The potential effects of global warming as a result of rising carbon dioxide concentration is superimposed on a considerable amount of natural variability within the earth-atmosphere climate system. As discussed in Section 1.2, we define natural variability of the climate system as that which would take place without any intervention by humans. Some of this natural variability is forced by fluctuations in the amount of solar energy absorbed at the earth's surface (for example, the seasons can be viewed as a form of natural variability) while some of the natural variability arises from the nonlinear, chaotic behavior of the climate system. We will begin our discussion of natural climate variability with the effects of volcanoes on the climate system. In this chapter, we limit ourselves to climate variations that last a few years. In the next chapter, climate variations on much longer time scales will be discussed.
Volcanoes have played an important role in the earth-atmosphere climate system since the formation of the earth itself. The emissions from volcanoes constitute the original source of the atmosphere. These emissions continue to play a role in our climate system and may have contributed to a slight global cooling of the atmosphere in recent years. We deviate first from the main theme of this chapter to review how the atmosphere was formed.
The earth is generally accepted to have been formed roughly 4.6 billion years ago without an atmosphere. The atmosphere slowly grew over the ages from the gases emitted by volcanoes. While the emissions from volcanoes depend to a small degree on the location of the volcano, they, on average, can be categorized as having the following gaseous composition:
* 1% nitrogen
* 80% water vapor
* 12% carbon dioxide
* 7% sulfur dioxide and other gases
Consider for the moment the early earth with its volcanoes. The gasses emitted by them begin to accumulate to form an atmosphere. Since most of the emission is water vapor, the atmosphere (which then, as now, can hold only a very little water vapor) reaches rapidly the point at which it is saturated with water vapor. After that time, the water vapor emitted by the volcanoes condenses to liquid water, and this begins to form the global oceans.
The present atmospheric composition can be summarized as:
* 78% nitrogen
* 21% oxygen
* less than 1% argon
* .4% water vapor
* .036% carbon dioxide
* tiny traces of other gases, including sulfur
How do we get from the composition characterizing volcanic emissions to the present day composition of the atmosphere? The simple fact that the earth and the atmosphere are in contact with each other at the ocean surface permits molecules of atmospheric gas to enter the ocean. The totality of each species of molecule (nitrogen, oxygen, etc.) that has found its way into the ocean in this way and remains there constitutes a dissolved gas. So, we find in the ocean dissolved nitrogen, dissolved oxygen, etc. This is obviously what allows life in the ocean: the gills of fishes are devices for extracting dissolved oxygen, and dissolved carbon dioxide feeds plant life in the ocean. Ocean water at the present time on average contains the following amounts of dissolved gases:
* .9% nitrogen
* .5% oxygen
* 4.5% carbon dioxide
The interesting aspect of these concentrations is that carbon dioxide dissolves in ocean water much more readily than nitrogen and oxygen (compare the amount of carbon dioxide gas in the ocean versus that in the atmosphere). The reason for this is that nitrogen and oxygen do not react chemically with water but carbon dioxide does.
Although carbon dioxide dissolves readily in the oceans, the present concentration of carbon dioxide gas dissolved in the ocean is an insignificant fraction of the total amount of carbon dioxide emitted by volcanoes over the eons. The prehistoric oceans soon became saturated with respect to dissolved carbon dioxide gas in the same way that the ancient atmosphere became saturated with respect to water vapor. However, there is another molecule present in the oceans that affects the concentration of carbon dioxide in the oceans. That molecule is the mineral calcium that is washed into the oceans. In ocean water saturated with dissolved carbon dioxide, the compound calcium carbonate forms and precipitates (falls) to the ocean floor. By this process, dissolved carbon dioxide has been removed from the ocean and the carbon atoms deposited in sediments. This opened the way for more atmospheric carbon dioxide to dissolve in the ocean. The net result is that the carbon dioxide emitted by the volcanoes over the ages has become locked up for the most part in the earth itself. We will discuss this process further in Chapter 6.
