Some Basics of Water and the Hydrosphere

1. Introduction

2. Important Properties of Water

3. Brief History of Water

4. Role of Water in the Climate

5. Intro to the Oceans -- Size and Structure

6. Surface Circulation

7. Thermocline and Mixed Layer

8. Deep Circulation

9. El Nino - Southern Oscillation

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Introduction to the Hydrosphere

As seen from outer space, Earth is dominated by water; most of the planet is covered by oceans and clouds (droplets of water or ice crystals).

Water (along with life) is what sets our planet apart from our neighbors and it is the topic of this chapter. The hydrosphere is the watery part of the Earth, but this is a little misleading since it gives the impression that there is one place where water occurs. Instead, water is everywhere, or nearly so. Water is present in the atmosphere, in the form of vapor, liquid, and solid. It is tied up in rocks, locked away in the crystal lattices of minerals that make up rocks; it occurs in the innumerable small pore spaces of rocks, from the surface to depths greater than 5 km. Water is also tied up in living organisms; humans, for example, contain about 70% water. Water is stored in ice sheets large and small, it occurs in various surface water bodies, and, of course the oceans, the reservoir of the great majority of Earth's water. The hydrosphere, then, is intimately tied up with all of the other spheres that make up our whole Earth system.

Inventory of Water

Here is a partial breakdown of where water occurs and how much is there.

Total amount of water:  1,385,990.5 x 1015 kg

Reservoirs	Mass of Water in 1015 kg	Approximate %
Oceans	1,350,000	97.4
Marine atmosphere	11	0.0008
Land atmosphere	4.5	0.0003
Surface Water	275	0.02
Ground Water	8,200	0.59
Snow & Ice	27,500	1.98

Data from Chahine, 1992, The hydrological cycle and its influence on climate, Nature, v. 359, p. 373-380;

Gleick, P.H., 1993, Water in Crisis, Oxford Univ Press, N.Y

These data, along with some data on the rates of transfer between different reservoirs, provide the necessary information to create a model, which will be explored in a later chapter.

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Important Properties of Water

Water is a simple enough compound, just two hydrogens and an oxygen, but it has some interesting properties that are important. Relative to other compounds of similar molecular weight, the boiling and freezing temperatures of water are unusually high, which enables water to exist in all three phases - solid, liquid, and vapor - at the surface of the Earth. As shown in Figure 1.13, water's latent heat of fusion and latent heat of vaporization are also unusually high, which means that water is capable of very efficient heating and cooling as it cycles from the surface to the atmosphere and back.

To understand why water has some of these special properties, we have to look at its molecular structure, shown in Figure 1.14. The two hydrogen atoms are bonded to the oxygen by shared electrons, but the sharing is not equal. Oxygen draws these shared electrons closer to its nucleus, and since the hydrogens are not oppositely arrayed around the oxygen, the molecule has a weak negative charge on the oxygen side, and a weak positive charge on the hydrogen side, forming what is known as a polar molecule. This polarity leads to a number of important characteristics.

Two water molecules will form weak bonds in which the positive side of one is attracted the negative side of another (this is known as hydrogen bonding).

This hydrogen bonding tends to limit the movement of water molecules, which means that when you add heat to water, its kinetic activity does not respond too readily - you have to put a lot of energy in to get a rise in temperature. This is why water has a high heat capacity or specific heat. Having a high heat capacity, water is capable of storing heat energy more so than any other material on the surface (see table below).

HEAT CAPACITY OF EARTH MATERIALS
Substance		Heat Capacity (Jkg-1K-1)
Water	                             4184
Ice	                                2008
Average Rock	                2000
Wet Sand (20% water)	   1500
Snow	                              878
Dry Sand	                     840
Vegetated Land	                 830
Air	                                 700

The polarity and resulting hydrogen bonding between water molecules is also the reason why water has high freezing and boiling temperatures relative to other molecules of similar weight. The hydrogen bonding also means that water is reluctant to change from one state to another. When ice changes to liquid water, this breakdown can occur once a certain fraction of the hydrogen bonds are broken, but to vaporize water, all of the hydrogen bonds in the vicinity must be overcome, which is why the latent heat of vaporization is so much greater than the latent heat of fusion or melting.

