El Niño advisory takes effect: What it means for Michigan weather

An El Niño Advisory was issued on Thursday, which means the climate pattern that has taken effect is likely to continue through the spring.

NOAA's Climate Prediction Center says the El Niño is expected to be weak, but it may bring wetter conditions across the southern half of the U.S. in the coming months.

El Niño is a natural ocean-atmospheric phenomenon marked by warmer-than-average sea surface temperatures in the central Pacific Ocean near the equator.

How will it impact Michigan weather?

Typical El Niño patterns during winter and early spring include below-average precipitation and warmer-than-average temperatures along the northern tier of the U.S. and above-normal precipitation and cooler conditions across the South.

Impacts can vary during each El Niño event. The influence of El Niño on U.S. winter climate is a matter of probability, not certainty. The Climate Prediction Center takes El Niño into consideration in their monthly and seasonal outlooks, which describe the likelihood that temperature and precipitation will be well-above or well-below average.

Previous: Metro Detroit winter outlook 2018-2019: Here's what to expect

What happens during El Niño

During El Niño, the surface winds across the entire tropical Pacific are weaker than usual. Ocean temperatures in the central and eastern tropical Pacific Ocean are warmer than average, and rainfall is below average over Indonesia and above average over the central or eastern Pacific.

Rising air motion (which is linked to storms and rainfall) increases over the central or eastern Pacific, and surface pressure there tends to be lower than average.  Meanwhile, an increase in sinking air motion over Indonesia leads to higher surface pressure and dryness.  

What's La El Niña?

During La Niña, it’s the opposite. The surface winds across the entire tropical Pacific are stronger than usual, and most of the tropical Pacific Ocean is cooler than average. Rainfall increases over Indonesia (where waters remain warm) and decreases over the central tropical Pacific (which is cool). Over Indonesia, there is more rising air motion and lower surface pressure. There is more sinking air motion over the cooler waters of the central and eastern Pacific.

How do El Niño and La Niña affect weather patterns?

El Niño and La Niña alternately warm and cool large areas of the tropical Pacific—the world’s largest ocean—which significantly influences where and how much it rains there. The primary location of moist, rising air (over the basin’s warmest water) is centered over the central or eastern Pacific during El Niño and over Indonesia and the western Pacific during La Niña.

This shift disrupts the atmospheric circulation patterns that connect the tropics with the middle latitudes, which in turn modifies the mid-latitude jet streams. By modifying the jet streams, ENSO can affect temperature and precipitation across the United States and other parts of the world. The influence on the U.S. is strongest during the winter (January-March), but it lingers into the early spring.

During El Niño, the southern tier of Alaska and the U.S. Pacific Northwest tend to be warmer than average, whereas the U.S. southern tier of state—from California to the Carolinas—tends to be cooler and wetter than average.  During La Niña, these deviations from the average are approximately (but not exactly) reversed.

What causes El Niño and La Niña to occur?

The winds near the surface in the tropical Pacific usually blow from east to west. For reasons scientists don’t yet fully understand, these relatively steady winds sometimes weaken or strengthen for weeks or months in a row. 

Weak winds allow warm surface waters to build up in the eastern Pacific. Sometimes, but not always, the atmosphere responds to this warming with increased rising air motion and above-average rainfall in the eastern Pacific. This coordinated change in both ocean temperatures and the atmosphere begins an El Niño event. As the event develops, the warmed waters cause the winds to weaken even further, which can cause the waters to warm even more.

El Niño is often (but not always) followed by La Niña the following year, particularly if the El Niño is strong. During La Niña conditions, the easterly trade winds near the equator get even stronger than they usually are. Stronger winds push surface water into the western Pacific. Meanwhile, cool water from deeper in the ocean rises up in the eastern Pacific. If the cooling persists, it can inhibit rising air movement and rainfall in the eastern Pacific, beginning a La Niña event. As the event develops, the cooled waters cause the winds to strengthen even further, which can cause the waters to cool even more.

How long do El Niño and La Niña typically last?

El Niño and La Niña episodes typically last 9-12 months. They both tend to develop during the spring (March-June), reach peak intensity during the late autumn or winter (November-February), and then weaken during the spring or early summer (March-June).

Both El Niño and La Niña can last more than a year, but it is rare for El Niño events to last longer than a year or so, while it is common for La Niña to last for two years or more. The longest El Nino in the modern record lasted 18 months, while the longest la Niña lasted 33 months. Scientists aren’t sure why the duration of the two types of events can be so different.  

Does global warming affect El Niño and La Niña? 

There are many ways in which global warming could affect the frequency and intensity of El Niño / La Niña (see this ENSO blog post, for example), but scientists currently have low confidence in their ability to predict exactly how a warmer world affect the ENSO.  Scientists have high confidence, however, that ENSO itself has been occurring for thousands of years, and will continue into the future.  Global warming is likely to affect the impacts related to El Niño and La Niña, including extreme weather events.

More FAQ here from Climate.gov.

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