Coastal Trapped Waves Frequently Asked Questions:
Q: What is a Coastal Trapped Wave (CTW)?
A: A coastal trapped wave is a propagating disturbance characterized by higher-than-normal sea levels. These waves are “trapped” to the coast due to the rotation of the Earth and the Coriolis force, which deflects elevated surface waters back into the coastline as they try to move offshore and poleward.
Q: How do CTWs form?
A: Coastal trapped waves are often associated with equatorially trapped Kelvin waves, which form in the far western equatorial Pacific in response to westerly wind bursts. These wind bursts weaken the background trade winds, allowing warm water and elevated sea states found in the western equatorial Pacific to “slosh” back toward the eastern equatorial Pacific as equatorially trapped Kelvin waves. These Kelvin waves are a critical component of El Niño development. When the waves arrive at the South America, they continue to propogate up the coastline as CTWs in both the Northern and Southern Hemispheres. The CTWs will continue to follow the contour of the coast, around bays, coves, and inlets, for thousands of miles. In total, these waves travel +25,000 km over ~120 days, from the western equatorial Pacific all the way to the Gulf of Alaska.
Q: Why do we care about CTWs?
A: Coastal trapped waves can raise background sea levels by 6-12 inches, depending on the strength of the wave. When combined with high tides and/or storms, CTWs can significantly increase the risk of coastal flooding and beach erosion. Additionally, CTWs suppress upwelling along the US west coast, warming the ocean and contributing to marine ecosystem impacts. In total, we usually expect 5-6 major CTWs during the life of an El Niño, with the largest waves typically occurring during the winter.
Q: How do we track CTWs? Are CTWs predictable?
A: We currently track CTWs using observational products that combine real-time ocean measurements with computer simulations to generate a complete picture of how sea surface heights are evolving around the globe. CTWs are predictable! They travel at a known speed of ~2.7 m/s or ~6 mph. As a result, once we identify that a wave has kicked off, we usually have a good sense for when it will arrive at a particular location.
Q: Where can I download the CTW animations?
A: You can download the latest CTW animations here and here. They update daily when new data rolls in. You can also download a classic CTW example from the 1997/1998 El Niño here. If you use these animations for publications, I ask that you please email me to let me know and to properly credit me as the source.