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September 1999
Science
Aside from the now-famous El Niño and La Niña, the Pacific Ocean is prone to other sea-surface temperature (SST) anomalies. Large areas of above- or below-normal SSTs recur from winter to winter across the North Pacific. These anomalies often vanish during the summer, but they sometimes reappear in the same area the next fall. Clara Deser (CGD) and two other scientists have completed the most thorough study to date of these annually recurring patterns. The paper, "The re-emergence of SST anomalies in the North Pacific Ocean," written with lead author Michael Alexander and Michael Timlin (both of NOAA Climate Diagnostics Center and the Cooperative Institute for Research in Environmental Sciences), appears in the August issue of the Journal of Climate.

In 1970, Jerome Namias (Scripps Institution of Oceanography) and colleague Robert Born first recognized the SST persistence in the North Atlantic. They theorized a "re-emergence mechanism" in which the anomalies are stored 50 to 150 meters (160-490 feet) beneath the surface during the summer, when light winds and a warm atmosphere create a shallow mixed layer extending only about 25 meters below the surface. As fall and winter arrive, and stronger winds help deepen the mixed layer, the stored anomalies return to the surface and SSTs trend toward their values of the previous winter.

A lack of observations below the surface of the North Pacific has hindered in-depth study of the re-emergence mechanism. Clara and her colleagues filled the data voids between 1950 and 1996 by using output from the dynamical ocean model of the National Centers for Environmental Prediction, and by using statistical interpolation for periods that precede the NCEP period of record. They found evidence for the re-emergence process throughout the basin, as well as some clear differences between the eastern, central, and western Pacific. In the east, the anomalies stay closer to the surface during the summer and only descend for about three months. In the western Pacific, a deeper mixed layer tends to bring the anomalies further down and delay their return to the surface until as late as December.

Could multiyear atmospheric patterns be forcing the re-emergence cycle? The authors refuted this idea by correlating the ocean-atmosphere heat fluxes in winter and spring with those in the following fall and winter at each point across the North Pacific study area. The correlations were negligible, which hints that atmospheric persistence alone cannot explain the re-emergence.

Clara and colleagues also strengthened an interesting thread between the North Pacific anomalies and the El Niño/Southern Oscillation (ENSO). Scientists have long noted that during an El Niño winter, showers and thunderstorms displaced to the central tropical Pacific tend to rearrange atmospheric patterns and cool down the North Pacific surface waters. Clara and colleagues found that these cool anomalies appear to be stored the following summer beneath the surface of the central North Pacific. However, they add, "The extent to which the atmosphere responds to the ocean temperature anomalies which return to the surface in the North Pacific in the following fall and winter remains an open question."

Correction: In last month's Science Briefing, we indicated that new senior scientist Doug Nychka was the organizer of the 1998 colloquium Statistics for Understanding the Atmosphere and Ocean. The lead organizers were Rol Madden and Liz Rothney rather than Doug, and the colloquium was sponsored by the Geophysical Statistics Project rather than ASP. Staff Notes Monthly regrets the errors.


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