1. Mechanisms for Lagged Atmospheric Response to ENSO SST Forcing Hui Su **, J. David Neelin ** and Joyce E. Meyerson * Dept. of Atmospheric Sciences *, Inst. of Geophysics and Planetary Physics **, U.C.L.A. Tropical Tropospheric Temperature Anomalies ( ´ ) Lag ENSO SST Anomalies by 1-3 months QTCM Experiments with Prescribed SST and with a Slab Mixed- layer Ocean Model Phase and Amplitude of ´ Dependence on:  Mixed-layer Depth  ENSO SST Frequency  Fraction of Mixed-layer ocean Region A Simple Analytical Atmospheric Model Coupled with a Mixed- layer Ocean Model Free decay time scale of the coupled system depends on flux exchanges at the surface, heat loss at TOA and energy transports between tropics and mid-latitudes^^

2. Composite Anomalies of T and SST

3. Zonal Avg. of Tropospheric Temperature Anomaly Regression on Niño3.4 SSTa

4. Lead/Lag Regression ´-Niño3.4 SSTa --comp ´ lags Niño-3.4 SSTa NCEP  4 Months OBS SST  3 Months OBSPAC SST+ML  2 Months CLIM+OBSPAC SST  1 Month ^^

5. Lead/Lag Regression ´-Niño3.4 SSTa Lead/Lag Regression ´-Niño3.4 SSTa depth Phase Lag is not a monotonic function of mixed-layer ocean depth Amplitude of ´ decreases as mixed-layer ocean depth increases ^^

6. Precipitation response Little lag behavior for precipitation inside or outside ENSO SST forcing regions Tropical-mean precipitation shows a much longer lag than tropospheric temperature

7. Experiments with Sinusoidal SST forcing

8. QTCM Experiments with prescribed sinusoidal SST forcing Phase lag of ´ is not a monotonic function of MLD Phase lag increases as SST forcing period increases Amplitude of ´ decreases as MLD increases Amplitude increases as SST forcing period increases ^ ^

9. A Simple Analytical Model

10. Free Decay Rate of the Coupled System Free Decay Rate of the Coupled System

11. Analytical Results Analytical results are consistent with the model results At low and high values of MLD, the lag is close to that determined by atmosphere internal heat capacity

12. Lead/Lag Regression ´-Niño3.4 SSTa with and without Advection Anomaly Lag and amplitude increase when anomalous advection of T and q are suppressed ^

13. With and Without Advection Anomalies Lag and amplitude increase when advection anomalies suppressed

14. Summary The lag of ' relative to ENSO SST exists even when no air- sea coupling is allowed, but coupling with a slab mixed-layer ocean significantly increases the lag. The lag and amplitude of ' depend on mixed-layer ocean depth, ENSO SST forcing period and areal fraction of mixed- layer ocean. The behavior of phase and amplitude variations of ' can be explained by a simple analytical model. The free decay time scale of the coupled atmosphere-ocean system depends on various flux exchange coefficients at the surface, at the top-of-atmosphere, and between tropics and mid-latitudes. It indicates the extent of the contribution of atmosphere-ocean coupling to the lagged response of ' to ENSO SST. ^ ^ ^

15. Coupled Atmosphere-Ocean Response in a switch-on SST anomaly experiment Atmosphere  Fast and coupled slow modes Mixed-Layer Ocean SST  Slow mode Precipitation  Fast and slow varying