 Temperature Distribution of Oceans

 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, etc.

 How does deep water marine organisms survive

 Photic zone -only about few hundred meters.- depends on lot of factors
like turbidity, presence of algae etc
 no enough primary producers below few hundred meters
 At sea bottom- bacteria -make use of heat -supplied by earth’s interior-
to prepare food.-are the primary producers.
 Other organisms-feed on these primary producers
 But productivity -too low compared to ocean surface.

 diurnal range of ocean temperatures too small?, Why

 Heating and cooling of oceanic water-slower than land
 due to vertical and horizontal mixing
 high specific heat of water.
 (More time required to heat up a Kg of water compared to solid-at same
temperatures and equal energy supply).

 Source of Heat in Oceans

 Absorption of sun’s radiation.
 conventional currents:ocean water at great depths -heated faster
than upper water layers-convectional oceanic circulations develop-
causing circulation of heat
 friction caused by surface wind and tidal currents

 ocean water is cooled by

 Back radiation (heat budget)-reradiated as long wave radiation
 Evaporation:
 Exchange of heat between sea and atmosphere

 Factors Affecting Temperature Distribution of Oceans

 Insolation:average daily duration of insolation and its intensity.
 Heat loss:by reflection, scattering, evaporation and radiation.
 Albedo:of the sea (depending on the angle of sun rays).
 salinity-[Salinity increased == Boiling point increased ==
Evaporation decreased].
 presence of submarine ridges and sills [Marginal Seas]:due to lesser
mixing of waters
 shape of the ocean:latitudinally extensive seas-warmer surface water
than longitudinally extensive sea [Mediterranean Sea higher
temperature than Gulf of California].
 enclosed seas (Marginal Seas – Gulf, Bay etc.)-in low latitudes- record
relatively higher temperature than open seas;-in high latitudes -lower
temperature than open seas
 Unequal distribution of land and water:-oceans in northern
hemisphere -more heat-contact with larger extent of land
 Local weather conditions such as cyclones.
 Prevalent winds-(off-shore winds- drive warm surface water away
from the coast-upwelling of cold water from below
 Ocean currents: Warm ocean currents raise temperature in cold areas -
while cold currents decrease temperature in warm ocean areas
 eg-Gulf stream (warm current) raises temperature near eastern coast
of North America

 Vertical Temperature Distribution of Oceans

 Photic or euphotic zone-from upper surface to ~200 m.-receives
adequate solar insolation.
 Aphotic zone-from 200 m to ocean bottom;-not receive adequate
sunrays.

 Thermocline-
 boundary region between surface waters and deeper layers.-around
100 – 400 m below sea surface-
 boundary region, from where - rapid decrease of temperature-approach
0° C.

 Three-Layer System
 first layer-500m thick-top layer of warm oceanic water-temperatures
between 20° and 25° C.
 within tropical region- present throughout the year -but in mid-latitudes
-develops only during summer.

 second layer-thermocline layer-characterized by rapid decrease in

temperature with increasing depth.-500 -1,000 m thick.
 third layer-cold-extends up to deep ocean floor.

 General behavior
 rate of decrease of temperature with depths -greater at equator
than at poles.
 surface temperature and its downward decrease-influenced
by upwelling of bottom water
 In cold Arctic and Antarctic regions-cold water sinks + moves towards
lower latitudes
 In equatorial regions -surface water sometimes -lower temperature
and salinity -due to high rainfall- whereas layers below it -higher
temperatures.
 enclosed seas in both lower and higher latitudes -higher temperatures
at bottom
 enclosed seas of low latitudes -like Sargasso Sea, Red Sea
and Mediterranean Sea -high bottom temperatures -due to high
insolation throughout year + lesser mixing of warm and cold’ waters.
 high latitude enclosed seas- bottom layers of water -warmer- as water
of higher salinity and temperature -moves from outer ocean -as a sub-
surface current.
 presence of submarine barriers-lead to different temperature conditions-
two sides of barrier. -example, at Strait of Bab-el-Mandeb,-submarine
 barrier -subsurface water in strait -at high temperature compared to
water at same level in Indian ocean.

 Horizontal Temperature Distribution of Oceans

 average temperature-27°C-decreases from the equator towards poles.
 rate of decrease of temperature -with increasing latitude -generally
0.5°C per latitude
 horizontal temperature distribution -shown by isothermal lines, ie.,
lines joining places of equal temperature.
 example-in February- isothermal lines closely spaced in Newfoundland
-and then isotherms widen out- make; a bulge towards coast of Norway
 cause -cold Labrador Current-reduces temperature of region more
sharply than in other places

 Range of Ocean Temperature

 even if solar insolation [maximum at noon[ ocean surface
temperature highest at 2 p.m.
 average diurnal range of temperature-barely 1 degree in oceans and
seas.
 diurnal range-highest-if sky -free of clouds and atmosphere is calm.
 annual range-Ifluenced by annual variation of insolation- nature of
ocean currents and prevailing winds.
 maximum in August and minimum in February
 northern Pacific and northern Atlantic oceans - greater range of
temperature -than southern parts -due to a difference in force of
prevailing winds from land


 Ocean Salinity
 Salinity-amount of salt (in gm) dissolved in 1,000 gm (1 kg) of
seawater.
 Factors Affecting Ocean Salinity
 depend mainly on evaporation, for example, Mediterranean sea.
 precipitation-eg-equaltorial regions-low salinity
 in coastal regions-influenced by fresh water flow from rivers
 in polar regions-processes of freezing and thawing of ice.
 Atmospheric pressure and Wind direction:anti-cyclonic conditions with
stable air and high temperature increase salinity
 ocean currents-warm currents near equatorial region -push away salts
from eastern margins -accumulate them near western margins of
oceans
 Temperature and density:Salinity, temperature and density of
water- interrelated-any change in temperature or density-
influences salinity of an area.

