Climate patterns and variability
The Colorado River Basin contains incredible climatic diversity; the high mountains of the Upper Basin have annual average temperatures below 32°F and receive over 40” of annual precipitation, while the desert lowlands of southwestern Arizona average 60°F warmer and receive less about one-tenth the precipitation (Figure 1). The complex spatial patterns in climate on both local and regional scales are mainly driven by several consistent and predictable mechanisms. Similarly, the typical seasonal distribution of precipitation is quite variable from place to place, but there is regularity in the climatic drivers of the seasonality.
Underlying those averages for a given location is considerable climatic variability over time. Each month, season, year, and decade of what we call climate is the aggregation of a unique sequence of weather that is loosely bounded by the past but never repeats it. The Western U.S. is well known for high variability in precipitation especially, and the Colorado River Basin is no exception. The wettest years, averaged across the basin, have seen more than twice as much precipitation as the driest years, and extremely wet years may follow extremely dry ones, and vice versa.
The spatial and seasonal patterns of climate drive the distribution of water resources across the basin. Climate patterns also largely determine the distribution of ecological habitats and species across the basin, as well as the suitability of different areas for agriculture and other land uses. The hydrologic consequences of year-to-year climate variability in the basin has required societal (and ecological) adaptations to buffer those swings, for example with reservoir storage.
Topography and elevation
The most important spatial gradients and patterns in average climate arise from the topography and resulting differences in elevation. In general, seasonal and annual precipitation is significantly greater at higher elevations in a given area, due mainly to orographic lift: moist air masses are forced upslope by the terrain, causing water vapor to cool, condense and precipitate (Figure 1). Conversely, rain shadows occur in basins downwind (typically, east) of mountain ranges as downslope flow leads to warming and drying of air masses. Altogether, the crests of mountain ranges receive 2 to 5 times more precipitation on an annual basis than the basins or valleys below. Higher elevations also experience cooler temperatures as a consequence of the lower atmospheric pressure; air masses (whether moist or dry) expand and cool at higher elevation (Figure 1).
Other mechanisms create gradients in climate at broader scales. Average temperatures increase dramatically from north to south, not only due to the lower elevations in southern Arizona and California compared to Wyoming, but also because of increasing proximity to the tropics and the greater solar heating there. Precipitation also is also lower in the southern parts of the basin because of the increasing influence of the subtropical high pressure belt that tends to deflect storm systems, especially in the cooler months.
Dynamics and seasonality of precipitation
All precipitation over land requires two things: (1) the horizontal transport of water vapor from a moisture source--typically an ocean--to that area, and (2) a mechanism to vertically lift that water vapor so that it can cool, condense, and fall as rain or snow. For the Colorado River Basin, seasonally varying atmospheric dynamics and weather patterns bring different moisture sources into play while also influencing the mechanisms of lift that dominate in a particular season and location (Figure 2).
Overall, the climate feature most important to the basin's hydrology is the series of mid-latitude cyclonic storms (i.e., low-pressure systems) and lesser disturbances, entraining moisture from the Pacific Ocean, that track across the interior West throughout the cool season from October-May. The frequency and specific track of these systems, which generally follow the jet stream, are the main determinants of water year precipitation in the headwaters and thus, of the basin's annual streamflow as well. In mid-winter (Dec-Feb), the moisture delivery of some of these storms is greatly enhanced by accompanying “atmospheric rivers” (ARs) that periodically penetrate inland into the basin. The Lower Basin has a distinct peak in storm activity in mid-winter, while the Upper Basin tends to get a more even distribution of storms throughout the cool season, at least on average.
In summer and early fall (Jun-Sep), the jet stream weakens and shifts off to the north, setting the stage for the North American Monsoon (NAM). The NAM is a pattern which brings moist subtropical air northward from the Gulf of California and also the Gulf of Mexico, firing up regular if not daily convective storms (i.e., thunderstorms) across the Lower Basin and into parts of the Upper Basin. In the spring and summer, moisture for storms may also be “recycled” evapotranspiration (ET) from the land surface. From late summer into October, periodic landfalling Pacific tropical storms may douse the Lower Basin as intact systems, or at least juice the monsoonal storms with additional moisture.
