SW Climate 

Regional Climate: The Four Corners


This is the second of two discussions presenting an overview of Earth’s climate and its manifestation in the Four Corners region. The first reviews the basics of processes controlling the global climate.



Figure 2: Click for full size


Desert and mountains

The equator-to-pole spatial variations discussed in Part I still do not describe Earth’s climate very well, because they say nothing about the seasons. Just as climate variations in space are relevant to the Four Corners, so are changes in time. These are most easily described with graphs, in which time runs from left-to-right and the climatic quantity that’s under discussion runs up and down. Figure 2 is an example of such a time series plot. This graph shows temperatures averaged over two sub-regions of the Southwest; these subregions are shown on the inset map. The Southern Rocky Mountain sub-region, which includes the mountains of Colorado, Southern Wyoming, Northern New Mexico, and Utah, is representative, in a general way, of the higher altitude parts of the Southwest. The Southern Desert sub-region, which includes southern New Mexico, much of Arizona, and the Mojave Desert in California east of the coastal range, represents the lower altitudes. Each set of temperatures on Figure 2 shows the averaged high and low temperatures (dashed) and the average of those two (solid) in each sub-region for the four seasons shown on the horizontal axis at the bottom. The temperatures are shown in degrees Celsuis; for reference, 0oC is freezing and 25oC is 77oF.


The temperature differences between the two sub-regions illustrate the range of climate over the Southwest. An easy way to think of these variations is to note that summer in the mountains is about the same temperature as spring and fall in the desert; and winter in the desert is about the same as spring and fall in the mountains. Note also that the range of extremes—the spread between the dashed curves—is largest in winter and spring. This reflects the weather systems that bring snow to parts of the Southwest in these seasons.


Temperature Extremes


The sub-region/seasonal averages in Figure 2 smooth out the temperature variations considerably, because the temperatures over these large areas are averaged together and the years from 1901-1986 are averaged together. One way to examine the spatial variations in greater detail is to look at temperatures from two locations. An example of this is shown in Figure 3. The two time series plots shown give temperatures averaged over a number of years for Yuma, Arizona (top), and Gunnison, Colorado. These two cities were chosen as extreme representatives of the two sub-regions in Figure 2; their locations are noted with dots on the map inset in Figure 2. Although Gunnison is farther north, that is, farther from the equator, than Yuma, the climatic differences are due more to altitude than to the north-south distance.


The plot for each city has five temperature time series on it. The middle one is the averaged temperature for each day of the year; that is, the January 1st temperature in the middle plot is the average temperature for all the January firsts averaged together. The two plots just above and below this average temperature are the averaged high and low temperatures for the date in question. The two outside plots are the extreme high and low temperatures for the date; that is, these are the “record” temperatures for the date. The years included in these averages are noted; they represent all of the data available in the Historical Climate Data Set used for these plots.



Figure 3: Click for full size


Between these two graphs and among all the plots on them, there is a lot of information in Figure 3. Some things are pretty obvious: it's a lot hotter in Yuma than in Gunnison, year ’round; the winters in Gunnison can get pretty rough—the record low temperature looks to be somewhere around -45oC (almost 50o below zero Farehheit!). And summer in Yuma is no picnic, with record highs over 100oF (about 38oC) for over four months of the year. On average, each city has an annual cycle—that is, the summer-winter change in average temperature—of about 20-25C (or as much as 45oF). The daily temperature range, which, on average, is the difference between the average high temperature and the average low temperature, is nearly this large.


What about Global Warming? To assess the effect of Global Warming on the temperatures in the Southwest, we need to turn to the most sophisticated climate models. The calculation discussed above in which we found the Earth’s average temperature to be 0oF represented the use of an extremely simple climate model. Of course, the best climate models in use today are far, far more complicated, and accurate, than this one was. Even though they are far from perfect, they do quite a good job of calculating Earth’s present climate. They also include as many of the climatic feedback loops as scientists can figure out ways to include, such as the one about cold temperatures, snow, and the reflection of sunlight noted above, as well as loops involving the oceans, cloud processes, and on and on.


