Current Surveys: How NOAA Predicts the Tide's Push, Years in Advance

A kayaker who knows the tide will be flooding hardest at 1 pm can plan to leave at another time. That kind of warning comes from a tidal current prediction — and every prediction is only as good as the survey behind it. Current surveys are how NOAA keeps those predictions accurate.

Why surveys are repeated

NOAA's Center for Operational Oceanographic Products and Services provides tidal current predictions to commercial and recreational mariners who depend on them to navigate safely. To keep the predictions as accurate as possible, NOAA periodically resurveys coastal and estuarine locations. It chooses where to survey based on mariners' needs, oceanographic analysis, and how long it has been since the last survey.

Modern measurement and computing make a new survey worth a great deal. In 2017, for example, NOAA finished a three-year current survey of Puget Sound, Washington, to update the region's tidal current predictions. Until then, the predictions there had rested on limited data collected in the 1930s-1960s.

How the currents are measured

Each year NOAA places dozens of temporary instruments at coastal and estuarine sites. The main one is the Acoustic Doppler Current Profiler (ADCP), which sends out beams of sound that bounce off particles in the water and return to the instrument. ADCPs measure currents throughout the water column for one to four months. At some sites NOAA adds sensors for salinity, temperature, oxygen and pressure, to follow the movement of different masses of water.

From measurements to predictions

Oceanographers then run the data through a statistical process called harmonic analysis, which separates out the part of the current caused by the tides. That tidal current can be predicted at the location for many years into the future. Other influences, such as wind, cannot be forecast more than a few days ahead, so they are left out of the prediction.

A prediction for an example harbor might say the current will flood — flow inland — at 1 pm, go slack at 4 pm, and ebb — flow out to sea — at 7 pm, with the fastest currents at peak flood and peak ebb. Predictions like these are available online for thousands of coastal and estuarine locations.

A second use for the same data

The survey data also supports NOAA's Operational Forecast Systems. These models give nowcasts and forecasts, up to 48 hours ahead, of water levels, currents, salinity, water temperature and winds in critical ports, harbors and estuaries along the coasts and in the Great Lakes.

Sources

Based on "What is a current survey?" from NOAA's National Ocean Service, a work of the United States government in the public domain.