The polar nature of the water molecule is also important because it enables water to hold both positive and negative ions in solution and thus it can transport them. In this role, water is absolutely key to the operation of all of the biogeochemical cycles that keep our Earth system functioning. This transporting ability of water is why it is such an important part of living organisms.

Another peculiar feature of water is that its solid form is less dense than its liquid form (Figure 1.15), which turns out to be a very good thing. Imagine what would happen if ice sank - before too long, it would accumulate on the sea floor and eventually, the whole ocean would be one frozen mass and Earth would be completely different. The same hydrogen bonds that occur in water are what hold the ice crystal together, but in the case of ice, each hydrogen on each water molecule is bonded to a neighboring oxygen from another molecule, while in water, only about 80% of the hydrogens are bonded, thus giving the molecules more freedom to arrange themselves into a denser packing. The rigid structure of ice, where all the molecules are geometrically arranged leaves a lot of open space between the molecules.

Most of the water on Earth, as seen in the table above, is in the oceans, and here, it's behavior is slightly different due to the salinity. The average chemical composition of water is shown in the table below.

Important Constituents of Sea Water

Where did all of these dissolved species come from? Weathering of rocks on the continents and the delivery of the soluble weathering products via rivers has produced a salty ocean. In addition, exchange of seawater with young, hot volcanic rocks on the seafloor contributes to the chemistry of the oceans. As an aside, it is interesting to remember that one of the early attempts to determine the age of the Earth involved a calculation of how long it would take to create an ocean with the observed present salinity given a rate of input of these salts by rivers. This seems like a good idea, this simple calculation gave a surprisingly young age for the Earth - 90 million years. Why does this approach give the wrong answer? It does not recognize that the oceans represent a system that has both input and outputs - the delivery of dissolved salts by rivers is somewhat offset be the deposition of sedimentary rocks, especially those called evaporites, which include halite (NaCl), and also by interaction with hot volcanic rocks on the sea floor. The rate of removal of the various components of seawater relative to the size of the oceans give rise to something called the residence time for each component, shown above. We will look into this concept in greater detail later, but it is really quite simple - it represents that average length of time that a molecule stays in the ocean before moving on to some other place, usually to the bottom of the seafloor, as a sedimentary particle. Note the vast range of residence times, reflecting a range of different removal processes.

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Brief History of Water

Where did all of this water come from? The most probably source for this water is the material that was accreted together to form the Earth, which is assumed to be roughly similar in composition to some of the meteorites that continue to fall down to the surface. Many of these meteorites contain minerals that contain minute quantities of water bound up in their crystal lattices. When these minerals melt, as most of them would in large impacts during the early history of the Earth, the water is released. In this scenario, the Earth would have acquired most of its water very early in its history, which is in agreement with what we know about the very early history, which is unfortunately limited. Some of the very oldest rocks are sedimentary rocks, or they were derived from sedimentary rocks, and those sedimentary rocks bear evidence of having been deposited by flowing water - either waves, or a stream. The sedimentary particles in these rocks are were produced by the erosion of some pre-existing rock and they have been rounded during their transport to the site of deposition. This erosion and transportation almost certainly took place above sea level; this means that there had to be water flowing on the surface as it does today. This implies then, the operation of a water cycle to get the water from the oceans to the continents to do the work necessary to create these sedimentary rocks.

Throughout the geologic record, there is abundant evidence in the form of sedimentary rocks that tell us that the hydrologic cycle has been operating more or less the same for as much of Earth's history as we know about. The ocean basins and continents have themselves grown and shifted about, but the water has remained as one of the most important substances on the Earth.

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Role of Water in the Climate

We have previously seen that water is one of the most important greenhouse gases in our atmosphere, accounting for nearly half of the total greenhouse warming that makes out planet a nice place to live. but water is also a key component of the climate in many other respects.

