How mountains impact El Niño winters and snowfall amounts

In their pursuit of improved water conservation strategies along the Colorado River, scientists are now examining the influence of mountainous regions on how El Niño and La Niña events affect precipitation in the western part of North America.

This new research focuses on refining the accuracy of winter rainfall forecasts in the Intermountain West region, amidst the shift from a pronounced La Niña to a strong El Niño.

New research indicates that understanding the impact of mountainous terrain on the variations in precipitation caused by El Niño and La Niña could be key to enhancing water conservation efforts along the Colorado River.

Mountainous terrain affects Colorado River

The study, which aligns with the recent transition from a significant La Niña to a powerful El Niño, aims to bring a new level of precision to the prediction of winter precipitation in the Intermountain West by analyzing 150 years of historical rain and snowfall data alongside past El Niño-Southern Oscillation patterns.

The findings reveal a pattern of increasing winter precipitation in the northern regions and a decline in the southern areas, especially during the latter half of the 20th century. The research also highlights the role of mountains in intensifying and blocking precipitation, resulting in higher rainfall west of the mountains and reduced precipitation to the east.

By providing a more accurate assessment of the historical influence of El Niños on winter precipitation, the study offers valuable insights for the future management of water resources in western North America, which is among the regions most challenged by water scarcity.

"Because of the seasonality of precipitation in the West, most of it falls during the winter. If you can predict how much precipitation you'll have in the winter, you'll have a good sense of what your summer dry period will look like in terms of your water allocation," said James Stagge, lead author of the study and an assistant professor of civil, environmental and geodetic engineering at The Ohio State University.

"We can do anything to improve our ability to predict how much water we will get during this critical period. This allows cities, farmers, water managers, and member states of the Colorado River Compact to prepare for upcoming drought and potentially start conserving water ahead of time, so they won't get caught flat-footed."

El Niño, La Niña impact weather

The two phases of the El Niño Southern Oscillation (ENSO), El Niño and La Niña, characterize by warmer (El Niño) or cooler (La Niña) than average sea surface temperatures in the central and eastern tropical Pacific Ocean. These temperature anomalies cause far-reaching effects on global weather patterns, leading to significant fluctuations in temperature and precipitation, including extreme increases or decreases.

In their research, Stagge and his team chose to focus on the Intermountain West, a region that ENSO-related studies have historically overlooked, to explore how El Niño and La Niña influence winter precipitation. The team used water gauge readings dating back to 1871 to correlate actual precipitation amounts with not just the location but also the elevation of the area.

The National Oceanic and Atmospheric Administration (NOAA), which maintains the Multivariate ENSO Index, then compared these historical precipitation measurements with ENSO trends recorded by the index. The index provides both current and historical data on ENSO events, enabling a detailed analysis of their impact on regional precipitation.

"Rather than using climate models, we're using only observations, which allow us to be a little bit closer to reality," Stagge said. "We didn't use averages -- we showed more precise information about where precipitation fell between the designations of El Niño and La Niña. We put each gauge in its specific location, assigned it an elevation, and looked at how it changed depending on whether it was an El Niño or La Niña year: Was it wetter or drier than normal?"

Elevation complicates tracking weather patterns

This method revealed more intricate historical patterns, particularly in the northern part of the Intermountain West, where varying elevations have complicated the tracking of ENSO’s influence on winter precipitation.

The research indicates that in this region, mountainous terrain is likely to enhance the increase in precipitation associated with El Niño by a factor of 2 to 6 times. However, this increase is predominantly observed on the western slopes of mountains due to the orographic effect. As moist air from the Pacific travels from west to east, it ascends the mountains into cooler air, releasing precipitation and leaving the air dry as it descends on the eastern side.

Stagge explained, “All the rain falls on the west side, and by the time the air reaches the east side of the mountains, there’s no moisture left to precipitate. The ENSO effect acts as a multiplier, making the wet side significantly wetter during El Niño in the south, and much drier during La Niña.”

The study also confirmed the dipole effect of ENSO on regional precipitation, with contrasting impacts in the north and south: Winter precipitation tends to rise in northern Utah and Wyoming during La Niña, while New Mexico and Arizona experience wetter winters during El Niño.

South reacts linearly, north binary

However, the analysis revealed that the two regions react differently to ENSO. In the southern areas, each increment of El Niño temperature difference corresponds to a proportional change in precipitation. In contrast, in the northern regions, precipitation changes do not scale linearly with ENSO intensity but rather behave like a binary switch—either occurring or not.

Stagge noted that the complexity of the terrain might be a factor in the observed differences. In the southern regions, the Sierra Nevada mountains do not obstruct airflow as they do for Utah and Wyoming.

This insight is significant for water managers who need to anticipate winter conditions. In the northern areas, forecasts that focus on the intensity of El Niño or La Niña events are more predictive, whereas in the south, precise estimates of temperature changes would be more beneficial.

Stagge is looking forward to collaborating with NOAA to integrate data and modeling tools to enhance short-term forecasts.

“Water is a critical resource in western North America, underpinning the economy and supporting large urban populations,” he stated. “A deeper understanding and, in some cases, the ability to predict precipitation in this region, would greatly improve our preparedness for potential water shortages.”

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