The waves that most of us are familiar with are the waves at the beach—waves that endlessly curl and crash on the shore. But the ocean and atmosphere also have what are called “planetary waves”—waves of immense scale. Kelvin waves are a kind of planetary wave.
Unlike the waves you see at the beach, Kelvin waves do not curl over and then crash. They are more like the waves in your bathtub, which slowly slosh around. They don’t break, but they still have broad peaks and valleys that change the depth of the water (the ocean equivalent is “sea surface height”).
According to the National Oceanic and Atmospheric Administration (NOAA), Kelvin waves that are relevant to ENSO only move eastward and along the equator. Like all planetary waves, the geographic extent of an equatorial Kelvin wave is huge, often stretching over much of the Pacific Ocean (thousands of miles).
At this point, you may be asking yourself, what happens after this wave hits the coast of South America? Well, they can bounce back (slightly off the equator) as a westward moving Rossby wave. Also, there is a second type of Kelvin wave in the ocean that is not as directly applicable to ENSO prediction, which is called a coastal Kelvin wave that travels with the coast to its right in the Northern Hemisphere.
Equatorial Kelvin waves have two phases, which can lead to very different changes in subsurface and sea surface temperature (SSTs) in the eastern tropical Pacific:
Downwelling Phase
Normally, winds blow from east to west across the tropical Pacific, which piles up warm water in the western Pacific. A weakening of these winds starts the surface layer of water cascading eastward. The thick warm layer sloshes east, pushing down the thermocline as it goes, thus we call this a “downwelling” wave.
The thermocline is the boundary between the warmer, near surface mixed layer and colder deeper water. The thermocline is often defined by subsurface temperatures at 20°C. Around the 20°C layer, ocean temperatures change rapidly (a strong temperature gradient).
Because of this downward push as the wave travels eastward, it is harder for the colder, deeper water to affect the surface so near-surface temperatures are often above average. This will often (not always) warm the surface temperatures and plant the seeds for an El Niño.
The appearance of a downwelling Kelvin wave does not automatically mean an El Niño event is coming. Sub-surface temperatures can get quite warm, but they do not necessarily manifest themselves at the surface of the ocean in a 1:1 fashion. This is because it is “easier” to achieve large anomalies near the thermocline (a large temperature gradient can lead to big anomalies) and not necessarily right at the surface. However, downwelling Kelvin waves are one sign of a possible El Niño and are why it is important to monitor below the surface of the ocean in addition to the surface.
Upwelling Phase
After the downwelling part of the wave goes by, we sometimes see a rebound or upwelling where there was once downwelling. There does not have to be a rebound upwelling wave. The waves entirely reflect the wind forcing: if the winds remain westerly, there will be no rebound. But it happens that typically westerly wind forcing (e.g. say from the MJO) is then followed by easterly anomalies. This process is entirely external to the ocean— ultimately, the ocean will respond to sustained wind forcing.
Here, the colder water at depth upwells and the thermocline comes closer to the surface. We often will see below average temperatures near or at the surface.
After it gets started, a Kelvin wave takes 2-3 months to cross the tropical Pacific, which allows some lead time to anticipate a possible El Niño event. NOAA saw a big downwelling Kelvin wave in March/April 2014, but then saw an upwelling phase go through in June/July, which helped to reverse and cool temperatures in the eastern Pacific. The subsurface temperature changes are not always perfectly equal and opposite. Just because there is a strong downwelling phase does not mean there will be a strong upwelling phase.
Stay tuned to the the NOAA website for future ENSO updates.
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Photo Credit: Nikola Obradovic / Shutterstock.com