• Oceans absorb more than 80 percent of the solar radiation that reaches the earth
• Water has a remarkable capacity for absorbing heat
• The uppermost 10 percent of the oceans contain more heat than the entire atmosphere
• Ocean temperature is not uniform and differs from latitude to latitude and from the surface to the bottom
• Major determinants of ocean temperature are:
  • Latitude: Surface temperature of oceans declines from the equator towards the poles due to vertical sun rays at the equator and slanting rays towards the poles
  • Prevailing Winds: Direction of the prevailing winds, such as Trade Winds and Westerlies, determines surface temperature of ocean waters. For instance, eastern edges of oceans along the trade wind belt have cooler waters due to pushing of warm waters away from the coast causing upwelling of bottom waters.
  • Unequal distribution of Land and Water: The Northern Hemisphere has more land area than the Southern Hemisphere, leading to warmer oceans in the Northern Hemisphere.
  • Evaporation Rate: The rate of evaporation is not uniform across different latitudes, with warmer tropical waters having higher evaporation rates than cooler temperate waters.
  • Density of Water: Density of ocean water is mostly a function of temperature and salinity. Density also varies from latitude to latitude, with high salinity areas having relatively higher temperature and vice versa.
  • Ocean Currents: Surface temperature of oceans is also controlled by cold and warm currents. Warm water increases temperature and the rate of evaporation, leading to more rainfall. Cold currents reduce the temperature of moisture-laden wind, resulting in more fog but less precipitation.
  • Local Factors: Submarine ridges, local weather conditions such as storms, cyclones, winds, fogs, cloudiness, evaporation rate, lapse rate, condensation, and precipitation affect the distribution of ocean temperature.
• The study of ocean temperature is important for determining:
  • Movement of large volumes of water (vertical and horizontal ocean currents)
  • Type and distribution of marine organisms at various depths of oceans
  • Climate of coastal lands
• The sun is the principal source of energy (Insolation) for heating the ocean
• The ocean is also heated by the inner heat of the ocean itself, but this heat is negligible compared to that received from the sun

Vertical Distribution of Temperature

• Energy and sunlight decrease with depth in the oceans.
• Only about 45 percent of light energy striking the ocean surface reaches a depth of about one meter, and only 16 percent reaches a depth of 10 meters.
• The ocean depths may be divided into the following three zones based on temperature:
  • Surface Zone or Mixed Zone (Photic zone or Euphoric zone): The upper layer of the ocean with relatively constant temperature and salinity. It contains about 2 percent of the total volume of water in the ocean and is limited to a depth of about 100 meters.
  • Thermocline: Lies between 100 meters and 1000 meters with a steep fall in temperature. The density of water increases with increasing depth. It contains about 18 percent of the total volume of water in the ocean.
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  • Deep Zone: Lies below 1000 meters in the mid-latitudes with constant temperature. It contains about 80 percent of the total volume of water in the ocean. The ocean bottom always has a temperature which is one or two degrees Celsius above the freezing point.

Horizontal Temperature Distribution of Oceans

• Surface water temperature in lower latitudes is about 26 degrees Celsius, decreasing towards poles
• Northern Hemisphere oceans have an average temperature of 19.4 degrees Celsius
• Average temperature varies with 22 degrees Celsius recorded at 20 degrees latitude and 14 degrees Celsius recorded at 40 degrees latitude in the Northern Hemisphere
• Temperature drops to 0 degrees Celsius at the poles
• Maximum and minimum annual temperatures of ocean water in the Northern Hemisphere are in the months of August and February respectively
• Average annual range of temperature is about 12 degrees Celsius
• The North Atlantic Ocean has the highest annual range of temperature
• Annual range of temperature is higher for inland seas compared to open oceans
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