Climate patterns and variability

Overview

There is enormous diversity in the average climate over the basin, with the the high mountains of the Upper Basin seeing annual average temperatures below freezing and over 50” of annual precipitation, while the desert lowlands of southern Arizona average 60 F warmer and receive one-tenth the precipitation (Figure 1). The complex spatial patterns in climate on both local and regional scales are largely 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 climate drivers of the seasonality.

Underneath those averages for a given location, however, the climate varies widely and for the most part, unpredictably. Each month, season, and year is the aggregation of unique sequences of weather that (usually) fall into the range of past conditions. The Western US is well known for high variability in precipitation from year to year and decade to decade, and the Colorado River Basin is no exception. The wettest years, averaged across the Upper Basin have seen more than twice as much precipitation than the driest years.

Relevance

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 technical adaptations to buffer those swings, most notably 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).

Figure 1. Colorado River Basin, showing close correspondence of elevation (left) with annual average temperature (1981-2010; center) and annual average precipitation (1981-2010; right). (Elevation map: Reclamation 2020; Climate maps: Lukas and Payton 2020, based on gridded climate data from Livneh et al. 2013.)

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

Any precipitation over land requires, (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).

Figure 2. Schematic showing the predominant oceanic moisture sources in each season (blue arrows), the main dynamical mechanisms for precipitation (purple text), and average seasonal precipitation (blue contours) for the Colorado River Basin. (Design: Jeff Lukas; precipitation maps: Climate Engine/gridMET; https://app.climateengine.org)

Overall, the most important dynamic for the basin are the mid-latitude cyclonic storms (i.e., low-pressure systems) bearing Pacific Ocean moisture that track across the interior West throughout the cool season of October-May. The frequency and specific track of these systems, which generally follow the jet stream, are the main determinants of that water year’s precipitation in the basin’s headwaters and thus, of annual streamflow as well. In mid-winter (Dec-Feb), the moisture delivery of these storms may be 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 sourced (“recycled”) from 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 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 such as El Nino-Southern Oscillation (ENSO). However, there are no regular and consistent--and thus readily predictable--cycles in the regional climate. The predictability of the basin’s climate, especially precipitation, is limited to the climate “memory” imparted by conditions that may persist for months or years, such as the ENSO state, or feedbacks from soil-moisture anomalies. Most of the variability 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 likely the North American monsoon was much weaker as well, which disproportionately affects the Lower Basin.

Figure 3. Upper Colorado River Basin annual (water-year) precipitation, 1900-2020. (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)

Figure 4. Lower Colorado River Basin annual (water-year) precipitation, 1900-2020. (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)

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 (17.68”) was almost 20% higher than for the preceding 10 years, 1968-1977 (14.82”), and then the period from 2000-2009 (14.49”) was yet drier than 1968-1977. The presence of this substantial natural decadal variability makes it difficult to discern long-term trends, or ascribe meaning to any trend.

Temperatures in the basin also vary from year to year (Figures 5 & 6), but the effects of this variability on water resources and ecosystems are much less obvious than those of precipitation variability. The two variables are physically and statistically related; drier years 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.

Figure 5. Upper Colorado River Basin annual average temperature, 1900-2020. (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)

Figure 6. Lower Colorado River Basin annual average temperature, 1900-2020. (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)

The most obvious feature of the observed temperature record in the basin is the substantial warming trend, of about 2 degrees 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 and its consequences for hydrology are described in greater detail on XX pages.

Additional resources

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.

New and Notable Research (2020-present)

[<URL FOR PAPER>]

Summary