Kelvin Waves and California Sea Level: Understanding the Kelvin Wave Phenomenon

Overview

In this Science Friday episode Flora Lichtman discusses Kelvin waves, a form of internal gravity wave that travels along the equator and can affect sea level on the US west coast during El Niño. The guest, Dr. Andrew Lysing from NOAA, explains how these deep waves differ from surface waves and why they matter for California's coast.

• Kelvin waves are deep ocean internal waves that propagate from west to east along the equator.
• They can thicken the warm water layer at the surface, contributing to higher sea levels near the coast.
• During El Niño the pulses can become stronger, potentially stacking with other warming signals.
• Upwelling, ecosystem health, and coastal storms interact to shape local impacts.

Introduction and context

The podcast opens with Flora Lichtman introducing a coastal ocean phenomenon on California's coast and the term Kelvin waves, described as invisible underwater pulses connected to El Niño. Dr. Andrew Lysing, an oceanographer at NOAA's Southwest Fisheries Science Center, joins to explain the physics, implications, and timing of these waves. The discussion frames Kelvin waves as a long-known feature of El Niño dynamics that has gained pop-press attention due to climate variability and recent observations.

What Kelvin waves are and where they come from

According to Lysing, Kelvin waves start at the western Pacific along the equator. In non-El Niño conditions, trade winds push warm surface water toward the western Pacific, creating a bulge that resembles a mountain of water. When El Niño disrupts that wind pattern, the surface water stops pushing west and sloshes back east. This sloshing generates a deep, internal gravity wave, not a surface wave, that propagates along the equator. These internal waves move energy, not mass, and travel long distances while thickening the warm surface layer, which effectively expands the entire water column from the surface downward.

Mechanism and consequences

The thickened layer of warm water at depth leads to higher sea level due to thermal expansion. Kelvin waves are described as planetary gravity waves because of their vast scale and eastward travel. As the wave travels toward the US West Coast, it reaches the coast and breaks apart, propagating along the coastline. Lysing emphasizes that the phenomenon should be understood as a heat content and vertical structure change rather than a traditional surface wave. The energy carried by the wave thickens the warm layer, contributing to elevated sea level and modifying the vertical temperature profile of the ocean.

Frequency, timing, and why this is happening now

The guest notes that Kelvin waves can occur at any time during the year; one kicked off in February, prior to the formal declaration of El Niño on June 11. As the El Niño season progresses, pulses tend to become stronger and more capable of transmitting warm water toward the surface. A typical El Niño cycle might feature five to six Kelvin wave pulses over several months. The discussion also addresses media emphasis around the idea of a “super El Niño,” clarifying that while the event is strong, the term is inaccurate for scientific framing. Observations in 2023-2024 show unique behavior, with the Kelvin waves adding to a preexisting marine heat wave affecting Southern California and extending north toward San Francisco.

Impacts for California and coastal environments

One key concern is sea level rise along the California coastline. The thickened warm-water layer can increase coastal sea levels by a measurable amount, potentially exacerbating high-water events, especially when paired with storm activity. Lysing notes that the overall warming signal is already lifting baseline sea levels, and Kelvin waves can push levels higher than typical El Niño-affected baselines. Beyond sea level, the thickened warm layer can influence upwelling, a critical process that brings cold, nutrient-rich water to the coast. Suppressed upwelling can affect coastal ecosystems and food webs, with potential downstream effects on fisheries and marine life. The discussion also covers how Kelvin waves interact with marine heat waves and broader climate patterns, including the 2015 Blob event, which helped elevate attention to marine heat dynamics on the West Coast.

Aftereffects and the longer arc

After a Kelvin wave passes, sea surface height can remain elevated due to the thickened warm water layer. Upwelling suppression can persist, reducing nutrient flux and biological productivity in coastal zones. There can be partial rebound when parts of the wave’s energy interact with the coastline, but the dominant trend tends to be deeper warm layers rather than a quick return to prior conditions. The conversation also distinguishes marine heat waves that arise from atmospheric conditions separate from El Niño, noting pre-existing warmth during December and record warm Aprils that set the stage for ongoing warming. The Blob and other events are referenced as context for how these oceanographic processes shape long-term trends in California waters.

What to watch and takeaways

The episode emphasizes that Kelvin waves are a standard feature of El Niño, not an extraordinary anomaly, though the current season’s conditions are producing noteworthy observations. The waves continue as long as El Niño persists, with pulses potentially continuing monthly through March or April. The overall risk landscape includes elevated water temperatures, higher base sea levels, and storm-driven surges that could compound coastal impacts. The podcast frames this understanding as part of a larger effort to monitor marine heat waves, warm-water pulses, and their ecological implications, highlighting the value of ongoing observations and climate-informed coastal management.