Catching a wave
Posted by sogasex on March 12, 2008
By Christopher Zappa, LDEO
I sit writing this entry late at night, listening to the howling wind outside, and feeling the ship being tossed about by the waves on the ocean. Nearly everyone on the ship by now has their sea legs, the ability to “roll” with the motion of the ship on the ocean. And it occurred to me — many times it’s difficult for me to know when the waves are big and the winds are strong unless I can actually see the waves or feel the wind on my face. Even when the computer display tells me the waves are big, I wonder why the ship isn’t pitching and rolling more. One thing I definitely notice is that when the wind and waves are at our backs, the ride is a LOT more comfortable than when a gale force wind blows and waves crash over the bow.
I am part of the onboard air-sea interaction group consisting of collaborating scientists from LDEO, University of Connecticut, University of Hawaii, and NOAA PSD in Boulder. One of the topics our group is studying is how the energy from the wind goes into making waves, how these waves grow, and how they eventually whitecap, or break. Wave breaking and whitecapping are important processes in air-sea interaction. The lengths of these breaking waves range in size from a few feet (microbreakers that have no visible whitecap) to hundreds of feet (whitecapping). These breaking waves are crucial in enhancing gas transfer over the oceans and important to the overarching goal of Southern Ocean GasEx–Gas exchange in high winds and big waves. Microbreakers are everywhere on the windswept ocean and break through the resistance to gas transfer at the air-sea interface. Whitecapping generates bubbles and mixing that contribute even more to gas transfer.
Measuring waves from ships is a difficult task, especially since the ship is influenced by the motion of the ocean. Until recently, most instruments for measuring waves from ships were one-dimensional. This means we were only able to get an idea of the height of the waves. During this experiment, we are using WaMoS II (Wave Monitoring System)—an advanced system that uses a ship’s radar along with some fancy software to produce images of the waves. This state-of-the-art instrument provides not only information about the height of the waves, but also their frequency, their wavelength, their “age,” their steepness, and their direction.
WaMoS II can also tell us if there are various “types” of wave systems at the same time that will cause changes in the motion of the ocean. The other day we had a five-knot wind but very long ocean swell waves that measured roughly 10-12 feet high. The ship was able to follow along the swell rather easily and the ride was smooth. Later in the day the wind picked up, and overnight the short-wavelength wind-generated seas had grown and were coming from a different direction than the long ocean swell. The complicated wind sea and swell conditions made it difficult to find that “smooth” ride that the captain looks for by heading with the waves. The long swell eventually went away as the wind seas continued to grow. However, the wind produced short-wavelength steep waves of 6-8 ft high. Even though smaller in height than the longer swell, the wind seas did not allow the ship to navigate the smooth ride because they were steeper. WaMoS captured data on the steepness and direction of all these wave systems that greatly effect the motion of the ocean we feel.
Just to the south of us are some of the largest waves on the world’s oceans. Think of it… no land masses to stop the waves from circumnavigating the globe. The wave forecast for that region this week shows a potential of over 30 feet high! We might even experience waves that high during our cruise. For now, it’s for sure a smoother ride up here without them.
The ship chasing a packet of whitecapping breaking waves.
An oncoming whitecapping breaking wave.
A wave imagery from WaMoS II.