As mentioned above, water, because of its molecular structure, plays a very important role in the global climate. Because of its high heat capacity (see table above), it is the most important storage reservoir for solar energy. The high heat capacity wouldn't mean much if the water reflected all of the sunlight, but in fact, water is among the most absorbent materials on the surface of the earth, so it does indeed operate as a tremendous heat energy reservoir. The storage of this heat energy is important in helping the earth moderate abrupt changes in the solar. For instance, without any heat storage, the surface of the Earth would grow extremely cold every night when the sun went down.

Water is also the primary medium for transporting energy. When water evaporates from the surface of the oceans, it removes heat from that surface and carries that heat along until the point that it condenses to form a droplet of liquid water; then it releases that latent heat. The rapid circulation of air masses in the atmosphere transfers some of this latent heat from the equatorial region to the poles.

Another important transfer of heat from the equator to the poles comes about through the circulation of the oceans. Warm surface currents flow to high latitudes and bring with them a tremendous amount of heat; cold currents flowing down to the equator represent the other end of this energy exchange loop. The importance of this heat transfer to climate is appreciated by people in the British Isles, which enjoy an unexpectedly mild climate considering how far north they are - 50 to 55°N. Warm currents reaching the surface near Iceland contribute something like 30% of the heat for the entire polar region.

Water in its solid form is another key component of the climate system. In contrast to liquid water, ice is the most reflective material on Earth's surface. So, when large ice sheets grow, as they have numerous times in the last few million years, they cover a large area with highly reflective material, which prevents the absorption of solar energy, thus tending to encourage further cooling of the climate. This is a classic example of what is called a positive feedback mechanism. Positive, not in the sense that it necessary is good, but in the sense that this mechanism, triggered by some change, operates to add to that change as opposed to counteracting it.

The waxing and waning of ice sheets and other processes, contribute to the global climate by altering global sea level. When large ice sheets grow, sea level drops, and this reduces the surface area covered by the highly absorbent water, which in turn affects the total amount of solar energy absorbed by the Earth, and thus the global climate is also affected. Ice ages are capable of changing sea level by perhaps two hundred meters, and this does not change the ratio of land to sea area by a significant amount. Other processes, involving plate tectonics, which we will discuss a bit later, are capable of greater sea level changes, perhaps over 350 m, which is enough to cover a significantly larger surface area (as much as 80 to 85%), so this has the possibility of affecting climate more strongly.

The oceans also represent a major reservoir for CO₂, containing something like 300 times as much as is in the atmosphere. There is a relatively rapid exchange of CO₂ between the atmosphere and the surface waters of the oceans and the oceans appear to be absorbing a significant fraction of the CO₂ that humans have released into the atmosphere through biomass burning and fossil fuel burning. There are also deep currents in the oceans that take store and exchange CO₂ on a longer time scale; if these currents were inactive, all of that CO₂ would have to reside in other parts of the global carbon cycle, and much of it would end up in the atmosphere, producing a much more potent greenhouse.

It should be clear from this brief summary that water is versatile, omnipresent, and very important to the operation of the global climate system. Because the oceans are such a major part of the global climate system, it is important to understand some things about the structure and dynamics of the oceans.

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Introduction to the Oceans

Size and Structure of the Ocean Basins

Standing at the edge of an ocean, or better yet, sailing on an ocean, you can't avoid being impressed with the immensity and seemingly infinite expanse of water. Images of Earth from space confirm this, as do the basic statistics. A little over 70% of the Earth's surface is covered by the oceans and they have an average depth of about 3750 meters, with a maximum depth of about 11,000 meters (deep enough to submerge Mt. Everest) in the Marianas Trench. If the Earth had a perfectly smooth surface, it would be covered with a blanket of water about 2,430 meters deep. Although it is not apparent at first, the oceans also play host to a vast community of organisms and as we shall see later, these organisms also play important roles in the global climate.

