La Nina, the cool phase of the El Nino-Southern Oscillation (ENSO) cycle, is forecast to appear weakly this winter and last into next spring. However, not all signs are consistent, and scientists are facing many unanswered questions about the link between ENSO and global climate change.
La Nina occurs when sea surface temperatures in the central and eastern Pacific Ocean drop below average, with a difference of at least 0.5 degrees Celsius. This causes major changes in atmospheric flows and global weather. In contrast to El Nino (the warm phase), La Nina typically brings colder winters in North America, more rain in Australia, and affects the Atlantic hurricane season.
Although the official threshold for a La Niña event has not been reached, atmospheric conditions have exhibited familiar characteristics. First, stronger-than-normal trade winds have reduced rainfall in the Pacific. Second, an active Atlantic hurricane season, with increased storm numbers and intensity. Third, some areas have been unusually dry, while others have been experiencing prolonged periods of heavy rain.
These signs are giving scientists a headache because they do not completely match the traditional ENSO norms.
The US National Oceanic and Atmospheric Administration (NOAA) predicts that a weak La Nina will transition to a neutral state in the spring and last through the summer of 2025. A neutral state occurs when sea temperatures in the ENSO region range from -0.5 degrees Celsius to +0.5 degrees Celsius, leaving the weather not strongly influenced by La Nina or El Nino.
In this context, local weather patterns tend to dominate, rather than the large fluctuations seen when ENSO is strong. However, recent neutral summers have coincided with unusually high global temperatures, raising questions about the role of climate change in modulating ENSO.
One of the big challenges is understanding the interactions between ENSO and climate change. The unprecedented rise in global ocean temperatures over the past year is complicating traditional forecasts. Current ENSO indices may no longer accurately describe the phenomenon as the ocean warms, according to NOAA.
Scientists are also debating whether climate change will increase the frequency of El Nino and La Nina, or make them stronger. "ENSO is a complex system where the ocean and atmosphere constantly interact. Climate change is like an invisible hand, tweaking the system's knots in ways we don't yet fully understand," said Tom Di Liberto, a NOAA meteorologist.
While weak La Ninas do not bring historically cold winters, the uncertainties in ENSO models pose a major challenge. The phenomenon can cause extreme weather, mixed with unexpected fluctuations, especially when combined with climate change.
While we await more research and data, people and states need to prepare for unpredictable weather events, from extreme temperatures to fluctuating rainfall patterns, to minimize risk and damage this winter, NOAA said.