Climate variability over time
The previous sections describe the average tendencies of climate over the historical record. But as all water users and water managers know, the basin’s climate can vary dramatically from year to year, and also from decade to decade, especially precipitation and related drought and moisture variables. This regional climate variability is associated, at least in part, with identified modes of global climate variability, principally El Nino-Southern Oscillation (ENSO). The predictability of the basin’s climate on seasonal and interannual timescales mainly stems from ENSO events and the "memory" imparted by long-term soil-moisture anomalies. For temperature, the long-term warming trend also imparts predictability; i.e., it is likely that every year will be warmer than a historical average. But most of the variability in precipitation from year to year appears to be unpredictable.
As noted previously, the annual precipitation of the Upper Basin has varied two-fold between the wettest and driest years (Figure 3), while the Lower Basin is more variable, with a three-fold difference between the wettest and driest years (Figure 4). The wettest years are those in which the tracks mid-latitude cyclones over the basin were especially active throughout the cool season (Oct-May), while in the driest years those storm tracks were unusually inactive and/or shifted north of the basin, and often the North American Monsoon was much weaker as well, disproportionately affecting the Lower Basin.
The basin’s precipitation also varies on decadal time scales, and these multi-year and longer excursions towards wet and dry can lead the large reservoirs on the mainstem Colorado River to fill and spill, or experience great stresses. Average annual precipitation in the Upper Basin from 1977-1986 was almost 20% higher than for the preceding 10 years, 1968-1977. The presence of this substantial natural decadal variability makes it difficult to discern long-term trends, and one cannot assume that a trend, if detected, will continue.
Temperatures in the basin also vary from year to year, though this temperature variability is more spatially coherent than precipitation variability; accordingly, a single time-series for the entire Colorado River Basin tells the story for both the Upper Basin and Lower Basin (Figure 5). The effects of temperature variability on water resources and ecosystems are more subtle than the effects of precipitation variability. The two variables are physically and statistically related; drier years, especially over the April-October warm season, tend to be warmer than average, while wetter years tend to be cooler, since the underlying weather patterns tie together dry-sunny-warm conditions, and conversely, wet-cloudy-cool conditions.
The most obvious feature of the observed temperature record in the basin is the substantial warming trend, of about 2°F over the past 40 years. This trend has a magnitude similar to the interannual variability in temperature, meaning that the range of temperatures, not just the average, is shifting away from the past climate. The warming trend is described in greater detail in Recent climate change.
Data and tools
There are several climate tools that are useful for plotting and examining time-series and recent trends in temperature, precipitation, and other climate variables over specific areas (states, counties, river basins, etc.). Each tool depicts one or more gridded climate datasets, so users of the tools may want to familiarize themselves with these datasets as well; see Weather and climate monitoring and Chapter 4 of the State of the Science report.
The “CAG” tool is a versatile tool that can be used to generate many types of charts, maps, and analyses from NOAA’s official nClimGrid monthly gridded climate dataset. Selecting "Regional" and "Time series" at top brings up several dozen region options, including the Upper Basin and Lower Basin. Selecting "Regional" and "Mapping" allows data to be mapped with river basin boundaries.
This tool, developed by researchers at the U. of California-Merced and partners, generates climographs of average (1981-2010) monthly temperature and precipitation for any point, county, HUC8 watershed, or user-selected area, from the gridMET gridded (4 km) climate dataset.
State of the Science Report
Chapter 2 of the State of the Science report describes these patterns, mechanisms, and trends in much greater detail, in sections 2.2, 2.3, 2.4, 2.7, 2.8, and 2.10.
Assessment of Climate Change in the Southwest United States
Chapters 4 (Present Weather and Climate: Average Conditions) and 5 (Present Weather and Climate: Evolving Conditions) of the 2013 Southwest Climate Change Assessment cover the climate processes and patterns and climate variability and trends, respectively, of the six states of the Southwest region (CA, NV, UT, CO, NM, AZ).