Marine heatwaves (MHWs) are periods of unusually warm ocean temperatures that can last for weeks or even years. They can seriously impact marine ecosystems and coastal economies. In this study, we focus on the Gulf of Alaska (GOA), where we found that these heatwaves tend to follow a pattern that repeats twice a year—what scientists call a semi-annual harmonic.

At first, this pattern didn’t seem to be directly tied to how surface ocean temperatures typically vary throughout the year. But we discovered that it likely comes from how the atmosphere and the ocean’s mixed layer (the upper layer of water that mixes due to wind and temperature) interact. Specifically, strong winter winds mix the ocean deeply, while summer conditions lead to a shallower, calmer mixed layer. This seasonal difference, driven by the atmosphere, combines with the ocean’s internal rhythms to produce a twice-a-year signal.

In short, the semi-annual pattern of marine heatwaves in the Gulf of Alaska is not just a result of surface temperature changes—it’s caused by complex interactions between the atmosphere and the ocean’s mixing behavior throughout the year.