Temperature of ocean waters: factors and horizontal distribution

Detail

1. Why ocean temperature matters: water heats and cools slowly

• Oceans heat up and cool down more slowly than land.
• Water needs more heat than land to warm up by the same amount.
• Sunlight goes several metres down into water, so the heat is spread through a thick layer. On land it stays in a thin top layer.

• Water moves (waves, currents, mixing), so it carries heat away and spreads it out.

• Result: coastal climates are mild, or equable (equable climate: small difference between summer and winter temperatures).

• Mumbai (on the Arabian Sea coast, Maharashtra) has a small gap between its summer and winter temperatures.

• Nagpur (in the interior of Maharashtra, far from the sea) has very hot summers and cool winters. This is a continental climate, with a large yearly range of temperature.

• The sea acts like a heat store. It takes in heat in summer and gives it back slowly in winter.

2. Factors affecting ocean temperature

2.1 Latitude

• Insolation (incoming solar radiation: the sunlight that reaches the earth) is strongest near the equator. There the sun's rays fall almost straight down.
• Towards the poles the rays fall at a slant. The same energy is spread over a bigger area and passes through more air.
• Result: surface water gets colder from the equator towards the poles.

2.2 Unequal distribution of land and water

• The Northern Hemisphere has much more land than the Southern Hemisphere.
• Northern oceans touch large land masses. In summer this land heats up and passes heat to the nearby sea.
• Result: oceans of the Northern Hemisphere are warmer than those of the Southern Hemisphere.

2.3 Prevailing winds

• Prevailing wind means the wind that blows most often in a place, from one main direction.
• Offshore winds (blowing from land to sea):
• They push warm surface water away from the coast.
• Cold water rises from below to take its place. This rising of cold, deep water is called upwelling.
• Result: the coastal sea is colder than other places at the same latitude. So temperature changes along the longitude (east to west), not only along latitude.
• Examples: the coast of Peru (west coast of South America, Pacific Ocean) and Namibia (south-west Africa, Atlantic Ocean).

• Upwelled water brings up nutrients, so these are rich fishing grounds.

• Onshore winds (blowing from sea to land):

• They pile warm surface water against the coast.
• Result: the coastal water there gets warmer.

2.4 Ocean currents

• Ocean current: a large, steady flow of sea water in one direction, like a river within the ocean.
• Warm currents carry warm water from low latitudes towards the poles. Cold currents carry cold water from high latitudes towards the equator.

Current	Type	Where it flows	Effect on temperature
Gulf Stream (and its extension, the North Atlantic Drift)	Warm	From the Gulf of Mexico, north-east across the North Atlantic	Warms the eastern coast of North America and the west coast of Europe
Labrador Current	Cold	From the Arctic, south along the Labrador coast (Canada)	Chills the north-eastern coast of North America

• Where the warm Gulf Stream meets the cold Labrador Current (near Newfoundland), the water is very rich in fish.

2.5 Enclosed seas

• Enclosed sea: a sea almost fully surrounded by land, with only a narrow link to the open ocean.
• Water in an enclosed sea mixes little with the open ocean. So it takes on the climate of the land around it.

Enclosed sea	Latitude zone	Temperature compared to open sea	Reason
Red Sea (between Africa and Arabia)	Low	Warmer	Hot desert land on both sides, strong sun
Persian Gulf (between Iran and Arabia)	Low	Warmer	Same as above
Baltic Sea (northern Europe)	High	Colder	Cold land around it, weak sun, little warm water comes in

3. Horizontal distribution of surface temperature (Figure 12.4, Class 11)

• Horizontal distribution means how surface temperature changes from place to place across the ocean, mainly from the equator to the poles.
• Average surface temperature near the equator is about 27°C.
• It falls by about 0.5°C for every 1° of latitude towards the poles.

