Which Current Moves Cold Water To Warm Latitudes

Figure 1.6. The intertropical convergence zone, a belt of rising air heated by the equatorial

Air descends further away

Air rises at zone of maximum heating from sun being directly overhead

Air descends further away the earth is rotating, and in every 24 hour rotation the equator has a lot farther to travel round than the poles. So, the closer you are to the equator, the faster you are traveling as the earth turns. When wind comes from a slightly higher latitude, it comes from a part of the earth that is rotating more slowly. As it nears the equator, it gets "left behind"—and the closer to the equator it gets, the more it lags behind. So, because it is getting left behind the wind follows a curving path sideways. This lagging effect of differences in the earth's rotation speed with latitude is known as the "Coriolis effect'', and any wind or ocean current that moves between different latitudes will be affected by it. It also explains, for example, why hurricanes rotate.

Although it has been moving towards the equator, much of this wind does not get there because the Coriolis effect turns it sideways. It ends up blowing westwards as two parallel belts of winds, one belt either side of the equator (Figure 1.7a). These are the trade winds, so-called because in the days of sail, merchant vessels could rely on these winds to carry them straight across an ocean.

There is another related effect—the "Ekman spiral''—when a wind bent by the Coriolis effect blows over the rough surface of the earth, the friction of the earth's surface—which remember is rotating underneath it at a different speed—will drag the wind along with the rotating earth, canceling out the Coriolis effect (Figure 1.7b). This causes the wind direction to change near the earth's surface, and is part of the reason why winds by the ground can be blowing in one direction, while the clouds up above are being blown in a different direction. Between the air nearest the ground and the air way above, the wind will be blowing at an intermediate angle; it is "bent" around slightly. The closer it gets to the surface the more bent off course it gets.

The Ekman Spiral Describes

Higher altitude wind direction is (b) dominated by the Coriolis effect

But dragging of wind near surface changes its direction to follow the rotation of the part of the Earth it is blowing over

Figure 1.7. (a) The Coriolis effect. (b) The Ekman spiral.

There are many other aspects to the circulation pattern of the world's atmosphere, too many to properly describe here in a book that is mainly about vegetation. For instance, there is another convection cell of rising and sinking air just to the north of the outer tropical belt, and driven like a cog wheel by pushing against the cooling air that sinks back down there. A third convection cell sits over each of the poles.

Outside the tropics, air tends to move mostly in the form of huge "blobs" hundreds of miles across. These are known as "air masses''. An air mass is formed when air stays still for days or weeks over a particular region, cooling off or heating up, and only later starts to drift away from where it formed. You might regard an air mass as resembling a big drop of syrup poured into a pan of water. It tends to spread out sideways, and also mix sideways with what is around it. The collision zone between an air mass and the air that it is moving into is known as a "front". When a front passes over, you get a change in the weather, and often rain.

In a sense, the detailed patterns of moving individual air masses are controlled by thin belts of higher altitude winds (at between 9 and 12 km altitude) in the atmosphere at the edge of the polar regions, and also at lower latitudes where the air from the ITCZ starts descending.

These eastward-trending winds are the jet streams. They "push around'' the lower-level air masses like chess pieces. There is the subtropical jet stream and the polar jetstream in each hemisphere. That makes four jet streams in all. The jet streams are fed by air rising up into them moving in a polewards direction, and they are propelled east by the Coriolis force because the air comes from the faster-rotating lower latitudes.

1.3 THE OCEAN CIRCULATION

Just as winds move through the atmosphere, there are currents in the oceans. These too transport an immense amount of heat from the equator towards the higher latitudes. For the most part, ocean currents only exist because winds blow them along, pushing the water by friction. But part of the reason winds blow is that there are temperature differences at the surface, and ocean currents sometimes bring about such contrasts in temperature (especially if there is upwelling of cool water from below). So the water moves because the wind blows across it, yet the wind may blow because of the very same temperature contrasts that are brought about by the water moving!

Wind skimming across the surface will drive the top layer of water as a current in a particular direction, and if it moves towards or away from the equator the current will eventually get bent round by the Coriolis effect. So, for example, in each of the world's main ocean basins there are eastward-curving currents that travel out from the equator because of this mechanism (see below). But below the surface of a current being bent by the Coriolis effect, the deeper part of the current is being dragged by contact with the still waters below it. That dragging tends to move it along in the direction that the earth is rotating locally. So because of this dragging, this deeper water in the ocean ends up traveling in a slightly different direction. The deeper you go, the more the angle of the current is diverted by dragging against water below, and different layers in the ocean can be traveling in quite different directions. This is the same Ekman spiral effect as occurs in the atmosphere.

