What Are El Niño and La Niña
El Niño and La Niña are both effects of the El Niño Southern Oscillation (ENSO) cycle. About every 3-5 years, the surface temperatures in the Pacific equatorial region fluctuate to the point where the temperatures are either above the average (El Niño) or below the average (La Niña). A long time ago, fishermen in South America noticed the warmer Pacific water temperatures that peaked around December. They named it El Niño de Navidad which was shortened to just El Niño (the little boy) later. La Niña (the little girl) is just the opposite and is marked by a period of colder waters in the Pacific equatorial region.
El Niño
El Niño Pattern (https://oceanservice.noaa.gov/facts/ninonina.html)
El Niño weakens the trade winds which pushes the warmer waters to the east causing the warming of the Pacific waters which in turn causes the jet stream in the Pacific to move south of its normal position. This movement causes the northern US and Canada to experience warmer and drier weather than normal. It also causes the southern US to experience warmer and wetter weather than normal.
La Niña
La Niña Pattern (https://oceanservice.noaa.gov/facts/ninonina.html)
La Niña strengthens the trade winds pushing the warmer waters to the west. This pushing of the waters to the west causes the colder water to be drawn up from deeper in the Pacific causing a cooling of the eastern Pacific. This cooling pushes the jet stream to the north causing wetter weather in the northern US and Canada and warmer, drier weather in the southern US. Sometimes the La Niña conditions will even cause widespread drought in the southern US. La Niña also causes ideal conditions for the formation of tropical cyclones.
El Niño to La Niña Transition
The transition from El Niño to La Niña usually predicts a busier hurricane season than normal. As the gap between the end of El Niño and the beginning of La Niña shrinks, the intensity of that season is predicted to increase. The 2024 season was one of those events.