El Niño and La NiñaTwo climate mysteries

What causes them?

Why do they occur?

What is El Niño?What is El Niño?

• A disruption of the A disruption of the ocean-atmosphere ocean-atmosphere system in the Tropical system in the Tropical Pacific having Pacific having important important consequences for consequences for weather and climate weather and climate around the globe around the globe

• Typically, the ocean Typically, the ocean surface warms up by a surface warms up by a few degreesfew degrees

• At the same time, the At the same time, the place where huge place where huge thunderstorms occur thunderstorms occur on the equator moves on the equator moves eastward eastward

What is El Niño?What is El Niño?

• Generally affects Generally affects Northern Hemisphere Northern Hemisphere in the winterin the winter

• Originates around Originates around equatorequator

• Historically, it has Historically, it has occurred at irregular occurred at irregular intervals of 2-7 yearsintervals of 2-7 years

The first signs of an El NiñoThe first signs of an El Niño

• A rise in air pressure in the Indian Ocean, A rise in air pressure in the Indian Ocean, Indonesia, and AustraliaIndonesia, and Australia

• A fall in air pressure over Tahiti and the rest of the A fall in air pressure over Tahiti and the rest of the central and eastern Pacific Ocean central and eastern Pacific Ocean

• The trade winds in the South Pacific weaken or The trade winds in the South Pacific weaken or head easthead east

• Warm air rises in PeruWarm air rises in Peru

• Warm water spreads from the West Pacific and the Warm water spreads from the West Pacific and the Indian Ocean to the East Pacific. It takes the rain Indian Ocean to the East Pacific. It takes the rain with it, causing rainfall in normally dry areas and with it, causing rainfall in normally dry areas and drought in normally wet areas. drought in normally wet areas.

Under normal conditions, easterly trade winds move water and air warmed by the sun towards the west

This also creates an upwelling of nutrient-rich cold water along the coasts of Peru and Ecuador

What the heck is a thermocline?What the heck is a thermocline?

• A layer within a body of water or air where the A layer within a body of water or air where the temperature changes quickly with depth temperature changes quickly with depth

• a heating cycle often fueled by the sun in which a a heating cycle often fueled by the sun in which a liquid or gas within a closed system is heated and liquid or gas within a closed system is heated and

risesrises

What about a convective loop?

Under El Niño conditions, warm, nutrient-poor water slumps towards the east because of weakening winds

The nutrient-rich cold water does not get pulled up as much

El Niño’s warm current of nutrient-El Niño’s warm current of nutrient-poor tropical water replaces the poor tropical water replaces the cold, nutrient-rich surface water cold, nutrient-rich surface water where Peru’s fishing industry where Peru’s fishing industry usually is vibrantusually is vibrant

But it doesn’t stop there!But it doesn’t stop there!The warmer ocean then affects the winds, and makes the winds weaker. So if the winds get weaker, then the ocean gets warmer, which makes the winds get weaker.

This is called positive feedback, and it’swhat makes El Niño grow.

But it doesn’t stop there!But it doesn’t stop there!What stops positive feedback are Rossby waves

When El Niño gets going in the middle or eastern part of the Pacific, it creates Rossby waves that drift slowly towards southeast Asia. After several months of traveling, the finally get near the coast and reflect back. The changes in interior ocean temperature that these waves carry with it “cancel out” the original temperature changes that made the El Niño in the first place

The Effects of El Niño are The Effects of El Niño are WorldwideWorldwide

The Effects of El Niño are The Effects of El Niño are WorldwideWorldwide• Wet winters over southeastern United Wet winters over southeastern United

StatesStates

• Drought in Indonesia and Australia, and Drought in Indonesia and Australia, and other places that are typically wetother places that are typically wet

• Flooding in South America in areas Flooding in South America in areas typically drytypically dry

• Wildfires and mudslidesWildfires and mudslides

• Warmer winters in the upper Midwest Warmer winters in the upper Midwest states and Canadastates and Canada

El Niño historicallyEl Niño historically• ENSO conditions seem ENSO conditions seem

to have occurred at to have occurred at every two to seven every two to seven years for at least the years for at least the past 300 years, but past 300 years, but most of them have most of them have been weak.been weak.

