kmfkais
7th August 2006, 05:31 PM
'Scuse Me While I Kiss the Sky
The interface between the sky and the sea is one of the most intensely studied places on our globe. If you consider that the Earth's surface is nearly two-thirds water, then it should be obvious that this "skin" where they sea and the sky meet covers an area of vast importance. In this lecture, we examine the structure of the atmosphere and consider how atmospheric forces impact ocean processes. As the sun and oceans heat the atmosphere, the resultant winds drive the motions of the ocean currents. In turn, the heat capacity of the oceans plays a role in buffering the heat fluctuations of the atmosphere. Through our examination of this topic, we shall see how the sea and the sky interact to create the Earth's climate and distribution of heat on the planet.
The Air-Sea Interface
The air-sea interface acts as a kind of boundary between two media, namely, air and water. It serves as a transition zone for the exchange of gases, both inert and biologically active; the exchange of chemical compounds, such as those released by the activities of marine organisms, (i.e. dimethyl sulfide); and the exchange of energy, such as wind energy or radiation.
On global scales, atmospheric winds, governed by the heat of sunlight and the oceans and bent by the Coriolis forces, drive the circulation of the oceans. Global winds are responsible for the major gyres of oceanic circulation; they cause the Gulf Stream and the California current. Changes in global atmospheric pressure lead to episodes of El Nino, the phenomenon of ocean warming that occurs along the eastern boundaries of the Pacific which can be quite destructive both to ocean wildlife and to humans.
In turn, the oceans act as a kind of thermal distribution system, releasing heat to or removing heat from the atmosphere and distributing it around the globe. The waxing and waning of the polar ice caps act to moderate temperatures on the planet, both by reflecting greater or lesser amounts of sunlight back into space, and/or by removing heat from the atmosphere as the sea ice melts. The "long memory" of the oceans, its ability to retain its temperature for hundreds of years, also acts to dampen any short-term swings in atmospheric temperature.
The net result of air-sea interactions is the climate. Our global weather and its changes over the days, weeks, years, centuries, and millennia is are influenced by the interaction between the air and the sea. Major climatic changes such as ice ages or global warming are a direct result of exchanges of heat, gases, and chemicals between the oceans and the atmosphere.
Atmospheric and oceanographic scientists studying air-sea interactions must contend with enormous scales of change and hundreds of factors that contribute in some way to exchanges between the atmosphere and the oceans. The daily cycle of heating and cooling, the lunar cycle of the tides, the seasonal cycles, and longer-scale phenomena influence the nature of air-sea interactions and have a direct effect on our climate. By understanding these processes, we can hope to predict major climatic changes and lessen their impact on man.
When was the last time you stopped to consider that we live at the bottom of a vast ocean of air, extending higher into the sky than the oceans are deep? In fact, although the mass of the atmosphere is only about 1% that of the oceans, its weight is enough to exert 14.7 pounds of pressure for every square inch of surface immersed within it. If you consider that the average person carries around 6500 square inches of surface area on their body, then the total weight of the atmosphere exerted on each of us amounts to something around 95,500 pounds! That's a lot of weight to carry around. The reason we don't feel this weight is because air within our bodies exerts an equal amount of pressure and, thus, we don't perceive the weight.
The atmosphere extends nearly 54 miles above the surface of the Earth, with about 90% of all the gases contained within the first 9 miles. For the most part, our atmosphere consists of nitrogen and oxygen, comprising 78.08% and 20.95%, respectively. Trace elements, including carbon dioxide, make up less than 1% of the atmosphere. However, as we learned with the composition of elements in seawater, these trace constituents can be quite important. Consider that small increases in carbon dioxide have raised concerns about global warming and you get the idea.
http://www.oceansonline.com/images/atmostruct.jpg
Like water, the density of air plays a direct role in the circulation of air through the atmosphere. Increases in temperature lower the density of air, just as in water. The net effect is a lowering of air (or atmospheric) pressure, the force exerted by air molecules on a surface. The differential heating and cooling of the atmosphere results in pockets of high and low pressure. As shown in every weather map in every newspaper in the world, regions of high and low pressure form across the globe. The circular-like lines that surround these high and low pressure cells are called isobars. Just like the isolines we discussed previously, isobars represent areas of equal atmospheric pressure. These contour plots of atmospheric pressure, especially the colored USA Today-style maps, are the workhorse of every weatherman.
