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Chapter 3: Climatic determinants of global patterns of biodiversity

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Edited by Peter Moyle & Douglas Kelt
By Douglas A. Kelt, September 2004

The major patterns of ocean circulation
Wind drives the surface waters of the oceans, and therefore is largely responsible for the major patterns of oceanic circulation (compare Fig. 5 with Fig. 6). This results in the clockwise circulation patterns observed in the northern hemisphere, and the counter-clockwise patterns in the southern hemisphere. However, there is one very important exception to this, and it results in a particularly important phenomenon. We will use the coast of northern California to exemplify this, as it is a regional event and has important ramifications for both the ecology and the economy of our state.

If we look at currents along the coast of California, we note that they run parallel to the coast of the northern third of the state. However, the southern two thirds of the state begin to curve eastward, while ocean currents continue southward. As ocean water moves south along the coast of northern California, it is subjected to the Coriolis Effect, and it is shifted to the right (which is towards the west), resulting in surface waters being moved offshore (Fig. 7).

Fig. 6. Main patterns of circulation of the surface currents of the oceans. In each ocean, water moves in great circular gyres, which move clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. These patterns result in warm currents along the eastern coasts of continents and cold currents along the west coasts.

This water has to be replaced, however, and the water that replaces it wells up from deeper levels off the continental shelf. This deeper water has two important characteristics. First, it is colder than the surface water. Cold water holds more oxygen than warm water (another physics fact that you need to accept at the moment) so it provides a basic nutrient needed by plankton. Second, it is very rich in nutrients; this is because it comes from deeper regions where light does not penetrate, and where photosynthesis does not occur. Therefore, all of the nutrients that are needed by plankton are in relatively high abundance. As a result, areas characterized by upwelling waters also are characterized by having cool and nutrient rich near-shore water. How does this affect these areas?

First, the high concentrations of oxygen and nutrients make these areas very productive. Plankton develop and occur at very high concentrations, feeding fishes which in turn feed larger fishes, birds, and marine mammals. The famous anchovy fisheries off the coast of Peru are a direct result of upwelling. In the Peruvian case, waters are moving north along the coast, and are deflected westward by the Coriolis Effect. The same is true along the northeast Pacific, where upwelling provides rich waters that support tremendous populations of salmon.

Fig. 7. Upwelling (top) and down-welling (bottom) caused by winds blowing along a shoreline.

Second, when cold water comes in contact with warm air, fog develops. Thus, along the coast of northern California we have very productive forests that are largely dependent upon summer fog to provide moisture through the dry season. Since this region is particularly interesting to us, let's spend a bit of time on this.

Climate in California
California is hailed internationally as a haven for sun worshippers. We have wonderful weather along much of our southern coast, and the beaches are a haven for bikinis, muscle-bound show-offs, and many marine mammals as well. However, the coast of northern California calls a very different image to our minds – one of fog, lush forests, and rain. Additionally, the climate of California is very seasonal, with relatively dry air and clear skies throughout the summer and fall, and clouds and rain during the winter. A famous song proclaimed “it never rains in California, but girl don't they warn ya, it pours, man it pours.” When it pours in California, it can really pour. But, what causes the rather substantial change in weather between summer and winter?

To understand this, we need to digress briefly to understand a couple of additional features of climate in the North Pacific. At about the latitude of Hawaii there is a zone of high pressure (air is subsiding, causing the famous weather characterizing Hawaii). This area is called the Hawaiian High Pressure Region, or the Hawaiian High. Air that descends here then moves either south or north. Air that moves north reaches about the Aleutian Islands before encountering polar air that is moving south. When these meet, two things happen. First, they have nowhere to go but up, so we find air rising in this region. This creates a low pressure region that is referred to as the Aleutian Low, and the associated clouds and storm development. However, because we have warm southern air meeting cold northern air, there is an additional degree of turbulence, so that the storms here also tend to be somewhat violent. Because mid to upper troposphere winds generally move eastward (the so-called “westerlies”), any storms that form along the Aleutian Low tend to be displaced eastward, and they often drop a lot of rain and snow on the North American continent. These storms also are shifted south by a high pressure region in Canada, so these storms usually end up hitting North America somewhat south of their origin.

Now, this all gets interesting (and particularly relevant) when we superimpose the seasonal shifting of the earth relative to the sun onto the position of the Aleutian Low and the related weather patterns. Recall that the position of the sun relative to the earth shifts seasonally, from as far north as the Tropic of Cancer to as far south as the Tropic of Capricorn. As the sun shifts north and south, the major bands of regions of rising and subsiding air masses also shift north and south, although not as much as the sun does. However, the major storm generator for the Pacific northwest, the Aleutian high pressure region, shifts from about 60° N in the summer to about 50° N in the winter. So, in summer this low pressure region lies sufficiently north that the storms generated there collide with North America fairly far north. However, winter brings a double whammy to this scenario.

First, the air that is descending from the Polar regions is colder in the winter than in the summer. When this air meets the air moving north from the Hawaiian High, the greater difference in their temperatures results in even stronger storms than in the summer.

Additionally, the low pressure zone itself has shifted south in response to the suns southerly migration, and the storms that are generated plow right into the northern coastal regions of our fair country. This is why northern California gets so much rain in the winter while staying warm and dry in the summer.

