Trends in Ocean Temperature: Daily and Annual Range

The temperature of the oceanic water is important for phytoplanktons as well as zooplanktons. The temperature of sea water also affects the climate of coastal lands and plants and animals. The study of both, surface and subsurface temperature of sea water is thus significant.

Measurement of Temperature

Standard type of thermometer is used to measure the surface temperature while reversing thermometers and thermographs are used to measure the subsurface temperature. These thermometers record the temperature up to the accuracy of ±O.02° centigrade.

Layers of Temperature in tropics

Oceans absorb more than 80% of the solar radiation and water which has highest specific heat is the remarkable capacity of storing the heat. The uppermost 10% of the oceans has more heat than the entire atmosphere of earth!

With respect to temperature, there are three layers in the oceans from surface to the bottom in the tropics as follows:

• The first layer represents the top-layer of warm, oceanic water and is 500m thick with temperature ranging between 20° and 25°C. This layer is present within the tropics throughout the year but it develops in mid· latitudes only during summer.
• The thermocline layer represents vertical zone of oceanic water below the first layer and is characterized by rapid rate of decrease of temperature with increasing depth,
• The third layer is very cold and extends upto the deep ocean floor. The polar areas have only one layer of cold water from surface (sea level) to the deep ocean floor.

The radiant energy transmitted from the photosphere of the sun in the form of electromagnetic short waves and received at the ocean surface is called insolation. Besides, some energy, though insignificant, is also received from below the bottom and through the compression of sea water. The amount of insolation to be received at the sea surface depends on the angle of sun’s rays, length of day, distance of the earth from the sun and effects of the atmosphere. The mechanism of the heating and cooling of ocean water differs from the mechanism on land because besides horizontal and vertical movements of water, the evaporation is most active over the oceans.

Daily Range of Temperature

The difference of maximum and minimum temperature of a day (24 hours) is known as daily range of temperature. The daily range of temperature of surface water of the oceans is almost insignificant as it is around 1°C only. The daily range of temperature is usually 0.3°C in the low latitudes and 0.2° to 0.3°C in high latitudes.

The diurnal range depends on the

• Conditions of sky (cloudy or clear sky),
• Stability or instability of air and
• Stratification of seawater.

The heating and cooling of ocean water is rapid under clear sky (cloudless) and hence the diurnal range of temperature becomes a bit higher than under overcast sky and strong air circulation. The high density of water below surface water causes very little transfer of heat through conduction and hence the diurnal range of temperature becomes low.

Annual Range of Temperature

The maximum and minimum annual temperatures of ocean water are recorded in August and February respectively in the northern hemisphere. Usually, the average annual range of temperature of ocean water is -12°C but there is a lot of regional variation which is due to regional variation in insolation, nature of seas, prevailing winds, location of seas etc.

Annual range of temperature is higher in the enclosed seas than in the open sea (Baltic Sea records annual range of temperature of 4.4°C or 40°F). The size of the oceans and the seas also affects annual range of temperature e.g., bigger the size, lower the annual range and vice versa. The Atlantic Ocean records relatively higher annual range of temperature than the Pacific Ocean.