The Blue Planet

"Beautiful, sublime, and glorious;
Mild, majestic, foaming, free, -
Over time itself victorious,
Image of Eternity!"

-- Bernard Barton "The Sea"

We wish to learn:

• How and why do ocean waters circulate?

• What role does the ocean play in climate change?

• What is the role of the ocean for nutrients and for carbon dioxide?

1. Composition and Salinity of the Oceans

Planet Earth has been called the "Blue Planet" due to the abundant water on its surface. Here on Earth, we take liquid water for granted; after all, our bodies are mostly made of water. However, liquid water is a rare commodity in our solar system. There is no liquid water on the Sun nor anywhere else in the solar system, save Earth. Nor has a drop of water been observed in interstellar space. Only a planet of the right mass, chemical composition, and location can support liquid water. Only on such planets could life flourish.

Liquid water covers most of the surface of our planet. This water comes in many forms, each with it's own special properties. Rain is essentially pure water (consisting only of H₂O), while spring water contains dissolved salts (~0.02-0.4%). Ocean water is the saltiest, with several ppm dissolved salts. The word "brine" is used to describe water that is saturated with salts. These different types of water are found in different environments on Earth.

The largest and most dramatic bodies of water are our oceans. The oceans cover ~71% of the globe and have an average depth of 3,729 meters. They have an average salinity (amount of dissolved salt) of 3.47% or 34.72 parts per mill (ppm). Seawater also contains lots of solutes in addition to salt. The mean residence times for ions that are involved in biological cycles (e.g. bicarbonate HCO₃-) are shorter than those for ions that are created by primary production and lost by undersea vulcanism and wet subduction (e.g., Na+, Cl-).

The oceans are extremely important in regulating climate. Over half of the solar radiation absorbed by our planet is taken up by the oceans. This energy, once absorbed, is redistributed by ocean currents. The mass movement of water performed by the oceans plays a key role in the control of climate.

Temperature and Salinity

When water evaporates, it leaves behind dissolved salts, making the remaining water more dense and likely to sink to the ocean floor. In this way important nutrients (such as dissolved oxygen) are cycled to the ocean floor in a process known as "bottom-water formation". Also, the sinking action of saline water can drive large-scale ocean circulation (see below). Subsurface ocean circulation produced by a combination of temperature and salinity is called "thermohaline circulation". This type of circulation is distinguished from the surface circulation, which is wind-driven.

The salinities of the Atlantic and Pacific oceans are shown in Figure 1 for north-south cross-sections of the Atlantic and Pacific oceans. Note the effect of enhanced salinity due to outflow from the Mediterranean.

The North-South thermal structures of the Atlantic, Pacific, and Indian oceans are shown in Figure 2. Several features are of interest in this figure. Temperature tends to maximize at the surface, with the deep waters being largely isothermal (having a constant temperature with depth). As a consequence, the temperatures are high in the top 1000 meters and drop thereafter. The warmest temperatures are near the equator, as expected, with the polar water being relatively cold and isothermal. Because of the nature of the thermal structure, it is often useful to think of two separate ocean reservoirs

2. Ocean Circulation

Unlike the atmosphere, which is characterized by turbulent weather systems, the ocean is a fairly stable place. This is because it is heated from above, in contrast to the atmosphere, which is heated from below.

Warm water is less dense (heavy) than cold water. The relatively warm surface waters, therefore, tend to remain floating on top of the colder and denser deeper waters. This tendency leads to a generally stable situation which inhibits turbulence. If the deep ocean were never to move, however, we would find no oxygen and no aerobic life at deep levels. Since life is plentiful there, it is evident that some large-scale motion occurs in the oceans, allowing deep waters to become oxygenated.

The ocean has four types of motion:

• surface currents

• deep circulation

• tides

• tsunamis

Different sources provide energy for these different types of motion. Surface and deep currents are powered by solar radiation. The energy source for the tides is gravitational attraction of the Earth and Moon. The Earth's internal heat provides energy for tsunamis. We will focus on the first two types of circulation: surface currents and deep currents.

Large-Scale Surface Currents

Surface currents are forced by frictional interaction between the ocean surface and the prevailing atmospheric wind. The wind imparts its momentum to the top layer of the ocean.

In each ocean, the prevailing trade winds drive equatorial ocean currents from east to west. When these currents encounter land, they divide to flow north and south along the eastern borders of the continents. As they progress, they are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere by the Coriolis Effect (see climate patterns lecture), creating large vortices or gyres.

Figure 3 illustrates the large-scale surface currents. Notice the large clockwise gyres in the Northern Hemisphere and the counter-clockwise gyres in the Southern Hemisphere. Small west-to-east counter-currents are seen near the equator. The famous Gulf Stream (currents number 1 and 2) carries more than 20 times the total amount of water in all the rivers and streams in the world!