Now you should begin to see how the present atmosphere was formed. Since nitrogen is a relatively inert (non-reacting) gas in the atmosphere and ocean, the small amounts of nitrogen emitted by volcanoes accumulate in the atmosphere and the ocean. In a similar manner, the tiny amounts of the very inert gas argon emitted by volcanoes events accumulated to a detectable amount in the present day atmosphere. In contrast, the history of carbon dioxide has not been one of continuous increase, but rather one of continuous removal.
Our last remaining puzzle is how the present abundance of oxygen was created in the atmosphere, since volcanoes emit none. This puzzle can be explained by the process of photosynthesis, by which plant life in the oceans and on land consume carbon dioxide and eject oxygen. As we will see in a subsequent chapter, the origin of the oxygen in the present atmosphere is photosynthesis in the ocean.
While the central role of volcanoes in the formation of the atmosphere is accepted widely in the scientific community, volcanoes are often viewed as an external agent in the climate system. As we discussed in Section 1.2, the only true external forcing mechanism for the earth-atmosphere climate system is the forcing imposed by the sun. Nevertheless, an "external" forcing agent (such as a volcanic eruption) is often defined as one which affects the climate system without itself being affected by climate variations.
The eruption of a volcano always has an immediate impact on the weather in surrounding regions. As the water vapor cools and forms a cloud of water and ash high above and downwind of the volcano, the daytime temperatures drop noticeably and the nighttime temperatures increase (this is another example of the radiative effect of high clouds). This immediate effect of volcanoes on the weather of surrounding regions is unquestioned. However, the eruption of a major volcano has long been thought to be associated with changes in the global climate, most noticeably a cooling of the surface that lasts for a year or two after the eruption takes place[*].
First, what do we mean by a major volcanic eruption? Volcanic eruptions of varying intensity take place around the globe during every year. However, only a few fall into the major eruption category that are likely to have had an appreciable impact on the global climate. These include:
* Mount Pinatubo, Philippines- 1991
* El Chichón, Mexico- 1982
* Agung, Indonesia- 1963
* Santa Maria, Guatemala- 1902
* Krakatoa, Indonesia- 1883
* Tambora, Indonesia- 1815
Other scientists that are involved in research in this field might want to include a few more that took place during the past 200 years, but the five above are the best documented. Notice that missing from the list is the most recent volcanic eruption in the continental United States: Mt. Saint Helens in Washington during May 1980. Like all volcanoes, Mt. Saint Helens had an immediate impact on the weather downwind over the state of Washington and parts of Idaho, but it did not have any appreciable effect on the global climate.
The composition of the gaseous output from volcanoes was listed earlier in this chapter. However, there is another significant aspect of volcanic emissions: heat and the explosive energy of the eruption. The massive amounts of energy involved in major volcanic eruptions are enough to send the plume of water vapor, sulfur dioxide, and ash upwards more than 20 to 30 km vertically into the atmosphere. As the plume is carried laterally by the prevailing winds, most of the heavier particles (the ash, liquid water droplets, etc.) fall out within a few hundred kilometers of the volcano.
We just mentioned that the volcanic plume can extend to heights in excess of 20 or 30 kilometers. Remember from Fig. 1.6 that the troposphere lies roughly in the lowest 10 kilometers while the stratosphere extends upwards another 30 kilometers. In other words, volcanic plumes are lifted far into the stratosphere. One way to distinguish major eruptions from others is the great height to which the major eruptive plumes extend into the stratosphere. The stratosphere is characterized by stable conditions; once a light particle reaches this elevation, it is likely to remain suspended in the stratosphere for a lengthy time. Put another way, the residence time (see Section 3.2) of the particles in the stratosphere is much longer than that of similar particles in the troposphere.
A significant aspect of the list of major volcanoes above is that they all took
place equatorward of 30[o ]latitude. The reason that this is
significant is that once a volcanic plume extends into the tropical
stratosphere, it spreads out laterally and eventually poleward into both
hemispheres. It is for this reason that the major volcanic eruptions for
climate purposes are those that take place in the tropics. The area covered by
a plume from a similar volcano in the extratropics is limited by the prevailing
tendency of the atmospheric circulation in the stratosphere to carry material
poleward and then downward in higher latitudes.