Latitude	Average surface temperature
Equator (0°)	about 27°C
20°	about 22°C
40°	about 14°C
Near the poles	about 0°C

• Thermal equator: the belt of the hottest surface water. It lies slightly north of the geographical equator.

• Reason: there is more land in the Northern Hemisphere, and this land adds heat.

• Hemisphere averages:

• Northern Hemisphere oceans: about 19°C.
• Southern Hemisphere oceans: about 16°C.

• Reason: unequal distribution of land and water (see 2.2).

• Isotherms (lines on a map that join places with the same temperature):

• They bend towards the poles along warm currents, because warm water is carried into higher latitudes. Example: the Gulf Stream in the North Atlantic.
• They bend towards the equator along cold currents, because cold water is carried into lower latitudes. Examples: off Peru and off Namibia.
• In the Southern Hemisphere the isotherms are more regular and run more east–west, because there is little land to disturb them.

4. Sea surface temperature (SST) as a driver of weather and climate

4.1 What SST is

• Sea surface temperature (SST): the temperature of the top layer of the ocean.
• Satellites measure SST from space. Ships and floating buoys measure it directly.

4.2 SST and tropical cyclones

• Tropical cyclone: a large, violent storm with very low pressure at its centre. It forms over warm tropical seas. Winds spiral around the centre.
• Tropical cyclones need SST above about 26.5°C.
• Warm water → a lot of evaporation → warm, moist air rises.
• The rising air cools, and its water vapour condenses. This releases latent heat (the hidden heat stored in water vapour, given out when it turns back into water).

• This heat powers the storm and keeps it growing.

• SSTs of 28–29°C and above make conditions very good for cyclones to form and grow in both the Arabian Sea and the Bay of Bengal [7].

Feature	Bay of Bengal	Arabian Sea
SST	Warmer	Cooler (traditionally)
Number of cyclones	More	Fewer
Recent trend	Still the main cyclone zone	Warming fast; more and stronger cyclones

• The Arabian Sea is warming.
• The SSTs at which cyclones form in the Arabian Sea are now 1.2–1.4°C higher than four decades ago [7].
• Cyclones are now gathering over the eastern Arabian Sea, near India's west coast [7].
• The Arabian Sea is becoming a cyclone hotspot. Severe cyclones have struck India's west coast in recent years (reported 2021) [8].

4.3 SST and the Indian monsoon: ENSO and IOD

• ENSO (El Niño–Southern Oscillation): a natural cycle of warming and cooling of the surface water in the central and eastern equatorial Pacific Ocean. It comes with a shift in air pressure across the Pacific.
• El Niño: the Pacific surface water becomes unusually warm. It usually weakens the Indian monsoon.

• La Niña: the Pacific surface water becomes unusually cool. It usually helps the Indian monsoon.

• How ENSO is measured:

• The Niño 3.4 region lies between 5°S–5°N and 120°W–170°W in the Pacific [6][5].
• Oceanic Niño Index (ONI): a 3-month running average of the SST anomaly (difference from normal) in Niño 3.4 [6].

• It is El Niño when this anomaly is +0.5°C or more for five 3-month periods in a row. It is La Niña when it is –0.5°C or less for five in a row [6][5].

• IOD (Indian Ocean Dipole): a see-saw of SST between the western and eastern Indian Ocean [6].

• Western box: 50°E–70°E, 10°S–10°N (Arabian Sea side, off East Africa) [6].
• Eastern box: 90°E–110°E, 10°S–0° (off Sumatra, Indonesia) [6].
• Positive IOD: the west is warmer than the east. It usually helps India's monsoon and can make up for an El Niño.

• Negative IOD: the east is warmer than the west. It usually weakens the monsoon.

• IMD issues a regular ENSO and IOD bulletin to track these patterns for monsoon forecasts [6].

