High School Ocean Lesson Plans: Trade Winds and Horse Latitudes

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Trade Winds and Horse Latitudes

Topics Covered in this Lecture:


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 today's 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 Earth's Heat Budget

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, either in the form of heat or as sunlight penetrating into or scattered from the sea surface.

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.

In this lecture, we will attempt to gain some appreciation for the magnitude of air-sea interactions. 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.

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.

As discussed in your book, 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.

Which Way Do The Winds Blow?

Just like the deep-ocean currents, the circulation patterns of air in our atmosphere is density-driven; that is, differences in the density of air at different locations on our planet causes movement of air. The winds, in effect, are an attempt by the air to "balance" its density. In the same way that denser water moves in a vertical direction (i.e. deeper) to achieve isostatic equilibrium with the surrounding water, denser air moves to maintain an equilibrium across the horizontal realm of the globe.

As with seawater, density differences in air are caused by heating of the atmosphere. More heating in one region of the globe produces "lighter" (less dense) air; cooling produces "heavier" (more dense) air. This dense air commonly occurs as high pressure cells; colder, denser water moves rapidly from the high latitudes and sweeps across the globe where it mixes with warmer air. Conversely, lighter, warmer air, commonly occurs as low pressure cells. These low pressure cells give rise to storms and high winds as denser air rushes to "fill the gap" caused by the expanding, low pressure air. Through a combination of circumstances, low pressure cells may even form hurricanes.

Let's take a look at a very simple model of the circulation of the winds and the processes that drive that circulation. Your book illustrates the formation of a convection cell, formed when warm air rises in one location and is cooled in another location. A good example of this is the local circulation of air in coastal environments. Heating by the land (why does land heat more rapidly) causes air to rise. To replace this rising air, cooler air sweeps off the ocean, leading to a "circle", or convection cell, of wind blowing from the ocean across the land, where it rises and returns seaward at a higher altitude.

The land-sea interaction that causes coastal winds often exhibits a diurnal (or daily) cycle. Heating of land during the day creates onshore winds, as we just learned. However, because the land cools more rapidly at than the oceans, the situation is reversed at night (Why does land cool more rapidly than the oceans?!). Warm air rises over the oceans which is replaced by the cooler denser air that forms over land. In this case, an offshore ocean breeze is created.

Now let's consider this process on global scales. In our previous lecture, we learned that the seasonal cycle of the Earth orbiting around the sun creates global differences in the rate at which our planet is heated. Specifically, the equator is heated most directly, and higher latitudes (both north and south pole) experience seasonal changes depending on the angle of the hemisphere in relation to the sun. These global and seasonal differences create an unequal distribution of heat across the globe and, as we learned above, an imbalance in the density of air at any given latitude.

You should recall two things about the heat budget we talked about earlier. First, most of the heating of the atmosphere comes from the Earth's surface. Because most of the Earth's surface is oceans, it stands to reason that the oceans heat the Earth's atmosphere primarily. Second, you should recall that most of this heat is transferred to the atmosphere as a result of evaporation. Remember the 540 calories of energy required to "evaporate" a gram of water? Well, this latent heat of vaporization is transferred to the atmosphere when water evaporates. Thus, heat from the oceans is transferred to the atmosphere through water vapor. That's why the tropics are so muggy; there's lots of evaporation and lots of heat being pumped into the atmosphere as a result of evaporation of the surface of the oceans.

Okay, we've got a stationary globe heated at the equator and cooled at the poles; we've got convection cells that circulate air through the atmosphere; now let's get real. The Earth rotates, and as much as I would like the Earth's rotation to have no effect on the winds (because it would make teaching this subject much easier), the rotation of the Earth does have a profound influence on the atmospheric circulation patterns that we observe.

Here's the tricky part: the atmosphere is not attached to the Earth. Therefore, the Earth can spin independently of the atmosphere. But we've got these convection cells of air rotating from the equator to the poles, and an Earth spinning beneath it. Thus, the air appears to be moving eastward as the Earth moves eastward at a rate of 1050 miles per hour along the equator. However, the Earth doesn't spin at the same rate at the equator as it does at the poles (because the Earth is a sphere and the diameter of the Earth at higher latitudes is less than the diameter of the Earth at lower latitudes). In fact, the Earth moves half as slow at 60 degrees N as it does at the equator. The net result of all this spinning about is that the air moving at the equator northward at the eastward rate of 1050 miles per hour, travels north with this same eastward speed, and travels over land that is slowing down. The net result is that the air is moving eastward faster than the Earth and thus it appears to be deflected to the right (in the northern hemisphere). In the southern hemisphere, it appears to be deflected towards the left.

This apparent deflection of the moving air is known as the Coriolis effect. This effect "influences" objects moving in the northern hemisphere to deflect towards the right and all objects moving in the southern hemisphere to deflect towards the left. This is the basis sense of the Coriolis force and if you remember anything at all about it, just remember what it does.

