Buoyancy (oceanography)
Buoyancy (including English upwelling ) denotes the rise of water in the oceans , side seas and lakes from underlying layers into the near-surface light-filled layer. The water in the deeper layers is usually colder and richer in nutrients than the water in the surface layer. Buoyancy therefore generally leads to a cooling and nutrient enrichment of the surface water.
The vertical velocities associated with the buoyancy are of the order of 10 m / day. They are therefore smaller than the measurement errors of currently available flow meters . The ascending movements last longer than one period of inertial vibration .
causes
The cause of the buoyancy of the oceanic deep water is in most cases the divergence of the wind-driven Ekman transport in the turbulent cover layer of the sea surface. Divergent Ekman transport is excited in the top layer of the open ocean when the field of wind shear stress on the sea surface divided by the Coriolis parameter shows a positive rotation . In addition, a spatially constant easterly wind blowing over the equator generates equatorial lift. On the coasts of the seas, buoyancy is stimulated when a spatially constant wind blows parallel to the coast in the north (south) hemisphere, so that when looking in the direction of the wind vector, the coast lies on the left (right) side.
Effects of Buoyancy
Abiotic effects
When the surface layer is warmer than the deeper layers of the sea, upwelling leads to a regional cooling of the sea surface temperature . This is usually the case towards the equator of the oceanic polar front. The atmosphere above is influenced in different ways. The atmospheric boundary layer over a colder surface water is stabilized and thus the turbulence in the boundary layer is reduced. The consequence of this is that the impulse of the gradient wind in the higher air layers cannot be transmitted as strongly to the maritime boundary layer and thus the wind speed is lower over cold water than over warm surface water. The influence of the surface temperature on the wind speed at the sea surface causes the rotation of the wind vector to be increased by a temperature gradient perpendicular to the wind direction, while a temperature gradient parallel to the wind direction increases the divergence of the wind field. The cool surface temperature in upwelling areas lowers the dew point of the layers of air above, so that more fog forms over the upwelling area. The sea wind that forms during the day advances the fog from the coastal upwelling area up to a few 10 km inland, where it contributes to the water supply of the flora and fauna, especially in deserts .
Biotic effects
The nutrients in deep water have an important effect on the ocean and its environment. On the one hand, these are nutrient salts such as nitrates and phosphates , which dissolve again in the water of the deeper layers when the organic material, detritus or also called sea snow , decomposes . On the other hand, the buoyancy water is also rich in inorganic carbon. The nutrients that swell with the buoyancy in the euphotic zone cause a strong increase in phytoplankton , which often takes on the dimensions of an algal bloom , which can even be seen from space. This high primary production is the basis for the oceanic food chain . Therefore, the population density of higher species of the marine ecosystem in permanent upwelling areas is comparatively large. These upwelling areas with an extraordinarily high rate of biomass production are of great economic importance, especially since they are often located off the coast of less productive regions.
Appearance of buoyancy
Due to the properties of the planetary circulation, there are only certain areas on earth in which permanent or seasonal buoyancy can be stimulated. In the open ocean, long-term upwelling is mainly observed in the subpolar regions, in which the cores of the low pressure areas move, and along the equatorial ocean, over which the southeast trade wind blows (Sverdrup & Fleming 1942, Tomczak & Godfrey 1994).
Coastal upwelling areas are primarily located on the west coasts of the continents in the area of influence of the trade winds (Sverdrup & Fleming 1942, Tomczak & Godfrey 1994). The most important upwelling areas are found on the west coasts of South America ( Peru , Chile ) and North America ( California , Oregon ) as well as the west coasts of North Africa ( Morocco , Mauritania and Senegal ) and southern Africa ( Namibia , South Africa ). All these areas represent rich fishing grounds, which are an essential economic factor of the neighboring countries. In addition, the permanent coastal upwelling areas in the trade winds strengthen the desert climate of the adjacent land areas through the interaction of the cold surface water on the coast with the atmosphere.
In interannual periods, the otherwise very stable upwelling phenomena in the trade winds change due to long-distance effects from the western parts of the equatorial Pacific and the Atlantic . A temporary weakening of the trade winds in the western equatorial ocean triggers the propagation of an equatorial Kelvin wave along the equator, which crosses the entire ocean and finally spreads as a coastal Kelvin wave towards the poles on the east coasts of the oceans. The Kelvin wave carries the warm surface water from the western equatorial ocean with it and lowers the thermal thermocline along its path of propagation. This process prevents the buoyancy of nutrient-rich, cold water into the surface layer. The fish population then collapses and the increase in surface temperature changes the interaction with the atmosphere in such a way that heavy rainfall occurs in the otherwise arid coastal areas. This phenomenon is called El Niño in the Pacific and occurs on average every 5 years. In the Atlantic it is called Benguela Niño and only occurs here every 10 years.
The southwest monsoon causes buoyancy during the northern summer in the Arabian Sea , on the Somali coast , the southern coast of the Arabian Peninsula (Tomczak & Godfrey 1994), and on the coast of Vietnam .
Short-term upwelling can form on all coasts if the wind blows parallel to the coast, so that the coast on the north (south) hemisphere is on the left (right) looking in the direction of the wind vector and it lasts longer than a period of inertia .
See also
literature
- CM Risien, DB Chelton: A Global Climatology of Surface Wind and Wind Stress Fields from Eight Years of QuikSCAT Scatterometer Data . In: Journal of Physical Oceanography . tape 38 , 2008, p. 2379-2413 , doi : 10.1175 / 2008JPO3881.1 .
- HU Sverdrup, RH Fleming: The Oceans: Their Physics, Chemistry, and General Biology . Prentice-Hall, 1942, p. 1087 .
- M. Tomczak, JS Godfrey: Regional Oceanography: An Introduction Pergamon . 1994, p. 422 .
Web links
- Wind Driven Surface Currents: Upwelling and Downwelling: Background. NASA, Physical Oceanography Program, accessed December 7, 2009 (Statement of Buoyancy).
Individual evidence
- ^ Risien & Chelton 2008
- ↑ N. Jiao, Y. Zhang, K. Zhou, Q. Li, M. Dai, J. Liu, J. Guo, B. Huang: Revisiting the CO 2 "source" problem in upwelling areas - a comparative study on eddy upwellings in the South China Sea . In: Biogeosciences . tape 11 , no. 9 , 2014, p. 2465–2475 , doi : 10.5194 / bg-11-2465-2014 ( PDF ).