4.4 Monitoring SST in India: INCOIS and Oceansat-3

• INCOIS (Indian National Centre for Ocean Information Services) is in Hyderabad, Telangana. It works under the Ministry of Earth Sciences.
• Oceansat-3 (EOS-06) was launched by ISRO in 2022. It is the third-generation satellite of the Oceansat series and continues the work of Oceansat-2 [2].
• Its instruments [2]:
  • OCM-3 (Ocean Colour Monitor): measures ocean colour, which shows chlorophyll (green pigment of tiny sea plants called phytoplankton).
  • SSTM (Sea Surface Temperature Monitor): measures SST.
  • SCAT-3 (Ku-band scatterometer): measures wind over the ocean.
  • ARGOS: collects data from buoys and other platforms.

• It gives daily data, which helps to find Potential Fishing Zones [2].

• Potential Fishing Zone (PFZ) advisory: a forecast that tells fishers where fish are likely to gather.

• How it is made:
  • Satellites give SST and chlorophyll maps.
  • Fish gather at thermal fronts (lines where warm and cold water meet) and where there is a lot of chlorophyll (food).
  • INCOIS marks these zones and sends the advisory to fishers.
• INCOIS issues PFZ advisories three times a week, using SST and chlorophyll data from satellites such as Oceansat-2 [3].
• They are given with reference to 586 fish landing centres along the Indian coast [4].
• Benefit: fishers save fuel and time and get bigger catches.

Prelims Hooks

• Surface temperature falls about 0.5°C per degree of latitude: about 27°C at the equator, 22°C at 20°, 14°C at 40°, and 0°C near the poles.
• The thermal equator lies slightly north of the geographical equator, because there is more land in the Northern Hemisphere.
• Average temperature: Northern Hemisphere oceans about 19°C > Southern Hemisphere oceans about 16°C.
• Offshore winds → upwelling → cold coastal water: off Peru (Pacific) and Namibia (Atlantic). Onshore winds pile up warm water.
• Isotherms bend poleward along warm currents (Gulf Stream) and equatorward along cold currents (Labrador, Peru).
• Trap: enclosed seas are not always warmer. The Red Sea and Persian Gulf (low latitude) are warmer than open seas, but the Baltic (high latitude) is colder.
• A tropical cyclone needs SST above about 26.5°C. The Bay of Bengal has more cyclones than the Arabian Sea.
• El Niño and La Niña are judged by the ONI: a ±0.5°C SST anomaly in the Niño 3.4 region (5°S–5°N, 120°W–170°W) [6].
• Oceansat-3 = EOS-06 (2022). Its payloads are OCM-3, SSTM, SCAT-3 and ARGOS [2]. PFZ advisories come from INCOIS, Hyderabad.
• PFZ advisories use SST + chlorophyll data [3].

Mains Points

• Distribution pattern (GS-I): ocean temperature mainly follows latitude, but land and water, winds, currents and enclosed seas change it. So isotherms are not straight lines. Examples: warm Gulf Stream vs cold Labrador Current; upwelling off Peru and Namibia; the Red Sea vs the Baltic.
• The sea's effect on climate (GS-I): water heats and cools slowly, so coasts like Mumbai have an equable climate while inland Nagpur has a continental one. Warm currents keep the west coast of Europe mild in winter.
• SST and disasters (GS-III): SST above about 26.5°C feeds tropical cyclones. The Arabian Sea is warming, and the SSTs at which cyclones form there are 1.2–1.4°C higher than four decades ago [7]. This means more risk for Gujarat, Maharashtra, Goa and Kerala. It calls for better early warning, cyclone shelters and coastal planning on the west coast as well as the east.
• SST and the economy (GS-III): SST patterns in the Pacific (ENSO) and the Indian Ocean (IOD) shape the monsoon, and so farm output and food prices. IMD's ENSO and IOD bulletins [6] help with monsoon forecasts. Satellite SST from Oceansat-3 [2] supports INCOIS PFZ advisories [3][4], which raise fishers' catches and cut fuel use. This is an example of space technology helping the blue economy.