If you set up one of these models to calculate what the temperature changes in the Southwest will be as the CO2 in the atmosphere increases, you find that the temperatures tend to warm up. By the end of this century, the models suggest that the temperature increases could be as much as 4oC (assuming that the CO2 content of the atmosphere continues to rise, and that nothing strange—such as an increase in the number of volcanic eruptions—happens). The models also suggest that the wintertime temperatures could rise more than the summertime temperatures (much to the relief of people in both Yuma and Gunnison, one might imagine). This means that the plots in Figure 2 or 3 would not simply lide upward in a uniform way—their shapes would change somewhat as well.


Now, compared to the annual and daily ranges of some 20oC, 4oC (just over 7oF) isn’t a huge temperature change by any means. Further, different climate models give different answers, some suggesting hardly any average change at all. However, there is more to consider than simply this comparison. First, changing the shapes of the curves in Figure 3 could change such things as the length of the growing season. This would affect both agriculture and the sustainability of natural ecosystems. Second, the strength of extreme events could be affected. That is, a small increase in average summer temperature could translate into new record high temperatures on some days. The two 50oC (over 120oF!) record highs in Yuma could become relatively commonplace. This would affect both the type of ecosystems that could survive as well as people's health. Consequently, even small average changes are reason to study the problem more.


Temperature, of course, is only part of the story. In the semi-arid Southwest water is always a concern as well.


Precipitation


It has already been noted that the Southwest is a semi-arid region, and this is reflected in the precipitation records at the two cities. Figure 4 shows the daily precipitation averages (left column of panels) and the extreme precipitation events (right column) for Yuma and Gunnison. The bottom row shows the contribution of snowfall to the Gunnison precipitation (there is no snow in the precipitation record for Yuma, at least in this dataset).


In the daily averages (left column of panels), the heavy lines show 31-day running averages, included here to smooth out the day-to-day variations. These represent “:monthly” averages, at least for the 15th of each month. That is, the first point at the left ends of the heavy lines is the monthly average for April.


As with the temperature records in Figure 3, these precipitation time series plots contain a large amount of information. One thing that is obvious is that neither of these cities receives very much in the way of water—there are 25.4 millimeters in an inch, and so the daily averaged precipitation in both locations is always well below a tenth of an inch per day. Several other points are worth emphasizing here.



Figure 4: Click for full size


  • The records for both cities exhibit two peaks in the annual precipitation, a several-month period of enhanced precipitation from about July to October and another from about October to April. These peaks are most obvious in the smoothed data (heavy lines). In the Southwest generally, these two peaks be called the summer “rainy season” associated with the Southwest Monsoon, and the winter rainy season, associated with storms coming off the Pacific Ocean. The Southwest Monsoon is an extremely important climatological feature of the region that provides badly needed summertime rainfall, particularly in the southern deserts. Even though there is significant snowfall in Gunnison, the summer rainy season provides more water to the region: note the higher summertime peak.
  • Although there is no snow in Yuma, there is still the secondary rainy season due to winter storms. In Gunnison, the secondary enhanced period of precipitation in the winter and spring is due to snowfall. This is true generally in the mountains of the Southwest (for example, the area of sub-region 1 in Figure 2 above), and the spring runoff from the melting snowpack is also quite important to the Southwest's water resources. Not only is the amount of water important, but the delay associated with the snowfall’s remaining on the ground for weeks to months exerts a strong influence on water resources.
  • The day-to-day variations in the daily averages, combined with the scale changes from the left column to the right in Figure 4, suggest that extreme precipitation events play an strong role in the averaged rainfall. For example, consider the extreme event of 109 mm (over 4 inches—no doubt from a large thunderstorm) at Yuma on about August 1. If it never rained on any of the other August firsts (or whatever date this is) in the 76-year record, the daily average for this date would be 1.4 mm/day. It appears that most of the “daily averaged” precipition for this day of the year may be due to this one event.
  • Although this particular extreme event is the most extreme of the entire 76-year record, other large rainfall events at Yuma are also evident in the upper right-hand panel of Figure 4. Undoubtedly, much of the “daily averaged” precipitation for Yuma shown in the upper left-hand panel is associated with these large events. The same is likely true for Gunnison as well.

The point here is that extreme precipitation events, like record high and low temperatures, will be affected by climate change, but we do not know how. Will there be more 4-inch thunderstorm events in Yuma? How will the Southwest Monsoon—which is just lots of thunderstorms day after day for several months—be affected by global warming? Will there be less or more snow in the mountains?


Answering these questions is the focus of current scientific research, and this work is critical to the future of the Four Corners region.



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