The 4-year drought from mid-1998 through mid-2002 affecting the United States and large portions of southern Europe and Asia has been linked to remarkably persistent cold SST anomalies in the east tropical Pacific and remarkably warm SST anomalies in the west tropical Pacific during this period. Either of these anomalous SST conditions could have caused the drought; their simultaneous occurrence apparently made it much worse. The cold east Pacific SSTs were clearly associated with a lingering La Nina event, but the warm west Pacific SSTs have been argued to be "consistent with greenhouse gas forcing".

In this talk, it will be argued from observations and GCM simulations of the last 50 years that the 4-year average precipitation anomaly pattern during 1998-2002 was mostly caused by the persistent La Nina forcing. It will be stressed that the anomalous tropical SST pattern during La Nina events typically includes both cold east Pacific AND warm west Pacific SST features. There is thus no need to invoke greenhouse gas forcing to explain the west Pacific warming, and hence the 4-year drought. The average tropical SST anomaly pattern during this period may indeed be viewed to a first approximation as the superposition of a La Nina pattern and a spatially uniform "global warming" trend pattern. Our point is that while this warming trend component may have contributed substantially to the forcing of, say, the extratropical 200 mb height anomalies in 1998-2002, it was probably much less important in causing the extratropical surface temperature anomalies, and even less so in causing the precipitation anomalies, than the La Nina component.