These plots of isobars of pressure help us to understand the circulation of the atmosphere. Just like the oceans, air moves from areas of higher density to areas of lower density; the "weight" of the air forces movement through the atmosphere. The interface between these areas of high and low pressure are called fronts. That's why "cold fronts", cells of cold, dense air, move across the country in such a fury. It is in the regions of fronts that much of the weather as we know it is produced, as mixing of air masses creates wind, rain, lightning, snow, hail, and other weather phenomena.
Heating and cooling of the Earth's surface by solar radiation from the sun drives the circulation of the atmosphere and the oceans. To understand these circulation patterns and how they interact, we first need to know what happens to solar radiation when as it enters the atmosphere.
The balance between incoming solar radiation and outgoing radiation is called the heat budget. Obviously, if the temperature of our planet is to remain stable, then the losses of heat radiation must be balanced by the gains in heat radiation. Using this concept, we can form a budget for heat transfer to the Earth.
As solar radiation penetrates our atmosphere (at the top of the mesosphere), it begins to experience absorption and scattering. Absorption is the process by which light energy is absorbed by chemical compounds; scattering (for our purposes) is the process by which light energy is re-directed by chemical compounds.
Nearly 26% of the radiation reaching our planet is reflected back into outer space before it hits the Earth's surface. This reflection of solar energy is primarily due to clouds, but also includes scattering by dust and other particles. (Remember Mount Pinatubo). Another 19% of this radiation is absorbed directly by the atmosphere, but 51% is absorbed by the Earth's surface, primarily the oceans. Reflection by the Earth's surface, including glaciers and the polar ice caps, adds about 4% to the loss column.
This translates into a transfer of about 70% of the impinging solar radiation to the heat budget of the Earth (i.e. 51% absorbed by the Earth and 19% absorbed by the atmosphere). Thus, to balance our budget, we must find ways to remove an equal amount of radiation. These losses primarily occur as reradiation of longer wavelength radiation back into outer space. Reradiation from the atmosphere into space accounts for the bulk of this loss (70%); direct radiation from the Earth's surface to outer space, unimpeded by absorption in the atmosphere, accounts for 6%.
Note that 45% of the heat absorbed by the Earth's surface is reradiated directly to the atmosphere before leaving into outer space. Transfer of heat from the oceans to the atmosphere accounts for a far greater percentage of the heat input into the atmosphere than does direct heating of the atmosphere by the sun alone. Compare the 19% absorbed by the atmosphere directly with the 51% that comes from the Earth's surface. Thus, most of the heating (and cooling) of the atmosphere is moderated by the ocean and land surfaces.
The transfer of heat from one compartment to the next is accomplished through the processes of conduction, evaporation, and reradiation. Conduction of heat is the transfer of heat within the same medium from a point of higher energy to a point of lower energy. If we heat (or cool) the surface of a bowl of water, the heat will eventually be distributed through the entire bowl. Heat in the oceans is accomplished both by diffusion and by currents. Evaporation, as we learned in an earlier lecture removes heat as water is transformed from a liquid to a gas (How much heat is removed?). Reradiation of heat transfers heat energy from one medium to the other; the oceans radiate heat to the atmosphere (or vice versa) and eventually into outer space.
It is important to remember that all of these processes are dynamic. The heat contained in any one compartment is a balance between the sources and sinks of heat. If the heat input is greater, such as during summer, the temperature will rise (this doesn't mean that heat loss stops, just that heat gain is greater than heat loss). Conversely, if the losses of heat are greater, the oceans will cool. Near the equator, there is a net surplus of heat. This heat is transferred to higher latitudes through the movements of the winds and currents. At higher latitudes, there is a net loss of heat. Obviously, for the Earth to maintain a stable temperature, these processes must remain in balance.
