La Nina
Climate Change Daily Earth Science Hydrosphere Repost

A Cooling Trend Drives Rain, Wind, and Fire

El Viejo — the old man — AKA La Niña

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Easterly winds in Puerto Ayora gained strength, and seawater in the town’s harbor cooled. Positioned on the equator, the Galapagos Islands are front-and-center as La Niña conditions develop in the eastern equatorial Pacific Ocean. Sea surface temperatures may drop by up to 9 degrees Fahrenheit and remain chilly throughout the autumn and winter. Our planet is a dynamic, inter-connected web of cause-and-effect, and cold waters in Puerto Ayora may mean flooding in Louisiana and fires in California.

It’s true, California is already burning, and Louisiana is currently being inundated with wind and water as the eighteenth tropical storm of the season hits the coast. But La Niña can extend the pain. How do events over the equatorial Pacific Ocean spread misery to communities many thousands of miles away?

The appearance of La Niña is not a surprise to weather forecasters who predicted its development last spring. But even with predictions, there is little to do in the way of practical preparations, other than building a psychological readiness for a rough ride. La Niña has visited 24 times in the past 120 years — sometimes appearing over multiple consecutive years and other times disappearing for up to ten years. The winter of 2017–2018 was its last previous visit.

ENSO

La Niña and her brother El Niño are alternating phases of a natural weather pattern called the “El Niño-Southern Oscillation” (ENSO). La Niña is also referred to as El Viejo — the old man. During the La Niña phase, easterly trade winds strengthen over the Pacific Ocean, leading to an upwelling of cooler waters from the deep ocean. The arrival of these deep waters to the surface is why Puerto Ayora is chilling out.

The equatorial trade winds in the Pacific blow from east to west. When the winds are weak, they don’t effectively move the surface waters, and heat from the sun accumulates, warming the ocean’s surface. But when ENSO oscillates to the La Niña stage, stronger trade winds push the warmed water into the western Pacific Ocean.

The process starts off the coast of western South America, and as the warm surface waters move westward, colder water from the deep ocean upwells to fill the void. A cold-water wedge is then pushed westward behind the retreating warm water, forming an elongate plume, stretching from South America to the western Pacific.

Maps of sea surface temperature anomaly in the Pacific Ocean during a strong La Niña (top, December 1988) and El Niño (bottom, December 1997). Maps by NOAA Climate.gov, based on data provided by NOAA

Cold there wet here

Cooler water in the equatorial Pacific has a knock-on effect. The oceans chill the atmosphere, and because temperature changes drive air circulation, La Niña changes the jet stream’s position pushing it northward. Even though the jet stream rides high above us in the upper atmosphere, its reach extends downward to the surface affecting weather throughout North America. One of the connections La Niña makes is between colder Pacific water and increased tropical storms in the Atlantic Ocean 5,000 to 10,000 miles away.

The connection works through a phenomenon called “wind shear.” Somewhere between North Africa and the Caribbean, warm moist air is rising from the ocean’s surface. As the warm air leaves the surface, a sea breeze develops to fill the space left by the rising air. The heat from the ocean warms the breeze, and this air also rises. The greater the volume of rising air, the faster the winds blow, and a tropical storm is born.

The storm’s moist, warm air needs to go somewhere, and the only option is up. The higher up it travels, the stronger the circulation cell, and the more intense the storm — this where wind shear comes into play. Wind shear describes conditions where mid and upper atmosphere winds travel at different speeds and directions than the surface and lower atmosphere winds. Imagine the rising air from a tropical storm reaching a point where upper-level winds blow away the top of the circulation cell. It takes the oomph out of the storm and keeps it from growing stronger.

Simply put, more wind shear equals fewer tropical storms. The antithesis of this is true also. Less high-level atmospheric activity reduces wind shear and promotes more tropical storms. Enter La Niña. She pushes the jet stream northward, leaving calmer mid and upper-level atmospheric conditions over the Atlantic’s hurricane alley. The result is more tropical storms form, and more of these storms turn into hurricanes. Voila, while Puerto Ayora residents are donning their fleeces, homeowners in Lake Charles, Louisiana, are surveying flooded homes and missing roofs as the latest hurricane strikes.

Typical influence of La Niña on Pacific and Atlantic seasonal hurricane activity. Map by NOAA Climate.gov, based on originals by Gerry Bell.

Cool and dry, not as good as cold and wet

Meanwhile, in California, the Creek Fire is sending a smoke plume 55,000 feet into the atmosphere, and pockets of flame reach up over 25,000 feet. Fire tornados are spawned from this inferno, wreaking havoc on surrounding communities. Cold, wet weather would be a welcome relief, but unfortunately, the dry season is in full swing. An average dry season runs through October, but La Niña generally brings dryer conditions and raises the dry season’s risk of extending into winter. The jet stream has moved farther north, and higher vertical wind shear over the Pacific means fewer storms and less rain.

Neither El Niño nor La Niña are direct indicators of individual storms and weather events. The effect is more subtle. These ENSO end-members alter the probabilities and change the odds — like adjusting a slot machine to provide either more or fewer jackpots. No individual pull on the lever notices this change, but your probability of winning or losing is changed. After a thousand pulls of the lever, you may notice.

As La Niña conditions develop this year, we don’t exactly know what will happen. What we do know is hurricanes will probably continue later into the season, and California’s dry season has a good chance of extending into the winter. Even though La Niña is part of a natural cycle, it serves as a stark reminder of how interconnected our environment is. Changes in the Pacific Ocean impact cities on the Gulf Coast. Warming in the Arctic affects property owners in Miami as sea levels rise. Climate change is a global process, and there are no hidden places where the effects of climate change do not reach. Therefore, real solutions must be global solutions.

Sources:

El Niño and La Niña: Frequently asked questions (Source: NOAA)


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William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.