Monthly Sea-Level Anomalies and El Niño

This webpage and its subpages present monthly sea-level anomalies for three NOAA tide stations along the Northern California coast: Crescent City, North Spit (Wigi or Humboldt Bay), and Arena Cove. A sea-level anomaly measures how much higher or lower sea level was in a given month than expected for that time of year. These anomalies highlight oceanographic and climate influences, particularly El Niño, that can raise or lower coastal water levels for months at a time. Sustained positive anomalies can increase coastal flood risk.

Monthly mean sea level at a tide station reflects three main components: a long-term trend in relative sea level (regional sea-level change combined with vertical land motion), a seasonal cycle (typically higher water levels in fall and winter and lower levels in spring and summer), and month-to-month variability driven by oceanographic and atmospheric conditions. To isolate this variability, we remove the long-term trend and average seasonal cycle from NOAA's monthly mean sea levels.

We first reference the monthly means to mean sea level for the 1983–2001 National Tidal Datum Epoch. We then remove the long-term trend using each station's relative sea-level rate (NHE, 2026), referenced to 1992.5. Finally, we remove the average seasonal cycle, estimated by fitting annual and semiannual cycles to each station's complete years of record through 2025 using least squares. Zero (normal) represents the sea level expected for that month after accounting for the long-term trend and typical seasonal cycle. We do not remove atmospheric pressure effects, so the anomalies retain water-level variability associated with atmospheric pressure.

The analysis uses NOAA's verified monthly mean sea levels for each station's available record, beginning in 1933 at Crescent City and in the late 1970s at North Spit and Arena Cove, where the record contains gaps. Because NOAA releases verified monthly means after a delay, we calculate recent monthly means from NOAA's 6-minute water levels and apply the same adjustments. We identify these values as preliminary and include the current month once at least seven days of data are available. For months with both verified and calculated means, the two methods agree within approximately 0.5 mm.

To examine the relationship between sea-level anomalies and El Niño, we use the Multivariate ENSO Index (MEI.v2) from NOAA's Physical Sciences Laboratory, supplemented by MEI.ext before 1979. Positive MEI values indicate El Niño conditions, while negative values indicate La Niña conditions. We identify El Niño periods when MEI reaches or exceeds 1.0 for at least five consecutive index periods. Large positive sea-level anomalies along the Northern California coast have coincided with major El Niño events, notably 1982–83, 1997–98, and 2015–16.

Each station page provides two interactive plots:

• History: Displays the full monthly anomaly record and a centered 13-month running mean, with El Niño periods shaded and MEI shown below. Red highlights the current July–June season, while open circles identify preliminary monthly values. Use the range slider or time buttons to explore different periods.

• This Season: Compares monthly anomalies for the current July–June season with historical conditions. The top panel uses box plots to show the distribution for each month across all past years, including El Niño years, and ranks the current month's anomaly (1 = highest). The bottom panel compares the current season with historical El Niño seasons and their average.

An automated R script retrieves the latest NOAA data and MEI and updates the plots every 6 hours. The plots are available in English or metric units, and users can save individual plots as images using the camera button.

For daily water levels and tidal residuals, visit the Adjusted Tide Station pages. 
To see how this year's sea-level anomalies, tidal residuals, and skew surge relate,
visit the Sea-level Synthesis pages.