Figure 4.1. Incoming solar radiation (heavy arrow) is shown being intercepted by aerosols in the atmosphere. The solar radiation is scattered (thin arrows) in all directions.
So, how do volcanic eruptions affect the climate of the global earth-atmosphere system? This question is still under investigation. There is one very strong candidate to explain this phenomenon. Volcanoes emit a significant amount of sulfur dioxide, which mixes with the water vapor in the plume and in the stratosphere to form tiny droplets and particles of sulfuric acid (referred to collectively as sulfuric acid aerosol). These aerosols remain suspended in the stratosphere and are spread over the globe by the stratospheric circulation.
One consequence of the additional sulfuric acid aerosol suspended in the
stratosphere is brilliant red and orange tinted sunsets over Salt Lake City
from several months after a major eruption until a couple of years later. The
delay of a few months results from the time that it takes to carry the aerosols
from the tropics to the midlatitudes. Most recently, the eruption of Mt.
Pinatubo led to frequent vivid sunsets over Salt Lake during 1991 and 1992. We
can infer from these facts that the residence time of sulfuric acid aerosols in
the stratosphere is on the order of a couple of years.
Figure 4.2. The distance that light travels through the atmosphere is shorter at noon when the sun is overhead compared to that at sunrise or sunset.
Why are the sunset colors more vivid after an eruption? The volcanic sulfur aerosols scatter additional amounts of sunlight. Scattering is a process by which sunlight incident on a gas or small solid particle that is suspended in the atmosphere is reflected in all directions as shown schematically in Fig 4.1. Scattering from air molecules by these molecules or small particles depends on the wavelength of the solar energy, and short (that is, the blue end of the visible spectrum) wavelengths are preferentially scattered at the expense of the long (red) wavelengths. Scattering causes the normally bluish tint to the clear sky during the middle of the day. However, at sunrise and sunset, the sunlight travels farther through a thicker portion of the atmosphere, as shown in Fig. 4.2, so that the blue light is scattered out and the sky appears red or orange. The additional scattering by volcanic aerosols contributes to especially vivid sunsets. It was possible after the eruption of Mt. Pinatubo to see lighted bands and layers of the aerosols over Salt Lake City.
Now we finally finish our answer regarding the climatic impact of volcanic eruptions. It is hypothesized that the additional sulfuric acid aerosols in the stratosphere reflect more sunlight back to space than they absorb in infrared radiation. This hypothesis has climatic implications similar to the albedo effect of increasing low clouds. As the amount of sulfuric acid aerosols in the stratosphere increases after an eruption, the amount of sunlight reaching the earth's surface diminishes and the globally averaged temperature should decrease.
Notice that we have stressed at many points that the above statements are a hypothesis-- the link between volcanic eruptions and climate is not as firmly established as other links in the climate system. Statistical analyses of the global climate record suggest that the globally averaged temperature of the earth may drop a couple of tenths of a [o]C after a major eruption for a year or two. The effects of major volcanic eruptions are difficult to assess because so few have occurred.
Figure 4.3. Globally averaged surface temperature anomalies with major volcanic eruptions indicated by arrows.
Let's do our own crude analysis of the link between global climate and major volcanic eruptions. Figure 1.7 is repeated on this page with the time of occurrence of Krakatoa, Santa Maria, Agung, and El Chichón indicated by arrows. Dips are evident in the temperature record after the Krakatoa eruption in 1883 and the Agung eruption in 1963. However, there is no such dip after the 1982 eruption of El Chichón; it occurred prior to some of the warmest years on record. In fact, this global warming was initially underestimated from satellite measurements as a result of contamination of the measurements by the additional aerosols in the stratosphere emitted by El Chichón. Records of the globally averaged temperature that extend to the present time show that the much warmer than normal conditions during the 1980's have diminished somewhat after the time of the Mt. Pinatubo volcanic eruption.
