Chapter 5: The circulation of the Oceans

Layers of the ocean

- Surface Zone: (60-100 m) lower-density zone, interacts with the overlying atmosphere. Takes place through evaporation, precipitation, exchanges of kinetic energy, radiative exchanges, and the exchange of heat. Well mixed by wind action, and so the surface zone is often referred to as the mixed layer. - Pycnocline (Transition) zone: characterized by a rapid increase in density with increasing water depth (pycnocline). In some regions this density gradient is dominated by salinity changes, and salinity rises rapidly with increasing depth. In this case, the salinity gradient is specifically referred to as the halocline. In most other regions, temperature drops rapidly with increasing depth. This transition is called the thermocline. STABLE LAYER (limits vertical movements and insulates deep ocean from seasonal changes in temp and salinity). - Deep ocean: contains about 80% of volume of oceans. Stable (little vertical movement).

Constituents of Sea Salt

Chloride (Cl), sodium (Na+), sulfate (SO), magnesium (Mg2+), calcium (Ca2+) and potassium (K+)

The vertical structure of the oceans

Determined by water density. - the highest densities tend to occur in the deepest layers, while the lowest densities are found near the surface. - Water density is controlled by temperature and salinity - Usually, density increases as salinity increases or temperature decreases - density decreases as salinity decreases or as temperature increases.

Divergence

Divergence can occur at the ocean surface when two ocean currents split. Upwelling will occur at that location. The southeast trades produce the westward-flowing South Equatorial Current and the net Ekman Transport is to the left of the wind flow, toward the South (hence, divergence occurs near the equator. Important areas of divergence: off the southwest coast of North America and the West Coast of North America due to easterly winds and southward-moving currents. Important areas of divergence: off the west coast of South America and Southern Africa. Where northward-moving currents have same effects.

The Ocean

Have high heat capacity. So, slight differences in incoming solar radiation from place to place have little impact on surface temperature of ocean (lateral temperature and density differ slightly).

NON-ENSO Year

Low pressure (rising air) at the surface over Australia and Indonesia and high pressure (subsiding air) at the surface in the central and eastern Pacific.

El Nino - Souther Oscillation Events

Major shift in oceanic circulation that occurs every 2-10 years. It is associated with large changes in articulation of the tropical atmosphere (gives rise to significant climate anomalies).

If such great volumes of salts reach the oceans each year, are the oceans still getting saltier with time?

No. Because.. 1) Evaporation of seawater from shallow seas. 2) Biological processes. For example, some marine microogranisms remove the elements calcium or silicon from seawater to form their shells. 3) Chemical reactions between seawater and newly formed volcanic rocks on the sea floor. 4) The formation of sea spray. As small droplets of sea-water become airborne, salts, especially sodium and chlorine, are removed when the spray is deposited on land.

Ocean Circulation

Persistent easterly winds at surface in the Pacific ocean produces a westward-flowing ocean current --> water piles up in western part of ocean. - This east-to-west movement of water thickens the warm surface layer in the west and thins it in the east. - The thinner surface layer in the east allows the upwelling of colder, nutrient-rich water below, which promotes high levels of biological productivity and large fish populations.

ENSO Year

Pressure increases over Australia and decreases in the central pacific year

Thermohaline Circulation

The circulation of temperature and salinity that deep-ocean currents depend on. In deep oceans, horizontal changes in density are small, but vertical changes can be larger. - but, the densest water is at the bottom, so the structure is very stable.

Bottom-water formation

The densest water produced in the oceans. Near the poles, surface waters are cooled below the normal freezing point bc of cold overlying atmosphere. When water freezes, it forms a layer of sea ice. When ocean surface freezes, most of the sea salt is excluded bc the salt doesn't fit into the crystal structure of the ice. Because of this, water just beneath the sea ice becomes saltier and an underlayer of very cold, highly saline water forms. Sinks, floats down to equator as the bottom layer of water in the deep-ocean basins.

Oceans vs Rivers

The oceans are much saltier than river water because, when ocean water evaporates, the salt is left behind (increasing the salt concentration) in the ocean.

Upwelling

The rising of cooler water to the surface to replace warm, divergent surface water. It is rich in nutrients, and brings them to the surface.

Salinity

The salt content of a water mass. - deep-ocean circulation is driven by differences in water density. - These differences are caused by variations in temperature and salinity. - sodium and chloride (Na, Cl) - avg salinity of world's oceans is 35%

The Ocean

Therefore, the ocean does not circulate as a direct response to surface heating.

Convergence

Water piles up (or converges) in the middle of the gyre. This happens because of wind-driven surface-ocean currents, Earth's rotation, and ultimately, friction. Downwelling

Wind Stress

Wind over ocean causes friction at surface --> wind-drift currents (force of wind acting on surface).

Downwelling

the process of accumulation and sinking of higher density material beneath lower density material, such as cold or saline water beneath warmer or fresher water or cold air beneath warm air. It is the sinking limb of a convection cell.