Journal of Climate: Vol. 12, No. 8, pp.
2419-2433.
The Reemergence of SST Anomalies in the North Pacific Ocean
Michael A. Alexander
Climate Diagnostics Center, CIRES, University of Colorado, Boulder,
Colorado
Clara Deser
National Center for Atmospheric Research
P. O. Box 3000
Boulder, CO 80307
Michael S. Timlin
Climate Diagnostics Center, CIRES, University of Colorado, Boulder,
Colorado
(Manuscript received 29 October 1997, in final form 24 July 1998)
Sea surface temperature (SST) data and two different upper-ocean temperature analyses
are used to study the winter-to-winter recurrence of SST anomalies in the North
Pacific Ocean. The SSTs recur when temperature anomalies that form in the deep ocean
mixed layer in late winter/early spring are isolated from the atmosphere in the
summer seasonal thermocline and then reemerge at the surface when the mixed layer
deepens during the following fall/winter. This “reemergence mechanism” is
evaluated over the basin by correlating the time series of the leading pattern of
ocean temperature anomalies in the summer seasonal thermocline
(~60-85 m in August-September) with SST anomalies over the course of the year. The results
indicate that the dominant large-scale SST anomaly pattern that forms in the North
Pacific during late winter, with anomalies of one sign in the central Pacific and
the opposite sign along the coast of North America, is sequestered in the seasonal
thermocline in summer and returns to the surface in the following fall, with little
persistence at the surface in summer.
Regions in the east, central, and west Pacific all show signs of the reemergence
process but indicate that it is influenced by the timing and amplitude of the mean
seasonal cycle in mixed layer depth. The maximum mixed layer depth increases from
east to west across the basin: as a result, the thermal anomalies are shallower and
return to the surface sooner in the east compared with the west Pacific. At some
locations, the reemerging signal is also influenced by when the SST anomalies are
created. In the east Pacific, SST anomalies that are initiated in February-March
extend through a deeper mixed layer, persist at greater depths in summer, and are
then reentrained later in the year compared with those initiated in April-May.
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Hongjun Zhang:
zhangho@ucar.edu