Most people that have visited or lived in the Sierra Nevada mountain region have observed that the climate pattern of this region tends to be warm and dry in the summer time and cooler and wetter in the wintertime. But have you ever noticed that this is rather unusual in comparison to most weather patterns throughout the world? If you looked at a seasonal precipitation map for the planet, you would likely observe that the majority of the earth receives most of its precipitation in the summertime. As it turns out, a large part of California is considered to be a Mediterranean Climate; one of only four locations in the world other than the region for which this climate is named in southern Europe and Asia (See Figure 1).
Figure 1
Only 2% of all land on Earth is considered part of a Mediterranean climate! All the incredibly diverse grasslands, wildflowers, oak woodlands, and even evergreen forests have adapted over millions of years to be able to grow in this specific climate region. So why is that, and should you care? Let’s take a look at the why, and through this a sense of caring and pride just might sneak up on you, if it hasn’t already!
Figure 2
Through an ongoing interaction between the sun, air, ocean, land, and life, climates as we know them are formed. How do these interactions create California’s Mediterranean climate and how can we see these processes? Let’s start with those hot, dry summers that make us so unique. Why so hot, why so dry? If you look at Figure 3 you can see that in the summertime a huge high-pressure mass of air sits directly off the coast of California.
Figure 3
This air mass is part of a global circulation pattern that creates different pressure systems (high and low) which is driven by differential heating of the Earth’s atmosphere by the sun from the equator to the poles. Check out Figure 4 to get the big picture on how this generally operates.
Figure 4
California’s high-pressure system (called the “North Pacific High”) is dry, warm, and (this is the key) pretty stable which results in very little opportunity for other wetter systems to displace it. All Mediterranean climates are located between 30-40º north or south of the equator, where the sun’s rays penetrate the atmosphere enough to create a lot of heat; not as much as at the equator, but much more than at the poles.
Another big role-player in the formation of climates are ocean currents. Mediterranean areas have a cold current that parallels their coast, and like all currents they are driven by the wind. Prevailing winds in California come from the northwest, and the cold “California Current” is brought down from the north in a clock-wise direction. The cold water is from upwelling cold-water coming from a deep ocean-floor current. Check out Figure 5 and 6 to see the ocean currents and winds that are described. This cold current serves as one more barrier for moisture reaching inland in the summer via the westerly winds; any moisture that may have managed to gather in the North Pacific High condenses into fog when hitting the current as the westerly winds blow across the ocean and onto land.
Figure 5
Figure 6
So, now the basic processes have been covered that create the dry and warm summer climate that draw so many people to live and visit California! But what about that winter rain? What has changed that allows moisture to reach our thirsty land? The amount of heat received by the sun is the main force behind the shift. As summer turns into fall the intensity of the sun shifts to the southern half of our planet, which in turn weakens the high pressure system that is so prominent in the summer, and shifts it south. This makes room for the polar jet stream to bring moist, unstable air masses further south and with it precipitation. Almost 80% of precipitation falls in the wintertime in regions that are Mediterranean! The moist air has an additional assistant in ensuring that the rain is dropped in our region and not pushed further inland by the westerly winds in the form of the Sierra Nevada. When the wind pushes an air mass into a mountainous barrier it moves the air up hill. The higher the elevation the cooler the air, and cooler air has less ability to hold moisture, and when it reaches its capacity it drops much of it in the form of rain or snow! Check out Figure 7 to get a basic visual of how this “Rain Shadow Effect” works.
Figure 7
Figure 9
All the rain and snow melt that runs down the mountain not only provides one the major sources of drinking water for California, they also provide the water needed to make the Central Valley agriculturally productive. Before humans started irrigating and damming these mountain rivers, the Valley seasonally flooded which did two major things. First, massive underground aquifers that lay below the valley were refilled. Secondly, nutrient-rich minerals that were broken down from the rock materials of the mountains were washed down into the Valley floor, creating an incredibly fertile soil that built up over centuries. Although these natural processes can no longer occur anywhere near the extent that they formerly did because of irrigation, the aquifers and soil resources still are still available and utilized today, although both are being depleted at a rate much faster than they are being replenished.
Figure 10
Climate has changed many times shifting and evolving throughout the history of earth. The Mediterranean region in California has at different points in the past been warmer, colder, more humid, and dryer than it is now. These changes have happened gradually over time, but recently much validation has been given by an overwhelming portion of the scientific community that a global warming trend that is occurring is happening at an accelerated rate and is likely being caused by pollutants being released into the air (atmosphere) from human activities. How much climate change will affect the climate that we are accustomed to here in central California is still unknown. However, now that an overview of the amazing weather patterns that define the Mediterranean climate has been covered, you have to admit, you can’t help but appreciate a little bit more how specific, unique, and complex our little corner of the world is.
Did anything you read in this blog create any questions you’d like to know more about? Is there anything that seemed inaccurate? What is your favorite part of the central Sierra Nevada region? Why don’t you comment and tell me?
*All images were obtained through www.creativecommons.org .
Julia Stephens
Environmental Associate