Sea-Level Synthesis: Base Sea Level, Tidal Residuals,
and Storm Surge

This webpage and its subpages bring together three measures of non-tidal water-level variability for three NOAA tide stations along the Northern California coast: Crescent City, North Spit (Wigi or Humboldt Bay), and Arena Cove. Each station page displays monthly sea-level anomalies, daily mean tidal residuals, and daily maximum skew surge over the station's available historical record, using a common time axis and vertical scale. Together, these measures illustrate how sustained sea-level changes, such as those associated with El Niño, interact with short-term weather and oceanographic conditions to elevate coastal water levels.

Water levels during high tide reflect the predicted astronomical tide plus non-tidal influences that operate over different timescales: a slowly varying base sea level and shorter-term fluctuations associated with weather and ocean conditions. When base sea level remains elevated for weeks or months, as can occur during El Niño, high tides and moderate storms can produce higher coastal water levels. The three panels help distinguish these influences:

• Monthly sea-level anomaly (top): Represents departures in base sea level after removing the long-term relative sea-level trend and average seasonal cycle from monthly mean sea level, as described on the Sea-level Anomaly pages. Horizontal steps show monthly values; dashed steps identify preliminary months calculated from NOAA's 6-minute data.

• Tidal residual (middle): Represents adjusted observed minus adjusted predicted water levels, as described on the Adjusted Tide Station pages. Bars show the daily mean residual for each local day with at least 80% of the expected observations. A centered 7-tidal-day (approximately 7.2-day) running mean highlights sustained non-tidal influences, including persistent winds, atmospheric pressure, and oceanographic conditions. Because the running mean is centered, it ends approximately 3.5 days before the latest data.

• Daily maximum skew surge (bottom): Shows the largest skew surge among each day's high tides. Skew surge is the difference between the highest observed water level during a tidal cycle and the corresponding predicted high water level, even when the peaks occur at different times. It provides a measure of non-tidal influences on high water levels that is particularly relevant to coastal flooding.

All three panels use the same vertical scale, allowing direct comparison of their magnitudes. During relatively calm periods, monthly anomalies, daily mean residuals, and skew surge often have similar values, reflecting sustained changes in base sea level. During storms, daily residuals and skew surge may rise above the monthly anomaly as short-term effects become more pronounced. When all three remain elevated for weeks without large storm peaks, they indicate sustained higher-than-normal sea levels, as can occur during El Niño.

The monthly anomalies and tidal residuals account for seasonal variability differently. Monthly anomalies remove the average seasonal cycle observed at each station, while tidal residuals reflect the seasonal cycle represented in NOAA's tidal predictions. Both account for the same long-term relative sea-level trend referenced to 1992.5. The difference between their seasonal adjustments is expected to be small.

The historical analysis uses NOAA verified hourly water levels before January 1996 and 6-minute water levels thereafter. Grey bars in the skew surge panel identify historical tsunami days from NOAA's Global Historical Tsunami Database. These annotations help distinguish tsunami-related water-level changes from those associated with weather and longer-term sea-level variability. The plots also identify preliminary NOAA data and gaps in the historical record.

Each station page provides an interactive plot covering the station's available historical record through the latest data. The plot initially displays the most recent two years. Use the time buttons (1, 2, 5, or 10 years, or All), the range slider, or click and drag to explore different periods. The vertical scale adjusts to the displayed data. Hover over the plot to view values, select English or metric units, and save the plot as an image using the camera button.

An automated R script retrieves the latest NOAA data and updates the plots every 6 hours. Each station page also provides a downloadable ZIP file containing annotated CSV files of the daily values in feet and meters, along with a README describing the data columns. The downloadable files update with the plots.

For the full monthly sea-level anomaly record and comparions with past
El Niño events, visit the Sea-level Anomaly pages. 
For yesterday’s and today’s water levels, tidal residuals, and skew surge,
visit the Adjusted Tide Station pages.