Smaller-Scale Surface Currents:

The Coriolis Effect forces surface water at an angle of ~45^o to the direction of the wind. As momentum passes downwards through ocean layers, the currents rotate further under the influence of Coriolis and moderate. The net result is what is known as an "Ekman Spiral" motion (shown in Figure 4). On average, the surface waters, taken as a whole, tend to move perpindicular to the prevailing wind (to the right in the Northern Hemisphere and to the left in the Southern Hemisphere).

• Figure 4 shows the Ekman Spiral for the Northern Hemisphere. For a wind blowing southward along the coast of California, the ocean current will be seaward.
  

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• The region over which the current actually reverses direction is known as the friction layer (~100 m depth).
  

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• The Ekman Spiral helps explain why the waters off the coast of California are so cold (see Figure 5).
  
  

• The cold waters off the coast of California are clearly unusual. Water temperatures closer to what one might expect for the latitude (about 10^°F higher) are found to the north and the south of the localized cold zone.

The Ekman Spiral has a different direction in the Southern Hemisphere than in the Northern Hemisphere, where the rotation is towards the right.

The cold waters off California are produced by the same process that results in cold surface waters off the western coasts of South America and Africa. The process is related to the offshore surface current driven by the prevailing winds, which blow parallel to the coast.

Figure 6 shows the geometry for the case of California surface waters. The prevailing wind blows from north to south along the coastline and, in conjunction with the Coriolis Effect, forces the surface waters west and out to sea. The waters in this seaward current are replaced from below. The forced upwelling of cold deep water cools the surface waters and brings vital nutrients upwards to support a rich food chain, including large numbers of photosynthetic phytoplankton. This situation in Californian coastal waters is repeated off the coast of Peru, with the difference that the Peruvian cold waters are in the Southern Hemisphere. The Peruvian waters support an incredibly rich food chain, including the world's most important anchovy fishery.

El Niño

Since the upwelling currents are forced by prevailing winds, they are affected by changes in the strength of these winds. A phenomenon known as El Niño occurs periodically in Peruvian coastal waters. This term refers to the occasional situation when the trade winds slacken sufficiently to cut off the Peruvian upwelling, leading to an increase in the temperature of the surface waters. It is called El Niño (the Christ-Child in Spanish), since it often occurs around Christmas time. During these events, most of the photosynthesizing (phytoplanktic) sea creatures are starved of the nutrients normally brought up from the deep waters and die away. Sometimes, the die-off is so rapid and intense that the decomposing bodies of the sea creatures leave a foul smell on the ocean.

Figure 7. El Niño influence in the Pacific Ocean.

Figure 7 shows the region of the Pacific Ocean that is anomalously warmed during El Niño events. The size and extent of surface-water warming affects the Earth's climate on a truly global scale, with certain areas of the world experiencing drought while others experience greater rainfall.

El Niño events occur sporadically every 3-5 years or so and last for a year or two. Some of the unusual weather we experienced in 1997-98 has been ascribed to an El Niño event. A new and interesting theory attempts to correlate El Niño events with equatorial volcanic eruptions (which would affect the strength of the trade winds).

• El Niño events are also sometimes referred to as "ENSO" events (standing for El Niño, Southern Oscillation events). One of the effects of an ENSO event is a rise in sea level off the coast of South America and a drop in sea level off Australia.
  
  

• El Niño events occur sporadically but fairly often (one every 3-5 years or so). They are almost impossible to predict.
  
  

• The opposite (and more normal) situation to El Niño is sometimes called "La Niña" - good fishing time!

Deep Water Circulation - The Thermohaline Circulation

A large-scale circulation of the oceans involving a deep current carries more than 30 times the volume of all the rivers of the world combined. We will call this 3-dimensional current the "Conveyor" (Figure 8).

The conveyor belt is a globe-straddling ocean current that keeps Northern Europe unusually warm for its latitude. The Gulf Stream is one leg of this current system. It functions to transfer heat from the relatively warm tropics to the relatively cold polar regions.

The extra heat received by Northern Europe is brought by the conveyor-belt-like ocean current. The part of the conveyor near the ocean surface moves to the North in the Atlantic and to the south in the Pacific. In the Atlantic, the surface of the conveyor carries warm water to a region near Iceland, where it cools and sinks, returning at great depth. The convergence of warm surface current and cold Arctic air allows for effective transfer of heat to the atmosphere, keeping Europe warm. The amount of heat involved is very large: about 30% of the total solar energy received by the entire Atlantic! As the waters cool due to this heat transfer conveyor, they become more dense and sink, feeding the lower part of the conveyor.

