Failed teleconnections (of drought) appear to be • compensated by increased drought occurrence • elsewhere – how?

17. Sobel et al. 2002, J. Climate Tropical SSTs warm in response to El Nino... Convection transfers this heat to the atmosphere...

18. But convective heating is not a linear function of SST... CMAP (DJF)

19. ...and averaged across the tropics, SST and PRCP are NOT well correlated... (GPCP, TRMM) Su and Neelin 2003, J. Climate

20. ...but the net tropical heating is still linearly related to the tropical average SST Linearity ~holds regardless of the location and configuration of SST anomalies, although some regions show greater sensitivity in response (Su et al. 2003) Su and Neelin 2003, J. Climate

21. Drought Forcing? Consider the Reanalysis... “CIN” Lyon, 2004 J. Climate (in review)

22. Su and Neelin, J. Climate 2003

23. Rainfall anomalies averaged over the tropical oceans (12-month running average) CMAP OLR

24. Summary • A remarkably robust relationship is found between the spatial extent of • drought in tropical land areas and El Niño “strength”, with tropical land area • in drought increasing by roughly a factor of 2 between weak and strong • events – for all levels of severity • In many teleconnected tropical land regions, the occurrence of drought • (i.e., the total months experiencing drought) is greater during El Niño than • for all other times (1950-1998) – and the occurrence increases with • increasing drought severity • There are significant asymmetries when comparing the behavior of • La Niña and excessively wet conditions with El Niño and drought – ENSO • has many asymmetric characteristics... • The results indicate the importance of separating land from ocean areas • when considering the rainfall response of tropical SST anomalies – a • warmer tropics does not necessarily imply a wetter tropics everywhere... • The behavior of tropical rainfall variations in different GCMs is currently • being investigated with diagnostic studies to follow...