The wind, rain and storm surge of a hurricane can be incredibly destructive to those of us on shore, and the processes that build them are no less disruptive to the oceans that build them. We often hear reports about the “warm waters” that fuel hurricanes, but when a storm draws on that energy, it will churn the ocean in the process, changing conditions as deep under the water. The water that doesn’t get sucked up into a cloud is left with different temperatures, depths and salinity, all of which can take weeks to years to truly restore everything to normal.
Stirring up the depths of the seas
Normally, ocean water is slightly layered according to salinity, or salt concentrations, and temperature. Colder, saltier water goes down, while fresher, warmer water sits closer to the surface. When the storm comes along, it stirs all this up like a meteorological egg-beater, creating a more uniform mix of water as it goes. By drawing warmer air and water into the air, and pulling cold water up from the depths, the wake the hurricane leaves behind is significantly colder than the ocean around it, to the point that the “cold wake” of a storm can be detected well after the hurricane has passed by.
The are numerous lasting effects of these changes in temperature and salinity. Marine animals generally expect a degree of stability in their environments, and changes in water temperature can be dangerous to many species that can’t flee the area. The upside of this is that the colder water brought to the surface has less energy to fuel the storm, and so in some cases this process can actually reduce the hurricane’s strength.
Tossed by turbulence
Beyond changes in bringing cold water up, hurricanes trigger a lot of lateral movement as well. Currents and turbulence underwater can slam things around, almost like a marine version of the winds we experience on land. Oil pipelines have been broken, sunken ships moved and sand resettled, even close to 1,000 feet below the water’s surface. Some animals will just get shoved around in all this, but sedentary marine life, like corals, can take a beating during a hurricane, suffering from both impacts with other objects as well as being choked by fine particulate stirred up by the water.
Sucking water to build a storm surge
Finally, there’s some significant “mixing” with the air above the water too. High winds and strong currents help whip the water up into waves and sprayed water, sometimes to the point where the boundary between “air” and “water” gets pretty blurred. Quite importantly, the center of hurricane is actually a place with very low air pressure, meaning there’s less air molecules to push down against the water. In a full vacuum, this can lead to water essentially boiling away into vapor, and while that’s not exactly what’s in a hurricane, there’s certainly a bit of a void to be filled.
With air being pushed away from the eye of the storm, the most abundant material to fill that space is even more water. The combination of currents, wind and low air pressure essentially shoves more and more water from elsewhere in the ocean into a big bulge of water under the hurricane. In big storms, this can lead to dramatic situations where water has been drawn away from shorelines, draining beach fronts and bays. As with the other processes that build a hurricane, this can disrupt marine life, leaving creatures like manatees beached when the water evacuates from underneath them.
The water isn’t technically being sucked by the storm, of course, but is instead rushing to the point of lower air pressure creating what’s known as a storm surge. When conditions change and the eye of the hurricane begins releasing more of this water, the surge can come rushing back to shores very quickly, often exceeding normal sea level by over ten feet. As fascinating as it may be to walk the ocean floor for a few minutes, you can’t be sure when the surge will return, and it can certainly rush inland with enough force to damage roads, buildings and of knock you off your feet.
Altogether, hurricanes reshape the oceans as much as they do life on land. We don’t have to the same reference points when watching the ocean, but it’s still pretty clear that these storms effectively upend normal water flow, even before that water starts getting dumped on the land.
Source: What Happens Underwater During a Hurricane? by Brian McNoldy, Rosenstiel School of Marine & Atmospheric Science