Reading path: Coastal water observation →
The basic process
Upwelling occurs when water from below rises toward the surface to replace water that has moved away. Along some coasts, persistent winds and the rotation of Earth can help move surface water offshore, allowing deeper water to rise. The process can also occur around features such as headlands, though the details depend on local circulation and geography.
Deeper water is often cooler than sun-warmed surface water, so a nearshore temperature drop may be consistent with upwelling. But temperature alone cannot identify the process. A change in tide, wind-driven mixing, a passing water mass, or freshwater input can also produce cooler readings. Treat upwelling as a hypothesis to examine, not a conclusion drawn from one cold patch.
Look for a pattern along the shore
Take temperature readings at a repeatable nearshore location and, if safe and practical, compare them with another nearby location or a trusted coastal observation. Note the time, depth, tide stage, wind direction, and recent weather. Do not wade into surf or onto exposed rocks to collect data. A shore-based log is useful when its limits and measurement conditions are clear.
Suppose a sheltered pool is cooler than it was the previous day. That observation alone says little about the source. If several accessible nearshore readings also cool, the wind has persisted in a direction associated with offshore surface transport, and broader observations show a regional cooling pattern, upwelling becomes a possibility worth investigating. Each piece adds context; none should be overstated.
Check competing explanations
Compare temperature over several days and at more than one position, while keeping instrument and measurement depth consistent. A tidal exchange can bring different water into a cove on a predictable schedule. Wind mixing can cool the surface by spreading heat downward. Rain or stream flow can make a shallow shoreline site cooler than nearby seawater. Record these possibilities alongside the observed pattern.
Regional data or coastal research can help show whether conditions extend beyond a single beach. NOAA’s ocean science resources offer general context for coastal processes, but broad information does not prove what happened at a particular station. A time-stamped set of observations from your site is the evidence to compare with those larger-scale conditions.
Report observations without overclaiming
Separate what you measured from what you infer. A useful entry might say: “Nearshore surface readings were lower at three visits over two days; winds were persistently from the same direction, and the nearby protected cove showed less change. Possible upwelling; tide and water exchange still need checking.” This wording preserves useful detail while acknowledging unresolved causes.
The most informative follow-up is a repeat measurement under contrasting conditions, such as after the wind shifts, while noting tide stage. If the cooling pattern recurs with similar winds and appears across multiple sites, the case becomes stronger. A shore observer can contribute careful evidence by describing the pattern, method, and uncertainty together.
Sources & further reading
Have a correction or a source to suggest? Send a note →