1. 1 Changes in low-latitude radiative energy budget - a missing mode of variability in climate models? Richard P. Allan, Tony Slingo Hadley Centre for Climate Prediction and Research In collaboration with Bruce Wielicki (NASA Langley) and coauthors. 1) Introduction 2) Satellite Instruments 3) Interannual Variability and comparison with Climate Model 4) Spatial Signature 5) Additional Evidence 6) Conclusions

2. 2 1) Introduction Unexpected difference in low- latitude radiation budget between 1998 (CERES) and 1985-89 (ERBE) Independent confirmation by additional satellite instruments Is this explained by: –satellite calibration error? –El Nino Southern Oscillation? –previously unidentified decadal variability? Is the variability captured by current climate models? Links with changing cloud radiative effect, temperature lapse rate and atmospheric circulation

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4. 4 2) Satellite instruments ERBS - scanner, 72-day precess, res~40km, 1985-89, accuracy~1% ScaRaB - scanner, 213-day precess, res~60 km, 1994/5, accuracy~1% CERES - scanner, 47-day precess, res~20 km, 1998, accuracy~0.5% ERBS WFOV - non-scanner, 72-day precess, res~1000 km, 1985-1998. Scanners: absolute and stability of calibration. Non-Scanner: stability of calibration only Anomaly comparison: instruments consistent to within 1 Wm -2 or 0.5% of tropical OLR

5. 5 3) Observed interannual flux variability and comparison with climate model From Wielicki et al. (2001), submitted.

6. 6 40S-40N TOA Fluxes, seasonal cycle removed. Observations vs model.

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8. 8 4) SPATIAL SIGNATURE OF ANOMALIES: EOF, annual mean ocean-only WFOV data.

9. 9 EOF, annual mean ocean-only HadAM3 data.

10. 10 Is the satellite data wrong? Consistent variability shown by 7 independent instruments Inconsistent variability from NOAA AVHRR and HIRS OLR - but these data have serious flaws Checked many possible calibration inaccuracies: none likely to be larger than 0.2-0.5 Wm -2 –WFOV: 2 active cavity sensors: TOTAL,SW –TOTAL stability solar calibration: 0.1% –SW expected exponential drop in gain, mostly before 1990 - accounted for in processing –Offset of TOTAL and SW not correlated with OLR anomalies –Solar calibration consistent with independent MFOV instrument

11. 11 Some evidence from ISCCP of a decrease in total cloud amount… Conflicting evidence from HIRS/SAGE II+III 5) Independent evidence for changes in the tropical climate

12. 12 IPCC (2001), Ch 2, Fig 12: Temperature Lapse rate changes?

13. 13 Is there an important physical process that we do not yet understand and is therefore not resolved in current GCMs? Piecing together the evidence...

14. 14 6) Conclusions Unexpected variability of radiation budget robust Related to changes in cloud? Links to changes in temperature lapse rate/ atmospheric circulation Not reproduced by GCMs Unknown forcing/ missing physics? Greenhouse effect/ unforced natural variability? Value of long-term well calibrated monitoring from multiple platforms