Winds blow fast but per volume of air they don't carry very much heat. The heat-carrying capacity of ocean water is much greater, but the ocean currents move much more slowly than the winds. In fact, both ocean currents and winds are important in transporting heat around the earth's surface.

1.3.1 Ocean gyres and the "Roaring Forties" (or Furious Fifties)

The most prominent feature of the world's ocean circulation are currents that run in big loops, known as gyres. They start off in the tropics moving west, and curve round eastwards in the higher latitude parts of each ocean basin, eventually coming back down to the tropics and completing a circle.

These gyres originate from the powerful trade winds that blow towards the west in the outer tropics. The winds push against the surface of the ocean producing these currents. But why does an ocean gyre eventually turn around and flow eastwards? It happens because the ocean currents are slammed against the shores on the west sides of ocean basins by the trade winds that blow west along the equator. Both the winds and the currents bounce off the western side of the basin, and start to head away from the equator. Because they are traveling with the same rotation speed as the equatorial zone, the Coriolis effect bends them off towards the east, diagonally across the ocean towards higher latitudes.

The winds that follow the outer parts of these ocean gyres, and help drive them, are powered by the big contrast in temperature created as the ocean currents move polewards and cool off. In the southern hemisphere these winds are known as the Roaring Forties, blowing west-to-east just south of South Africa and Tasmania, and hitting the southern tip of South America with a glancing blow. The nickname that generations of sailors have given these winds comes from their unrelenting power and their tendency to carry storms, and the fact that they stay within the 40s latitudes. In the northern hemisphere, the equivalent belt of winds is located more in the fifties and low sixties, hitting Iceland, the British Isles and the southwest Norwegian coast. These winds, even stormier, are known as the "Furious Fifties''.

1.3.2 Winds and ocean currents push against one another

As I've implied above, surface ocean currents are driven by winds, but to some extent the winds are responding to pressure and temperature differences created by ocean currents beneath them. So it is a rather complex circular chicken-and-egg situation.

Actually, there is something peculiar about the North Atlantic circulation, beyond just the push of equatorial trade winds, which partly explains why it is strong enough to produce the Furious Fifties. As well as being pushed, it is also pulled along by another mechanism, the thermohaline circulation.

1.4 THE THERMOHALINE CIRCULATION

Ocean currents do not just move around on the surface. In some places, the upper ocean waters sink down into the deep ocean. This happens for example in the North Atlantic off Greenland, Iceland and Norway. Where the surface water sinks, this sends a "river" of surface water down into deep ocean. A similar sinking process happens off Antarctica, and in a small patch of the Mediterranean Sea (just south of Marseilles, France) in winter.

The reason these waters sink is that they are denser than the surrounding ocean. But why are they denser? It is mostly due to their higher salt content. Pour a dense brine solution into a bowl of fresh water and it will sink straight down to the bottom, and the same principle applies here. These denser, saltier ocean waters are derived from areas that undergo a lot of evaporation, because the climate is hot. Evaporation of water leaves a more concentrated salt solution behind, and this is the key to the whole mechanism. So, for example, the waters in the north Atlantic gyre are derived from the Gulf Stream that comes up from the Caribbean. Heated by the tropical sun, it has lost a fair amount of water by evaporation. After water vapor is transported away, the remaining seawater is left saltier and denser as it leaves on its path northwards across the surface of the Atlantic (Figures 1.8a and 1.9). But the water is not yet dense enough to sink because the Gulf Stream is still warm as it is transported northwards. Warm water tends to be less dense than cold water. Even though it is saltier, its extra warmth is keeping its density quite low and it can still float over the less salty but cold water below.

Only when it reaches northern latitudes does the Gulf Stream water cool off drastically, giving up its heat to the winds that blow east over Europe. Because it has cooled, the Gulf Stream water is now left heavier than the surrounding waters and it finally sinks, as "pipes" of descending water about a kilometer across that lead down to the ocean floor. These pipes tend to form in the spaces between sea ice floes when a cold wind skips across the surface. On reaching the bottom, the sunken waters pan out to form a discrete layer that eventually spreads through all the world's ocean basins (Figure 1.8b).

