The ocean is made up of several layers. The uppermost layer has the highest temperatures and it is called a mixed layer, because it is continuously being mixed and thus kept in contact with the atmosphere.

Below this is the thermocline layer, which gets colder as depth increases. During the summer, the mixed layer is shallow, often only about 10-20 meters deep, but during the winter, cooler atmospheric temperatures and stronger winds can cause the mixed layer to expand to more than 100 meters.

As a result, the mixed layer extends into what used to be the upper part of the thermocline, called the seasonal thermocline, thereby re-absorbing any warm anomaly that was deposited in this layer. Only anomalies in the part of the thermocline that remains below the mixed layer for the entire year, known as the permanent thermocline, will remain after the winter.

When a hurricane passes over an ocean, its powerful winds stir and mix the warm surface water with the colder, deeper water. This mixing results in warm water being forced down into the deep ocean and cold water being brought to the surface layer.

Scientists know that the cold water near the surface is reheated by the atmosphere to pre-hurricane temperatures within a few weeks, but they have been less clear on what happens to the warm water mixed into the deep ocean. It has been suggested that this heat is transported toward the poles by ocean currents and contributes to the ocean heat transport, the process by which oceans regulate our climate by transporting warm water away from the equator and cold water toward the equator.

It has also been speculated that the heat pumped into the ocean by hurricanes strengthens subsequent storms that pass over the same part of the ocean, because ocean heat is the energy source that powers hurricanes. Stronger storms would then mix even more heat into the ocean driving a positive feedback loop for hurricane intensity.

A new MIT analysis suggests that previous studies have overestimated the amount of hurricane-induced ocean heating and its overall impact on climate. The analysis indicates that previous estimates have failed to consider how the oceans change with the seasons.

Most of the heat from the warm water that hurricanes mix deep into the oceans during the summer and early fall is returned to the atmosphere in the winter, meaning these “warm anomalies” don’t appear to affect the long-term state of the oceans, according to a paper published Feb. 10 in Geophysical Research Letters