“Rock Me Like A Hurricane”*
Tropical Storm Leslie has been hanging around since this leg of the mission started, like an annoying younger sibling who keeps interfering when there is work to be done. Every day we check the predicted path (and cone of uncertainty) as well as the marine forecast. Obviously if we thought Leslie was heading our way there would be no question – we would head to shore. She does not have us in her sights, however we will be feeling her impact in the waves and swells even though we are about 600 miles away.
The transfer of wind energy into the ocean creates waves. This transfer is done by friction between the air molecules and the water molecules, and tropical storms have lots of wind energy. This friction causes the water molecules to oscillate (move up and down). It really is similar to the aptly named “wave” that fans do in stadiums and arenas. A group of people stand up and raised their arms and then sit back down, much like the up and down motion of the water particles.
In general the water particles are moving up and down (vertically) not horizontally. If we examine the up and down motion a little more closely we see that each molecule actually makes a more circular route – a tiny little rotation as if it was on a small Ferris wheel. With each rotation it moves forward very slightly, so the individual water molecules out here with us will take an extremely long time before they reach shore as a breaking wave.
When we look at the typical wave shape we see the highest point of the wave, or crest and the lowest part or trough. The distance between two crests (or two troughs) is the wavelength. The height of the seas is the distance between the trough and the crest. Both of the distances – the wave height and wavelength can affect the way the ship handles, especially the recovery of gear such as the ROV.
In addition to rough waves from Leslie, we are concerned about the swells she is generating. Swells are large undulations of water. They can have waves on them, and are created by a build up of wave energy. Large swells that are very close together make the boat rock dramatically and will prevent us from putting any gear in the water.
Eventually the wave energy we see out here will hit the coast. The breakers along the coast happen where the depth of the water has decreased, allowing the water to pile up until it is unsteady and will start to fall over, or break. These waves help shape our coastlines. They create the longshore (or littoral) current as they recede from the shore and run into more waves that are heading in. They move sand, pebbles, and sediment along the oceanfront, eroding some places and depositing material at others.
Our plans have changed as the seas and the predictions have dictated. Initially we expected big swells from Leslie to hit on Friday, and discussed pulling back into port. However Friday morning we awoke to smooth seas so we continued with our work. Saturday was sunny but the seas were rough. We had to end the ROV dive early in order to get it back on the ship safely. The seas were rough overnight, as many people discussed over breakfast, however the seas settled down and we were able to launch the ROV this morning. The seas are confused today, with the wind and the swells coming from different directions. They are not big, so our ride is rather smooth. The forecast for tomorrow does not look so good. As Leslie moves northward it appears that she will continue to disrupt our work.
*Song by the Scorpions, 1984