OCEAN LAYERS
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.
EVOLUTIONARY
CONSIDERATIONS
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
