Climate patterns and variability - Revision history

JeffreyJLukas: /* Topography and elevation */

2026-01-05T15:26:47Z

Topography and elevation

← Older revision		Revision as of 11:26, 5 January 2026
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	[[File:Elev_Temp_Precip maps Figure.png\|thumb\|900px\|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.)]]		[[File:Elev_Temp_Precip maps Figure.png\|thumb\|900px\|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.		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 greater influence of the subtropical high pressure belt that tends to deflect storm systems, especially in the cooler months.

	==Dynamics and seasonality of precipitation==		==Dynamics and seasonality of precipitation==

JeffreyJLukas: /* Topography and elevation */

2026-01-05T15:22:39Z

Topography and elevation

← Older revision		Revision as of 11:22, 5 January 2026
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	==Topography and elevation==		==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).

	[[File:Elev_Temp_Precip maps Figure.png\|thumb\|900px\|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.)]]		[[File:Elev_Temp_Precip maps Figure.png\|thumb\|900px\|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.)]]

	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.		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.

JeffreyJLukas: /* Additional resources */

2024-10-31T20:34:31Z

Additional resources

← Older revision		Revision as of 16:34, 31 October 2024
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	Chapters 4 (Present Weather and Climate: Average Conditions) and 5 (Present Weather and Climate: Evolving Conditions) of the 2013 [https://www.swcarr.arizona.edu/ 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).		Chapters 4 (Present Weather and Climate: Average Conditions) and 5 (Present Weather and Climate: Evolving Conditions) of the 2013 [https://www.swcarr.arizona.edu/ 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).

			===[https://www.zotero.org/groups/4274378/colorado_river_science_wiki/collections/P8TWWJX9/items/Q6GTT4E5/item-list Wiki Library: Climate patterns and variability]===

			The Wiki library lists over 30 research publications on climate patterns and variability relevant to the Colorado River Basin.

JeffreyJLukas: /* Climate variability over time */

2024-10-28T17:53:34Z

Climate variability over time

← Older revision		Revision as of 13:53, 28 October 2024
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	[[File:LCRB_WY_Precip_1900-2024.png\|thumb\|700px\|Figure 4. Lower Colorado River Basin water-year precipitation, 1900-2024 (green dots and line), and a smoothing filter that emphasizes multi-year variability (dark red line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]		[[File:LCRB_WY_Precip_1900-2024.png\|thumb\|700px\|Figure 4. Lower Colorado River Basin water-year precipitation, 1900-2024 (green dots and line), and a smoothing filter that emphasizes multi-year variability (dark red line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]

	The previous sections describe the average tendencies of climate over the historical record. But as all water users 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 can be partly attributed with identified modes of global climate variability, principally El Nino-Southern Oscillation (ENSO). To the extent that [[https://coloradoriverscience.org/Weather_and_climate_forecasts#Seasonal_climate_forecasts seasonal climate forecasts]] have predictive skill in different parts of the basin, this skill mainly stems from ENSO events and also 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 basin precipitation from year to year cannot currently be explained by forecast models or other diagnostic tools.		The previous sections describe the average tendencies of climate over the historical record. But as all water users 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 can be partly attributed with identified modes of global climate variability, principally El Nino-Southern Oscillation (ENSO). To the extent that [https://coloradoriverscience.org/Weather_and_climate_forecasts#Seasonal_climate_forecasts seasonal climate forecasts] have predictive skill in different parts of the basin, this skill mainly stems from ENSO events and also 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 basin precipitation from year to year cannot currently be explained by forecast models or other diagnostic tools.

	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 of 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. Often the North American Monsoon was much weaker as well in the driest years, disproportionately affecting the Lower Basin.		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 of 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. Often the North American Monsoon was much weaker as well in the driest years, disproportionately affecting the Lower Basin.

JeffreyJLukas: /* Climate variability over time */

2024-10-28T17:53:17Z

Climate variability over time

← Older revision		Revision as of 13:53, 28 October 2024
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	[[File:LCRB_WY_Precip_1900-2024.png\|thumb\|700px\|Figure 4. Lower Colorado River Basin water-year precipitation, 1900-2024 (green dots and line), and a smoothing filter that emphasizes multi-year variability (dark red line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]		[[File:LCRB_WY_Precip_1900-2024.png\|thumb\|700px\|Figure 4. Lower Colorado River Basin water-year precipitation, 1900-2024 (green dots and line), and a smoothing filter that emphasizes multi-year variability (dark red line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]

	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~~.		The previous sections describe the average tendencies of climate over the historical record. But as all water users 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 can be partly attributed with identified modes of global climate variability, principally El Nino-Southern Oscillation (ENSO). To the extent that [[https://coloradoriverscience.org/Weather_and_climate_forecasts#Seasonal_climate_forecasts seasonal climate forecasts]] have predictive skill in different parts of the basin, this skill mainly stems from ENSO events and also 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 basin precipitation from year to year cannot currently be explained by forecast models or other diagnostic tools.

