Summary (tl;dr)

A large, slow-moving ocean phenomenon called a Kelvin wave, driven by an exceptionally strong El Niño, is approaching California's coast in late 2026, threatening to raise sea levels by up to a foot and exacerbating risks of coastal flooding and erosion.

Essential Background

Kelvin waves are immense, subsurface pulses of warm water that propagate across oceans, often along the equator or coastlines, and are not visible as typical crashing waves. They are a natural part of Earth's climate system and are typically generated during El Niño events when weakened trade winds in the Pacific allow warm water to "slosh" eastward across the ocean. When these equatorial waves reach continents, such as South America, their energy can turn and travel poleward along the coast as coastal Kelvin waves. Scientists have been observing and understanding Kelvin waves for about 150 years.

The Full Story

Currently, an unusually strong El Niño is developing in the Pacific Ocean, with some experts calling it potentially the largest in at least 1,000 years. This powerful El Niño has generated several Kelvin waves, one of which reached the northern tip of South America in late August 2026 and is now making its way northward along the North American coast. This particular Kelvin wave is expected to arrive along the California coastline in early to mid-October 2026. While not a visible, crashing wave, this phenomenon is a significant mass of warm water that will temporarily raise baseline sea levels along California's coast by several inches, potentially up to a foot, and its effects could linger for months.

Why It Matters

The impending Kelvin wave is significant because the projected temporary rise in sea levels, combined with already ongoing long-term sea level rise due to climate change, greatly increases the risk of coastal flooding, storm surges, and beach erosion. This is particularly concerning as the wave's arrival coincides with California's upcoming winter storm season and potential king tides, which could lead to compounded and more severe impacts on coastal communities and infrastructure. Additionally, the warm, less nutrient-rich waters associated with Kelvin waves can affect marine ecosystems and local fisheries. California Governor Gavin Newsom has already declared a state of emergency in anticipation of the potential impacts of this historic El Niño season.

Geographic Location

• Pacific Ocean (origin and propagation of Kelvin wave)
• Northern tip of South America (recent arrival of Kelvin wave)
• California coastline, United States (expected arrival and impact of Kelvin wave)
  • Southern California, California, United States (expected arrival in early October 2026)
  • San Diego, San Diego County, California, United States (expected arrival in early to mid-October 2026)
  • San Francisco, San Francisco County, California, United States (expected progression northward by early October)