How the weather changes from El Nino to La Nina

El Nino is likely to recede soon, signaling a rapid shift to the opposite climate and ocean pattern, La Nina.

La Nina can lead to powerful Atlantic hurricanes like Hurricane Matthew, which hit Haiti in 2016. Photo: NASA — La Nina can lead to powerful Atlantic hurricanes like Hurricane Matthew, which hit Haiti in 2016. Photo: *NASA*

The shift from El Nino to La Nina brings with it the risk of severe storms in the Atlantic and drier-than-normal weather in the southern United States. Globally, La Nina typically brings cooler temperatures, but before its impact, 2024 will be in the top five warmest years on record, according to Tom Di Liberto, a scientist at the National Oceanic and Atmospheric Administration (NOAA). "All signs point to 2024 being a hot year," Di Liberto told Live Science on April 20.

El Nino and La Nina describe opposing patterns in the trade winds that move around the equator, blowing westward from South America to Asia. In neutral years, when neither pattern is in play, the trade winds push warm water westward, replacing it with colder water from the deep ocean. When El Nino occurs, the trade winds weaken, so the eastern Pacific Ocean, along with the western coasts of North America and South America, is warmer. As a result, the jet stream moves southward, leaving Canada and the northern United States dry but bringing more moisture to southern regions of the United States, according to NOAA.

During a La Nina year, trade winds strengthen, pushing warm water toward Asia and increasing the upwelling of cold water off the Pacific coast of the Americas. The jet stream moves northward, causing drought in the southwest and southeast of the United States and bringing wetter weather to the northwest and Great Lakes.

El Niño officially begins in June 2023, but NOAA's Climate Prediction Center reports that the climate pattern is weakening, with an 85% chance of transitioning to a neutral phase by June. After that, La Niña will return, with a 60% chance of occurring between June and August, according to the National Centers for Environmental Prediction. "For an El Niño this strong, it's not uncommon for an event like this to end and transition to La Niña so quickly," Di Liberto said.

Current ocean measurements show that the surface temperatures in the Pacific Ocean are warm, but the water below is colder than average. As the cold water rises to the surface, the transition will happen quickly. The shift from El Nino to La Nina raises the prospect of a strong hurricane season ahead, says Alex DesRosiers, a doctoral student in atmospheric sciences at the University of Colorado. During El Nino, warm air rises from the eastern Pacific into the upper atmosphere, leading to stronger winds at high altitudes. This creates vertical wind shear, a difference in the speed and direction of winds at the surface versus in the atmosphere. Vertical wind shear can break up storms as they form.

During La Nina, winds in the upper atmosphere are calmer, reducing wind shear and allowing large storms to form through the convection of warm, moist air from the ocean surface. "As we transition to La Nina, the atmosphere becomes more favorable for storms to form and intensify," DesRosiers said.

Influenced by the impending La Nina and the current extremely warm Atlantic surface temperatures, the CSU Tropical Weather and Climate Research Group is predicting a very active Atlantic hurricane season, with an estimated 23 named storms (above the average of 14.4) and five Category 3 or higher hurricanes. This year could be similar to 2010 and 2020, both of which had active hurricane seasons, although it is uncertain whether the strong storms will affect land.

An Khang (According to Live Science )

Source link