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.

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Figure 4: Plots of selected physical parameters (Fig. 2) vs. latitude for all sample locations used in this paper (Fig. 1). Data from Levitus (1982). See text for discussion.

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