You may have learned in Oceanography that wind driven surface effects are limited to the first 70m of the ocean, in what’s commonly known as the Ekman layer. However, climate oscillations, even small ones, have a deeper impact on the oceans. They can influence oceanic conditions a mile below. The Telegraph UK calls one newly revealed mechanism a “freak” ocean wave because it travels underwater, with little or no surface expression. This was such a good spin I had to promote it to main title.
In reality, these are periodic Kelvin waves travelling eastward along the equator at the submerged boundary between warm surface water and colder deep water. The forcing mechanism is a small climate oscillation called Madden-Julian Oscillation (MJO). The MJO is El Nino- Southern Oscillation’s (ENSO’s) little brother. It’s a trigger for ENSO that drives the tumultuous weather in the Coral Triangle region around the Philippines and Indonesia.
The MJO is a one to two month cycle of high and low precipitation with associated winds that force a train of equatorial Kelvin waves. The waves cause periodic changes in temperature down to at least 600m, sometimes as deep as 1300m. The lag time between surface flux and deep temperature response is 1-2 weeks. The amplitude of the temperature change in the deep-ocean wave is 3-6 times that of the annual cycle. Currents are also intensified, roughly doubled.
Upon reaching the eastern coastline of the Pacific, these deep ocean waves propagate north and south along the coastline of the Americas. Which leads me to wonder… how do we surf a deep-ocean wave?
“They represent a substantial transfer of energy form the surface layer, where they are forced, to the deep ocean, where they must be dissipated,” writes author AJ Matthews of the University of East Anglia, UK in the recent issue of Science. The Telegraph UK covers the story with the subtitle “…origin of freak waves” in a nice perspective article here.
To make the discovery, scientists used the Argo ocean observing system of buoyancy controlled deep-water floats programmed to profile the water column from 2000m to the surface. Nearly 3000 of these jumbo sized yellow floats populate the ocean today. Upon breaching the surface, the floats transmitting their signal to satellite , about every ten days or so.
Matthews, AJ, P Singhruck, and KJ Heywood. 2007. Deep Ocean Impact of a Madden-Julian Oscillation Observed by Argo Floats. Science 318: 1765-1769.