US Globec and PFEL Research: ENSO |
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:
open ocean Ekman processes appear to govern much of the evolution of regional scale SSTAs in the North Pacific (Murphree and Reynolds 1995, Lynn et al. 1998)
the NPH and the Aleutian Low (AL) link the NEP to East Asia and the equatorial Pacific on intraseasonal to interannual scales via the Hadley-Walker circulation and low frequency atmospheric wave trains (Murphree et al. 1999)
the structure of the Asian - North Pacific upper tropospheric jet leads to
large intraseasonal to interannual variations in wind stress around the NPH and
AL in response to (i) East Asian forcing throughout the year and (ii)
equatorial Pacific forcing primarily during the winter
(Murphree and Schwing 1999, Murphree et al. 1999).
Download the The evolution of oceanic and atmospheric anomalies in the northeast Pacific during the El Niņo and La Niņa events of 1995-2001" manuscript (PDF, 4.6M)!.