The typical form of the ocean basins can be seen in a cross-sectional view of the Atlantic Ocean (Figure 1.16) The Atlantic has a symmetry to it that comes about because of the way it has formed -- by the rifting and separation of North America + South America from Europe + Africa, with each side moving away from the point of separation at more or less equal rates. The oceanic crust thus varies in age from 0 at the center to about 180 Ma -- the age of rifting -- at the edges. As ocean crust ages, it cools and becomes more dense, so it sinks a bit lower. The relationship is roughly:

Depth(m) = 2500(m)+ 338(age).5

where age is expressed in millions of years. The seafloor begins a relatively abrupt rise where the ocean crust is attached to the continental crust. This slope is usually draw at deceptively steep angles, but it is really quite gentle, at about 2° to 8° on average. Departures from these gentle slopes occur in submarine canyons, some of which dwarf the Grand Canyon. These slopes then flatten out to form the continental shelves, with depths of less than 200 m, where the slopes really are quite low -- just a fraction of a degree. The form of the shelf and slope is mainly controlled by processes that transport sediment once it is brought to the coast by rivers. Waves, tides, and storm currents move this sediment away from the shore and spread it out over the shelf, with the excess making it to the shelf edge, where it spills down the slope periodically in the form of turbidity currents -- dense slurries of sediment and water that race down the slope before dying out and depositing their sediment load at the base of the slope. Continental margins represent the former sites of stretching and rifting the crust, and this stretching causes the crust to subside. This subsidence creates space for the deposition of tremendous thicknesses of sediment, often reaching 10 km or more, derived from the weathering, erosion, and transport of nearby mountains.

In contrast, the Pacific Ocean is shaped somewhat differently, mainly due to a different plate tectonic history. The Pacific does have Mid-Ocean Ridges, just like the Atlantic, but these may have formed when oceanic crust was torn in two, though this it is difficult to be sure of this since the evidence has been destroyed. The Pacific Ocean is nearly surrounded by subduction zones, where oceanic crust is descending into the mantle; deep trenches mark the locations of these subduction zones and an arc of active volcanoes usually can be found on the overriding plate (the one not being subducted).

In general, the form of the ocean basins is determined by plate tectonic processes, and each ocean basin is expected to be somewhat different, but nearly all of them have mid-oceans ridges, which represent huge mountain chains that are unusually long and with very few breaks in them. These enormous ridges create constraints on the deeper circulation of the oceans, which we will explore in one of the upcoming sections.

Circulation of the Oceans

The circulation of the oceans is not so apparent to us because it does not occur as rapidly as atmospheric circulation, and it occurs on a vast scale, making it difficult to observe by most conventional means. But the oceans do indeed circulate, and as we did with the atmosphere, we'll look at this circulation in two general categories; surface circulation and deeper circulation.

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Surface Circulation

First, it's worth mentioning something about how surface circulation is figured out. Most of the data come from ships that drift off course. If a ship sets a bearing and a speed, they can predict where they will be if they have a known starting point, but they commonly find that they are not where they expect. The difference is due to the effects of ocean currents; analyzing all of the ship-drift data has led to a map like that shown in Figure 1.17 of the major surface currents in the oceans.

What gives rise to this pattern? First, let's consider what drives this motion, then we'll look into what modifies the motions to give the resulting circulation. Wind is about the only driving mechanism you can imagine for the ocean currents. If you look at Figure 1.8, showing the pattern of winds on the surface, you can see a pretty good correspondence between the two - at least to a first approximation - so it does appear that winds provide the primary driving force for the surface currents. In actuality, the direction of the surface current should not be parallel to the wind except in the very thin uppermost layer of the oceans. This thin sheet transfers motion to the underlying water, but the transfer is affected by the Coriolis Effect. This process of downward transfer of motion, coupled with the Coriolis Effect leads to a downward spiral in vectors that describe the motion of water, and the vectors grow smaller because of energy loss to friction between the individual layers of moving water (see Figure 1.18). The net result of this Ekman transport, as it is called, is that the surface current direction is about 45° to the right of the average wind direction (in the northern hemisphere; it goes to the left in the southern hemisphere). The Ekman transport typically involves a layer of water about 50 to 100 m deep, depending on the wind velocity and constancy and the net movement of water is about 90° to the right of the average wind direction.

