Annalisa Bracco, Georgia Institute of Tech.
For months, the North Atlantic Ocean has experienced unusually high surface temperatures. However, recent cooling in both the Atlantic and eastern Pacific might offer some relief, particularly for vulnerable coral reefs. This cooling is attributed to two climate phenomena: La Niña, which occurs in the tropical Pacific, and the less well-known Atlantic Niña. Both can influence the Atlantic hurricane season, though in different ways.
La Niña, a key part of the El Niño–Southern Oscillation, is a well-known climate pattern with global impacts. During La Niña, sea surface temperatures in the tropical Pacific drop below normal, strengthening easterly trade winds and promoting cooler water upwelling off South America. This cooling affects atmospheric patterns worldwide, often leading to stormier conditions in some areas and drier conditions in others. Notably, La Niña typically weakens wind shear in the Atlantic, which can help hurricanes form and intensify.
In contrast, Atlantic Niña is a smaller-scale phenomenon occurring in the Atlantic Ocean. It usually peaks in July or August and has a shorter duration and more localized effects compared to La Niña. Atlantic Niñas generally have the opposite impact of Atlantic Niños, which can reduce rainfall in Africa’s Sahel and increase it in Brazil and nearby regions.
While weaker than La Niña, Atlantic Niña can counteract some of La Niña’s effects by weakening summer winds that drive upwelling in the eastern Pacific. This July and August, meteorologists observed signs of an Atlantic Niña developing, marked by unexpectedly weak surface winds and warmer sea surface temperatures earlier in the summer. Simultaneously, eastern Pacific waters were cooling, with La Niña conditions expected by October or November.
Seeing both Niñas simultaneously is rare but not unheard of. This combination is akin to two pendulums swinging in opposite directions. More common pairings include La Niña with Atlantic Niño or El Niño with Atlantic Niña.
An Atlantic Niña might initially seem advantageous for hurricane-prone areas. Cooler waters off Africa can suppress the formation of African easterly waves, which can develop into tropical disturbances or hurricanes. Since tropical storms rely on warm sea surface temperatures for energy, cooling in the tropical Atlantic could reduce this energy, potentially lowering the likelihood of tropical cyclones.
However, NOAA’s August outlook for the Atlantic hurricane season remains highly active. This forecast considers multiple factors, including near-record warm sea surface temperatures in the North Atlantic, which can intensify hurricanes. Additionally, the anticipated La Niña in the Pacific is expected to weaken wind shear, further supporting hurricane development. Thus, La Niña’s significant effects might overshadow the Atlantic Niña’s impacts.
Recent years have seen exceptionally high ocean temperatures globally, and while the two Niñas might provide some localized cooling relief, it may be temporary. The ongoing trend of global warming, driven by rising greenhouse gas emissions, continues to elevate baseline temperatures, potentially fueling more intense hurricanes. [Abridged]
Annalisa Bracco, Georgia Institute of Tech.
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