aosc670 wk3 : ocean currents

1 atmosphere pressure = 100,000 pascal = 10^5 pa  = 10 dB
1 deciBar (dB) = 10,000 pascal (pa) = 10^4 pa
1 mbar = 100 pascal
1 hpa = 100 pascal



Mixed-layer: definition.

the mixed-layer is the layer between the ocean surface and a depth usually ranging between 25 and 200m, where the density is about the same as at the surface. The mixed-layer owes its existence to the mixing initiated by waves and turbulence caused by the wind stress on the sea surface. An effect of mixing is to make both properties of water, temperature and salinity, thus density, more uniform. The penetration of mixing to a certain depth (the mixed-layer depth) mostly depends on the stability of the sea water and on the incoming energy from the wind. The more stable is the surface water, the less mixing occurs, and the shallower is the mixed-layer. Sea water stability in the near-surface is determined by the atmospheric fluxes through the ocean surface (wind stress, heat and fresh water exchange). A typical unstable configuration is when water is denser (“heavier”) at the surface than below. The mixing that ensues, for example with some impulse from waves or turbulence, renders the density more uniform and deepens the mixed-layer. In certain conditions occurring only in a few areas of the high-latitude seas (e.g. Labrador Sea in North Atlantic, Weddell Sea in the Antarctic waters), instability is so strong that denser surface water literally sinks and mixes over large depths reaching more than 1000m.

In many situations, the mixed-layer can be identified with the layer of mixed temperature, when the salinity does not vary much with increasing depth in general. However, this becomes untrue as soon as for instance fresh water is exchanged between the ocean and the air above (evaporation or rain), which may create large salinity contrast.

The mixed-layer is the oceanic surface zone that responds the most quickly and directly to atmospheric fluxes, and it is through the mixed-layer that such influence is transmitted to the whole ocean in the long term. Conversely, the mixed-layer is the part of the ocean through which the ocean influences directly the atmosphere. Many important processes occur within the mixed-layer, whether physical (e.g. direct wind-forcing of the ocean circulation), chemical (e.g. dissolution of incoming CO2 from the atmosphere), or biological (e.g. phytoplankton production).



Thermocline
A thermoc-line (sometimes metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, such as an ocean or lake, or air, such as an atmosphere) in which temperature changes more rapidly with depth than it does in the layers above or below.

In the ocean, the thermocline may be thought of as an invisible blanket which separates the upper mixed layer from the calm deep water below. Depending largely on season, latitude and turbulent mixing by wind, thermoclines may be a semi-permanent feature of the body of water in which they occur or they may form temporarily in response to phenomena such as the radiative heating/cooling of surface water during the day/night. Factors that affect the depth and thickness of a thermocline include seasonal weather variations, latitude and local environmental conditions, such as tides and currents.



Note: Graph showing a tropical ocean thermocline (depth vs. temperature). Note the rapid change between 100 and 1000 meters. The temperature is nearly constant after 1500 meters depth.

Halocline

In oceanography, a halocline is a subtype of chemocline caused by a strong, vertical salinity gradient within a body of water. Because salinity (in concert with temperature) affects the density of seawater, it can play a role in its vertical stratification. Increasing salinity by one kg/m³ results in an increase of seawater density of around 0.7 kg/m³.


Note: Plot of temperature and salinity in the arctic ocean at 85,18 north and 117,28 east dated Jan. 1st 2010.


This is a good resource about oceanography.

http://www.goes-r.gov/users/comet/tropical/textbook_2nd_edition/print_3.htm

The direction of the currents is not an exact match of the wind. The surface stress decreases with depth leading to decreasing current speed and a spiraling of the water current vector (the Ekman spiral)23 shown in Fig. 3.21a. The mean surface current is about 45 degrees to the right of the wind in the Northern Hemisphere and to the left of the wind in the Southern Hemisphere; because like the air, the water is affected by Coriolis deflection. As surface water is removed, it is replaced with deep water, a process called upwelling (Fig. 3.21b).



Ocean circulations are also driven by differences in temperature and salinity (Fig. 1.10b) as temperature and salinity gradients lead to density variations and vertical circulations known as the thermohaline circulations. Cold water is denser than warm water and salty water is denser than fresh water so colder or saltier water tends to sink relative to warmer or fresher water. For example, in the north Atlantic, water sinks as the Gulf Stream cools and freezing increases the salinity of the upper ocean. The global ocean conveyor belt (Fig. 3.22a) is the resultant mean ocean transport of surface and deep ocean waters.