	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.		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 of 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. Often the North American Monsoon was much weaker as well in the driest years, 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.		The basin’s precipitation also varies on decadal time scales. These multi-year and longer excursions towards wet and dry can lead the large reservoirs on the mainstem Colorado River to fill and then get drawn down to low levels. 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 high natural decadal variability makes it difficult to discern long-term trends, such as those that might be caused by anthropogenic climate change.

	[[File:UCRB_WY_Tave_1900-2024.png\|thumb\|700px\|Figure 5. Annually averaged temperature over the Upper Colorado River Basin, 1900-2024 (orange dots and line), and a LOESS curve that emphasizes multi-decadal variability (green line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]		[[File:UCRB_WY_Tave_1900-2024.png\|thumb\|700px\|Figure 5. Annually averaged temperature over the Upper Colorado River Basin, 1900-2024 (orange dots and line), and a LOESS curve that emphasizes multi-decadal variability (green line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]

JeffreyJLukas: /* Climate variability over time */

2024-10-25T17:04:37Z

Climate variability over time

← Older revision		Revision as of 13:04, 25 October 2024
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	[[File:UCRB_WY_Tave_1900-2024.png\|thumb\|700px\|Figure 5. Annually averaged temperature over the Upper Colorado River Basin, 1900-2024 (orange dots and line), and a LOESS curve that emphasizes multi-decadal variability (green line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]		[[File:UCRB_WY_Tave_1900-2024.png\|thumb\|700px\|Figure 5. Annually averaged temperature over the Upper Colorado River Basin, 1900-2024 (orange dots and line), and a LOESS curve that emphasizes multi-decadal variability (green line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]

	[[File:~~UCRB_WY_Tave_1900~~-2024.png\|thumb\|700px\|Figure 6. Annually averaged temperature over the Lower Colorado River Basin, 1900-2024 (orange dots and line), and a LOESS curve that emphasizes multi-decadal variability (green line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]		[[File:LCRB_WY_Tave_1900-2024.png\|thumb\|700px\|Figure 6. Annually averaged temperature over the Lower Colorado River Basin, 1900-2024 (orange dots and line), and a LOESS curve that emphasizes multi-decadal variability (green line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]

	Temperatures in the Upper and Lower basins also vary from year to year (Figures 5 and 6). The effects of temperature on water resources and ecosystems are more subtle than the effects of precipitation, but still pervasive. Temperature and precipitation are physically and statistically related; drier years tend to be warmer than average, while wetter years tend to be cooler than average. 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 entire 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]].		Temperatures in the Upper and Lower basins also vary from year to year (Figures 5 and 6). The effects of temperature on water resources and ecosystems are more subtle than the effects of precipitation, but still pervasive. Temperature and precipitation are physically and statistically related; drier years tend to be warmer than average, while wetter years tend to be cooler than average. 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 entire 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]].

JeffreyJLukas: /* Climate variability over time */

2024-10-25T17:03:39Z

Climate variability over time

← Older revision		Revision as of 13:03, 25 October 2024
Line 26:		Line 26:

	==Climate variability over time==		==Climate variability over time==
			[[File:UCRB_WY_Precip_1900-2024.png\|thumb\|700px\|Figure 3. Upper Colorado River Basin water-year precipitation, 1900-2024 (green dots and line), and a smoothing filter that emphasizes multi-year variability (dark red line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]

			[[File:LCRB_WY_Precip_1900-2024.png\|thumb\|700px\|Figure 4. Lower Colorado River Basin water-year precipitation, 1900-2024 (green dots and line), and a smoothing filter that emphasizes multi-year variability (dark red line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]

	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.		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.		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.

	[[File:UCRB_WY_Precip_1900-2024.png\|thumb\|700px\|Figure 3. Upper Colorado River Basin water-year precipitation, 1900-2024 (green dots and line), and a smoothing filter that emphasizes multi-year variability (dark red line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]

	[[File:LCRB_WY_Precip_1900-2024.png\|thumb\|700px\|Figure 4. Lower Colorado River Basin water-year precipitation, 1900-2024 (green dots and line), and a smoothing filter that emphasizes multi-year variability (dark red line). (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 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.		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.

	Temperatures in the Upper and Lower basins also vary from year to year (Figures 5 and 6). The effects of temperature on water resources and ecosystems are more subtle than the effects of precipitation, but still pervasive. Temperature and precipitation are physically and statistically related; drier years tend to be warmer than average, while wetter years tend to be cooler than average. 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 entire 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]].

