Density structure of the Ocean

-Distribution of temperature and salinity in the ocean

Temperature

• Energy source for heating and cooling is the sun

• Sun emits electromagnetic spectrum of energy at many wavelengths

• Visible light (later) and longer wavelengths (infrared) that provide heat

• Measure water temperature with thermometers, CTD

Water density• A function of temperature and salinity• Density = mass/volume• Seawater weighs 2-3% more than fresh

water• Density of seawater is 1.020 – 1.030 g/cm3

(density of fresh water is 1.000 g/cm3)• Seawater’s density increases with

increasing salinity, increasing pressure and decreasing temperature

Seawater can have the same density at different temperature and salinity combinations

Density zones• Surface – mixed layer

– Temperature and salinity (thus density) relatively constant with depth because of wind mixing

– Least dense water– Exposed to atmosphere and sunlight– 2% of total ocean volume– Typically 10 - 100 m (book says ~150 m) but ranges from nearly 0 – 1000 m

• Pycnocline– Density increases steeply with depth– Separates surface and deep water– 18% of ocean water

• Deep zone– Little change in water density with depth– 80% of ocean water

• In surface, warm (maybe saltier), low-density water at the surface

• Rapid density increase within pycnocline mainly due to decrease in temperature (thermocline)

• In some regions (e.g., near rivers) may get a strong pycnocline due to vertical variations in salinity (or a halocline)

Stratification

• An assessment of water column stability• Temperature pretty constant in mixed layer• Rapid density increase with depth mainly due to

decrease in temperature (thermocline); e.g., the pycnocline is mainly due to the thermocline in the open ocean

• At depth is dense, cold, stable water. The bulk of the ocean water is < 1 – 3 oC bringing the average ocean temperature to 3.9 oC

The effect of temperature• Temperature varies with latitude due to

differential solar heating (future)• Radiation hits the earth in parallel rays• Incident angle varies with latitude• Energy is spread out over more area• Passes through more atmosphere• Poles are cooler because they receive lower

intensity solar radiation do to angle of incident radiation.

S

N

• Shape varies depending on latitude or other factors – most pronounced at mid- and low latitudes

• Surface temperature is regulated by sunlight and solar heating

• Thermocline varies with season and local conditions (storms, wind, etc)

Thermocline

Tropical Thermocline

• Surface layer is thicker in the tropics– Higher sunlight penetration is due to sun

angle and fewer suspended particles.• Tropical thermocline is deeper and more

pronounced (steeper)• Tropical surface later can warm to ~ 30oC• Deep waters still cold

Polar Thermocline

• Little solar heat• Often little change in surface temperature

(surface can actually be colder than deeper water!)

• Often lack a thermocline• If there is a thermocline it is less steep and

may be a shallower depth

•Surface temperaturevaries

•Deep water is coldeverywhere

TropicalTemperatePolar

Salinity also varies with latitude

• For different reasons than temperature• Global balance of precipitation and

evaporation• High salinity where little precipitation• At poles also have seasonal ice formation

and melting

The effect of salinity on density• Low salinity can also contribute to pycnocline (e.g.,

rivers)• Also, latitudnally,

– Where precipitation >> evaporation, salinity will be low– Where evaporation >> precipitation, salinity will be high

• Changes in salinity with depth produce the halocline (zone of rapid salinity increase with depth)

• Halocline often coincides with the thermocline and together produce pycnocline

Water masses• We describe “water masses” as discreet parcels

of water with characteristic temperature and salinity (therefore density)

• Even the densest water mass has origins at the surface (Deep water production in N Atlantic, mid-water production in Mediterranean)

• Water masses retain their identity for a long time• Water masses are layered by density and may

circulate differently – Surface circulation driven by atmospheric heating and

cooling and winds

Density stratification• Vertical movement of water is possible when

surface density increases to resemble deep water density

• Very stable water column in the tropics driven by temperature (solar heating outweighs storms and horizontal movement)

• Water column stability in northern polar regions driven by salinity (high runoff and ice formation)

• Southern polar ocean is more weakly stratified and mixes through the circumpolar current (little runoff). Turbulence encourages deep-water upwelling.

Summary - Ocean surface• Temperature shows effects of differential solar

radiation with latitude– More variation in temperature in the temperate zone

but salinity more stable • Salinity shows ratio of evaporation to

precipitation– Evaporation >> precipitation in tropics– Precipitation >> evaporation in temperate and polar

zones• Variations in temperature and salinity (and

density) confined to surface water.

Take home points

• Thermocline, halocline, pycnocline• Solar heating• Effect of ice (freezing and melting),

precipitation, evaporation• Relative distribution of temperature and

salinity with depth• Relative distribution of temperature and

salinity with latitude• Ocean is density stratified