Changes in pycnocline depth associated with the 1976-77

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Changes in pycnocline depth associated with the
1976-77 climate regime shift

• Antonietta Capotondi, Mike Alexander,
• Climate Diagnostics Center
• Art Miller
• Scripps Institution of Oceanography

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Pycnocline depth changes can be very relevant for
biological productivity in the northeast Pacific.
First of all, changes in pycnocline depth can be
an indicator of changes in upwelling, a process
responsible for exchange of nutrients from the
deep ocean to the upper ocean. Second, the
northeast Pacific is characterized by a
well-mixed fresh surface layer bounded at the
bottom by a halocline. Since at high-latitudes
density is controlled by salinity, the halocline
tends to coincide with the pycnocline. Thus,
pycnocline depth is a good approximation for
winter mixed layer depth in this area. The
changes in pycnocline depth associated with the
1976-77 climate regime shift, as diagnosed from
the output of the NCAR OGCM driven by NCEP winds
over the period 1958-1997 show a shoaling of the
pycnocline in the central part of the Gulf of
Alaska after the mid-seventies, and a deepening
in a broad band that follows the coast. The
deepening is particularly pronounced in the west
part of the Gulf of Alaska, to the southwest of
Kodiak Island, where the pycnocline deepened by
25-30m after 1976. We have examined which
component of the surface forcing is responsible
for these changes. Our results indicate that
changes in the local Ekman pumping can account
for a large fraction of the pycnocline depth
changes. In the band along the coast, and in
particular along the Alaska Peninsula,
propagation of pycnocline depth anomalies by
either advection or wave processes is also
important.
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Model

• NCAR Ocean Model (NCOM)
• Gent and McWilliams eddy flux parameterization
  along isopycnal surfaces
• Large et al. nonlocal planetary boundary layer
  parameterization (KPP)

Resolution 2.4º in longitude, ranging in
latitude from 0.6º in the Tropics to 1.2º
poleward of 30º

• Forcing Observed fluxes of momentum, heat and
  freshwater over
• the period 1958-1997.
• Wind Stress NCEP
• Solar fluxes Insolation and cloud cover from
  ISCCP (1984 6/1991)
• Precipitation Blend of MSU (1979-1993) and
  Xie-Arkin (1979-1996)

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Pycnocline (26.4??) depth changesPeriod2 (77-97)
Period1 (64-75)
P OWS P G GAK1
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Changes in Ekman pumping