Why has this winter been so warm?

While spring is about to come, this winter's abnormal fluctuations in temperatures are hard to ignore. 

This past Thursday, the high in Baltimore was 79 degrees — the warmest February 23rd in the region since records have been kept. In addition, the light snowfall on February 1 was the third-latest “first snow” recorded in the region. 

The El Niño-Southern Oscillation (ENSO), which refers to cyclical environmental conditions that occur across the Pacific Ocean near the equator, is the system that includes El Niño and La Niña. El Niño and La Niña are opposite extremes of the ENSO and exert a significant influence on winter climates across North America. 

La Niña is a naturally occurring climate phenomenon that results from the cooling of sea surface temperatures in the equatorial Pacific Ocean. This cooling can have far-reaching effects on weather patterns around the world, and the U.S. is no exception. In the winter of 2023, La Niña was expected to have a significant impact on the weather in the United States.

When surface water in the equatorial Pacific becomes warmer than average and east winds blow weaker than normal, the Pacific jet stream spreads further to the southeast, leading to wetter conditions in the southern part of the U.S. and warmer, dryer weather in the northern part; this is the phenomenon known as El Niño. 

On the contrary, La Niña is characterized by colder-than-average water temperatures in the equatorial Pacific, causing colder and stormier weather in the northern part of U.S. and warmer conditions across the South.

In an interview with The News-Letter, Ali Hasan Siddiqui, a PhD student in the Department of Earth and Planetary Sciences at Hopkins, discussed the causes and effects of El Niño and La Niña to this year’s warmer-than-average winter. 

“We normally experience the El Niño and La Niña effects because of the movement of large amounts of ocean water. The warmer western jet is, the larger La Niña phenomenon there is,“ he said. “These phenomena affect winter seasons specifically, since both have an onset during the winter and before the winter time.”

Winters in the southern U.S. and along the East Coast tend to be warmer during La Niña years due to global upper-atmospheric wind patterns. The jet stream, an upper-level wind where the warmer air to the south and the colder air to the north separate, is typically located north of Baltimore. It also provides a passage for approaching storm systems.

Siddiqui noted that what was special about this year’s La Niña was the fact that it was the third consecutive year of La Niña-affected winters, known as a “Triple Dip” La Niña pattern, which leads to the increase in magnitude and potential long-lasting and causal effects on the overall climate system. 

One of the most notable effects of La Niña in the U.S. is the increased likelihood of colder and wetter conditions in the Pacific Northwest and the Northern Great Plains, since La Niña tends to shift the jet stream farther south, bringing cold Arctic air down into these regions. As a result, these areas are likely to experience more snow and ice than usual, which can lead to transportation disruptions and other challenges for residents.

On the other hand, La Niña tends to promote drier, warmer conditions in the southern United States. This can be particularly problematic in areas that are already experiencing drought, as La Niña can exacerbate these conditions. In addition, warmer temperatures can increase the risk of wildfires, particularly in the West, where there is already a high risk of fires due to dry conditions.

La Niña can also have an impact on severe weather events in the United States. Because La Niña tends to bring warmer and moister air into the region, which can create unstable atmospheric conditions conducive to tornado formation, it increases their likelihood in the Southeast, particularly in the early part of the year. 

Based on data tracked by the World Meteorological Organization, advancements in weather modeling enable scientists to foresee ENSO and related phenomena one to nine months in advance with great accuracy, giving us the ability to better prepare for associated hazards — including heavy precipitation, floods and droughts.

Siddiqui reflected on how different levels of weather-monitoring and forecasting systems could play a crucial role in developing effective strategies for extreme weather precaution. 

“Our weather broadcast is pretty accurate for the most part for the last two decades at both local and national levels; there is more to expect in the future,” he said.