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northeast, east or southeast. South Pacific swells resulting from storms in
the Southern Hemisphere occur between April and October. They produce
swells (the southern swell) from a southern quadrant with long periods (14-
22 sec) and low amplitudes (approximately 1 m).
5.5.2.3. Water chemistry
Water chemistry and primary productivity studies in the region show
that the Northern Equatorial Current carries a distinct but diffuse plume of
nutrients and fine-grained materials westward from terrigenous and
upwelled sources off the North American continent. Due to the normal
increase of solar energy inputs to surface waters with decreasing north
latitude, primary productivity generally follows an increasing trend toward
the equator. However, because of the nutrient influx from the west in the
area of interest, the gradient here is dominated by a trend of decreasing
primary productivity to the west.
Seawater density varies inversely with temperature, directly with
salinity and, to a minor degree, directly with pressure. Seawater density
controls oceanic stratification. According to DOMES data, the sea-surface
density in this region is about 1.022 g/cm 3 . At depths of 1000 m and more,
it is about 1.0275 g/cm 3 . These small changes are significant, so density is
usually expressed by the quantity sigma-t (st), a dimensionless unit
calculated by subtracting 1.0 from the density and multiplying by 1000. In
the above case, st varies from 22.0 to 27.5. The rapid increase in density
that coincides closely with the thermocline and the halocline (the zone of
rapid salinity increase) is called the pycnocline.
The pycnocline/halocline/thermocline is an important midwater
layer because it retards vertical diffusion and sinking. Thus, as animals and
plants in the mixed layer die and sink, they tend to accumulate and decay in
this zone. This concentrated decomposition depletes available dissolved
oxygen, creating an oxygen minimum.
DOMES project data (figure 5) reveal typical oceanic salinity and
temperature profiles with little seasonality 49 . The average mixed layer
salinity was 34.3 g/kg of seawater. Below this are found two minima and
maxima within the upper 1000 m. A maximum range of about 2 g/kg
through the water column was typical.
In the ocean, pH is maximal at the surface due to the combined
effects of carbon dioxide uptake and oxygen evolution in the photosynthetic
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