We have presented here a fairly lengthy discussion of the role of volcanoes in the climate system as an example of a physical phenomenon that undoubtedly affects the climate system, yet for which it is difficult to estimate exactly what the effect is. There is no doubt that the amount of solar radiation transmitted through the atmosphere is reduced during and after a major volcanic eruption. We have satellite estimates of the reduction of the amount of solar radiation transmitted through the atmosphere during and after El Chichón and Mt. Pinatubo that indicate irrefutably that the amount of solar radiation is reduced. Thus, we have a strong causal link: volcanoes reduce the amount of solar radiation reaching the surface. However, we can not demonstrate irrefutably that this is associated with a significant cooling in the globally averaged temperature. Some scientists believe that the uncertainty associated with this cooling is small while others (including the authors) believe it is larger than the proponents of this hypothesis advocate. We can at least agree that volcanoes serve as one source of climate variability for which mankind does not need to be blamed.
We now turn to climate variations that arise as a result of the interactions of the atmosphere and ocean. The atmosphere and underlying ocean are coupled strongly on all time scales. If you have lived in the coastal regions of the United States (or visited there for any length of time), you are aware of how the ocean tends to moderate the coastal climate. The temperatures aren't as cold nor are they as warm compared to regions further inland. All water bodies have this effect on their environment and the strength of this effect depends on the surface area of the ocean, sea, or lake. Even the Salt Lake helps to moderate the climate in the Salt Lake Valley. Nearly every day during summer, a northerly lake breeze develops that carries cooler air over the valley.
Let's begin by developing a few basic ideas about the oceans.
Figure 4.4. A simple schematic of the layers of the ocean and an idealized profile of temperature with depth.
The oceans cover about 70% of the surface of the earth at an average depth of 3700 m. The ocean can be divided into two layers: the mixed layer, occupying on average the top 100 m (330 feet); and the deep ocean, which is everything below. So, a typical vertical cross-section of the ocean looks like that in Fig. 4.4.
One reason for this division into two layers is that sunlight does not penetrate more than about 100 m into the ocean. So, as shown schematically in Fig. 4.4, the mixed layer of the ocean is warmer than the deep ocean. In addition, the mixed layer is kept well stirred by the action of the winds blowing across the ocean surface. The mixed layer can be viewed as the part of the ocean that responds rapidly to changes in atmospheric conditions. The net effect of all of the ocean waves generated by the surface winds is that the top 100 m of the ocean tends to be uniformly warm. Since warm water is also light water, we can say literally that the mixed layer floats on top of the deep ocean. The rest of the ocean is dark and cold. Its only communication with the mixed layer (and the atmosphere above) is by the processes of upwelling and downwelling.
The simple sketch in Fig. 4.4 of the vertical structure of the ocean applies everywhere over the earth except in relatively small but important regions in high latitudes. There, the mixed layer of the ocean is in contact with very cold air during most of the year. This cold air and accompanying strong winds in those regions render the mixed layer waters so cold (and heavy) that they sink (downwell) into the deep ocean. There is no mixed layer in these regions, and the ocean is just one continuous body characterized by cold, dense surface water sinking into the depths.
This water that sinks into the ocean depths must come back up somewhere. The
rising of water from the ocean depths is called upwelling. We can
summarize the above information by modifying the previous diagram to consist
of a high latitude part and a "rest-of-the-world" part. Since the downwelling
occurs at high latitudes, this water must be carried equatorward by currents
in the deep parts of the oceans, until eventually the water is upwelled again
at lower latitudes. In Chapter 3, we explained why the winds must blow as a
consequence of the need to carry warm air north and cold air south to maintain
the earth's radiative equilibrium. We can view the schematic above in the same
context. The oceans transport cold water south (deep in the ocean) and carry
relatively warm, light water north. So, we could have labelled the left edge of
the schematic as the equator and the right edge as the pole[*].
Figure 4.5. Schematic of the global ocean circulation split into a high latitude region in which downwelling is taking place and the rest of the oceans in which upwelling occurs.
