Atmosphere Rivers describe the atypical years of El Niño and La Niña

El Niño and La Niña are climatic phenomena associated with wet, dry winter conditions in the southwestern United States, respectively. However, in 2023, La Niña year was found to be very wet in the southwest rather than dry.

A new study from scientists at the Oceanography Institute in San Diego, California found that atmospheric rivers account for most of the atypical El Niño Southern Oscillation (ENSO) such as 2023. It became the 10th wettest year in the state on record.

The study, published in the journal Climate Dynamics, shows that atmospheric rivers can overwhelm the effects of El Niño and La Niña on the total annual rainfall in the US West. This has important implications for water managers to rely on seasonal forecasts based on El Niño and La Niña to inform key planning decisions regarding reservoirs and water allocation.

Despite the widespread impacts on the global climate of El Niño and La Niña, the rivers in the atmosphere do not seem to follow their lead. “Atmosphere Rivers don’t dance to ENSO songs,” said Alexander Gershunov, a climate scientist at Scripps and co-author of the study.

While dancing towards the beat of your own drums, the Atmosphere River is key to California’s water supply, offering an average of 65% of annual rainfall in northern California and 40% in Southern California. However, their contributions vary widely from year to year. For example, in Southern California, the Atmospheric River reached 5% in 1977, and 71% in 1956.

“Atmosphere rivers are a wildcard for precipitation in the western United States,” said Rosa Lunanino, a postdoctoral scholar at Scripps and a leading author of the study.

“One or two atmospheric rivers can change into rainy years, but weak atmospheric river seasons can change into dry years, which is to make accurate water years predictions. It means we cannot trust El Niño and La Niña completely.”

Scientists expect these rivers in the sky to become an increasingly important source of annual rainfall in the western US under climate change, and could further distant El Niño and La Niña years from the typical patterns .

NOAA declares El Niño when the waters of the Central and Eastern Pacific Oceans near the equator are unusually warm for three months. La Niña is the opposite and is identified when it is colder than the average water temperature in the eastern equatorial Pacific.

The temperature of this patch in the tropical Pacific is closely monitored as it has a wide range of effects on atmospheric circulation and global climate.

El Niño and La Niña usually last for 9 to 12 months, but can also grow over multiple years. The two phenomena can be detected several months before the effect is felt, which is useful for long distance prediction.

Atmospheric rivers are ribbons of water vapor in the sky, which can result in large amounts of precipitation when they reach the land. Brewing atmospheric river landings can be predicted up to three weeks in advance (westerly weather and water extreme centres, or CW3E scientists are working to improve these predictions), but atmospheric river seasons The frequency is almost impossible to predict.

After 2023, despite the state of La Niña in the Pacific, the research authors learned whether there were other years that were anticipated but against grain, despite the state of La Niña in the Pacific. I wanted to. Furthermore, they wanted to investigate whether the activity of atmospheric rivers was higher or lower in those unusual years.

The team analyzed weather data over 70 years and compared predicted rainfall patterns based on ENSO with precipitation records. Researchers separated the contributions of the Atmospheric River into two categories: precipitation from the Atmospheric River and precipitation from other sources.

Team analysis revealed that about 32% of the ENSO years analyzed were what was called “heretic,” contrary to the standard pattern of precipitation expected from El Niño and La Niña. Of these heresies, unusually high or low atmospheric river activity explained about 70% of them.

The results suggest that atmospheric rivers can negate traditional El Niño/La Niña predictions. During these unusual years, only a few powerful atmospheric rivers can turn the expected dry La Niña year into wet years (1967, 2011, 2017, 2023), or their absence is predicted It could turn wet El Niño years into a dry spell (1964, 1977, 1987, 2007, 2013, 2015).

Findings suggest that water managers in the western United States cannot rely too much on El Niño or La Niña predictions for seasonal planning. Recently, the CW3E of Scripps was a major factor in seasonal precipitation in the western US, and to clarify that these predictions explain precipitation that is not related to atmospheric rivers, the CW3E of the Scripps was a seasonal forecast. I started with a disclaimer that includes one.

If climate change contributes more dominantly to atmospheric rivers to future annual rainfall patterns, research suggests, ENSO could be much more useful in predicting seasons.

In the meantime, Lunanino said she and her co-authors are trying to fuse seasonal predictions based on ENSO with short-term subseason predictions that predict atmospheric rivers.

“We need to continue to improve our ability to predict river landfall in the atmosphere, and the better we do that, the more we use that information to interpret seasonal predictions and vice versa.” Gershunov said.

Lunaniño and Gershnov from Scripps, F. In addition to Martin Ralph, Alexander Weyant, Kristen Gilgiss, Michael DeFlorio and Daniel Kayan of Scripps, Park Williams of UCLA co-authored the research.