Friday, September 15, 2017
OCEAN STRUCTURE AND EVOLUTION
The tropical and subtropical regions of the oceans have three layers, an upper warm layer, an intermediate layer of rapidly declining temperature (the thermocline), and the bulk of the ocean extending to the bottom. The intermediate layer is a zone where warm water mixes with colder deep waters.
The layers are maintained by density differences due to temperature and salinity; deep cold water is more dense or heavier than the upper layers. The surface layer is typically from one hundred to one thousand feet thick. It floats on top due to its expansion as it is warmed by the sun. Most of the energy of sunlight that enters the water is absorbed in the first meter; below a few meters only blue light continues until it too is absorbed almost completely in the top 100 meters.
The depth, typically between 50 and 500 meters, of the mixing layer varies with wave and current action depending primarily on wind speed and density differences between the layers. Constant winds produce higher waves and turbulence which induce deeper mixing. Thus, the energy of the sun is absorbed and transported throughout the upper layer. During a hurricane, the energy can be released from the water and strengthen the hurricane. As the hurricane moves on, a new one may form if the surface water has not cooled much below 80 degrees Fahrenheit.
The winds of storms presumably enable the surface layer to be warmed to greater depths and thus store energy sufficient to produce additional and or stronger hurricanes.
Does global warming contribute to warmer oceans and more or stronger hurricanes? Probably; a warmer air temperature increases the transfer of heat to surface water at the same time it reduces the rate of loss of heat from the water.
Do hurricanes contribute to ocean cooling, cooler weather, and a lull in hurricanes? Presumably, hurricane winds increase cooling effect at the surface as turbulence in the upper layer transfers heat to an increasing depth of the upper layer as well as increasing the rate of heat loss at the surface from radiant energy, heat of vaporization of water, and direct water to air transfer. The reduced water temperature increases the time needed to reach the temperature needed to generate a new hurricane.
THE CONSEQUENCES OF LAYERING
Layers make it more difficult to accurately measure the heat stored in the upper layer. As warm water is pushed toward the margin of a continent it increases in height and depresses the layers below and forces water of the middle layer into a bulge of middle layer that moves as an internal wave. The middle layer is squeezed between the lighter warm upper layer and the heavier lower cold layer, both being forced by gravity to seek an equilibrium of level layers. At the border of the warm and mixing layers, the internal wave (called a seiche by scientists studying freshwater lakes) can have much greater height (relative to the upper surface of the middle layer) than surface waves and move more slowly across the ocean (lake). I don’t know if oceanographers have studied hurricane induced seiches, but I would presume the initial movement would be a thickening of the near shore warm layer that would thin and push a bulge of the thermocline in a seaward direction.
The depth changes of the two interfaces, one of the mixing zone (thermocline) with the upper layer (epilimnion) and the other of the mixing zone with the lower layer (hypolimnion), may change by hundreds of feet in the ocean as an internal wave passes. So calculating the energy stored in the ocean from a single location of a depth and temperature profile, or determining if the ocean is warming by a few measurements, is not very accurate if the profile changes with a passing internal wave or warm water is built up along a coast by constant onshore wind that then subsides.
The flooding from a storm surge in coastal areas rises above normal ocean levels as enormous waves break into low-lying coastal areas as they peak in shallow beachfront areas. The magnitude of their intrusion is amplified by onshore winds, high tides, heavy rain, increased runoff from adjacent areas, and reduced runoff from normal channels or streams due to high ocean levels.
THE OXYGEN MINIMUM ZONE
Below the thermocline is an oxygen minimum zone where, light is insufficient for photosynthesis, and sinking dead organisms or their fragments provide nutrients for bacteria and other organisms to feed upon and deplete the oxygen in the process. At the bottom of the deeper parts of the ocean, cold, salty, well-oxygenated water replaces bottom water that is slightly warmer. The process of polar water replacing the deepest water continues until ten thousand or more years later the former bottom water approaches the oxygen minimum zone and becomes part of it until it enters the thermocline, eventually mixing with oxygenated surface water as well as being oxygenated by photosynthetic organisms when light is adequate.
The presence of oxygen gradually decreasing as water moves from the bottom to the oxygen minimum zone, and its long residence time, is evidence of the reduced biomass and long lives of individuals of the abyss.
It should be obvious that the rigors and size of a hurricane have a potential to eliminate individuals less well endowed with survival adaptations of structure, physiology, and behavior. It may be a tug-of-war between those survivors and ones better adapted to intervening weather and conditions.
The stratification of the non-polar portion of the ocean, with the cold, sunlight-lacking, enormous abyssal zone covering over half of the earth’s surface, provides a place of refuge for species adapted to those conditions. There are different ways of adapting, but one, the pogonophorans, have had security by living in a tube they secrete embedded in the sediments. Like many others in the abyss, they have extremely long lives, low metabolic rates, and ability to take up nutrients from very low levels in the water. Their circulatory system may be a critical component of their living deeply embedded in perhaps anoxic sediments while supplying oxygen to the embedded part from tentacles in the sea-water. http://evolutioninsights.blogspot.com/2013/06/evolution-in-deep-sea.html