 Horizontal distribution of salinity

 salinity for normal open ocean ranges between 33 and 37.
 land locked Red Sea- 41 o/oo
 hot and dry regions-salinity sometimes reaches to 70.
 estuaries and the Arctic-fluctuates from 0 – 35, seasonally

 Pacific-around 35-36
 Atlantic-average salinity-around 20-37.parts per thousand
 Indian Ocean-average salinity-35
 low salinity-Bay of Bengal due to influx of river water by river Ganga.
 Arabian Sea -higher salinity -high evaporation + low influx of fresh
water.

 Marginal seas
 North Sea-higher salinity-saline water brought by North Atlantic Drift.
 Baltic Sea- low salinity- influx of river waters in large quantity.
 Mediterranean Sea-higher salinity due to high evaporation.
 Black Sea-low Salinity-fresh water influx by rivers

 Inland seas and lakes-high-regular supply of salt by ‘ rivers falling into

them.
 Great Salt Lake , (Utah, USA), Dead Sea and Lake Van in Turkey is
220, 240 and 330 respectively.

 Sub-Surface Salinity
 With depth, salinity also varies,
 In high latitudes-salinity increases with depth
 In middle latitudes-increases up to 35 metres -then it decreases
 At equator, surface salinity is lower.

 Vertical Distribution of Salinity

 Salinity changes with depth- but changes depends upon location of the
sea
 Salinity at surface -increases by loss of water to ice or evaporation-
decreased by input of fresh waters,
 . Salinity decreases with increasing depth.
 Salinity at depth-fixed-no water is ‘lost’, or salt is ‘added.’
 lower salinity water rests above higher salinity dense water.
 High salinity seawater sinks below lower salinity water.
 Salinity, increases with depth -distinct zone called halocline -where
salinity increases sharply.
 leads to stratification by salinity.

 Role of Ocean Salinity

 determines-
 compressibility,
 thermal expansion,
 temperature
 , density
 , absorption of insolation,
 evaporation
 humidity.
 affects ocean circulation-Ocean water from bottom-from poles
to equator. Surface water- from equator to poles
 Biodiversity:Ocean water -heavier than fresh water-enables plankton to
floa

 Thermohaline Circulation
 deep-ocean currents-driven by differences in water’s density,-controlled
by temperature (thermo) and salinity (haline).known as
thermohaline circulation.
 called the ocean conveyor belt,
 -
 How Atlantic Meridional Overturning Circulation (AMOC) works?

 As warm water flows northwards in Atlantic- it cools-evaporation

increases its salt content.
 Low temperature + high salt content- raise density of wate-sink deep
into ocean.
 cold, dense water deep below slowly spreads southward.
 Eventually-gets pulled back to surface -warms again-circulation is
complete.

 Importance of Atlantic Meridional Overturning Circulation

(AMOC)
 aids in distributing heat and energy around earth (heat budget).
 Western Europe’s climate -less harsh in winters (Gulf Stream, North
Atlantic Drift).
 acts as a carbon sink

 Cause of concern
 AMOC-undergone exceptional weakening-last 150 years
 IPCC 6th Assessment Report- AMOC will weaken over 21st Century -
as greenhouse gases increase.
 because as atmosphere warms-surface ocean retains more heat. +
increases in rainfall and ice melt-lower salinity
 will make ocean water lighter + reduce sinking in conveyor belt
(weaker AMOC).
 weaker AMOC-bring less warm water northwards-will partly offset the
warming effect of western Europe.

 Causes:
 global warming-part of Arctic’s ice called “Last Ice Area” -also
melted.
 Freshwater inflow from melting of Greenland ice sheet
 Increasing precipitation and river run-off.
 Weakening of Gulf Stream
 Dilution dues to increased rainfall and river runoff

 Effects of AMOC Slowdown

 decrease in marine productivity in the North Atlantic
 Colder Europe:decline in warming effect of NAD + Gulf stream
 more storms in Northern Europe,
 less Sahelian summer rainfall
 less South Asian summer rainfall,
 shift monsoons in South America and Africa.
 reduced number of tropical cyclones in the Atlantic
 Socio-economic impacts: On agriculture, wildlife, transport,
 increase in regional sea level along northeast coast of North America.
 Collapse of AMOC: AMOC is one of the nine “tipping points” -could
push parts of
 Earth system into abrupt or irreversible change

 may induce changes in -

 ENSO [El Niño–Southern Oscillation] characteristics,
 dieback of Amazon rainforest
 shrinking of the West Antarctic Ice Sheet
 southern migration of ITCZ
 large warming of Southern Ocean etc

 Indian Ocean’s role

 rising temperatures in Indian Ocean- can help boost AMOC
 As Indian Ocean warms faster- additional precipitation.
 draws more air -from other parts of world -to Indian Ocean-including
the Atlantic.
 will lead to less precipitation in Atlantic Ocean.
 Less precipitation -lead to higher salinity-tropical portion of Atlantic.
 saltier water-will get cold much quicker than usual -sink faster.
 would act as jump start for AMOC-intensifying the circulation.

 Pycnocline
 Pycnocline -a boundary separating two liquid layers of different
densities.
 exists in oceans at depth of 100-1000 m- because -large density
difference between surface waters and deep ocean water.
 effectively prevents vertical currents except in polar regions.
 almost absent in polar regions-because of sinking of cold water near
poles.
 Formation of pycnocline -result from changes in salinity or temperature.
 pycnocline zone -extremely stable-acts as a barrier for surface
processes.
 changes in salinity or temperature -very small -below pycnocline- but
are seasonal in surface waters