There are several different sinking regions that feed water down into the deep (the North Atlantic being just one of them), and they each produce their own mass of water. These different waters sit above one another in a sort of "layer cake'' arrangement, that shows up in a cross section down through the ocean. Each layer has its own density, temperature/salinity balance, chemistry and is travelling in its own particular direction!

Just about all the world's deep ocean waters—those below about 300 metres—are cold (about 2 to 4° C), even though most of the ocean surface area is warmer. Even in the tropics, where surface water temperatures may be 32°C, the water below 300 m depth is about as cold as it would be in a domestic refrigerator. Why then are these

Thermohalin Circulation The Atlantic
Figure 1.8. Thermohaline circulation in the Atlantic.
Figure 1.8 (cont.). Relatively salty warm water (a) comes north from the tropics, then (b) cools off and sinks down into the deep ocean, pulling more water in behind it.
Equator

Figure 1.9. Ocean gyres.

deeper waters so cold? Because they originate as water that sinks in winter in the high latitudes, when the sea surface is cold. If other warmer waters at other temperatures had instead been filling the deep ocean, the mass of ocean water would reflect their particular temperature instead.

In fact, at other times in past (e.g., the early Eocene period, around 55 million years ago) the whole deep ocean was a mild 12°C instead of about 3°C at present. Why? Perhaps because the "feeding" of sinking water was occurring not only in chilly sub-polar seas but down in tropical latitudes, from places similar to the Arabian Gulf at present where warm but salty water (concentrated by evaporation) spills out into the Indian Ocean. What did this opposite circulation system do to climate? The climate scientists have no idea, really. But it could perhaps help explain the warmer world at such times, a world that, for example, had palm trees and crocodiles living near the poles.

The atmosphere tends to trap heat, through a process known as the "greenhouse effect''. The gases in the atmosphere are mostly transparent to visible light, which is the main form in which the sun's energy arrives on earth. But many of these same gases tend to strongly absorb the invisible infra-red light that the earth's surface radiates to loose heat back to space. Some of the infra-red captured by the gas molecules in the atmosphere is sent back down to earth (as infra-red again) where it is absorbed by the surface once more and helps keep it warm. This is known as the "greenhouse effect''.

Box 1.1 The greenhouse effect

If it were not for the combined greenhouse effect of naturally occurring gases in the atmosphere, the earth's temperature would naturally be somewhere around -200C to —300C on average. Thus this extra warming is very important in keeping the earth at a moderate temperature for life.

At present there is a lot of concern about an ongoing increase in the atmospheric levels of certain greenhouse gases due to human activities. For instance, carbon dioxide is building up at around 1 % a year due to it being released by fossil fuel burning and forest clearance around the world (Chapter 7). It is set to reach double the concentration it was at 250 years ago some time during the mid-21st century. The worry is that the increase over the background level of this and other greenhouse gases will lead to major climate changes around the world over the coming centuries. Already, detectable warming does seem to be occurring and the likelihood is that this will intensify. Since plants are strongly affected by temperature, it is likely that global warming will change the distribution of biomes (see Chapters 2 and 3). Shifts in rainfall that result from the changing heat balance and circulation of the atmosphere may also turn out to be important. And because of the many "feedbacks" discussed in the later chapters of this book, a change in vegetation may in itself amplify an initial change in climate, resulting in a bigger change than would otherwise have occurred (Figure 1.10).

VISIBLE LIGHT GETS THROUGH TO WARM THE EARTH

VISIBLE LIGHT GETS THROUGH TO WARM THE EARTH

Figure 1.10. How the greenhouse effect works. (a) Visible light from the sun can

BUT SOME OF THE INFRA-RED get through the FROM THE GROUND IS CAPTURED AND BOUNCED BACK BY GASES IN THE ATMOSPHERE

atmosphere. (b) Some of the infra-red leaving the earth's surface is absorbed and sent back down to earth.

1.5 THE GREAT HEAT-TRANSPORTING MACHINE

The decrease in temperature towards the poles forms the basic pattern of the earth's climates. But this pattern is greatly altered by the global circulation of two fluids: air and water. Factoring in the circulation of air and water enables us to understand the present-day patterns of climate in more detail.