The presence of land versus ocean is also is important to the heat gains and losses. The range of temperatures experienced over land is much higher than the range for oceans. In addition, higher latitudes vary more in temperature than tropical regions. One other interesting observation in this graph is that temperatures in the northern oceans vary more than temperatures in the southern oceans. This is because of the unequal distribution of the continents on the globe; there is more land mass in the northern hemisphere. Thus, because of the greater heat capacity of the oceans, as compared to land, the southern hemisphere, which is dominated by oceans, experiences less of a range of temperatures than the northern hemisphere.
These cycles of heat gain and loss are played out on many time scales. Over the course of a day, heat from the sun may be moderated by clouds and wind. At night, the oceans cool. Thus, there is a diurnal cycle to heating and cooling of the oceans. Over the annual cycle, the relationship of the Earth to the sun creates seasonal differences in the heating and cooling of the oceans. And if that wasn't enough, wobbles in the Earth's orbit creates cycles called Milankovitch cycles which are responsible for the ice ages! Unraveling these cycles and determining their influence is a complicated task for oceanographers studying air-sea interactions.
The annual cycle of solar radiation has, perhaps, one of the most profound influences not only on the oceans and heat budget of the Earth, but also on our whole way of living. The change of the seasons is familiar to us all, but how many times do we stop to think what a profound impact it has on what we do and how we live our lives.
In the oceans, the differential heating of the northern and southern atmospheres maintains a kind of gradient that drives the weather patterns on our planet. A "permanent" disequilibrium is maintained, such that the oceans and the atmosphere are constantly in motion trying to balance to flow of heat across the globe.
Over the annual cycle, we all know that the angle of the Earth, in relation to the sun, causes the apparent motion of the sun north and south with the seasons. During spring and summer in the northern hemisphere, the sun's rays "march" northwards, adding more heat to regions above the equator. On the longest day of the year, the first day of summer (June 21 or the summer solstice), the sun reaches its highest point north of the equator and begins to "march" southwards again. At the beginning of fall, the Earth "descends" below the equator and spring begins in the southern hemisphere, our days get shorter, and air (and ocean) temperatures get cooler.
The interface between the sky and the sea is one of the most intensely studied places on our globe. If you consider that the Earth's surface is nearly two-thirds water, then it should be obvious that this "skin" where they sea and the sky meet covers an area of vast importance. In this lecture, we examine the structure of the atmosphere and consider how atmospheric forces impact ocean processes. As the sun and oceans heat the atmosphere, the resultant winds drive the motions of the ocean currents. In turn, the heat capacity of the oceans plays a role in buffering the heat fluctuations of the atmosphere. Through our examination of this topic, we shall see how the sea and the sky interact to create the Earth's climate and distribution of heat on the planet.
The Air-Sea Interface
The air-sea interface acts as a kind of boundary between two media, namely, air and water. It serves as a transition zone for the exchange of gases, both inert and biologically active; the exchange of chemical compounds, such as those released by the activities of marine organisms, (i.e. dimethyl sulfide); and the exchange of energy, such as wind energy or radiation.
On global scales, atmospheric winds, governed by the heat of sunlight and the oceans and bent by the Coriolis forces, drive the circulation of the oceans. Global winds are responsible for the major gyres of oceanic circulation; they cause the Gulf Stream and the California current. Changes in global atmospheric pressure lead to episodes of El Nino, the phenomenon of ocean warming that occurs along the eastern boundaries of the Pacific which can be quite destructive both to ocean wildlife and to humans.
In turn, the oceans act as a kind of thermal distribution system, releasing heat to or removing heat from the atmosphere and distributing it around the globe. The waxing and waning of the polar ice caps act to moderate temperatures on the planet, both by reflecting greater or lesser amounts of sunlight back into space, and/or by removing heat from the atmosphere as the sea ice melts. The "long memory" of the oceans, its ability to retain its temperature for hundreds of years, also acts to dampen any short-term swings in atmospheric temperature.
The net result of air-sea interactions is the climate. Our global weather and its changes over the days, weeks, years, centuries, and millennia is are influenced by the interaction between the air and the sea. Major climatic changes such as ice ages or global warming are a direct result of exchanges of heat, gases, and chemicals between the oceans and the atmosphere.
