Ocean Temperature

MICRO-REALM 01

Insolation Engine

SOLAR RADIATION IS THEPRIMARY SOURCE OF HEAT

• Ocean water is heated by solar energy just like land
• However, water heats up and cools down much slower
• Due to water's high specific heat capacity

MICRO-REALM 02

Average Surface Temp

27°CGLOBAL AVERAGE

• The average temperature of surface water of the oceans
• Is about $27^\circ\text{C}$ (NCERT explicit fact)
• It gradually decreases from the equator towards the poles

MICRO-REALM 03

The Rate of Decrease

EQMIDPOLE

• Rate of decrease of temperature with increasing latitude
• Is generally $0.5^\circ\text{C}$ per latitude (NCERT)
• Averages: $22^\circ\text{C}$ at $20^\circ$, $14^\circ\text{C}$ at $40^\circ$, $0^\circ\text{C}$ near poles

MICRO-REALM 04

Ocean Isotherms

ISOTHERM

• Lines drawn on a map joining places of equal temperature
• Over oceans, isotherms are much straighter than over land
• Due to the homogenous nature of the water surface

MICRO-REALM 05

Northern Hemisphere Temp

AVERAGE: 19°CWARMER DUE TO MASSIVE LANDMASSES

• Oceans in the NH record relatively higher temperatures
• Average temp is around $19^\circ\text{C}$
• Because they are in contact with vast, hot landmasses

MICRO-REALM 06

Southern Hemisphere Temp

AVERAGE: 16°CCOOLER DUE TO WATER DOMINANCE

• Oceans in the SH are slightly cooler
• Average temp is around $16^\circ\text{C}$
• Due to the overwhelming presence of open water & ice

MICRO-REALM 07

Enclosed Seas (Low Latitudes)

HOT

• Enclosed seas in low latitudes (e.g., Red Sea, Persian Gulf)
• Record HIGHER temperatures than the open ocean
• Due to intense land heating and lack of cold mixing currents

MICRO-REALM 08

Enclosed Seas (High Latitudes)

COLD

• Enclosed seas in high latitudes (e.g., Baltic Sea)
• Record LOWER temperatures than the open ocean
• Cut off from the warming influence of major ocean currents

MICRO-REALM 09

Three-Layer System (NCERT)

1. TOP LAYER (0-200m)2. THERMOCLINE (300-1000m)3. DEEP LAYER (>1000m)

• The temperature-depth profile for the majority of oceans
• Is described as a three-layer system from surface to bottom
• Dictated by sunlight penetration and density

MICRO-REALM 10

1. Top Layer (Epipelagic)

WARM, SUNLIT & MIXED~500M THICK IN TROPICS

• Represents the warm top layer of the ocean
• Temperatures range between $20^\circ$ and $25^\circ\text{C}$
• Present throughout the year in tropics (thins in mid-latitudes)

MICRO-REALM 11

The Mixing Zone

〰

• The top layer is constantly agitated by wind and waves
• This mechanical mixing creates a uniform temperature
• Distributes solar heat downward to ~200 meters depth

MICRO-REALM 12

2. The Thermocline (NCERT)

BOUNDARY REGIONRAPID DECREASE IN TEMPERATURE

• The boundary region below the first layer
• Characterized by a RAPID decrease in temperature with increasing depth
• Acts as a thermal barrier between warm and cold water

MICRO-REALM 13

Thermocline Depth

STARTS ~300MENDS ~1000M

• Begins where wind-mixing stops (around 300m down)
• Extends roughly to 1,000 meters depth
• Holds about 18% of the total volume of ocean water

MICRO-REALM 14

3. The Deep Layer

EXTENDS TO OCEAN FLOORNEAR FREEZING CONSTANTLY

• The third layer, extending from 1,000m to the ocean floor
• Temperatures are near freezing ($0^\circ\text{–}3^\circ\text{C}$) year-round
• Contains 80% of all ocean water!

