| Next revision | Previous revision |
| lecture2:buoyancy [2020/10/12 23:04] – created - external edit 127.0.0.1 | lecture2:buoyancy [2024/02/27 20:21] (current) – [Stratification] admin |
|---|
| {{ :lecture2:summer_stratification_in_lakes.png?120|}} | {{ :lecture2:summer_stratification_in_lakes.png?120|}} |
| |
| This stratification in lakes also leads to the well-known effects of oxygen depletion in deep water, since diffusion is limited through the distinct strata. However, in the ocean, due to salinity the function for temperature becomes much more complex (state equation). The ocean is over all seasons mainly well stratified (except e.g. unstable situation in polar regions due to caballing (process in which two water parcels of the same density but differing temperatures and/or salinities mix and produce a water parcel of higher density, which subsequently sinks), which leads to deep water formation) and thus stable. It is noted that the steeper the stratification, the more stable the ocean. This is because the higher stratification means that more potential energy need to be overcome by a parcel of water to travel vertically. This energy can be supplied by wind stress or internal turbulences. | This stratification in lakes also leads to the well-known effects of oxygen depletion in deep water, since diffusion is limited through the distinct strata. However, in the ocean, due to salinity the function for temperature becomes much more complex (state equation). The ocean is over all seasons mainly well stratified (except e.g. unstable situation in polar regions due to cabbeling (process in which two water parcels of the same density but differing temperatures and/or salinities mix and produce a water parcel of higher density, which subsequently sinks), which leads to deep water formation) and thus stable. It is noted that the steeper the stratification, the more stable the ocean. This is because the higher stratification means that more potential energy need to be overcome by a parcel of water to travel vertically. This energy can be supplied by wind stress or internal turbulences. |
| |
| In saline water (salinity > 1000 ppm) water is in its densest form when below zero degrees. However, salinity and the temperature and pressure distributions determine the level of stratification. Stratification occurs when water masses with different properties form different small layers (<30m thickness) whose density increase with depth. These layers act as barriers to water mixing. The different properties of the water include salinity and temperature, and thus density. The salinity of the surface water is mainly determined by the balance between evaporation and precipitation. In the subtropics evaporation is clearly higher than precipitation, which leads to the highest surface salinity in these regions. The warmest surface water is in low latitudes due to the more direct sunlight in these regions. | In saline water (salinity > 1000 ppm) water is in its densest form when below zero degrees. However, salinity and the temperature and pressure distributions determine the level of stratification. Stratification occurs when water masses with different properties form different small layers (<30m thickness) whose density increase with depth. These layers act as barriers to water mixing. The different properties of the water include salinity and temperature, and thus density. The salinity of the surface water is mainly determined by the balance between evaporation and precipitation. In the subtropics evaporation is clearly higher than precipitation, which leads to the highest surface salinity in these regions. The warmest surface water is in low latitudes due to the more direct sunlight in these regions. |
| |
| |
| ====== Buoyancy Freuquency ====== | ====== Buoyancy Frequency ====== |
| |
| Another feature in stratified water is the buoyancy frequency, which is directly associated with static stability. In a stably stratified water a water parcel may be displaced upwards, but due to it being denser, and hence heavier, than its new surroundings it will sink and due to inertia overshoot its initial position. However, beneath the original position the water around the parcel is denser and the parcel will be forced upwards again. | Another feature in stratified water is the buoyancy frequency, which is directly associated with static stability. In a stably stratified water a water parcel may be displaced upwards, but due to it being denser, and hence heavier, than its new surroundings it will sink and due to inertia overshoot its initial position. However, beneath the original position the water around the parcel is denser and the parcel will be forced upwards again. |
