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dominant pattern of Pacific sea-level pressure
variability. The basic relationship is that coooler
(warmer) than average sea surface temperatures
tend to occur during periods of lower (higher)
than average sea-level pressure over the central
North Pacific. The PDO is in turn related to the
North Pacific Oscillation (NPO), which can be
described from a simple index based on the area-
weighted mean of sea-level pressure over the
extratropical North Pacific. The PDO time series
represents a good measure of the strength of the
Aleutian Low. Since about 1976 the PDO has
shown a general downward trend, meaning a
stronger Aleutian Low, accompanied by stronger
than normal westerly winds across the central
North Pacific and enhanced southerly to south-
easterly flow along the west coast of North
America. In a larger context, variability in ENSO,
the PDO and NPO link to variability in the
so-called Pacific North American (PNA) tele-
connection pattern. The PNA describes variations
in the atmospheric longwave pattern spanning
the equatorial Pacific through the northwest of
North America and to the southeastern part
of North America. The positive mode of the
PNA is characterized by a strong Aleutian Low, a
strong upper-air ridge along the west coast of
Canada, and a concurrent strong trough over the
southwestern United States.
evaporation, density changes and wind stress. The
effect of these processes is to produce a vertical
layering in the ocean that is of great climatic
significance:
1 At the ocean surface, winds produce a
thermally mixed surface layer averaging a few
tens of meters deep poleward of latitude 60°,
400m at latitude 40
°
and 100-200m at the
equator.
2 Below the relatively warm mixed layer is the
thermocline , a layer in which temperature
decreases and density increases (the pycnocline )
markedly with depth. The thermocline
layer, within which stable stratification tends
to inhibit vertical mixing, acts as a barrier
between the warmer surface water and the
colder deep-layer water. In the open ocean
between latitudes 60°N and 60°S the thermo-
cline layer extends from depths of about 200m
to a maximum of 1000m (at the equator from
about 200 to 800m; at 40
latitude from about
400 to about 1100m). Poleward of 60° latitude,
the colder deep-layer water approaches the
surface. The location of the steepest tempera-
ture gradient is termed the permanent thermo-
cline , which has a dynamically inhibiting
effect in the ocean similar to that of a major
inversion in the atmosphere. However, heat
exchanges take place between the oceans and
the atmosphere by turbulent mixing above
the permanent thermocline, as well as by
upwelling and downwelling. Below the surface
mixed layer in the Arctic there is also a salinity
gradient or halocline .
°
D OCEAN STRUCTURE AND
CIRCULATION
The oceans occupy 71 percent of the earth's
surface, with over 60 percent of the global ocean
area in the Southern Hemisphere. Three-quarters
of the ocean area are between 3000 and 6000m
deep, whereas only 11 percent of the land area
exceeds 2000m altitude.
During spring and summer in the mid-
latitudes, accentuated surface heating leads to the
development of a seasonal thermocline occurring
at depths of 50 to 100m. Surface cooling and wind
mixing tend to destroy this layer in autumn and
winter.
Below the thermocline layer is a deep layer
of cold, dense water. Within this, water move-
ments are mainly driven by density variations,
1
Above the thermocline
Vertical
The major atmosphere-ocean interactive pro-
cesses ( Figure 7.26 ) involve heat exchanges,
 
 
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