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Ocean Currents 

Reading Assignment
Gillespie, Netoff and Tiller, eWeather & Climate, as applicable (also note the search function on the CD).
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01. Sixth on the list of climatic controls is Ocean Currents. As you might expect, surface water temperature varies considerably between the Equator and the poles. As the ocean water is distributed across the Earth's surface by drifts and currents (see the infrared satellite photo of the warm Gulf Stream moving north along the east coast of Florida), warm water from the lower latitudes is moved into the higher latitudes and cold water is moved Equatorward. If you will remember, we noted in the section on Latitude that this redistribution of heat by ocean water is one of the major ways excess energy is moved poleward (and cold water is moved Equatorward).


02. We can divide ocean currents into two categories based on temperature: warm and cold currents. Think of cold currents as currents moving toward the Equator. These waters are colder than the water they are moving into. If you were swimming in the ocean whose temperature was 70 degrees F and you swam into a current whose temperature was 60 degrees F, the sensation would be cold. You would be swimming into a current colder than the surrounding water -- thus a cold current.
A warm current is just the reverse. A warm current is moving away from the Equator toward the poles. The water in a warm current is warmer than the surrounding water.

03. In this section we will take a look at the various ocean currents that influence weather in North America. As we consider each current, we will also touch on other controls that work together with the individual currents to create our weather patterns. Keep in mind that if you are experiencing a temperature of 120 degrees F, and you have a cold ocean current just off your coast, short of jumping in the ocean, you are not likely to benefit from the current unless something (the wind) can bring the cooling temperatures to your location. And we don't want to forget Continentality and Mountain Barriers -- all work together to create our weather.
We will begin on the west coast of North America where we find the North Pacific Drift colliding with the continent at about the border between the United States and Canada. Upon contact, the drift splits sending a current of warm water poleward (the Alaska Current) and current of cold water Equatorward (the California Current).

04. The presence of the warm Alaska Current off the west coast of Canada and the southern coast of Alaska provides welcome relief from the cold temperatures one might normally expect at such high latitudes. However, keep in mind that the current alone would do little to moderate temperatures on adjacent landmasses. Because this portion of North America lies between 30 and 60 degrees N latitude, the warmth associated with the north-flowing Alaska Current is spread to the adjacent landmass by the Prevailing Westerlies wind belt.
However, the moderating influence of the current is not felt for any great distance inland. As you can see on the accompanying map, the Canadian Rockies effectively block the movement of the winds inland. Just over the mountains the temperatures are extremely cold (Continentality) and precipitation drops off sharply.

05. As you can see in Photo 1 below, the Canadian Rockies often come right to the coast. Wind moving across the Alaska Current picks up not only the current's warm temperatures but a great deal of moisture as well. As the winds strike the mountains, the air is forced to rise. The resulting expansion and cooling delivers to the mountains much rain and snow. The entire region, in addition to its general warmth for such latitudes, is noted for its high atmospheric humidity and general dampness (Photo 2).


06. While the Alaska Current moves north and provides much needed warmth to the high latitudes of western North America, the cold California Current moves south along the west coast of the United States. And though the current gradually warms as it moves toward the Equator, no one in his right mind goes swimming in the ocean even as far south as San Francisco (remember Alcatraz) -- Los Angeles maybe, San Diego, no problem.

07. We want to take a look at how the California Current impacts the weather of two of the larger west coast cities: San Francisco and Los Angeles. While the current off the coast is similar in that it is cold and both cities are found in the belt of Prevailing Westerlies, there are other controls at work that make the weather of these two cities strikingly different.

08. Let's first consider the general physical geography of San Francisco. You will note the cold California Current off the west coast. The city of San Francisco is actually situated not on a plain, but rather on a low range of hills. To the east of the city is the bay, and to the east of the bay is a broad plain that stretches eastward to the high Sierra Nevada range.
As the Prevailing Westerlies move over the warm Pacific Ocean they pick up not only the ocean's general warmth but a great deal of water vapor as well. As the winds move across the cold California current, the lower-most portions of the air mass are chilled to the point of condensation and thus a fog forms off the coast and rolls inland on the winds.
Is the fog there all of the time? No! For the most part, the fog is only present during the evening and into the early morning hours. What happens to it during the day? Well, if cooling the air creates the fog, would it stand to reason that warming the air (during the mid-morning and afternoon hours) would dissipate the fog? In contrast to Los Angeles, San Francisco is relatively smog (a combination of the words smoke and fog) free. Why -- because there are certainly enough people in the area to generate plenty of pollutants. Consider the landscape. The city is open to the east and the Prevailing Westerlies simply carry the pollutants eastward to lodge against the west side of the Sierra Nevadas.

