Pacific Ocean Warming Can Predict East Coast Heat Waves

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Not all weather disasters are photogenic storms sweeping across the Plains. Heat waves regularly go down as some of the deadliest and costliest weather disasters around the world. One strong heat wave in a populated area could claim hundreds if not thousands of lives, not to mention the financial toll of cooling millions of buildings and acre upon acre of crops withering under the hot summer sun.

Meteorologists understandably have a strong incentive to predict heat waves as early as possible to give people time to prepare, but current technology limits our ability to see potential heat waves more than a couple of weeks ahead of time. A study published today in Nature Geoscience claims there's one reliable tool: abnormal sea surface temperatures in the Pacific Ocean. The researchers say that what they term Pacific Extreme Patterns (PEPs) can be used to predict heat waves on the east coast of the United States. The East Coast is an important region when it comes to predicting extreme heat, as it’s home to both large cities and dense agriculture.

A team of scientists from the National Center for Atmospheric Research, the University of Washington, Penn State, and Harvard University studied dozens of the hottest days recorded on the U.S. East Coast between 1982 and 2015 to determine if their existence was influenced by unusually warm sea surface temperatures in the Pacific Ocean between North America and Asia. They found that sea surface temperatures in this region of the Pacific could serve as a predictor of a streak of hot days on the east coast up to 50 days in advance, with predictive accuracy getting better closer to the heat wave.

"The Pacific Extreme Pattern appears to provide a cohesive framework for improving seasonal prediction of summer precipitation deficits and high temperature anomalies in the eastern U.S.," the researchers write.

As a test case, the researchers looked at the extreme heat waves of 2012 to demonstrate the predictive nature of abnormally warm Pacific sea surface temperatures, concluding that this metric did a better job of predicting extreme heat events than the National Weather Service did in its seasonal outlooks in the months leading up to that brutal summer.

What's going on here? Unusually warm sea surface temperatures can affect the weather by aiding in the formation of a ridge of high pressure downstream from the warmer water, which can alter weather patterns in such a way that a region of high pressure sets up over the eastern U.S. A center of high pressure fosters sinking air, leading to calm weather and often warmer-than-normal temperatures. These stubborn highs can last a week or longer during the summer, leading to prolonged heat waves.

The phenomenon studied in this paper is an example of a teleconnection, a field of meteorology that links two seemingly unrelated atmospheric events in different parts of the world to explain the existence of a certain weather pattern.

One of the most common teleconnections meteorologists use to spot weather patterns in advance is El Niño and La Niña—formally known as the El Niño-Southern Oscillation (ENSO)—which uses abnormal sea surface temperatures in the equatorial Pacific to explain unusual weather events around the world. Another is the Arctic Oscillation (AO), which can be used to predict cold snaps and periods of stormy weather in North America. A positive AO usually leads to calmer weather, while a negative AO can lead to a wavy jet stream, allowing blasts of cold to flow south from the Arctic and big snowstorms to spin to life.