Tuesday, 11 February 2003: 9:00 AM
On the delayed atmospheric response to ENSO SST
Tropical tropospheric temperature (and related variables) lag ENSO SST by 1-2 seasons as noted by a number of previous investigators. The mechanisms for the delayed response are not yet clear. Two factors conjectured by Kumar and Hoerling (2002) are (i) the seasonal cycle of SST affecting sensitivity to ENSO anomalies; and (ii) the SST anomalies outside the eastern Pacific, such as the western Pacific and Indian Ocean. We use the intermediate complexity tropical circulation model (QTCM) to explore the mechanisms for the delayed atmospheric response to ENSO SST forcing. The model reproduces the delayed tropical atmospheric response when forced by observed global SST anomalies, with maximum correlation of tropical tropospheric temperature with Nino3-4 SST occurring at 4 month lag. A simulation that includes only SST anomalies within the tropical eastern Pacific produces smaller and shorter-lived tropospheric temperature response, and the maximum correlation occurs at 1 month. This clearly shows that SST anomalies outside the eastern Pacific play an important role in the delayed response. Besides the prescribed SST experiments, a mixed-layer ocean combined with prescribed SST anomalies in the eastern Pacific is used to further decompose the contribution of SST anomalies outside the main ENSO anomaly regions to the delayed response of the tropical atmosphere. Experiments shifting anomalies relative to the seasonal cycle suggest that while the seasonal cycle has some impact, it is a secondary factor.