MICRO-REALM 15

Density & Temp Relationship

WARM = LIGHT
(FLOATS)

COLD = DENSE
(SINKS)

• Cold water is heavier (denser) than warm water
• Hence, freezing water generated at the poles sinks to the bottom
• And slowly creeps along the abyssal plains toward the equator

MICRO-REALM 16

The Polar Exception Trap

0°C TOP TO BOTTOM

• NCERT: In Arctic and Antarctic circles, surface temps are close to $0^\circ\text{C}$
• Therefore, the temperature change with depth is negligible
• The Thermocline DOES NOT EXIST in polar regions!

MICRO-REALM 17

Factor 1: Latitudes

POLE -- EQUATOR -- POLE

• The angle of sun's rays becomes slanting towards poles
• Amount of insolation decreases from equator to poles
• Hence, surface temperature decreases poleward

MICRO-REALM 18

Factor 2: Unequal Land/Water

N. HEMI:
LAND HEATS OCEAN

S. HEMI:
WATER COOLS

• Oceans in the Northern Hemisphere receive more heat
• Due to contact with larger extent of landmasses
• Makes the NH isotherms bend erratically compared to SH

MICRO-REALM 19

Factor 3: Offshore Winds (Upwelling)

←

↑ UPWELL

• Winds blowing from land to ocean (Offshore)
• Push warm surface water away from the coast
• Causes cold water from below to rise (Upwelling), lowering temps

MICRO-REALM 20

Factor 4: Onshore Winds

→

PILES WARM WATER

• Winds blowing from ocean to land (Onshore)
• Pile up warm surface water near the coast
• This raises the local ocean temperature (e.g., El Niño effects)

MICRO-REALM 21

Factor 5: Cold Currents

DECREASE COASTAL TEMPSUSUALLY ON WEST COASTS

• Ocean currents act as massive conveyor belts
• Cold currents bring frigid water from poles toward equator
• Significantly chill the western coasts of continents (e.g., Peru Current)

MICRO-REALM 22

Factor 6: Warm Currents

INCREASE COASTAL TEMPSUSUALLY ON EAST COASTS

• Warm currents bring heated equatorial water poleward
• Significantly warm the eastern coasts of continents (e.g., Gulf Stream)
• Keep high-latitude ports ice-free in winter

MICRO-REALM 23

Salinity & Temp Relationship

HIGHER SALINITY = HIGHER DENSITYWATER SINKS (COOLS)

• Salinity influences density, which drives vertical mixing
• Heavily saline water sinks, displacing deeper waters
• Creating a complex interplay called Thermohaline Circulation

MICRO-REALM 24

Location of Highest Temp

NOT AT THE EQUATOR!SLIGHTLY NORTH OF IT

• NCERT Fact: The highest temperature is NOT recorded at the equator
• But slightly towards the north of it
• Due to maximum insolation during summer solstice and landmass heat

MICRO-REALM 25

The Thermal Equator

THERMAL EQUATOR SHIFTS NORTH

• The line of maximum temperature around the globe
• It does not align perfectly with the geographic equator ($0^\circ$)
• It averages around $5^\circ$ North due to NH landmass bulk

MICRO-REALM 26

Diurnal Temp Range

NEGLIGIBLE(~1°C)

• The daily difference between max and min temperature
• In oceans, it is incredibly small (around $1^\circ\text{C}$)
• Because water requires immense energy to change temp

MICRO-REALM 27

Annual Temp Range

OPEN OCEAN: 3°C

ENCLOSED SEA: 12°C

• Difference between summer max and winter min temp
• Small in open oceans (~$3^\circ\text{C}$ at equator)
• Much larger in enclosed seas (e.g., Mediterranean, Red Sea)

MICRO-REALM 28

Master Ocean Temp Matrix

SURFACE = 27°CTHERMOCLINE (DROP)UPWELLING (COLD)NH WARMER THAN SH

• Completes Ocean Temperature Dynamics (NCERT Ch 13)
• Sets up the density differences required for Salinity
• Transitions to Realm 89 (Ocean Salinity)