The sinking action of the conveyor at its northernmost extreme in the Atlantic is aided by its extra salinity. There is a slight imbalance between evaporation and runoff in the Atlantic, which causes it to be saltier than the Pacific. (Some of the water evaporated from the Atlantic ends up precipitating into the Pacific, either directly or from runoff via rivers and streams. Water evaporates in its pure form, leaving behind salt and other solutes. The net effect of this imbalance is to make the Atlantic more saline than the Pacific.) This difference in salinity between the Atlantic and the Pacific is a driving force behind the Conveyor. This type of circulation, powered by differences in temperature and salinity, is referred to as Thermohaline Circulation (Heat-Salt Circulation).

The importance of the conveyor to climate is impossible to overstate. Without its moderating influence on the climate of Europe, Paris would have roughly the same climate as the Hudson Bay and human history would have been quite different! Figure 9 illustrates the effect of the transfer of energy from the warm surface current to the air blowing from Canada towards Europe.

To recap: at the northern Atlantic limit of the ocean conveyor belt, surface waters release heat into the atmosphere, greatly moderating Europe's climate. The subsequent plummeting of the cooled water to greater depths simultaneously enriches the bottom waters with oxygen, allowing life in the deep ocean. The cycle is powered, in part, by the extra salinity of the Northern Atlantic waters, due to the higher rate of evaporation compared to the Pacific.

There is evidence that the conveyor has stalled once within the past 10,000 years during the brief cooling period known as the Younger Dryas.

3. An Example of Rapid Climate Change Caused by Air-Sea Interactions: The Younger Dryas

The Younger Dryas was a period of dramatic cooling that occurred during the warming trend to the current interglacial - ~10,000 years ago. The Younger Dryas period lasted ~700 years and led to enormous but short-lived changes in the climate of Northern Europe. These changes were first noted in the pollen records, which indicate a change-over from forests to herbaceous plants (such as Dryas, a plant that thrives on glacial tundras) and back to forest again. In the early 1980's, evidence for the Younger Dryas was obtained from CO₂ ice-core bubble measurements also, confirming that it was an event of global significance (since carbon dioxide is well-mixed throughout the global atmosphere).

The Younger Dryas period has caused much debate, since it challenged the previously-held idea that climate could only change very gradually. It had been thought that the thermal inertia of the ice sheets was so large that significant advances or retreats could only happen over long periods of time. The Younger Dryas demonstrated unambiguously that change can be abrupt. Climate appears sometimes to respond in a manner similar to earthquakes where stress and strain builds up over years, leading to sudden abrupt changes, rather than slow incremental changes.

Figure 10 provides a synopsis of what we suppose happened to the temperature of the North Atlantic Basin during the Younger Dryas. The top panel shows the changes in solar radiation expected from a consideration of the Milankovitch cycles over the past 30,000 years. The bottom panel shows the sluggish (lagging) response of the melting ice caps. The middle panel shows the paleoclimate temperature record.

Consider the melting of the polar ice cap as it retreated (Figure 11). Initially, all the runoff waters would have been channeled into the Gulf of Mexico along the Mississippi and its tributaries. At some point, when the glaciers had retreated to about the latitude of Northern Lake Superior, the runoff waters would have been diverted to flow out along the St. Lawrence into the Northern Atlantic. Since these fresh waters had very low salinity, they would have inhibited the downward plunge of the conveyor belt off the coast of Labrador. This sudden release of vast amounts of fresh water into the Northern Atlantic might have been sufficient to stall the Conveyor belt completely, leaving Europe without its moderating influence. After the initial shock of the fresh water runoff had been absorbed, the Conveyor would have started up again, returning the climate to its ordained "Milankovitch cycle" path. The timing of the retreat of the polar cap ice past Lake Superior has been shown to be coincident with the onset of the Younger Dryas, within errors.

The example of the Younger Dryas is a warning that climate change can be unpredictable and abrupt. Stresses on the system can build up until, almost without warning, a sudden and fundamental change occurs on a short time scale.

Summary

1. The oceans play an important role in controlling the climate of the Earth. Both temperature and salinity contribute to the establishment of the 3-dimensional thermohaline currents that encircle the globe.
   
   

2. The ocean is fundamentally stable since it is heated from above. The temperature structure defines a shallow warm surface region (~5% of the total) and a deeper cold layer (~95% of the total).
   
   

3. Surface water gyres and the Conveyor Belt both effectively transport energy from low to high latitudes. The Conveyor moderates the climate of Northern Europe.
   
   

4. Upwelling of cold, nutrient-rich waters on the west coasts of continents is forced by a combination of prevailing winds and the Coriolis Effect. When upwelling off the coast of Peru occurs, we have the El Niño condition, which can cause global climate changes.

Review

Take the Self-Test for this lecture.

All materials © 1996-2000 by the University of Michigan.