El Niño and La Niña Events:

Changes in the North Pacific associated with El Niño (EN) and La Niña (LN) events are dynamically similar to decadal events, so they are useful analogs of climate change in this region (e.g., Latif and Barnett 1996, Mantua et al. 1998). Sea level pressure and wind stress variations over the NEP during April-June 1997 contributed to the rapid development of the 1997-98 EN in the tropical Pacific during this period (Lynn et al. 1998, Murphree and Schwing 1999). These and similar variations in NEP winds during July-September 1997 also led to dramatic SST anomalies (SSTAs) in the region. NEP wind stress anomalies during July-September were primarily due to atmospheric wave trains emanating from East Asia, and not to EN processes in the equatorial Pacific. Ekman processes driven by anomalous winds during much of 1997 played a role in creating positive SSTAs in the NEP warm pool, a triangular region of relatively warm water between Hawaii, British Columbia, and Baja California. The warm pool is located under the North Pacific High (NPH) and appears to be the result in part of open ocean Ekman processes associated with the NPH.

The first major impacts of the EN on the NEP did not commence until November 1997 and ended abruptly in April-May 1998. Large-scale EN impacts occurred primarily via atmospheric wave trains emanating from both the equatorial western and central Pacific. The large NEP anomalies during the 1997-98 EN and composite EN and LN events have led to several new insights into climate variations in this region: