3.2 The Oceans Water

Ocean Waters Characteristics

A. Salinity
B. Temperature
C. Density

Seawater – An Introduction

Seawater, or salt water, is water from a sea or ocean.

Average density at the surface is 1.025 kg/L. Seawater is denser than both fresh water and pure water (density 1.0 kg/L at 4 °C (39 °F)) because the dissolved salts increase the mass by a larger proportion than the volume.

The freezing point of seawater decreases as salt concentration increases. At typical salinity, it freezes at about −2 °C . The coldest seawater ever recorded (in a liquid state) was in 2010, in a stream under an Antarctic glacier, and measured −2.6 °C .

Seawater pH is typically limited to a range between 7.5 and 8.4. However, there is no universally accepted reference pH-scale for seawater and the difference between measurements based on different reference scales may be up to 0.14 units.

Ocean Salinity

• Although the vast majority of seawater has a salinity of between 31 g/kg and 38 g/kg, that is 3.1-3.8%, seawater is not uniformly saline throughout the world.
• Where mixing occurs with fresh water runoff from river mouths, near melting glaciers or vast amounts of precipitation (e.g. Monsoon), seawater can be substantially less saline.
• The most saline open sea is the Red Sea, where high rates of evaporation, low precipitation and low river runoff, and confined circulation result in unusually salty water.
• The salinity in isolated bodies of water can be considerably greater still – about ten times higher in the case of the Dead Sea.
• Salinity levels – Freshwater (< 0.05%), Brackish water (0.05–3%), Saline water (3–5%), Brine (> 5% up to 26%-28% max)
• Although the vast majority of seawater has a salinity of between 31 g/kg and 38 g/kg, that is 3.1-3.8%, seawater is not uniformly saline throughout the world.
• Where mixing occurs with fresh water runoff from river mouths, near melting glaciers or vast amounts of precipitation (e.g. Monsoon), seawater can be substantially less saline.
• The most saline open sea is the Red Sea, where high rates of evaporation, low precipitation and low river runoff, and confined circulation result in unusually salty water.
• The salinity in isolated bodies of water can be considerably greater still – about ten times higher in the case of the Dead Sea.
• The line with same salinity is joined by Isohalines.
• Halodine indicates sharp increase in salinity.
• Salts brought by rivers are the main source of salinity. It contains 60% of calcium sulphate, 2% of sodium chloride.

Share of different salts in Ocean Water in %

• Sodium chloride — 77.7%
• Magnesium chloride—10.9%
• Magnesium sulphate — 4.7%
• Calcium sulphate — 3.6%
• Potassium sulphate — 2.5%

Role of Ocean Salinity

For two reasons you measure salinity – one is it tells you something about evaporation and precipitation at the surface of the ocean – so rainfall and evaporation, which are important to understand in the hydrological cycle – and the other reason you measure salinity is because of its impact on density, and basically warm water is less dense, and freshwater is less dense, and cold water is heavier, and salty water is heavier.

Salinity helps us understand the health of a water body, and what animal and plant species we expect to find (or not find) there. Some animals tolerate changes in salinity well, and are found in both freshwater and saltwater (eels, alewives, invasive Chinese Mitten Crabs). Others are healthy only when living within a certain range of salinity (oysters).

Ocean salinity is affected by the following factors :

• The salinity of water in the surface layer of oceans depend mainly on evaporation and precipitation.
• Surface salinity is greatly influenced in coastal regions by the freshwater flow from rivers, and in polar regions by the processes of freezing and thawing of ice.
• Wind, also influences salinity of an area by transferring water to other areas.
• The ocean currents contribute to the salinity variations. Salinity, temperature and density of water are interrelated. Hence, any change in the temperature or density influences the salinity of water in an area.

Horizontal distribution of salinity

High salinity regions

Seas with salinity levels above the normal: They have higher levels of salinity because of their location in regions with higher temperatures leading to greater evaporation. They include the Red Sea (39 – 41 ppt), Persian Gulf (38 ppt), Mediterranean Sea (37 – 39 ppt) etc.

Saltiest Sea and Bodies of Water in the World

• The Mediterranean Sea has a high level of salinity because it is mostly closed off from the rest of the ocean. It also has warm temperatures that result in frequent humidity and evaporation.
• Red sea has 220%, Lake Van has 330% and Dead sea has 238% of salinity.
• If the salinity in a body of water changes, it can affect the water’s density.
• The higher the saline levels, the denser the water. For example, visitors are often astonished that they can simply float on their backs, without any effort, on the surface of the Dead Sea, due to its high salinity, which creates high water density.
• Even cold water with high salinity, such as that found in the northern Atlantic Ocean, is denser than warm, fresh water.

Normal salinity regions

Seas with normal salinity levels: These have a salinity in the range of 35 to 36 ppt. They include the Caribbean Sea, Gulf of Mexico, Gulf of California, Yellow Sea etc.

Low salinity regions

Seas with salinity levels below the normal: They have a low salinity due to the influx of fresh water. They include the Arctic Ocean, Southern Ocean, Bering Sea, Sea of Japan, Baltic Sea etc. Their surface salinity can be as low as 21 ppt.

Vertical Distribution of Salinity

• Salinity at the surface increases by the loss of water to ice or evaporation, or decreased by the input of fresh waters, such as from the rivers. Salinity at depth is very much fixed, because there is no way that water is ‘lost’, or the salt is ‘added.’
• The lower salinity water rests above the higher salinity dense water.
• Salinity, generally, increases with depth and there is a distinct zone called the halocline (compare this with thermocline), where salinity increases sharply.
• Other factors being constant, increasing salinity of seawater causes its density to increase. High salinity seawater, generally, sinks below the lower salinity water. This leads to stratification by salinity.

Ocean Water Temperature

• The temperature of the oceanic water is impor­tant for marine organisms including plants (phytoplanktons) and animals (zooplanktons).
• The tem­perature of sea water also affects the climate of coastal lands and plants and animals therein.
• Sea surface temperature affects the behavior of the Earth’s atmosphere above, so their initialization into atmospheric models is important. While sea surface temperature is important for tropical cyclogenesis, it is also important in determining the formation of sea fog and sea breezes.
• Sea surface temperature (SST) is the water temperature close to the ocean’s surface. The exact meaning of surface varies according to the measurement method used, but it is between 1 millimetre and 20 metres below the sea surface.
• Air masses in the Earth’s atmosphere are highly modified by sea surface temperatures within a short distance of the shore.

Heating and Cooling Heating

Heating

• Absorption of sun’s radiation.
• The conventional currents: Since the temperature of the earth increases with increasing depth, the ocean water at great depths is heated faster than the upper water layers. So, convectional oceanic circulations develop causing circulation of heat in water.
• Heat is produced due to friction caused by the surface wind and the tidal currents which increase stress on the water body.

Cooling

• Back radiation (heat budget) from the sea surface takes place as the solar energy once received is reradiated as long wave radiation (terrestrial radiation or infrared radiation) from the seawater.
• Exchange of heat between the sea and the atmosphere if there is temperature difference.
• Evaporation: Heat is lost in the form of latent heat of evaporation (atmosphere gains this heat in the form of latent heat of condensation).
  Surface of ocean has highest temperature due to insolation and heat transmitted from below through conduction process.

Temperature Distribution
Vertical 

Solar energy effectively penetrates 20 m and seldomly reaches beyond 200 m depth.
​Increase in depth decrease the temperature.
​200 m divides the ocean floor into two vertically-

(i) ​Photic/ Euphotic zone is the upper surface upto the depth of 200 m and receives solar radiation.

(ii) ​Alphatic zone goes beyond 200 m depth to the bottom and receives no solar ray.

• Epilimnion Layer is the first layer which is upto 500 m from top having temperature of 20° – 25°C.
• Thermocline Layer is below 500 m to 1000 m where temperature decreases at a rapid rate with the increase in depth.
• Hypolimnion Layer is the third layer, very cold in nature and extend upto deep ocean floor from 1000 m. Only Polar region has this layer starting from surface to deep ocean form.

Mean Annual Temperature of Oceans 

• Pacific – 19.1 oC
• Atlantic – 16.9 oC
• Indian – 17.0 oC
• Average – 17.4 oC

Horizontal 

• ​Average temperature of the ocean is 26.7°C
• ​Gradual decrease from equator towards poles is 0.5° F per latitude.
• Variation of temperature in the northern and southern hemispheres is mainly due to unequal distribution of land and ocean water.
• The average annual temperature of all the oceans is 17.2°C(63°F).

2003–2011 SST based on MODIS Aqua data.

Factors Affecting Temperature 

• Insolation: The average daily duration of insolation and its intensity.
• Heat loss: The loss of energy by reflection, scattering, evaporation and radiation.
• Albedo: The albedo of the sea (depending on the angle of sun rays). The physical characteristics of the sea surface: Boiling point of the sea water is increased in the case of higher salinity and vice versa [Salinity increased == Boiling point increased == Evaporation decreased].
• The presence of submarine ridges and sills [Marginal Seas]: Temperature is affected due to lesser mixing of waters on the opposite sides of the ridges or sills.
• The shape of the ocean: The latitudinally extensive seas in low latitude regions have warmer surface water than longitudinally extensive sea [Mediterranean Sea records higher temperature than the longitudinally extensive Gulf of California].
• The enclosed seas (Marginal Seas – Gulf, Bay etc.) in the low latitudes record relatively higher temperature than the open seas; whereas the enclosed seas in the high latitudes have lower temperature than the open seas.
• Local weather conditions such as cyclones.
• Unequal distribution of land and water: The oceans in the northern hemisphere receive more heat due to their contact with larger extent of land than the oceans in the southern hemisphere.
• Prevalent winds generate horizontal and sometimes vertical ocean currents: The winds blowing from the land towards the oceans (off-shore winds-moving away from the shore) drive warm surface water away from the coast resulting in the upwelling of cold water from below (This happens near Peruvian Coast in normal years. El-Nino).Contrary to this, the onshore winds (winds flowing from oceans into continents) pile up warm water near the coast and this raises the temperature (This happens near the Peruvian coast during El Nino event)(In normal years, North-eastern Australia and Western Indonesian islands see this kind of warm ocean waters due to Walker Cell or Walker Circulation).
• Ocean currents: Warm ocean currents raise the temperature in cold areas while the cold currents decrease the temperature in warm ocean areas. Gulf stream (warm current) raises the temperature near the eastern coast of North America and the West Coast of Europe while the Labrador current (cold current) lowers the temperature near the north-east coast of North America (Near Newfoundland). All these factors influence the temperature of the ocean currents locally.

Horizontal Distribution of Temperature: Important Features

• On an average, the temperature of surface water of the oceans is 26.7°C (80°F) and the temperature gradually decreases from equator towards the poles.
• The rate of decrease of temperature with increasing latitudes is generally 0.5°F per latitude.
• The average temperatures become 22°C (73°F) at 20° latitude, 14°C (57°F) at 40° latitude, and 0°C (32°F) near the poles. The oceans in the northern hemisphere record rela­tively higher average temperature than in the southern hemisphere.
• The highest temperature is not recorded at the equator rather it is a bit north of it.
• The average annual temperature of all the oceans is 17.2°C (63°F). The average annual temperatures for the northern and southern hemispheres are 19.4°C (67°F) and 16.1°C (61°F) respectively.
• The variation of temperatures in the northern and southern hemispheres is because of unequal distribution of land and ocean water.
• The temperature of the surface water of the oceans is higher than the air temperature above the ocean surface which means ocean surface gives off heat to the atmosphere. This phenomenon influences the generation of oceanic circulation mainly sea waves and ocean currents.
• ‘The temperature for January is 22.2°C higher over the oceans between 20° and 80°N, while in July it is 4.8°C lower. The mean annual temperature is 7°C higher over the water meridian’. The difference between air and sea surface tempera­tures causes fogs over the seas and the oceans.
• This happens when warm air passes over a cold sea surface having the temperature below dew point of the air. Consequently the air over the sea surface is cooled from below and sea fog occurs. Generally, sea fogs are frequently formed during spring and early summer because air coming from over the land is warmer while the sea surface is still cold. Sea fogs are very common in the high latitudes but are generally absent in the tropics.

Regional Variations

Atlantic Ocean

The decrease of temperature with increasing latitudes in the northern Atlantic Ocean, is very low because of warm ocean currents. The average temperature between 50°-70°N latitudes is recorded as 5°C (41°F). The decrease of temperature with increasing latitudes is more pronounced in the southern Atlantic Ocean. According to Krumel the highest temperature of surface water of the oceans is at 5°N latitude whereas the lowest temperature is re­corded between 80°N and the north pole and between 75°S and the south pole. 

Pacific Ocean 

The average annual tempera­ture of the Pacific Ocean is slightly higher than the Atlantic Ocean (16.91 °C or 60°F) and the Indian Ocean (17°C or 60.6°F). The lowest (3.3°C or 35.94°F) and the highest (32.2°C or 89.96°F) temperatures of the oceans are recorded near New Scotland and in the western Pacific Ocean respectively. 

Indian Ocean 

The highest tem­perature of the Indian ocean (25°C or 82.4°F) is re­corded in the Arabian Sea and Bay of Bengal but the enclosed seas of the Indian Ocean record still higher temperatures (Red Sea = 32.2°C or 90°F and Persian Gulf = 34.4°C or 94°F). The average seasonal tempera­tures (February and August) of surface water of the oceans have been represented through isotherms.

Density of Ocean Water

• The density of pure water is 1000 kg/m3. Ocean water is more dense because of the salt in it. Density of ocean water at the sea surface is about 1027 kg/m3.
• There are two main factors that make ocean water more or less dense than about 1027 kg/m3: the temperature of the water and the salinity of the water.
• Ocean water gets more dense as temperature goes down. So, the colder the water, the more dense it is. Increasing salinity also increases the density of seawater.
• Less dense water floats on top of more dense water. Given two layers of water with the same salinity, the warmer water will float on top of the colder water.
• There is one catch though! Temperature has a greater effect on the density of water than salinity does.
• So a layer of water with higher salinity can actual float on top of water with lower salinity if the layer with higher salinity is quite a bit warmer than the lower salinity layer.
• The temperature of the ocean decreases and decreases as you go to the bottom of the ocean. So, the density of ocean water increases and increases as you go to the bottom of the ocean.
• The deep ocean is layered with the densest water on bottom and the lightest water on top. Circulation in the depths of the ocean is horizontal.
• That is, water moves along the layers with the same density.

Density stratification of Ocean

Density wise Ocean is a three layered structure

i) Surface layer
ii) Pychocline layer and
iii) Deep layer

(i)​Surface layer ​​It has lowest density, having thickness of 100 to 200 m and is called as photic zone. Only 2% of seawater is found there. The zone is important for the growth of phytoplankton through the process of photosynthesis. Phytoplankton is eaten by zooplankton, it initiates food chain process.

(ii)​Pycocline layer ​​It is a transition zone characterized by rapid change of density. It is found between 300 m – 1000 m depth of ocean water. 18% of the total volume of the ocean water lies in this zone.

(iii)​Deep layer ​​It has high density. Water mass beyond 1000 m depth to the ocean floor contain 80% of the total volume of ocean water. It denotes high density and low temperature.

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Ocean Water Movement

Types of Ocean Currents

Based on depth 

Surface currents constitute about 10 percent of all the water in the ocean, these waters are the upper 400 m of the ocean; 
Deep water currents make up the other 90 percent of the ocean water. These waters move around the ocean basins due to variations in the density and gravity. 

Based on temperature 

Cold currents bring cold water into warm water areas [from high latitudes to low latitudes]. These currents are usually found on the west coast of the continents (currents flow in clockwise direction in northern hemisphere and in anti-clockwise direction in southern hemisphere) in the low and middle latitudes (true in both hemispheres) and on the east coast in the higher latitudes in the Northern Hemisphere; 
Warm currents bring warm water into cold water areas[low to high latitudes] and are usually observed on the east coast of continents in the low and middle latitudes (true in both hemispheres). In the northern hemisphere they are found on the west coasts of continents in high latitudes.

The general movement of the currents in the northern hemisphere is clockwise and in the southern hemisphere, anti-clockwise. This is due to the Coriolis force which is a deflective force and follows Ferrel’s law. A notable exception to this trend is seen in the northern part of the Indian Ocean where the current movement changes its direction in response to the seasonal change in the direction of monsoon winds.General Characteristics of Ocean Currents 

The warm currents move towards the cold seas and cool currents towards the warm seas. In the lower latitudes, the warm currents flow on the eastern shores and cold on the western shores [food for imagination]. The situation is reversed in the higher latitudes. The warm currents move along the western shores and the cold currents along the eastern shores. Convergence: warm and cold currents meet. Divergence: a single current splits into multiple currents flowing in different directions.

Effects of Ocean Currents

Ocean currents have a number of direct and indirect influences on human activities.

• Desert formation – Cold ocean currents have a direct effect on desert formation in west coast regions of the tropical and subtropical continents.
• There is fog and most of the areas are arid due to desiccating effect (loss of moisture).
• Rains – Warm ocean currents bring rain to coastal areas and even interiors. Example: Summer Rainfall in British Type climate. Warm currents flow parallel to the east coasts of the continents in tropical and subtropical latitudes. This results in warm and rainy climates. These areas lie in the western margins of the subtropical anti-cyclones.
• Moderating effect – They are responsible for moderate temperatures at coasts. [North Atlantic Drift brings warmness to England. Canary cold current brings cooling effect to Spain, Portugal etc.]
• Fishing – Mixing of cold and warm ocean currents bear richest fishing grounds in the world. Example: Grand Banks around Newfoundland, Canada and North-Eastern Coast of Japan. The mixing of warm and cold currents help to replenish the oxygen and favor the growth of planktons, the primary food for fish population. The best fishing grounds of the world exist mainly in these mixing zones.

98.The most important fishing grounds of the world are found in the regions where[2013 – I] (a)warm and cold atmospheric currents meet (b)rivers drain out large amounts of freshwater into the sea (c)warm and cold oceanic currents meet (d)continental shelf is undulating
Ans 98.(c)The mixing of warm and cold current in the region where planktons are found, is food for fishes. The temperature is just right for them to survive. The temperature is just right for the growth of firsh food called planktons.

• Drizzle – Mixing of cold and warm ocean currents create foggy weather where precipitation occurs in the form of drizzle [Newfoundland].
  • Drizzle – Mixing of cold and warm ocean currents create foggy weather where precipitation occurs in the form of drizzle [Newfoundland].
  • Climate – Results in Warm and rainy climates in tropical and subtropical latitudes [Florida, Natal etc.], Cold and dry climates on the western margins in the sub-tropics due to desiccating effect, Foggy weather and drizzle in the mixing zones, Moderate climate along the western costs in the sub-tropics.
• Tropical cyclones They pile up warm waters in tropics and this warm water is the major force behind tropical cyclones. Navigation Currents are referred to by their “drift”. Usually, the currents are strongest near the surface and may attain speeds over five knots (1 knot = ~1.8 km). [At depths, currents are generally slow with speeds less than 0.5 knots].
• Ships usually follow routes which are aided by ocean currents and winds. Example: If a ship wants to travel from Mexico to Philippines, it can use the route along the North Equatorial Drift which flows from east to west. When it wants to travel from Philippines to Mexico, it can follow the route along the doldrums when there is counter equatorial current flowing from west to east.

Currents of the Atlantic Ocean 

To the north and south of the equator, there are two westward moving currents, i.e., the North and the South Equatorial Currents. Between these two, there is the counter equatorial current which moves from west to east. 

NORTHERN HEMISPHERE

• The South Equatorial Current bifurcates into two branches near the Cape De Sao Roque in Brazil and its northern branch joins the North Equatorial Current.
• A part of this combined current enters the Caribbean Sea and the Gulf of Mexico, while the remaining current passes along the eastern side of the West Indies as the Antilles Current.
• The part of the current which enters the Gulf of Mexico comes out from the Florida Straight and joins the Antilles current. This combined current moves along the south-eastern coast of the U.S.A and is known as the Florida Current up to Cape of Hatteras. Beyond Cape of Hatteras, it is known as the Gulf Stream.
• A cold current from the Arctic Ocean called Labrador Current, which flows along the eastern coast of Canada, meets the warm Gulf Stream near the north-east corner of U.S.A.
• The confluence of these two currents, one cold and the other warm, produce fog around the region and makes it the most important fishing ground in the world.
• The Gulf Stream then deflected eastward under the combined influence of the westerlies and the rotation of the earth.
• It then crosses the Atlantic Ocean as the warm North Atlantic Drift.
• In this journey, another cold current from the Arctic called as the East Greenland Current joins with the North Atlantic Drift.
• The North Atlantic Drift bifurcates into two branches on reaching the eastern part of the ocean.
• The northern branch continues as North Atlantic Drift; reaches the British Isles from where it flows along the coast of Norway as the warm Norwegian Current and enters the Arctic Ocean.
• The southern branch flows between Spain and Azores Island as the cold Canaries Current.
• The Canaries Current finally joins the North Equatorial Current and completes the circuit.

SOUTHERN HEMISPHERE

• The South Equatorial Current turns south and flows along the eastern coast of South America as Brazil Current.
• At about 350 south latitude, due to the influence of westerlies and the rotation of the earth, the current moves eastward.
• A cold current called as the Falkland Current which flows along the south-eastern coast of South America from south to north joins with the current at this time.
• The Brazil Current moves eastward and crosses the Atlantic Ocean as South Atlantic Current.
• A part of the west wind drift or the Antarctic Circumpolar Current merges with the South Atlantic Current while crossing the Atlantic.
• Near the Cape of Good Hope, the South Atlantic Current is diverted northward as the Cold Benguela Current.
• Benguela Current finally joins with the South Equatorial Current and completes the circuit.

Currents of the Pacific Ocean 

NORTHERN HEMISPHERE

• The North Equatorial Current turns northward and flows along the Philippines Islands, Taiwan, and Japan to form the warm Kuro Shio or Kuro Siwo current.
• Later, a cold current called Oya Shio or Oya Siwo which flows along the eastern coast of the Kamchatka Peninsula merges with the Kuro Shio Current (Okhotsk Current is a cold current which merges with the Oya Shio before its confluence with Kuro Shio).
• From south-east coast of Japan, the Kuro Shio current comes under the influence of westerlies and flow right across the ocean as the North Pacific Current.
• After reaching the west coast of North America, it bifurcates into two branches: the northern branch flows anti-clockwise along the coast of Alaska as warm Alaska Current and the southern branch moves southward along the coast of California as the cold California Current.
• California Current eventually joins with the North Equatorial Current and completes the circuit.

SOUTHERN HEMISPHERE

• In the South Pacific Ocean, the South Equatorial Current flows towards the west and turns southward as the East Australian Current.
• From Tasmania, it flows as the cold South Pacific Current from west to east and crosses the Pacific Ocean along with the West Wind Drift.
• On reaching the south-western coast of South America, it turns northward and flows as the cold Peru Current or Humbolt Current.
• The cold waters of the Peru Current are partially responsible for making the coast of the northern Chile and western Peru with very scanty rainfall.
• Peru Current eventually joins with the South Equatorial Current and completes the circuit.

Currents of the Indian Ocean 

The pattern of circulation of ocean currents in the Indian Ocean differs from the general pattern of circulation in the Atlantic and the Pacific Oceans. This is because the Indian Ocean is blocked by the continental masses in the north. The general pattern of circulation in the southern hemisphere of the Indian Ocean is anti-clockwise as that of the other oceans. In the northern hemisphere, there is a clear reversal of currents in the winter and summer seasons, which are completely under the influence of the seasonal changes of monsoon winds.

NORTHERN HEMISPHERE DURING WINTER

• In summer, the northern part comes under the influence of the South West Monsoon.
• It results in an easterly movement of water in the Bay of Bengal and the Arabian Sea in a clockwise direction.
• This current is called as the South West Monsoon Drift.
• In the Indian Ocean, the summer currents are more regular than those of the winter.

 SOUTHERN HEMISPHERE

• In the southern part, the South Equatorial Current which flows from east to west is strengthened by its corresponding current of the Pacific Ocean.
• It then turns southward along the coast of Mozambique in Africa.
• A part of this current moving in between the African mainland and the Mozambique is called as the warm Mozambique Current.
• After the confluence of these two parts, the current is called as Agulhas Current.
• Agulhas Current merges with the West Wind Drift when it crosses the Indian Ocean.
• A branch of this merged current flows along the western coast of the Australia as cold West Australian Current.
• It later joins with the South Equatorial Current to complete the circuit.

Which one of the following factors is responsible for the change in the regular direction of the ocean currents in the Indian Ocean?[1997]
(a)Indian Ocean is half an ocean (b)Indian Ocean has monsoon drift (c)Indian Ocean is a land-locked ocean (d)Indian Ocean has greater variation in salinity
Ans (b)Due to the monsoon drift of Indian ocean, its regular direction of the ocean currents changes twice an year.

Consider the following statements:[2002]
1.Ocean currents are slow-surface movement of water in the ocean.
2.Ocean currents assist in maintaining the Earth’s heat balance
3.Ocean currents are set in motion primarily by prevailing winds
4.Ocean currents are affected by the configuration of the ocean Which of these statements are correct? (a)1 and 2 (b)2, 3 and 4 (c)1, 3 and 4 (d)1, 2, 3 and 4
Ans (b)Ocean currents are not slow most of the time and can be subsurface current also. So, statement ‘1’ is wrong. But ‘2’, ‘3’ and ‘4’ statements are correct.

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