Atmospheric and oceanographic scientists studying air-sea interactions must contend with enormous scales of change and hundreds of factors that contribute in some way to exchanges between the atmosphere and the oceans. The daily cycle of heating and cooling, the lunar cycle of the tides, the seasonal cycles, and longer-scale phenomena influence the nature of air-sea interactions and have a direct effect on our climate. By understanding these processes, we can hope to predict major climatic changes and lessen their impact on man.
When was the last time you stopped to consider that we live at the bottom of a vast ocean of air, extending higher into the sky than the oceans are deep? In fact, although the mass of the atmosphere is only about 1% that of the oceans, its weight is enough to exert 14.7 pounds of pressure for every square inch of surface immersed within it. If you consider that the average person carries around 6500 square inches of surface area on their body, then the total weight of the atmosphere exerted on each of us amounts to something around 95,500 pounds! That's a lot of weight to carry around. The reason we don't feel this weight is because air within our bodies exerts an equal amount of pressure and, thus, we don't perceive the weight.
The atmosphere extends nearly 54 miles above the surface of the Earth, with about 90% of all the gases contained within the first 9 miles. For the most part, our atmosphere consists of nitrogen and oxygen, comprising 78.08% and 20.95%, respectively. Trace elements, including carbon dioxide, make up less than 1% of the atmosphere. However, as we learned with the composition of elements in seawater, these trace constituents can be quite important. Consider that small increases in carbon dioxide have raised concerns about global warming and you get the idea.
http://www.oceansonline.com/images/atmostruct.jpg
Like water, the density of air plays a direct role in the circulation of air through the atmosphere. Increases in temperature lower the density of air, just as in water. The net effect is a lowering of air (or atmospheric) pressure, the force exerted by air molecules on a surface. The differential heating and cooling of the atmosphere results in pockets of high and low pressure. As shown in every weather map in every newspaper in the world, regions of high and low pressure form across the globe. The circular-like lines that surround these high and low pressure cells are called isobars. Just like the isolines we discussed previously, isobars represent areas of equal atmospheric pressure. These contour plots of atmospheric pressure, especially the colored USA Today-style maps, are the workhorse of every weatherman.
These plots of isobars of pressure help us to understand the circulation of the atmosphere. Just like the oceans, air moves from areas of higher density to areas of lower density; the "weight" of the air forces movement through the atmosphere. The interface between these areas of high and low pressure are called fronts. That's why "cold fronts", cells of cold, dense air, move across the country in such a fury. It is in the regions of fronts that much of the weather as we know it is produced, as mixing of air masses creates wind, rain, lightning, snow, hail, and other weather phenomena.
Heating and cooling of the Earth's surface by solar radiation from the sun drives the circulation of the atmosphere and the oceans. To understand these circulation patterns and how they interact, we first need to know what happens to solar radiation when as it enters the atmosphere.
The balance between incoming solar radiation and outgoing radiation is called the heat budget. Obviously, if the temperature of our planet is to remain stable, then the losses of heat radiation must be balanced by the gains in heat radiation. Using this concept, we can form a budget for heat transfer to the Earth.
As solar radiation penetrates our atmosphere (at the top of the mesosphere), it begins to experience absorption and scattering. Absorption is the process by which light energy is absorbed by chemical compounds; scattering (for our purposes) is the process by which light energy is re-directed by chemical compounds.
Nearly 26% of the radiation reaching our planet is reflected back into outer space before it hits the Earth's surface. This reflection of solar energy is primarily due to clouds, but also includes scattering by dust and other particles. (Remember Mount Pinatubo). Another 19% of this radiation is absorbed directly by the atmosphere, but 51% is absorbed by the Earth's surface, primarily the oceans. Reflection by the Earth's surface, including glaciers and the polar ice caps, adds about 4% to the loss column.
