Oceanography

Kinetics §

• Most of the energy in the ocean is stored in the uppermost part of it.

• The ocean’s motion is in response to three things:

  • The wind which transfers its momentum to the water, forming waves.
    • Wind stresses vanish at a depth of about 100 m known as the null point.
    • If the planet were not rotating, water above the null point would flow parallel to the direction in which the wind is moving.
    • Currents largely reflect regional wind patterns.
  • Spatial variations in the density of water.
    • These variations are primarily due to changes in temperature.
    • Salinity also plays a role. More saline water is more dense
  • The Coriolis effect.
    • Because of the Coriolis effect, water above the null point moves at an angle to the direction of the wind. This causes the water to flow in an Ekman spiral pattern.

• Gyres pertain to large systems of ocean surface currents that move in a circular fashion due to the wind, the Coriolis effect, and spatial variations in the water.

• Ocean currents must turn to avoid continents. Thus the current is weak when near land (on top of winds not being strong near continents).

Temperature §

• Oceans can undergo phases of being warm and cool.

  • The surface water of the ocean circulates in huge currents that generally carry warmer water to the poles and colder water to the equator.
  • Generally, water is warmer near the equator and colder near the poles.
    • This is most obvious in open ocean with no obstructing continents.
    • However, the equator is not necessarily where the warmest waters are considering rising air near the equator which brings clouds that block insolation.
    • If an area of sea has a wider connection to the open sea, then the water can mix with or move to the open ocean, which also means their temperatures will tend to be the same as the open ocean.
  • Another thing to consider is upwelling where cold deep water flows towards the surface. This draws deeper water upward near the coast to replace the surface water blown offshore.

• Salinity decreases due to:

  • Fresh water flowing into the body of water.
  • More Precipitation compared to the amount of evaporation
  • Melting ice, which introduces more fresh water.

• Salinity increases due to:

  • More Evaporation of salt water relative to precipitation, which only removes the water.
  • Freezing ice which freezes only the water.

• The oceans vary in temperature and salinity, and therefore also in density, laterally across the surface and from shallower to greater depths.

  • High salinities occur in the subtropics — in areas with clear skies, intense sunlight and low precipitation
  • Low sanities occur near the poles due to low insolation. They also occur where major rivers deliver water.
  • As we move closer to the poles, the surface water temperature tends to be the same as that of the temperature in the depths.. This allows surface and deep waters to mix and for deep water to move up to the surface and shallow water to sink in a phenomenon called overturning
  • The warmest and most saline waters occur in the subtropics and tropics; Cold less saline waters occur near the poles.

• A warm pool is a region in the ocean where water temperature is higher than in adjacent regions. These region persist or seasonally reform. Their maintenance is described as follows:

  • Insolation generates warm air and a warm sea surface. Precipitation, however, means that there is no net loss of water to evaporation
  • The warm air rises and cools slowly. This forms a convection current between the tropopause and the troposphere. Additionally, the rising air causes the troposphere to be higher in these regions.
  • Cold, dry air moves laterally to the poles. At the same time, moist warm air moves to replace the unstable equatorial air. The incoming air releases latent heat which forms the warm pool.
  • In the poles, the process is reversed:
    • Cold air aloft moving poleward is dry. This air warms and sinks.
    • Because the air is cold and dry, there are few clouds. This means the water vapor loses heat.
    • Ice has high albedo which reflects any insolation. Coupled with the cooling of the water vapor, the surface and the vapor remain cold.
    • At the same time, cold polar air blows outward to replace the warm rising air.
    • Additionally, surface waters from lower latitudes flow to the poles which replace dense polar waters moving from the poles. This creates circulation.
    • Surface water sinks unstably due to the briny water in the poles, which forms due to the cold and saline water mixing with the incoming water. This water flows back to the equator as abyssal water.

• Ice shelves gain ice in three main ways:

  • Freezing of seawater below, but excluding salt.
  • Flow of glaciers from the land to the sea.
  • Snowfall, particularly that blown by the wind.

• Only the surface above the null zone is heated by insolation. This is called the photic zone. Water below the photic zone is consistently subzero due to the salinity and pressure.

  • The thermocline is a transition from the photic zone to deep ocean water. This marks a rapid change in temperature and density.
  • A marked vertical gradient in water density is a pycnocline, which commonly coincides with the thermocline

• Warmer, less saline and less dense waters “sitting” on top of cooler, more dense waters is an extremely stable arrangement which discourages mixing between the two sets of water.

• The thermohaline conveyor is the global circulation system driven by differences in temperature and salinity.

Links §

• Reynolds

• Water Resources - includes a discussion on the flow of water with oceans acting as one store.