Physical conditions follow certain trends on an ocean basin-wide
scale. The following discussion is based on plots of physical
parameters (Fig. 2) against latitude (Fig. 4). Seasonal variation
occurs at the sea surface. At 200 m depth the physical conditions
can be considered to be seasonally constant (Fig. 3).
The summer sea surface temperatures (Fig. 4) decline nearly
linearily from high latitudes towards low latitudes. Between
20°-30° the latitudinal temperature gradient becomes
slightly less steep towards the equator. In summer this latitudinal
belt is characterised by relatively uniform sea surface temperatures
mainly through an increase in sea surface temperatures in the
northern tropics and subtropics. Annual means show a broad zone
between about 30° N and 30° S of a decreasing latitudinal
temperature gradient towards the equator. The seasonality in
sea surface temperatures is largest in mid latitudes and lowest
in the tropical ocean near the equator. A difference exists between
the North Atlantic and the Indian Ocean in the amplitude of mid-
and high-latitude seasonality which is lower in the Indian Ocean.
Sea surface salinity shows distinct maxima in mid latitudes
and low values near the equator and in high latitudes. The pattern
remains about the same in summer and winter. Seasonality in sea
surface salinity is mainly characterised by spatial variability
in low latitudes while conditions are more uniform and stable
in mid latitudes.
Vertical temperature gradients increase towards the equator.
In winter the vertical temperature gradient is less than 2 °C
between the poles and 40° latitude and increases nearly linearily
from about 40° latitude to the equator. In summer the vertical
temperature gradients increase mainly in mid latitudes leading
to strong seasonal changes in these ocean areas. The steepest
gradients in the rate of change with latitude exist in the tropics,
between about 20° north and south of the equator.
The surface water density is highest near the poles and decreases
towards the equator. The stratification, expressed as the difference
in density between the sea surface and 200 m depth, shows geographic
variation in summer in the North Atlantic while in the southern
hemisphere the change with latitude seems to be more uniform.
In winter the stratification is low in high and mid-latitudes
and increases linearily from about 30° latitude towards the
equator.
Water temperatures at 200 m depth increase from the poles
towards about 20° latitude. Lower temperatures near the equator
reflect the updoming or upwelling of waters from greater depth
compared to the mid latitudes. This and the maximum in sea surface
temperature give rise to the strong vertical temperature and
density gradients in the tropical ocean.