The ocean breathes, and hurricanes swing on its breath like on a seesaw. Over the past seventy years, hundreds of tropical cyclones have crossed the North Atlantic, and not all of them behaved alike. Some decades brought a barrage of destructive storms to coastlines; others brought calm. Meteorologists have long suspected that two slow rhythms drive these shifts: El Niño in the Pacific and the Atlantic Multidecadal Oscillation, which alternately warms and cools the water between the Americas and Europe. A new study for the first time brings both rhythms together and tests them against a long observational record.
The Atlantic Multidecadal Oscillation, or AMO, acts like a giant thermostat. During warm phases, which last twenty to thirty years, the ocean surface warms, giving the atmosphere above it more fuel for storms. The study's author calculated that in such decades, an average of 22.5 hurricanes formed, compared with 17 in cold phases. Meanwhile, the cyclones' energy, measured by the ACE index, jumped by 400 units. In the 2000s, when the AMO was in a warm phase, this metric was nearly double its level from the 1970s. The ocean seesaw tilted upward, and hurricanes responded with greater strength.
But a storm's strength is only half the story. Where it goes matters just as much. This is where the second pendulum comes in. When the Pacific slips into La Niña while the Atlantic stays warm, hurricanes make landfall more often. In such seasons, maximum wind speeds reached 165 and 170 knots, whereas under El Niño the ceiling was 155 knots. The difference of fifteen knots is the gap between a strong storm and a catastrophic one. The coasts of the United States, Mexico, and the Caribbean islands feel it as the difference between torn-off roofs and demolished houses.
Sea surface temperature adds a third layer to this picture. The North Atlantic is warming faster than climate models predict. The actual values for 2023 match model estimates for 2040. That means the seesaw is swinging harder than expected, and the next warm AMO cycle could bring hurricanes we have never seen. From 2020 to 2023, five major hurricanes of category 3 or higher struck the United States, compared with three over the entire previous decade-long period.
The study is the first to combine three factors: AMO phases, El Niño phases, and sea surface temperature. Previously, each was studied separately, leaving unclear how they combine into a specific seasonal risk. Now it is clear: warm AMO phases increase the number of hurricanes, La Niña raises the likelihood of landfalls, and rising ocean temperatures boost the maximum strength of storms. These results can be used to improve seasonal forecasts of hurricane activity and to assess risks for coastal regions.
The ocean seesaw keeps swinging. The next warm AMO cycle will come, even if its onset cannot yet be predicted precisely. But now meteorologists have a map that shows both pendulums and their combined action. When the Pacific shifts into La Niña and the Atlantic remains warm, coastlines should brace for the worst. The ocean's breath will grow louder, and hurricanes will answer with doubled force.
From 2020 to 2023, five major hurricanes of category 3 or higher struck the United States, compared to three over the entire previous decade.
In Kim Stanley Robinson's novel 'Forty Signs of Rain', climate scientists try to predict abrupt changes in ocean currents and their weather impacts, echoing efforts to link slow ocean oscillations to hurricane activity.