One useful way to think of the world's climate circulation is to view it essentially as a heat-transporting machine that takes heat from the tropics and moves it to higher latitudes. It operates by movement of warm ocean currents, and also movement of winds and air masses (those great "blobs" of air) that move across the surface.

Heat is transported not just as the temperature that one can easily measure (known as "sensible" heat, because it can easily be "sensed"), but also in the form of "latent heat''. This latent heat is hidden energy that comes out only if you try to lower the temperature of moist air until a fog of water droplets appears. As you attempt to cool it, the air temperature drops, but nowhere near as fast as you would expect, because the water vapor condensing out as droplets gives off heat that keeps the air warm.

If it were not for this movement of heat in air masses and ocean currents, the high latitudes would be far colder than they actually are. Heat transport from the tropics "subsidizes" the higher latitudes, by as much as 40-60% more than the heat that they get from the sun (and the higher the latitude, the more important this heat subsidy is). This draining of heat away from the tropics also makes them cooler than they would otherwise be.

Places in the high latitudes that are close to the oceans, and receive especially strong ocean currents from the tropics, can be a lot warmer than places that do not. A warm current known as the Gulf Stream (mentioned above) crosses the Atlantic up from the Caribbean, and across to northwestern Europe. Largely because of the Gulf Stream, Britain has a much warmer climate on average than Nova Scotia, the eastern tip of Canada which is at the same latitude on the west side of Atlantic. In England, grass stays green in January and palmettos can be grown outdoors because the winters are so mild. In Nova Scotia the snow lies deep all winter long, and temperatures can dip to —■40 °C. As mentioned above, part of the reason that the Gulf Stream flows so strongly northwards and carries so much heat is that it is essentially "sucked" northwards by the sinking water of the thermohaline circulation in the north Atlantic.

There is a similar "gulf stream'' reaching the western side of North America (e.g., on rainy Vancouver Island) which has a very mild climate compared with the harsh winters of Sakhalin/northern Japan at the same latitude on the western side of the Pacific. However, because there is no strong sinking zone in the ocean to pull it in, its effect on climates is not as strong as in the north Atlantic.

High-latitude places that are isolated from tropical air masses and warm sea currents tend to be especially cold for most of the year. The most extreme example is Antarctica. It is cut off from the rest of world by the belt of swirling currents and winds known as the "Roaring Forties''. This prevents much heat transfer from lower

North Pole Air Mass

latitudes, so Antarctica is colder than the North Pole region which receives air masses and warmer ocean currents from low latitudes (Figure 1.11).

In some places an especially cold area of ocean just off the coast makes a difference to the climate inland. Although Nova Scotia is at a disadvantage for heat because it does not receive the Gulf Stream, the frigidity of its climate is added to by a cold sea current that comes down along the west side of Greenland, bringing water straight down from near the North Pole. Across the other side of North America, the remarkable climate of San Francisco in California, which almost never gets hot—and almost never has frost either—is caused by a zone of upwelling of cool deep ocean water just off the coast. A similar cool upwelling zone occurs off the coast of Peru, where it brings about the extreme aridity of the Atacama Desert (see below).

1.5.1 The "continental" climate

Areas far inland in the higher latitudes tend to experience wide seasonal swings in temperature, because they are cut off from the moderating influence of the oceans. Seas have a very high capacity to store up heat—so their temperature does not vary so much during the year (Figure 1.12a). In contrast, the land cools down or heats up far more quickly. An area far inland gets less oceanic influence and is more at the mercy of the amount of heat received from the differing sun angle and day length at different times of year. Hence in such places the seasonal differences in temperature can be extreme (Figure 1.12b). The coldest winters on earth outside Antarctica occur not at the North Pole but in the interior of northeastern Siberia, because of its isolation from the oceans. This is known as a "continental" climate, receiving little heat from the distant oceans, and not much warming water vapor in the atmosphere to release heat. The coldest temperature ever recorded in northeastern Siberia in winter was a bone-chilling —68°C. Yet, paradoxically, this same part of Siberia has warm summers too; temperatures can exceed 30°C. The summer warmth is the result of the same factor—isolation from cooling sea winds, which do not reach the interior of Siberia from the seas around its edges.

To a lesser extent, continental climates with wide seasonal temperature swings are found in central Canada and the USA, eastern Europe and central Asia.

Oceanic climate

Temperature

SPRING SUMMER AUTUMN WINTER

b) Continental climate

Temperature b) Continental climate

Temperature

Figure 1.12. The annual temperature cycle of an oceanic and a continental location compared.