• Archaeological Archaeological evidence in Peru that evidence in Peru that suggests it has suggests it has occurred for occurred for thousands of yearsthousands of years

• A recent study of A recent study of El NiñoEl Niño patterns suggests that patterns suggests that the French Revolution the French Revolution was caused in part by the was caused in part by the poor crop yields of 1788-poor crop yields of 1788-89 in Europe, resulting 89 in Europe, resulting from an unusually strong from an unusually strong El-Niño effect between El-Niño effect between 1789-931789-93

• The El Niño of 1997 - The El Niño of 1997 - 1998 was particularly 1998 was particularly strong and brought the strong and brought the phenomenon to phenomenon to worldwide attentionworldwide attention

How well can we predict El How well can we predict El Niño?Niño?

• Satellite readings from buoys Satellite readings from buoys strategically set in the Pacific Oceanstrategically set in the Pacific Ocean

• Complex computer modelsComplex computer models

El Niño vs. La NiñaEl Niño vs. La Niña

El NiñoEl Niño• Unusually warm ocean Unusually warm ocean

temperatures in temperatures in eastern Pacificeastern Pacific

• Usually more intense Usually more intense effects than La Niñaeffects than La Niña

• Summers in Omaha Summers in Omaha tend to be wetter and tend to be wetter and coolercooler

La NiñaLa Niña• Unusually cold ocean Unusually cold ocean

temperatures in temperatures in eastern Pacificeastern Pacific

• Usually follows El Usually follows El Niño, but not alwaysNiño, but not always

• Summers in Omaha Summers in Omaha tend to be drier and tend to be drier and warmerwarmer

El Niño vs. La NiñaEl Niño vs. La Niña

El NiñoEl Niño• Causes drier weather Causes drier weather

on West Pacific on West Pacific seaboards, and wetter, seaboards, and wetter, humid weather in the humid weather in the East PacificEast Pacific

• Warmer winters in the Warmer winters in the upper Midwest states upper Midwest states and Canadaand Canada

• Wet winters over Wet winters over southeastern United southeastern United StatesStates

La NiñaLa Niña• Causes wetter weather Causes wetter weather

on West Pacific on West Pacific seaboards and drier seaboards and drier weather in East Pacificweather in East Pacific

• Colder winters in the Colder winters in the upper Midwest states upper Midwest states and Canadaand Canada

• Dry winters over Dry winters over southeastern United southeastern United StatesStates

La Niña to blame?La Niña to blame?Recent droughts occurred during latest Recent droughts occurred during latest La NiñaLa Niña

Under La Niña conditions, unusually cold water pushes up in the east Pacific

Often follows El Niño

What the heck causes El What the heck causes El Niño and La Niña?Niño and La Niña?

Major Major theoriestheories

• Recharge oscillator:Recharge oscillator: Several mechanisms have been proposed where Several mechanisms have been proposed where warmth builds up in the equatorial area, then is dispersed to higher warmth builds up in the equatorial area, then is dispersed to higher latitudes by an El Niño event. The cooler area then has to "recharge" latitudes by an El Niño event. The cooler area then has to "recharge" warmth for several years before another event can take place. warmth for several years before another event can take place.

• Western Pacific oscillator:Western Pacific oscillator: In the western Pacific, several weather conditions In the western Pacific, several weather conditions can cause westerly wind anomalies. For example, cyclones north and south can cause westerly wind anomalies. For example, cyclones north and south of the equator force west-to-east winds between. Such wind may counteract of the equator force west-to-east winds between. Such wind may counteract the typical easterly flows across the Pacific and create a tendency toward the typical easterly flows across the Pacific and create a tendency toward continuing the eastward motion. A weakening in the westward currents at continuing the eastward motion. A weakening in the westward currents at such a time may be the final trigger needed to shift into an El Nino. such a time may be the final trigger needed to shift into an El Nino.

• Equatorial Pacific Ocean may tend to be near El Niño conditions, with Equatorial Pacific Ocean may tend to be near El Niño conditions, with several random variations affecting behavior. Weather patterns from several random variations affecting behavior. Weather patterns from outside the area or volcanic events may be some such factors. outside the area or volcanic events may be some such factors.

• Global warming??Global warming??