As with the atmosphere, the actual ocean circulation is not as simple as we have just described. Figure 4.6 shows a more accurate portrayal of the thermohaline circulation. The region of the world where much of the downwelling takes place is located in the North Atlantic Ocean. This is because the cold surface water found there is made even heavier by the fact that it is just about the saltiest water that can be found anywhere in the ocean (yet, still much less than the Salt Lake). The key words "cold" and "salty" are what give rise to the word "thermohaline."
The water sinks in the North Atlantic and flows southward into the Antarctic Ocean, where it turns eastward. At that point it is joined by some cold water, not shown in this diagram, which is sinking in a region of Antarctica known as the Weddell Sea. As we follow the stream of deep water eastward, we see that some of it upwells in the Indian Ocean and the rest (which is the largest fraction of the total) upwells in the Pacific. The water then makes its way in the mixed layer back through the Indonesian region, across the Indian Ocean, around the tip of Africa and back up into the North Atlantic. Not shown in this diagram is a feeder branch of this surface circulation that pipes surface water back to the Weddell Sea.
It is by this means that the deep ocean maintains contact with the mixed layer
and the atmosphere. It is possible to estimate that a water molecule in the
deep ocean transits this circulation in roughly 1100 years. That is, the time
that elapses between when the water sinks in the North Atlantic and the time it
upwells in the Pacific is on the order of 1100 years. In contrast, remember
that a water molecule in the atmosphere lasts on the order of 12 days.
Figure 4.6. A three-dimensional perspective of the global ocean basin. Continents are shown in black.
We now turn to discuss how the ocean and atmosphere interact on short (few year) time scales. Obviously, this type of interaction can have nothing much to do with the deep ocean and must involve interactions between the mixed layer and the atmosphere. In the next chapter, we will discuss longer climate fluctuations in which the deep ocean plays a more prominent role.
The atmosphere and ocean differ in many obvious respects from one another. However, one of the most significant differences between them for climate is the difference in their heat capacities. Heat capacity defines how much energy a medium can store for a given temperature. In fact, let's back up a moment and discuss a little what exactly heat is. Heat (which we should more accurately call heating), is a process by which energy is transferred from one body to another. We have already discussed in depth one such method, radiation. The sun transfers energy to the earth's surface via radiation. However, there are other methods of energy transfer as well. Conduction is another way in which energy is transferred from the ocean to the atmosphere and vice versa at the air-sea interface. Conduction is not a particularly efficient process for transferring heat and air is itself a very poor conductor of energy[*].
We now will introduce another method of transferring energy that operates across the air-sea interface. In Section 3.2, we determined that the evaporation of water from the earth's ocean is the primary source for water in the atmosphere. When water is evaporated, energy is removed from the ocean. It takes 2,500,000 J of energy to evaporate 1 kilogram of water. Since 336x10[15] kg of water are evaporated from the oceans each year, a tremendous amount of energy is extracted from the global oceans. In addition, when the water vapor condenses to form clouds, this energy is released to heat the atmosphere. In this fashion, vast quantities of energy are transferred from the oceans to the atmosphere.
Now let's return to the concept of heat capacity in order to understand where all of this energy to evaporate water in the oceans comes from. The heat capacity of water is 4186 J/(kg [o]C) while that of air is 1004 J/(kg [o]C). What these numbers represent is that it would take 4186 J of energy to raise the temperature of a kilogram of water an additional 1[o]C while it would take 1004 J to raise the temperature of a kilogram of air an additional 1[o]C. Or we can invert this concept and state when a kilogram of air receives 1004 J of energy its temperature will rise by one degree and when a kilogram of water receives 4186 J of energy its temperature will change by 1[o]C.
Another factor of significance here is that water is much more dense that air (1000 kg m[-3] versus 1 kg m[-3]). Thus, a square meter column of the mixed layer (100 m deep ) contains 100,000 kg of water while a square meter of the troposphere (10000 m deep ) contains 10,000 kg of air. Since density in the atmosphere decreases with altitude from the value of 1 kg m[-3] near the earth's surface, the figure of 10,000 kg is an overestimate.
