Evidence for a wind-driven intensification of the Kuroshio
Current Extension from the 1970s to the 1980s
Clara Deser
National Center for Atmospheric Research
P. O. Box 3000
Boulder, CO 80307
Michael A. Alexander and Michael S. Timlin
CIRES--Climate Diagnostics Center
University of Colorado
Submitted to J. Climate, 30 December 1997
In final form: 1 July 1998
The spatial and temporal characteristics of oceanic thermal variations
in the mixed layer and main thermocline of the mid-latitude north
Pacific are distinctive, suggesting different physical origins.
Within the main thermocline (400~m depth), the variability is
dominated by a westward-intensified pattern of decadal scale,
indicative of enhanced eastward geostrophic flow along the southern
flank of the Kuroshio Current Extension during the 1980s relative to
the 1970s. We argue that the decadal-scale change in the strength of
the Kuroshio Extension was a result of the dynamical adjustment of the
oceanic circulation to a decadal variation in wind stress curl
according to Sverdrup theory. Four-times daily wind stress fields
from the NCAR/NCEP reanalysis project are used to compute the decadal
change in Sverdrup transport associated with the ``1976/77'' climate
transition. It is shown that the decadal changes in Sverdrup
transport inferred from the wind stress curl field and in observed
geostrophic flow inferred from the upper ocean thermal field are
consistent both in terms of spatial pattern and magnitude. The
decadal change in depth-averaged geostrophic transport along the
Kuroshio Extension (referenced to 1~km) is 11.6~Sv, similar to the
Sverdrup transport change (11.5~--~13.9~Sv). The decadal-scale
thermocline variation along the western boundary between 30\dg~N and
40\dg~N exhibits a lag of approximately 4--5~yrs relative to the
decadal variation in the basin-wide wind stress curl pattern. This
delay may be indicative of the transient adjustment of the gyre-scale
circulation to a change in wind stress curl via long baroclinic Rossby
waves.
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Hongjun Zhang:
zhangho@ucar.edu