Density structure of the Ocean - Distribution of temperature and salinity in the ocean

1. Density structure of the Ocean-Distribution of temperature and salinity in the ocean

2. 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

3. 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

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

8. 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

9. 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)

10. 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

11. 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. N S

14. Thermocline • 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)

15. 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

16. 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

17. Surface temperature • varies • Deep water is cold • everywhere

18. Polar Temperate Tropical

19. 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

23. 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

24. 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

25. 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.

29. 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.

30. 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