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• Temperatures and salinity
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Ocean Temperature

• Introduction
• Global temperatures
• Vertical temperatures

• Virtually all the ocean heat comes from the sun. A minute amount comes from the earth's interior through gas vents at the ocean bottom.
• Because the equatorial areas are more faced toward the sun, the amount of heat absorbed per unit area is larger than at the poles. This causes the temperature to be higher at the equator than at the poles.
• The area of higher temperature shifts southward and northward with the season.
• The change from warmer equatorial areas to colder polar areas occurs largely in the form of fronts (=large transitions).
• The maximum mean monthly temperature is approximately 30 degrees, while the lowest temperature is less than 0 degrees.
• Antarctic ice is shown in white in the Figure. Note the change in ice cover from summer to winter.
• Note how the warm water off the east coast of southern Africa in summer penetrates southward, while off the west coast colder water bulges northward. This causes significant temperature differences between the east and west coasts (at the same latitude). The same is seen off South America.
• Similar east-west temperature differences are seen off North America in the northern summer.

Global temperatures measured by satellite

This image (above) shows the temperature of the ocean at the surface, measured by satellite in August 1998 (i.e. during the southern winter). The temperature scale is in Celsius. Cold water is shown in shades of blue, moderate temperatures in aqua to green and warm water in yellow to red.
Image courtesy of The National Oceanic and Atmospheric Administration (NOAA).

 

 

This image (above) shows the temperature of the ocean at the surface, measured by satellite in February 1998 (i.e. during the southern summer). The temperature scale is in Celsius. Cold water is shown in shades of blue, moderate temperatures in aqua to green and warm water in yellow to red.
Image courtesy of The National Oceanic and Atmospheric Administration (NOAA).

How does temperature change vertically?

• Because of the heat input (from the sun) at the surface, the surface layers of the sea are warmest (> 20 degrees), and the temperature decreases with depth (to < 5 degrees). This is especially the case in the equatorial and temperate latitudes. In the polar regions the heat input at the surface is low, so that there is not a large change in temperature with depth.
• The minimum temperature (at the sea bottom) is between -1 and -2 degrees. Shouldn’t sea water be frozen at such temperatures?
• Typical profiles of temperature show that the decrease in temperature with depth is not linear, but that the temperature undergoes a significant “jump” at a depth of a few hundred meters. This sharp decline is known as the thermocline.
• Between the surface and the start of the thermocline the water is very isothermal. This is referred to as the mixed layer, where the surface waves continuously churn the water. The higher the wind speed, the larger the waves, the deeper the mixed layer.
• Maximum temperature at the surface can be as high as 36° C (e.g. Persian Gulf).
• Normal sea-water freezes at -1.9° C because of the presence of salinity.
• The average temperature of the sea (over all depths) is (only) 3.8°C. This means that a thin layer (a few 100m thick) of warm water (say, > 15° C) overlays a deep, cold layer several kilometres thick.

In summer the surface temperature is higher, and the mixed layer shallower In winter the surface temperature is lower, and the mixed layer deeper These changes remain small compared to the average depth of the ocean.

coexploration.org/.../images/thermocline.jpg

Variations in temperature are primarily controlled by balances between solar heating and radiative cooling. More time in the sun = warmer Sun more vertical (like in mid-latitudes)= warmer Less time absorbing the sun’s heat = cooler Sun more oblique (like in polar regions)= cooler.

Map of The Indian Ocean with English captions (CIA FB 2002).

Winds are the main energy source for currents that flow across the ocean surface layers (~0-200m) and become the large ocean currents at continental boundaries. Currents that move deep in the oceans, far below the level of wind influence and large currents, are driven by variations in water densities. Higher density waters sink under less-dense water at the same depth. As noted previously density is largely a function of temperature and salinity.

The primary energy source for virtually all circulation is the sun. The sun heats the earth surface, which heats the atmosphere, causing winds that move much of the ocean waters around. In this rather peculiar way, the ocean waters are prevented from becoming too warm in equatorial regions and too cold (freezing) over larger areas at the poles. This would not have been possible without an atmosphere.

Introduction to Ocean Sciences, D. A. Segar, 1997