In addition to Ekman transport, the surface waters are affected by waves passing through the oceans. As a wave passes, particles of water move in elliptical orbits; the size of the ellipse is equal to the wave amplitude at the surface, but it diminishes to nothing at a depth of half the wavelength. But, since waves in the open oceans commonly have wavelengths of up to several hundred meters, they stir up the waters to a depth of a hundred meters or more.

Water, much more so than the wind, is confined to flow in a space that has major barriers - the continents. So while the tropical trade winds are blowing along the equator from east to west clear around the globe, the westward-flowing equatorial waters in the Atlantic and Pacific run into some difficulties at the western edges of the oceans; they are forced to flow to the north in the northern hemisphere (to the south in the southern hemisphere). The northward current is called the Gulf Stream in the Atlantic and the Kuroshio in the Pacific. These northward currents then are subjected to eastward blowing winds once they get past about 30°N, so parts of them get diverted off to the east. This motion, along with the northward bend from the equator are both aided by the Coriolis Effect, although it is less pronounced in the oceans because the Coriolis Effect is proportional to the velocity of flow. Much of the northward flowing water is eventually deflected first to the east and then back to the south when it runs into continental obstructions or other currents, although the northward extensions of the Gulf Stream make it all the way up to the Arctic Ocean. The southward flowing water is generally much cooler and forms a weaker, more diffuse current than the one that travels north along the western edges of the oceans in the northern hemisphere. This southward flowing water then joins the eastward flowing water along the equator and begins the circuit again. These circular or elliptical patterns are known as gyres and most oceans have one located near 30° N or S of the equator, offset from the center towards the western edge in the northern hemisphere and the eastern side in the southern hemisphere, where the strong currents are located. Gyres in the northern hemisphere thus rotate clockwise, while those in the southern hemisphere rotate counterclockwise, but in both cases, they are transferring warm water toward the poles and warming up the return flow. The major exception to this general pattern is the strong westward flowing current that goes around Antarctica. Here, the atmospheric winds blow consistently and strongly to the east, and the ocean current is not obstructed by a continental barrier, so it makes a complete loop, although some of this water will join the southern hemisphere gyres and vice versa. This water also comes into contact with the Antarctic ice sheet and sinks below the surface, initiating a major deep current in the oceans, which we will get to next.

It is worth pausing to try to get a sense for how big these ocean currents are. The Mississippi River, at its mouth, has a flow rate of about 8500 m3/s; the Amazon is greater still at around 200,000 m3/s. All of the rivers in the world total about 1,400,000 m3/s. In contrast, the Gulf Stream has a flow rate of about 150,000,000 m3/s near its "downstream" end - over one hundred times as great as the combined river flow from all of the continents!

The general pattern of surface circulation has a strong influence on the distribution of sea surface temperatures, which are shown in Figure 1.19, as measured by satellites. This may seem like a remarkable thing - a satellite measurement of the sea surface temperature. It certainly makes for a much more detailed picture than is available from ship-board measurements, which would be confined to the major shipping lanes. The satellite images take advantage of the fact that the spectrum of energy emitted by the surface is a function of the temperature of the surface. For the earth's surface, most of this emitted energy is in the infrared part of the spectrum, and most of this is absorbed by the atmosphere, so a satellite would never see it. There are, however, a few "windows" or parts of the emitted spectrum that the atmosphere does not absorb - satellites can observe these and by looking at the amount of energy coming out through these windows, you can calculate the temperature of the surface. Warm water masses can easily be seen moving northward along the western edge of the oceans in the northern hemisphere and southward along the western edges in the southern hemisphere. Likewise, the eastern edges of the oceans in both hemispheres have cold currents flowing to the equator. It is also interesting to see that the Atlantic is much more effective at heating the polar region than the Pacific, perhaps due to the absence of a continental barrier to northward flow in the Atlantic. Related to this is the outflow from the Arctic, which comes out of the Bering Sea and helps make the North Pacific generally colder than the North Atlantic.