The significance of the greater heat capacity combined with the greater density is that the mixed layer of the oceans serves as a giant reservoir of energy for the earth-atmosphere system. Much of the solar energy incident on the surface of the earth is absorbed in the mixed layer of the ocean. Because of the large heat capacity of water, this energy can be stored and released at a later time, maybe months after the energy was received. In a way, we can view a body with a large heat capacity as one that has a long memory. If a patch of the Pacific Ocean receives more solar energy during the summer (perhaps because of fewer low clouds covering the region), then the ocean will absorb and store more energy. That patch of the ocean may "remember" the excess energy the following winter. In this fashion, the ocean is able to force the atmosphere on time scales of a few months to a few years by releasing energy to the atmosphere months or years after it was received.
Contrast the behavior of the oceans to the desert landscape of Utah. The rocky soils of Utah have a low heat capacity (sand has a heat capacity of 1350 J kg[-1] [o]C[-1]). Moreover, sunlight cannot penetrate into the soil like it can into the relatively transparent water of the ocean. Thus, the ground temperature increases sharply during the day and cools off rapidly at night. The soils of Utah are not capable of forcing the atmosphere on time scales beyond a day. However, snow cover over Utah is one way for the earth-atmosphere system to remember conditions several months before. Snow cover affects the albedo of the earth and so it is possible for it to affect the climate long after the snow fell.
The strongest form of air-sea interaction is found in the equatorial Pacific Ocean. Let's begin by showing in Fig 4.7 the normal distribution of sea surface temperature across the tropical Pacific Ocean during the month of October. This field represents the climate state of the ocean surface during that month. The warmest region of the ocean is found in the western Pacific (mostly to the west of the dateline, 180[o] longitude); over that vast area, the sea surface temperature exceeds 28[o]C. As we look further east along the equator, the ocean surface becomes colder and colder, until we find the temperature along the coast of South America to be less than 23[o]C. Temperatures are even colder further south along the coast of South America.
This climate state of the ocean is determined primarily by the stress placed on the ocean surface by the winds in the atmosphere immediately above the surface. The climate state of the surface wind field is characterized by easterly winds (blowing from east to west) along the equator that help to drive ocean currents in the same direction. The cold waters found along the South American coast are then carried (advected) westward out along the equator. The easterly winds weaken considerably as they approach the dateline, so much so that the waters of the western Pacific remain warm year-round.
Every few years, the sea surface temperature in the equatorial Pacific undergoes significant flucutations. In some years, the ocean surface warms up; the largest warming during the past 100 years occurred during 1982. The top panel of Fig. 4.8 shows the distribution of sea surface temperature during November of that year. Notice that temperatures higher than 28[o]C extend much further east (to around 130[o]W along the equator) compared to the climate state. While it may be difficult to assess by comparing the top panel of Fig 4.8 and Fig. 4.7, the largest changes in sea surface temperature relative to the climate state occurred near the coast of South America, where the temperatures increased by as much as 3[o]C. One year later (the bottom panel of Fig. 4.8), the distribution of sea surface temperature is quite different. It looks like an enhanced version of the climate state, with a pronounced tongue of cold water extending eastward from South America towards the dateline.
Figure 4.7. The climate state of sea surface temperature across the tropical Pacific Ocean during October. Areas in which the temperature exceeds 28[o]C are shaded. The temperature field is contoured at an interval of 1[o]C.
The basin-wide changes in sea surface temperature exhibited in Fig. 4.8 went largely undetected until the past 40 years. However, sailors and residents of Peru and Ecuador during the late-1800s were aware of significant variations every few years in the currents and weather along the west coast of South America. Starting around Christmas, currents carrying warm water southward along the coast were observed. Torrential rainfall along the normally arid (desert) coastal plain often began as well at around the same time of year. Coastal areas which were unable to support even the hardiest vegetation during most years became covered by extensive fields of grass and other plants. The occurence of these events shortly after Christmas led them to be referred to collectively as El Niño in reference to the Christ child[*].
During the early part of the 20th Century, a negative aspect of El Niño
became apparent: widespread disappearance of some fish species and severe
mortality of bird populations. The fisheries in this region are remarkably
prolific as a result of strong upwelling and a relatively shallow mixed layer.