	[[File:UCRB_WY_Tave_1900-2024.png\|thumb\|700px\|Figure 5. Annually averaged temperature over the Upper Colorado River Basin, 1900-2024 (orange dots and line), and a LOESS curve that emphasizes multi-decadal variability (green line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]		[[File:UCRB_WY_Tave_1900-2024.png\|thumb\|700px\|Figure 5. Annually averaged temperature over the Upper Colorado River Basin, 1900-2024 (orange dots and line), and a LOESS curve that emphasizes multi-decadal variability (green line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]

	[[File:UCRB_WY_Tave_1900-2024.png\|thumb\|700px\|Figure 6. Annually averaged temperature over the Lower Colorado River Basin, 1900-2024 (orange dots and line), and a LOESS curve that emphasizes multi-decadal variability (green line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]		[[File:UCRB_WY_Tave_1900-2024.png\|thumb\|700px\|Figure 6. Annually averaged temperature over the Lower Colorado River Basin, 1900-2024 (orange dots and line), and a LOESS curve that emphasizes multi-decadal variability (green line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]

			Temperatures in the Upper and Lower basins also vary from year to year (Figures 5 and 6). The effects of temperature on water resources and ecosystems are more subtle than the effects of precipitation, but still pervasive. Temperature and precipitation are physically and statistically related; drier years tend to be warmer than average, while wetter years tend to be cooler than average. 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 entire 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==		==Data and tools==

JeffreyJLukas: /* Climate variability over time */

2024-10-25T17:02:40Z

Climate variability over time

← Older revision		Revision as of 13:02, 25 October 2024
Line 31:		Line 31:
	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.		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.

	[[File:~~UCRB_precip_WY~~.png\|thumb\|700px\|Figure 3. Upper Colorado River Basin water-year precipitation, 1900-~~2020~~ (green dots and line), ~~with~~ smoothing filter that emphasizes multi-year variability (dark red line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]		[[File:UCRB_WY_Precip_1900-2024.png\|thumb\|700px\|Figure 3. Upper Colorado River Basin water-year precipitation, 1900-2024 (green dots and line), and a smoothing filter that emphasizes multi-year variability (dark red line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]

	[[File:~~LCRB_precip_WY~~.png\|thumb\|700px\|Figure 4. Lower Colorado River Basin water-year precipitation, 1900-~~2020~~ (green dots and line), ~~with~~ smoothing filter that emphasizes multi-year variability (dark red line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]		[[File:LCRB_WY_Precip_1900-2024.png\|thumb\|700px\|Figure 4. Lower Colorado River Basin water-year precipitation, 1900-2024 (green dots and line), and a smoothing filter that emphasizes multi-year variability (dark red line). (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 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~~.		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.

	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~~.		Temperatures in the Upper and Lower basins also vary from year to year (Figures 5 and 6). The effects of temperature on water resources and ecosystems are more subtle than the effects of precipitation, but still pervasive. Temperature and precipitation are physically and statistically related; drier years tend to be warmer than average, while wetter years tend to be cooler than average. 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 entire 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]].

	[[File:~~CRB_ann_temp_1895~~-~~2020~~.png\|thumb\|700px\|Figure 1. Annually averaged temperature over the Colorado River Basin, ~~1895~~-~~2020~~, ~~shown with the blue~~ and ~~red bars as anomalies from~~ a ~~late 20th~~- ~~century~~ (~~1971-2000~~) ~~baseline. The gray line is a 10-year running average plotted on the 6th year. The dashed yellow line is the linear trend from 1980-2020, showing 2.1°F of warming over that period~~. (~~Design: Jeff Lukas, updated from Lukas and Payton 2020, based on gridded climate data from~~ NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/.)]]		[[File:UCRB_WY_Tave_1900-2024.png\|thumb\|700px\|Figure 5. Annually averaged temperature over the Upper Colorado River Basin, 1900-2024 (orange dots and line), and a LOESS curve that emphasizes multi-decadal variability (green line). (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&deg~~;~~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~~]].		[[File:UCRB_WY_Tave_1900-2024.png\|thumb\|700px\|Figure 6. Annually averaged temperature over the Lower Colorado River Basin, 1900-2024 (orange dots and line), and a LOESS curve that emphasizes multi-decadal variability (green line). (NOAA NCEI Climate-at-a-Glance; https://www.ncdc.noaa.gov/cag/)]]

	==Data and tools==		==Data and tools==

JeffreyJLukas: /* Additional resources */

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Additional resources

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	==Additional resources==		==Additional resources==

	===State of the Science ~~Report~~===		===[https://wwa.colorado.edu/resources/colorado-river-resources/CRBreport Colorado River Basin Climate and Hydrology: State of the Science report]===

	[https://wwa.colorado.edu/publications/reports/CRBreport/ColoRiver_StateOfScience_WWA_2020_Chapter_2.pdf 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.		[https://wwa.colorado.edu/publications/reports/CRBreport/ColoRiver_StateOfScience_WWA_2020_Chapter_2.pdf Chapter 2] of the 2020 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===		===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 [https://www.swcarr.arizona.edu/ 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).		Chapters 4 (Present Weather and Climate: Average Conditions) and 5 (Present Weather and Climate: Evolving Conditions) of the 2013 [https://www.swcarr.arizona.edu/ 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).

JeffreyJLukas at 20:15, 18 October 2023

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	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.		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.

	~~==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 ecological) adaptations to buffer those swings, for example with reservoir storage.		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.