An interesting anomaly in the temperature of the sea surface can be seen along the eastern side of Pacific along the equator, where there is a band of cold water. This is one of the areas in the ocean where an important process called upwelling occurs on a large enough scale to show up on this map. Here, the winds blow to the west, but Ekman transport tends to move water away from the equator. This divergence tends to sweep away the surface layer of water, and deeper, colder waters move up from below to replace the water that has flowed off to the side. This is an important process since the deeper waters that rise to the surface are nutrient rich in comparison to normal surface waters and they bring those nutrients up to the surface where enough sunlight exists for photosynthesis to occur; thus producing a biologically rich zone in the ocean.

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Thermocline and Mixed Layer

Ekman transport, driven by the wind and modified by the Coriolis Effect, combined with the motions induced by waves lead to a thoroughly mixed zone of the oceans, which typically has a depth of around 100 m. This is the region that undergoes significant seasonal temperature changes, as can be seen in Figure 1.20. This is an important observation since it tells us something about the size of the body of water that is actively involved in absorbing and releasing energy over the course of a year. This thickness, integrated over the whole ocean, gives us a sense of the thermal mass of the oceans. By contrast, just the top few centimeters of the land surface is involved in seasonal temperature changes, and this material has a much smaller heat capacity, so it's overall thermal mass is tiny in comparison to the oceans.

Below the mixed layer is a region where the temperature undergoes a rapid change with depth, ending at a depth of around 1000 m, below which the temperature changes very slightly. The thermocline apparently undergoes a circulation that begins near 40° N and 40°S, where the water sinks due to its density, travels down to the equator, and then rises up into the surface mixed layer. Thus there is an exchange between the mixed layer and the thermocline, and it is estimated that the thermocline changes its water every 130 years or so, which is pretty fast. This is pretty significant, because it helps define the size of the reservoir that is exchanging thermal energy on slightly longer timescales - it enlarges this reservoir, adding to the stability of our climate on somewhat longer timescales (though these are brief in comparison to the billions of years of Earth history).

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Deep Circulation

The waters of the oceans are also in motion at great depths below the surface; Figure 1.21 below shows the general, simplified pattern of these currents.

These deep currents are much slower than the surface currents and they take a longer time to mix the deep oceans, but they nevertheless do and provide for the ventilation of the world's oceans. Most of the deep circulation is driven by dense waters formed around Antarctica and in the North Atlantic, near Iceland; the waters generated at these sources then travel throughout the world's oceans. In the Weddell Sea area of Antarctica, dense water forms when sea water comes near the Antarctic ice sheet; it sinks to the bottom and spills down the continental slope of Antarctica, flowing north along the edge of South America. This flow, the Antarctic Bottom Water is the densest of all water masses so it hugs the bottom of the seafloor and is therefore constrained by the major topographic features such as ridges and trenches. The ABW continues up to 40°N and then turns back south at a higher level, mixing with the North Atlantic Deep Waters. The NADW forms near Iceland, where strong surface currents strip away the surface waters, allowing deeper water to rise. This deeper water comes from the thermocline, so it is not too cold, and it also comes from the gyre center, so it has a relatively high salinity. This water mass gives up a great deal of heat to the atmosphere where it rise up; the cooling of this water mass then makes it very dense, so it sinks down to the bottom of the ocean and travels south until it meets the ABW. At the meeting of these two water masses, the NADW is the less dense by a little bit, so it rises and travels southward above the ABW, making an impressive journey around the tip of Africa, and then splitting, with one branch surfacing in the Indian Ocean, and the other branch flowing into the Pacific, where it circulates and then rises to the surface and returns through Indonesia into the Indian Ocean.