As a result of the Coriolis force (and centrifugal forces talked about in your book), the global distribution of winds across the planet assume a much different pattern than might be expected. Rather than one giant convection cell from the equator to the poles, we now have three convection cells. These additional convection cells are caused by the deflection of the winds due to the Coriolis effect. As air moves towards the north from the equator, it is deflected to the right (in the northern hemisphere). The end result is that at about 30 degrees N, the wind is "turned around" such that it moves back towards the equator. The same thing happens to air moving equatorward from the poles; it deflects to the right and takes a turn at about 60 degrees N back towards the poles. This results in the formation of three giant atmospheric circulation cells in the northern hemisphere and three giant atmospheric circulation cells in the southern hemisphere.

These atmospheric circulation cells have been given names, although they don't appear in your book. The two equatorial circulation cells (one on each side of the equator) are known as Hadley cells, named after the man who came up with this scheme of atmospheric circulation. The atmospheric circulation cells between 30 degrees N/S and 60 degrees N/S are known as Ferrel cells, named after another man who contributed to our understanding of these circulation patterns. Finally, the poleward circulation cells are known as polar cells, which give rise to the polar easterlies at the north and south poles of our planets.

Trade Winds and Horse Latitudes

The wind patterns generated as a result of these circulation cells also have names, given to them by sailors who depended on these winds for transportation, exploration, and trade. On this latter note, the winds that blew ships across the Atlantic and Pacific Oceans near the equator are known as the trade winds. These winds are formed as poleward air bends right and circulates back towards the equator. Because of their predominant directions, they are known as the northeasterly trade winds in the northern hemisphere and the southeasterly trade winds in the southern hemisphere. Note that winds are named for the direction from which they originate. Thus, a north wind comes from the north.

As these trade winds converge and rise at the equator, they tend to cancel each other out. As a result, their is little wind in regions near the equator. Sailors have termed these areas the doldrums, to refer to a lack of air or wind that occurs here. The word doldrums has become synonymous with listless, gloomy, and apathetic, probably a reflection of the mood of sailors stuck in these areas, driven only by the slow-moving ocean currents.

At 30 degrees N and 30 degrees S, there is also a convergence of winds, as high pressure air descends from the poles. The cold air descending here tends to be rather dry and, like the doldrums, tends to be quite listless and variable. These regions are known as the horse latitudes. As Spanish sailing ships bound for America were becalmed in these areas, sometimes for weeks, the ships ran low on food and water. Rather than starve or die of thirst themselves, the sailors were forced to ditch their livestock. Thus, many a horse was thrown to a watery grave in these regions, and, as such, they came to be known as the horse latitudes.

Another important set of winds are the westerlies, which occur between the latitudes of 30 degrees and 60 degrees N/S. Westerlies are the surface winds of the Ferrel cells and they flow between the horse latitudes and the polar cells. Smart sailors learned of these winds and marked their return routes more northerly (or southerly) than usual to take advantage of these winds.

At the high altitudes, air that has cooled over the poles begins to descend towards the equator and turn west. At about 60 latitude, this air has warmed sufficiently to begin to rise, but it doesn't mix well with the warmer air mass of the westerlies. As a result, a frontal region is formed that is quite unstable. It is this frontal region that spins off most of the weather patterns that occur at these latitudes.

The three-cell circulation model for the global atmosphere presents a conceptual idealized description of the atmosphere. In actuality, several factors modify these patterns and complicate our wind and weather patterns on the globe. If we were talking a course in meteorology, we might study these in more detail. However, for the purposes of our discussion here, you should just be aware of the primary factors that modify the global wind patterns that we observe on a daily basis.

Two major factors that influence wind patterns on the globe. These are 1) seasonal changes in solar radiation; and 2) the presence of continents. This first factor we have discussed already and we won't repeat it here. This second factor is interesting, however, because it turns out that continents, which can block air patterns and create deserts due to the rain shadow effect, may have influenced the evolution of humans in Africa, as the continent moved over the last 4 million years.

Seasonal and topographic effects are discussed in your book and I won't repeat them here. Seasonal effects take the form of monsoons, hurricanes, typhoons, nor'easters, and a whole host of other weather phenomena associated with the changing of the seasons. Topographic effects include the rain shadow effect and orographic effects, changes in precipitation due to changes in elevation.

Longer term effects are evident in patterns of weather precipitated by changes in barometric pressure in the Pacific Ocean. While we are just beginning to understand these phenomena, the El Nino has dire consequences for humans and many forms of marine life. We will spend an entire lecture talking about El Nino later in the lecture.

A Few Final Thoughts

The interactions between the atmosphere and the sea occur over a multitude of times scales. From the daily cycle of heating and cooling of coastal areas to the global deep circulation conveyor belt, the time scales of change in air-sea phenomena span hours to centuries. Scientists are only now just beginning to unravel the many complicated factors that create weather and climate on our globe.

We should also not forget that our friendly neighborhood Gaia has the potential to play an important role here. If phytoplankton influence the formation of clouds, then global control of heat (recall the heat budget), winds (recall the circulation cells), and currents (see next lecture) could be under the control of phytoplankton. One only need apply a little air-sea interaction theory and a bit of imagination to appreciate the possibility that life controls our planetary processes.

from The Remarkable Ocean World