Vertical and Horizontal Distribution of Ocean Temperature

The distributional pattern of temperature of ocean water is studied in two ways viz.

• Horizontal distribution (temperature of surface water) and
• Vertical distribution (from surface water to the bottom).

Since the ocean has three dimensional shape, the depth of oceans, besides latitudes, is also taken into account in the study of temperature distribution. The following factors affect the distribution of temperature of ocean water.

Latitudes

The temperature of surface water decreases from equator toward the poles because the sun’s rays become more and more slanting and thus the amount of insolation decreases pole ward accordingly.  The temperature of surface water between 40°N and 40°S is lower than air temperature but it becomes higher than air temperature between 40°Latitude and the poles in both the hemispheres.

Unequal distribution of land and water

The temperature of ocean water varies in the northern and the southern hemispheres because of dominance of land in the northern hemisphere and water in the southern hemisphere.  As far as surface temperature is concerned, it has the following implications:

• The oceans in the northern hemisphere receive more heat due to their contact with larger extent of land than their counterparts in the southern hemisphere and thus the temperature of surface water is comparatively higher in the northern hemisphere than the southern hemisphere.
• The isotherms are not regular and do not follow latitudes in the northern hemisphere because of the existence of both warm and cold landmasses whereas they (isotherms) are regular and follow latitudes in the southern hemisphere because of the dominance of water.

The temperature in the enclosed seas in low latitudes becomes higher because of the influence of surrounding land areas than the open seas e.g., the average annual temperature of surface water at the equator is 26.7°C whereas it is 37.8°C in the Red Sea and 34.4°C (94°F) in the Persian Gulf.

Prevailing wind

Wind direction largely affects the distribution of temperature of ocean water.  The winds blowing from the land towards the oceans and seas (i.e. offshore winds) drive warm surface water away from the coast resulting into upwelling of cold bottom water from below.  Thus, the replacement of warm water by cold water introduces longitudinal variation in temperature.  Contrary to this, the onshore winds pile up warm water near the coast and thus raise the temperature.

Ocean currents

Surface temperatures of the oceans are controlled by warm and cold currents. Warm currents raise the temperature of the affected areas whereas cool currents lower down the temperature.

Other factors

Other factors include the following:

• Submarine ridges
• Local weather conditions such as storms, cyclones, hurricanes, fog, cloudiness, evaporation and condensation
• Location and Shape of area: The enclosed seas in the low latitudes record relatively higher temperature than the open seas whereas the enclosed seas have lower temperature than the open seas in the high latitudes.

Horizontal Distribution of Temperature

Average temperature of surface water of the oceans is 26.7°C and the temperature gradually decreases from equator towards the poles. The rate of decrease of temperature with increasing latitudes is generally 0.5°C per latitude. The average temperatures become 22°C at 20° N and S latitudes, 14°C at 40° N and S latitude, and 0°C near the poles. I have already mentioned above that the oceans in the northern hemisphere record relatively higher average temperature than in the southern hemisphere. Please note that the highest temperature is not recorded at the equator rather it is a bit north of it.

Also we should note that the average annual temperature of all the oceans is 17.2°C. The average annual temperatures for the northern and southern hemispheres are 19.4°C and 16.1°C respectively. The variation of temperatures in the northern and southern hemispheres is because of unequal distribution of land and water as Northern hemisphere is made up of more land, while the southern hemisphere is made up of more oceans.

In Northern Atlantic, there is a very low decrease of temperature with increasing latitudes towards north. This is because of the Gulf Stream currents which are warm currents. However, in southern Atlantic, the decrease of temperature with increasing latitude is more pronounced.  The table shows the variations of three major oceans:

Vertical Distribution of Temperature

The maximum temperature of the oceans is always on the surface because it directly receives the insolation. The heat is transmitted to the lower sections of the oceans through the mechanism of conduction.

Solar rays very effectively penetrate up to 20m depth and they seldom go beyond 200m depth. Consequently, the temperature decreases from the ocean surface with increasing depth but the rate of decrease of temperature with increasing depth is not uniform everywhere. The temperature falls very rapidly up to the depth of 200m and thereafter the rate of decrease of temperature is slowed down.

On this basis, oceans are vertically divided into three zones as follows:

Photic Zone or Euphotic Zone

This is the upper layer of the ocean. The temperature is relatively constant and is 100 meters deep.

Thermocline

Thermocline lies between 100-1000 meters. There is a steep fall in the temperature. The following graph shows the thermocline.

Deep Zone

Below 1000 meters is the deep zone. Here, the temperature is near zero °C. Please note that near bottom, the temperature of water never goes to 0°C. It is always 2-3°C.

Important Observations

• Sea temperature decreases with increasing depth but the rate of decrease of temperature is not uniform.
• The change in sea temperature below the depth of 1000m is negligible. The maximum change in temperature is between 100-1000 meters which is called Thermocline or Pycnocline.
• Diurnal and annual ranges of temperature cease after a depth of 30 feet and 600 feet respectively.
• The rate of decrease of temperature with increasing depth from equator towards the poles is not uniform.

Though, the surface temperature of the oceans decreases from equator to the poles, the temperature at the ocean bottom is uniform at all latitudes. However, some studies have shown that the coldest bottom temperatures, just below — 0.25°C, occur at 60-70°S, near the Antarctic continent.