Snowboarders and skiers often talk about whether the coming winter will be an El Niño or a La Niña year. In this article, I play around with D3 visualizations to understand for myself the forces that drive snowfall in different regions. I look at Mt. Annupuri in Niseko, Hokkaido, and Mt. Bachelor in Oregon to compare and contrast the two.
The cold temperatures in Siberia, relative to the warmer temperatures of the Sea of Japan, result in a wind blowing across the area. The wind picks up moisture and drops it as snow when it hits the mountain ranges on the west coast of Japan. The flow is also known to skiers as the Siberian Express, or more formally as the East Asian winter monsoon, and it happens every winter. Higher latitudes, with colder winters, get the powdery snow that resorts like Niseko are known for, while lower latitudes, with milder winters, get wetter snow. Over the years, the amount of snow varies with how strong the wind is, which depends on how extreme the temperature difference is between the cold air in Siberia and the warmer Sea of Japan.
While the Siberian Express is the winter monsoon, snowfall on the US Pacific coast depends on two sets of weather patterns, the jet stream and the trade winds. The jet stream occurs at higher latitudes and higher up in the atmosphere, traveling from west to east, while the trade winds occur nearer the equator and nearer the surface, traveling from east to west.
The jet stream sits where air south of the polar regions is warmer than the air to the north. That temperature difference pushes air from the warm side toward the cold side, from south to north. As Earth rotates, this becomes a west-to-east wind. It picks up moisture over the Pacific, and that moisture falls as snow when it hits the mountain ridges along the Pacific coast.
The trade winds begin where hot air over the tropical Pacific rises. Surface air flows in to replace it, from north to south, and as Earth rotates, that inflow blows from east to west.
As the trade winds blow, they pull the warmer surface water with them. In years with strong trade winds, that water is pulled all the way to the western Pacific. In years with weaker trade winds, it only reaches the central Pacific.
Warmer water in the central Pacific is what makes an El Niño year. Air rises over that warm water, and the disturbance travels north and piles up over the Gulf of Alaska. That pile is a high-pressure zone, and it pushes the usual snow jet south toward California instead of the Cascades. In a La Niña the warm water is farther west, so the air rises farther west and that high does not form over the Gulf of Alaska.
With a strong to very strong El Niño likely this winter, I wanted to investigate how much impact this could be in terms of snowfall at various popular ski destinations along the Pacific coast.
In particular, I looked at the last 8 La Niña and El Niño winters respectively to understand the snowfall range that we might be able to expect this winter. There are many other factors. One highlighted in the charts is the strength of the La Niña / El Niño impact, which is rated based on how much cooler or warmer the ocean temperatures are from what is considered normal.
(Disclaimer: This piece was written from someone trying to understand the climate patterns and practicing some D3 visualization, and not as an expert in climate, so please take the insights drawn with a grain of salt. Comments on what else to look at and consider are welcome!)
Directionally, all 4 resorts support increased snowfall for the Cascades in La Niña years and increased snowfall in California for El Niño years when we compare the median values. Yet, the ranges for La Niña and El Niño at the respective mountains do overlap quite a bit, suggesting that labeling a year as La Niña or El Niño alone is insufficient to predict how good or bad the snow season would be. We also observe outliers like in the 22–23 season, where a weak La Niña drove a significantly higher amount of snow at Mammoth and Snow Summit than the other La Niña years there. In the 2010–11 moderate La Niña season, Mammoth experienced an almost equally high amount of snowfall as in the 2022–23 weak La Niña season.