Nutrients are carried upwards from the deep ocean into the mixed layer where,
in the presence of sunlight, tiny plant life flourishes. These in turn provide
the food for a vigorous food chain. This process will be discussed further in
Chapter 6. During El Niño, fish species dependent upon this food chain
suffer, while a few species that can tolerate warmer, nutrient-deficient water
survive.
Figure 4.8. The distribution of sea surface temperature across the tropical Pacific Ocean during El Niño conditions (top panel) and La Niña conditions (bottom panel). Areas in which temperature exceed 28[o]C are shaded. The temperature field is contoured at an interval of 1[o]C.
The plight of the residents along the coast of Peru is interesting, but, it has little relevance to the issue of global climate change. Scientists during the late 1950's and early 1960's began to realize that the coastal El Niño phenomenon was associated with changes in the ocean across the entire Pacific Ocean as shown in the top panel of Fig. 4.8. So, the appearance of abnormally warm water across the Pacific began to be called El Niño, as well. Scientists also have a tendency to corrupt the meaning of terms for convenience. Years in which the temperatures are abnormally cold along the coast of Peru and the equatorial Pacific are now called La Niña years. The conditions found in November 1983 (lower panel of Fig. 4.8) can be considered to be representative of La Niña.
The temperature changes associated with El Niño and La Niña cover such a broad extent of the equatorial Pacific Ocean that they affect directly the surface temperature of the entire tropical strip as shown in Fig. 4.9. There are periods (El Niño years) when the tropical temperature is much higher than usual and periods (La Niña years) when it is much lower than usual. The magnitudes of the changes in temperature here from year-to-year are small, on the order of .5-1[o]C. However, these temperature swings have major effects on the climate of the earth-atmosphere system.
Figure 4.9. Month-to-month variations in surface temperature in the tropical strip (20[o]N-20[o]S) from 1961-1989. El Niño episodes are indicated by arrows.
Of even greater importance than the direct warming or cooling of the sea surface is the indirect effect of these changes on the atmosphere above and the ways in which the ocean and atmosphere interact. El Niño is initiated when the easterly surface winds across the Pacific weaken and La Niña begins when the winds strengthen. This represents a coupling between the ocean and atmosphere that has profound effects on the earth-atmosphere climate system. The coupled interactions between the atmosphere and ocean in the tropical Pacific are now referred to as the El Niño/Southern Oscillation phenomenon, which we will abbreviate as the ENSO phenomenon. The term Southern Oscillation refers to variations in the pressure field of the atmosphere that are associated with the weakening and subsequent strengthening of the winds during the El Niño/La Niña cycle. ENSO can be thought of as describing the extremes in the climate of the equatorial strip of the earth embracing both the years of abnormally cold and warm ocean waters in the equatorial Pacific.
The earth-atmosphere climate system is not like a flask in a chemistry laboratory where a complex chemical reaction can be studied under controlled conditions. Rather, our climate system is constantly undergoing changes on all time scales. One difficulty in understanding the potential climate effects of increasing carbon dioxide is that the present situation has not happened before; we therefore don't know what to expect. Scientists in such situations often look for other behavior that has occurred many times before and which can be used as a reasonable proxy to that they really want to study. The ENSO phenomenon can, in some respects, be viewed as a proxy for longer term climate variations. ENSO episodes occur every few years and the last few events have been studied in great detail.
We mentioned briefly in the last sub-section that the equatorial Pacific Ocean responds to changes in the strength of the easterly winds along the equator. In other words, we have viewed the surface of the ocean (and more accurately, the entire mixed layer) as being forced by the atmosphere. However, the greater heat capacity of the oceans' mixed layer compared to the atmosphere allows the mixed layer to force the atmosphere for long periods.
The primary means for the ocean to force the atmosphere is through changes in sea surface temperature, especially in the equatorial Pacific Ocean. What is the significance to the atmosphere of a 1[o] or 2[o]C increase in sea-surface temperature in the equatorial Pacific as shown in Fig. 4.8? First, the amount of water that evaporates from the ocean depends nonlinearly on the sea surface temperature. As the temperature increases slightly, the amount of water that can be evaporated increases sharply as shown schematically in Fig. 4.10.