Fortunately for people in southern California, the storm tracks generally reach no further south than the middle of the state. Of course, these storms also are responsible for the multi-million dollar skiing industry.

Fig. 8. Major summer and winter storm tracks affecting California. In summer (top) both the Hawaiian high pressure region and the Aleutian low pressure region are located relatively far north, and storms generated in the Aleutian low intercept North America in extreme northern California and further north. In winter (bottom) the Hawaiian high and the Aleutian low are located further south, and Aleutian-generated storms track a more southerly route, intercepting much of the state of California (modified after Miller and Hyslop 1983 “California: the geography of diversity” Mayfield Press)

Local influences
Finally, to fully understand the distribution of biodiversity in California we need to appreciate local dynamics that operate on relatively small scales. We discussed coastal upwelling already. This is responsible for the lush nature of northern California forests, the existence of the spectacular redwood trees, and the surreal beauty of the Old Man's Beard (often erroneously called Spanish moss, it is not really a moss) that hangs from many trees in the coastal range. Three other local influences are particularly important.

If you looked carefully at Fig. 1 you noticed that not all deserts occur right around 30° latitude. In fact, if you have ever traveled through much of Nevada, Utah, or eastern Oregon, you probably recall that this region seems very desert-like. Indeed, this is the Great Basin Desert, one of four deserts in North America (the others are the Sonoran, Chihuahuan, and the Mojave). But the Great Basin Desert extends from about southern Nevada (about 35°) to southern Alberta (about 55° N), and is not a product of air subsiding at about 30° N. The Great Basin Desert lies in the zone of westerly winds, which roll from the Pacific Ocean full of moisture and then begin crossing California. Before reaching Nevada, however, these air masses cross two mountain ranges, the Coast Range and the Sierra Nevada (Fig. 9). As air climbs up these mountains it cools with increasing altitude. As it cools, the air reaches its dew point, and rain forms. By the time this air crests the Sierra Nevada, much of the moisture that it carried from the ocean has been lost. This air then descends the eastern slope of the Sierra Nevada, and warms. Just like the subsiding air at 30° latitude, warming air holds more water, and results in a very arid region. The Great Basin Desert owes its existence largely to its geographic position in the “shadow” of the Sierra Nevada. Regions lying to the lee of a mountain range are therefore called rainshadows, and deserts that are products of such patterns are called rainshadow deserts. Some other rainshadow deserts are Patagonia and the Monte Desert of South America (constituting much of the country of Argentina), and much of inner Asia such as the Gobi Desert, which lies in the rainshadow of the Himalaya Mountain Range.

Figure 9

Fig. 9. Average annual precipitation is lower in the lee of a mountain range that is oriented at right angles to the prevailing winds because of the “rainshadow” effect. This is well illustrated by the Sierra Nevada, which blocks the movement of east-bound air masses. As these air masses climb the western slopes of the Sierra Nevada they cool and reach their dew point. Precipitation (denoted by the clouds) peaks around 6000-7000 ft elevation, declining thereafter. As this air descends the eastern slopes of the mountains it is too dry to condense and so little rain falls there.

North/south slope
In the northern hemisphere the sun warms south-facing slopes much more than it does north-facing slopes, resulting in greater temperatures and more rapid desiccation there. A common consequence of this is that very different plant formations characterize the southern and northern flanks of hills or mountains. This is particularly true and apparent in relatively dry regions (chaparral regions of southern California, for example), but is also apparent in temperate and even boreal areas. The greater moisture availability on north-facing slopes often leads to more lush and dense stands of plants. Animals that require dense vegetation, either for protection from predators or for food resources, often may be restricted to north-facing slopes. A drive along Berryessa Reservoir, not far from Davis, provides many opportunities to see this type of pattern.

Finally, the microclimate found at different points along a mountain may be quite different. Most campers know that cold air settles in the depths of valleys, and that a camp set up at the bottom of a ravine, while attractive during the heat of the day, may become quite cool at night. This is because when air cools at night, it descends the slopes of a ravine to meet cool, descending air from the other side of the ravine. These cool air masses merge and continue down the valley. If you are camped in the center of such a ravine you will have colder air at night, and you also will have stronger breezes than if you had camped slightly upslope. This has implications for the types of plants and animals you will find in these areas. If you have ever camped in such a spot, you may have noticed that in the evening, air descended the valley. Now you can see why. You also may have noticed that breezes reverse in the morning, and run up the valley. This is because the reverse dynamic occurs when the sun warms the landscape and the air, causing the air to rise and to flow uphill.

In the Santa Ana Mountains of southern California, fog funnels through passes and condenses on the needles of knob-cone pine trees. This adds as much as 10 cm of precipitation annually, allowing this tree to extend its growth into rainless periods. It also favors the local occurrence of Coulter pine and other species that require more moisture than would otherwise characterize this area.

Table of Contents

1. Roots of the modern environmental dilemma: A brief history of the relationship between humans and wildlife
2. A history of wildlife in North America
3. Climatic determinants of global patterns of biodiversity
4. Biodiversity
5. Natural selection
6. Principles of ecology
7. Niche and habitat
8. Conservation biology
9. Conservation in the USA: legislative milestones
10. Alien invaders
11. Wildlife and Pollution
12. What you can do to save wildlife

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