Despite the impressive journeys of these deep currents, they are smaller in flow rates than their surficial counterparts; the ABW flow at a rate of about 38,000,000 m³/s and the NADW flow at a rate of about 10,000,000 m³ /s, considerably smaller than the Gulf Stream (see above) but still impressive in comparison to the flow rate of all rivers combined. Collectively, the deep waters are capable of replacing all of the deep water in about 500 years. Figure 1.22 puts some of these numbers into a graphical context so that you can see just how massive some of the oceanic flows are in comparison to rivers.

It is important to realize that the deep circulation is created by a set of special conditions in two relatively small areas. If the Antarctic ice sheet did not come into contact with the oceans, as it does today, the ABW would probably not form, and it winds and surface currents in the North Atlantic changed, the NADW would not form. These changes would have serious implications for the global climate. The formation of NADW produces about 30% of the heat budget for the region above 65°N, where the major ice sheets grew during the recent series of glaciations (covered in more detail in a later chapter. These deep currents also lock create another storage mechanism for CO₂, preventing it from building up in the atmosphere. Water at the surface exchanges CO₂ with the atmosphere and the amount of CO₂ that the water can hold increases as the water gets colder. When this water sinks, it carries the CO₂ with it and removes it from the surface where it could be released back into the atmosphere.

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The El Niño - Southern Oscillation

The El Niño - Southern Oscillation phenomenon is an important example of the interaction between the atmosphere and the ocean; it turns out to have some unexpected, global consequences. In very simple terms, this phenomenon involves the sloshing back and forth of warm water along the equator in the Pacific ocean. The general conditions are shown in Figure 1.23 (from NOAA's El Niño web page). This phenomenon gets it name (referring to the young Jesus) because the warm water mass tends to show up off the coast of South America near December 25.

Under normal conditions, strong trade winds blowing to the west along the equatorial Pacific piles up the warm water in the western part of the Pacific. This movement of the warm, surface waters creates the conditions for strong upwelling in the eastern Pacific as deep, cold, nutrient-laden waters move up to replace the water that is pushed off to the west. The strong upwelling makes the eastern Pacific surface waters unusually rich in terms of biological productivity. The concentration of warm water over in the western equatorial Pacific means that there is more evaporation there (and more rainfall); in contrast, the eastern Pacific is quite dry.

This "normal" state is a dynamic condition, maintained by the trade winds and ocean currents, and so it is no surprise that changes can occur. Periodically, the trade winds weaken and the warm water mass from the western equatorial Pacific slides back to the eastern equatorial Pacific, snuffing out the upwelling that occurs there. The movement of this warm water mass also changes the locations of evaporation and precipitation along the equator; the western part, normally very wet, experiences droughts during an El Niño and the eastern region, normally so dry, experiences heavy rains. These changes in precipitation and biological productivity are the most obvious and direct results of the El Niño - Southern Oscillation phenomenon, but recently, a host of indirect, less-obvious effects have also come to light.

The meteorological effects of El Niño appear to be very widespread, and in fact, there is reason to believe that the Earth as a whole warms slightly during an El Niño. During El Niño years, the western coast of North America also experiences high rainfalls, there is a greater frequency of hurricanes in the Atlantic, and weather anomalies in Europe also correlate well with El Niño events. The precise reasons for these far-flung effects are not well known, but in a general sense, because El Niño involves changing the location where a great deal of water (and thus energy) enters the atmosphere, we should not be surprised if the consequences extend throughout a good portion of the atmospheric system. It is in a sense analogous to what happens when you create some diversion or disturbance at one point in a stream, triggering changes far downstream. Or alternatively, we might think about a system of linked reservoirs, initially in a steady state; if we make a change in one reservoir, that change propagates throughout the system and if it is a big enough change, it is not easily damped out as it moves through the system.

The El Niño - Southern Oscillation phenomenon seems to oscillate with an irregular period, but averaged over a very long period of time, the period seems to be in the range of 4-7 years. The recent history of oscillations shows up in Figure 1.24 (from NOAA's El Niño web page), a plot of the NINO3 index, which is the temperature of the sea surface averaged within a block of the eastern equatorial Pacific.

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