Figure 4.10. As the sea surface temperature increases from 26[o]C to 28[o]C, the number of molecules of water that can evaporate from the ocean surface increases markedly.
Figure 4.11. The equatorial atmosphere can respond sharply to regional changes in sea surface temperature as discussed in the text.
The increase in the rate of evaporation over the Pacific Ocean during El Niño years and the decrease in evaporation during La Niña years is not enough to explain how the ocean forces the atmosphere in this region. The schematic in Fig. 4.11 shows how the small increase in sea surface temperature leads to a large effect on the atmosphere. As a patch of the equatorial Pacific becomes warm relative to its surroundings, the amount of water that evaporates into the atmosphere increases and the air above warms as well. As described in Section 3.1, warm areas in the atmosphere tend to be associated with lower pressures. This leads to a circulation in the atmosphere where the air near the surface tends to blow towards the warm air, in order to attempt to fill that trough of low pressure.
As the air converges towards the region of warm air, it is forced aloft and
cools as it is lifted. Since the air is full of water vapor, some of it will
condense to form clouds and eventually, if there is enough lifting, rain will
occur. Remember that evaporation of water from the ocean extracts heat from it
while condensation of water vapor releases heat in the atmosphere. In this
manner, the formation of the clouds and rain leads to additional heating of the
atmosphere and the process continues until the ocean cools relative to its
surroundings or the atmosphere stabilizes the circulation in this region so
that additional clouds are unlikely to form. Finally, note in the diagram that
the air returns aloft and subsides over the colder waters. Thus, during La
Niña, the atmosphere over the central equatorial Pacific tends to be
less cloudy with less rainfall, since the oceans temperatures are colder than
normal.
Figure 4.12. Globally averaged surface temperature anomalies with major ENSO episodes indicated by arrows.
We will end this sub-section with a brief examination of the effects of ENSO episodes on the global surface temperature field. Earlier we have seen that the ENSO phenomenon controls the surface temperature in the tropical strip, but does it affect the global temperature as strongly and over the full length of the record? Figure 4.12 repeats the global temperature record with major El Niño episodes indicated by arrows. As with the major volcanic erruptions, the occurrence of El Niño episodes cannot explain all of the variations in the global climate record. However, it is possible to see that several recent periods of warming in the equatorial Pacific are related to warmer than normal conditions around the globe. In addition, El Niño years prior to 1930 tend to occur at times when the globally averaged temperature is higher relative to contemporary years.
1. How did the ancient atmosphere and the oceans form from volcanic emissions?
2. What are the processes that have made the chemical composition of the atmosphere so different from the composition of volcanic emissions from which it originated?
3. Why are sunsets redder than usual in Salt Lake City several months after major volcanic eruptions?
4. Why is more solar energy absorbed by the oceans than by the soils found in Utah?
5. What are the differences between the mixed layer of the ocean and the deep ocean that lies beneath it? Why is there no mixed layer in the high latitude oceans?
6. In what part of the world does surface water sink into the deep ocean and in what part o the world does deep ocean upwell to the surface? What causes surface water to sink?
7. What are the significant differences between El Niño and La Niña episodes?
8. Of what value is research on the ENSO phenomenon with respect to the global warming issue?
[*] There is also a "wind-driven" flow in the mixed layer of the major ocean basins such as the North Atlantic. Warm water moves northward there in the Gulf Stream. The circulation we are focussing on here and illustrated in Fig. 4.5 is referred to as the thermohaline circulation.
[*] That's why storm windows are constructed with two panes of glass having air trapped between them.
[*]Flooding at these times to the north in coastal Ecuador and Peru caused debris (alligators, snakes, huge tree trunks, bananas, coconuts, etc.) to be washed out to sea where it was carried southward and deposited eventually along the normally barren coast of Peru. It is in the context of this unusual bounty from the sea (and the proliferation of vegetation onshore) that led the local residents to call these periods "Years of Abundance" and attribute their origin to the Christ Child.