09. What a contrast Los Angeles provides! As with San Francisco, let's begin with a general overview of the area's physical geography. The NOAA infrared satellite view of the city clearly indicates the location of the heavily urbanized lowlands (blue color). In contrast, the surrounding hills (reds and browns) are sparsely populated. A more generalized view of the city may be seen on the trailing graphic.
Like San Francisco, Los Angeles has a cold current offshore and it is located in the Prevailing Westerlies wind belt, but unlike San Francisco that was open to the east, Los Angeles is surrounded by low hills. I might also point out that the city is located almost right on 30 degrees N.


10. As was the case with San Francisco, as the Prevailing Westerlies blow across the cold current, the lower-most levels of the air mass are chilled and fog forms -- although the fog is not as heavy in Los Angeles because the water is not as cold. And like San Francisco, the millions of people in the Los Angeles basin generate plenty of pollutants.

11. I often pose a question to my classes on campus: Where do rich people live? I get many answers: along rivers, away from rivers, in hills, on plains, here and there. In a nutshell, it usually all boils down to this: Rich people live wherever they want to live. In some places that means along streams, while in other places it means in the hills. By in large they live in the best, most desirable places -- the poor in the least desirable locations. Los Angeles is no different. Smog is a very important location factor in the city. Can you see why the wealthy would not want to live in east Los Angeles? If you were wealthy wouldn't you prefer to live along the coast where the air was cleaner? Or maybe in the hills to the north of the city where the smog problem is minimized?

12. But there is something else at work in Los Angeles. The hills to the east of the city are generally not high enough to trap the smog by themselves. The winds should have no difficulty moving the pollutants over the hills and into the desert beyond. Yet they don't. Why? What else is going on here that is holding the pollutants in the basin?
Do you remember the location of Los Angeles? 30 degrees N -- right under the semi-permanent high pressure cell. Both Los Angeles and San Francisco have the Westerlies wind belt, the cold current and millions of people. But San Francisco is not surrounded by hills, nor is it almost always under the subsiding semi-permanent high pressure cell. Los Angeles is -- and the high pressure cell creates an upper air inversion over the city that tends to keep the pollutants trapped in the basin.
How to solve the problem? Blow a hole in the mountains east of the city and provide a vent into the desert beyond (not likely). Then the only thing left is to try as best they can to live with the problem. This has brought about some of the most stringent anti-pollution regulations in the nation.

13. Moving south along the east coast of Canada is the cold Labrador Current. This current, which has its origins in the Greenland/Iceland area, is the one that brought the iceberg that sank the Titanic.

14. As is the case with both the Alaska and California currents, the Labrador Current lies in the belt of Prevailing Westerlies. As a result, there is a general tendency for the influence of this current to be minimal except immediately along the coast. But does it provide any assistance with Canada's weather temperature-wise?

15. In winter -- not likely. To begin with the Labrador Current is a cold current, and such a current would provide little relief from the severely cold temperatures brought from the interior of the continent by the Westerlies. Too, keep in mind the influence of Continentality. In the winter there is a high pressure cell over the cold land with winds moving generally from the land to the sea.

16. In summer -- maybe. After all Continentality does generate a low pressure cell over the warm land in summer with the winds moving from the cooler sea (and then over the cold current) onto the land. And would not a little cool/cold air feel good during the long summer days? Now you are thinking like a real Southerner! We here in the South may like to think about getting a little cool air during the August/September months, but we are talking here about Canada. About the last thing they want in summer is cool air. No, from a weather standpoint the Labrador Current has little effect on Canadian weather.

17. Finally, we have the Gulf Stream. This rapidly moving warm current located off the east coast of Florida would appear to be the answer to our cold winters in the southern United States. But is it?

18. Looks like the same problem here that we had with the Labrador Current. For openers, the prevailing wind pattern tends to direct the warmth associated with the Gulf Stream away from the American mainland.

19. And in winter, when the current could really do us some good, the high pressure cell over the land associated with Continentality tends to keep the influence of the current offshore.

20. And in summer -- well the winds are blowing right. The continental low pressure over the warm land does draw the warmth of the current onto the land. But if you have ever spent a summer in the southeastern United States, the last thing you want is more heat to go along with your humidity. No, the Gulf Stream really does us little good. Now Europe is another matter. As the Gulf Stream moves across the Atlantic, it, combined with the Prevailing Westerlies, does take a great deal of warmth to the continent. Compare the populations of Europe and Canada at similar latitudes.

21. While most of the United States receives little beneficial effect from the Gulf Stream, that is not to say that some parts of the country do not. Take a look at Florida. You will observe that most of the peninsula actually lies between 30 degrees N and the Equator -- in the belt of the Northeast Trades.
The east coast of Florida, long known for its mild winters and a haven for retirees, takes full advantage of the combination of the warm Gulf Stream and the Northeast Trades.

You have now completed Unit 7: Ocean Currents. You might wish to check your knowledge of the material presented in this section by working through the Multiple Choice, and True-False Quiz Questions as well as the essay-style Review Questions available through The Course dropdown located in the header of this page. To return to the top of the page.

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