This translates into a transfer of about 70% of the impinging solar radiation to the heat budget of the Earth (i.e. 51% absorbed by the Earth and 19% absorbed by the atmosphere). Thus, to balance our budget, we must find ways to remove an equal amount of radiation. These losses primarily occur as reradiation of longer wavelength radiation back into outer space. Reradiation from the atmosphere into space accounts for the bulk of this loss (70%); direct radiation from the Earth's surface to outer space, unimpeded by absorption in the atmosphere, accounts for 6%.
Note that 45% of the heat absorbed by the Earth's surface is reradiated directly to the atmosphere before leaving into outer space. Transfer of heat from the oceans to the atmosphere accounts for a far greater percentage of the heat input into the atmosphere than does direct heating of the atmosphere by the sun alone. Compare the 19% absorbed by the atmosphere directly with the 51% that comes from the Earth's surface. Thus, most of the heating (and cooling) of the atmosphere is moderated by the ocean and land surfaces.
The transfer of heat from one compartment to the next is accomplished through the processes of conduction, evaporation, and reradiation. Conduction of heat is the transfer of heat within the same medium from a point of higher energy to a point of lower energy. If we heat (or cool) the surface of a bowl of water, the heat will eventually be distributed through the entire bowl. Heat in the oceans is accomplished both by diffusion and by currents. Evaporation, as we learned in an earlier lecture removes heat as water is transformed from a liquid to a gas (How much heat is removed?). Reradiation of heat transfers heat energy from one medium to the other; the oceans radiate heat to the atmosphere (or vice versa) and eventually into outer space.
It is important to remember that all of these processes are dynamic. The heat contained in any one compartment is a balance between the sources and sinks of heat. If the heat input is greater, such as during summer, the temperature will rise (this doesn't mean that heat loss stops, just that heat gain is greater than heat loss). Conversely, if the losses of heat are greater, the oceans will cool. Near the equator, there is a net surplus of heat. This heat is transferred to higher latitudes through the movements of the winds and currents. At higher latitudes, there is a net loss of heat. Obviously, for the Earth to maintain a stable temperature, these processes must remain in balance.
The presence of land versus ocean is also is important to the heat gains and losses. The range of temperatures experienced over land is much higher than the range for oceans. In addition, higher latitudes vary more in temperature than tropical regions. One other interesting observation in this graph is that temperatures in the northern oceans vary more than temperatures in the southern oceans. This is because of the unequal distribution of the continents on the globe; there is more land mass in the northern hemisphere. Thus, because of the greater heat capacity of the oceans, as compared to land, the southern hemisphere, which is dominated by oceans, experiences less of a range of temperatures than the northern hemisphere.
These cycles of heat gain and loss are played out on many time scales. Over the course of a day, heat from the sun may be moderated by clouds and wind. At night, the oceans cool. Thus, there is a diurnal cycle to heating and cooling of the oceans. Over the annual cycle, the relationship of the Earth to the sun creates seasonal differences in the heating and cooling of the oceans. And if that wasn't enough, wobbles in the Earth's orbit creates cycles called Milankovitch cycles which are responsible for the ice ages! Unraveling these cycles and determining their influence is a complicated task for oceanographers studying air-sea interactions.
The annual cycle of solar radiation has, perhaps, one of the most profound influences not only on the oceans and heat budget of the Earth, but also on our whole way of living. The change of the seasons is familiar to us all, but how many times do we stop to think what a profound impact it has on what we do and how we live our lives.
In the oceans, the differential heating of the northern and southern atmospheres maintains a kind of gradient that drives the weather patterns on our planet. A "permanent" disequilibrium is maintained, such that the oceans and the atmosphere are constantly in motion trying to balance to flow of heat across the globe.
Over the annual cycle, we all know that the angle of the Earth, in relation to the sun, causes the apparent motion of the sun north and south with the seasons. During spring and summer in the northern hemisphere, the sun's rays "march" northwards, adding more heat to regions above the equator. On the longest day of the year, the first day of summer (June 21 or the summer solstice), the sun reaches its highest point north of the equator and begins to "march" southwards again. At the beginning of fall, the Earth "descends" below the equator and spring begins in the southern hemisphere, our days get shorter, and air (and ocean) temperatures get cooler.