How mountains impact El Niño winters and snowfall amounts

In the quest for better water conservation planning along the Colorado River, researchers have turned their focus to the role of mountains in shaping the impact of El Niño and La Niña on precipitation in western North America.

This new study emerges amid a transition from a strong La Niña to a potent El Niño, aiming to enhance the precision of winter precipitation predictions in the intermountain West.

Studying mountains and El Niño

The research team, led by James Stagge, an assistant professor at The Ohio State University, delved into 150 years of rain and snow data and its correlation with historic El Niño-Southern Oscillation (ENSO) patterns.

Their findings reveal a trend of increasing winter precipitation in the north and a decrease in the south, particularly during the late 20th century. The study highlights how mountains amplify or obstruct precipitation, causing heavier rainfall on their western flanks and reduced precipitation eastward.

Stagge emphasizes the significance of accurate winter precipitation forecasts for managing water resources in one of the world’s most water-stressed regions.

He points out, “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.” Improved predictions enable better preparation for droughts and conservation measures.

El Niño Southern Oscillation (ENSO)

ENSO, encompassing El Niño and La Niña, relates to temperature anomalies in the Pacific Ocean and significantly influences global weather patterns. In this study, the focus was on the intermountain West, a region historically overlooked in ENSO studies. Researchers used water gauge readings from 1871 and linked them to ENSO trends recorded by the NOAA’s Multivariate ENSO Index.

Stagge’s team prioritized observational data over climate models, seeking a closer alignment with reality. They scrutinized the specifics of precipitation changes during El Niño and La Niña years at different elevations and locations.

This approach revealed nuanced historic patterns, especially in the northern intermountain region, where elevation variations complicate the tracking of ENSO’s effects on winter precipitation.

Orographic effect and regional variations

The study underscores the orographic effect, where moist Pacific air ascends over mountains, releasing precipitation, and leaving drier air on the eastern side. This phenomenon, combined with ENSO, results in significant precipitation disparities. The research also noted a dipole effect, with opposing precipitation responses in the north and south during ENSO phases.

The findings suggest that ENSO forecasts are more indicative in the north, while quantitative temperature change estimates might be more beneficial in the south. This disparity is attributed to the varying topography and its influence on air flow and precipitation.

In summary, Stagge’s study offers a valuable tool for water managers and stakeholders in western North America. By enhancing the understanding and prediction of precipitation patterns, it paves the way for more effective water shortage preparation and conservation strategies.

The integration of this research with NOAA’s data and modeling tools could further refine short-term forecasts, offering a boon to the region’s water-dependent economy and large cities.

The full study was published in the journal Nature Water.

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