1.5.2 Patterns of precipitation

Not only temperature patterns depend on ocean currents and winds. Patterns in the wetness or aridity of large parts of the world's land surface can be understood as a product of circulation.

Why is it, for instance, that the tropics are so moist? Just as with temperature, this is ultimately a result of sun angle. The band of rising air along the equator (the intertropical convergence zone or ITCZ) occurs due to intense solar heating, from the sun being directly overhead. The heating sets up convection in the air, and this rising air sucks in moist ocean winds and water evaporating from the forests. As the air rises it cools, and water droplets condense out as clouds and then fall as rain. This gives the moist tropical rainforest climate down below.

A typical morning in the equatorial tropics begins clear and sunny. As the sun climbs high in the sky, the day becomes hot, but by mid-afternoon clouds begin to build and cover the sky as the heat of the sun sets off convection in the atmosphere. Eventually, by late afternoon the heat of the day is broken by a thunderstorm, leaving the air fresh and mild, and the vegetation moist with rain.

Hundreds of kilometers farther north and south, the air carried aloft in the ITCZ descends back down to earth. It has lost its moisture, which fell as rain as it first rose up from the surface, and now it also warms as it descends (Figure 1.13). The air is already dry, and the warming makes it hold onto its small amount of water vapor

Figure 1.13. How the rain-making machine of the tropics works. The heating of the ITCZ causes water to condense out and fall as rain. When the air descends again, no water vapor can condense out and there is an arid climate.

Sinking air in outer Rising air at equator tropics gives dry gives rainy climate climate

Moist air off sea

Figure 1.13. How the rain-making machine of the tropics works. The heating of the ITCZ causes water to condense out and fall as rain. When the air descends again, no water vapor can condense out and there is an arid climate.

Sinking air in outer Rising air at equator tropics gives dry gives rainy climate climate

Moist air off sea

Equator

Was this article helpful?

0 -1
Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Responses

  • PAUL CHAMBLESS
    Which current moves cold water to warm latitudes?
    7 years ago
  • miranda
    Which direction does cold water move in the ocean?
    7 years ago
  • swen
    What makes cold water move away from earths poles?
    7 years ago
  • ronja
    Which current moves cold water to warm latitutdes?
    7 years ago
  • fastred
    How does antartica effect North atlantic Current?
    7 years ago
  • mauro
    How will a wind blowing to the south in the Northern Hemisphere be affected by the Coriolis effect?
    7 years ago
  • amanuel
    What prevents wind from blowing directly from the North Pole to the South Pole?
    7 years ago
  • jyri
    WHAT HAPPENS WHEN WARM WATER MOVES ACROSS COLD WATER WHAT HAPPENS WHAT IS PRODUCED?
    7 years ago
  • alfred
    How will a wind blowing to the north in the Northern Hemisphere be affected by the Coriolis effect?
    7 years ago
  • Zemzem
    Why does north atlantic current flow clockwise?
    7 years ago
  • lucas
    Do cold fronts carry heat from tropics to higher latitudes?
    6 years ago
  • alison ritchie
    What vegetation is in cold water?
    5 years ago
  • lewis
    Why do the low lattitudes have warm water and high latitudes cold water?
    4 years ago
  • Giacobbe
    How does warm ocean currents reach higher latitudes?
    4 years ago
  • aaron
    How wind current moves at different latitudes?
    3 years ago
  • Bladud
    What is it warm water moves polewards?
    2 years ago
  • venanzio
    Which ocean currents move cold waters to warm latitudes?
    1 year ago
  • RAFFAELLA
    Which of the following currennts move cold water to warm latiudes?
    1 year ago
  • rodrigo
    How do warm tropical currents reach colder latitudes?
    5 months ago
  • uffo
    How do warm, tropical currents reach higher, colder latitudes?
    5 months ago
  • tracy
    Which current moves warm water to colder latitudes?
    5 months ago
  • tytti
    What latitudes do warm ocean currents originate from?
    2 months ago
  • feorie
    Why dose warm water move away from the equator and cold water moves to the equator?
    28 days ago
  • jukka-pekk
    Which way does thermal energy move when a cool breeze moves over a warm lake?
    9 days ago
  • mary elliott
    What happens to warm surface waters when they reach the poles?
    6 days ago

Post a comment