Atmospheric rivers can shape marine heatwaves, but their effect changes by season: they often precede winter heatwave peaks, while fewer atmospheric rivers tend to occur before summer peaks. Clouds and air-sea heat exchange drive the contrast. Summer cloud cover blocks sunlight and cools the ocean, while winter’s warm, humid air can reduce heat loss and promote surface warming. Tracking atmospheric river activity could improve marine heatwave forecasts, giving coastal industries and ecosystem managers more time to prepare for unusually warm ocean conditions. A marine heatwave can build for days before reaching its hottest point. Above it, a long corridor of warm, moist air may be moving across the ocean. These atmospheric rivers often make headlines when they bring heavy rain and flooding ashore. Over open water, their role looks more complicated. A Duke University study found that atmospheric rivers can influence marine heatwaves in the North Pacific and North Atlantic. Their effect changes with the season and location. During winter, increased atmospheric river activity often appeared before marine heatwaves reached their peak. In summer, heatwave …