Vertical Structure of Wind-Related Coastal Cooling in a Multi-Layer Buoy Array

The cooling of surface water in coastal seas is routinely detected by surface sensors, but a surface record alone cannot show whether the water below cooled at the same time or instead became warmer, and the two patterns may arise from different physical processes. We examined temperature records from seven coastal buoy stations operated by the Gyeongsangbuk-do Institute of Fisheries Technology on the east coast of the Republic of Korea, over a period from 2021 to 2025, each measuring at six to nine depths, to a maximum of 40 m. Records were averaged into one-hour bins and, after quality control designed to retain genuine oceanographic events rather than reject them, anomalies were formed by removing the fitted seasonal cycle of each sensor separately. Cases were grouped by the strength of the alongshore wind component, taken from a reanalysis and evaluated independently of the buoy records. The surface response shifted systematically with wind strength, from slight warming under the weakest forcing to about 3.5 °C of cooling in the strongest group. The vertical form of the response changed with it: among 183 cases of surface cooling under southerly wind, the proportion in which the 30 m layer cooled together with the surface rose from 45.0% to 71.3% across the three adequately populated positive-wind groups, so that stronger forcing was associated with surface cooling that was more often accompanied by cooling at 30 m, rather than simply with stronger cooling at the surface. Among cases in which both records cooled, the median ratio of the reanalysis cooling to the buoy cooling was about one-half overall and less than one-fifth in the strongest cooling group. Vertical structure of this kind is visible only where two or more depths are measured at the same time. These relationships are consistent with coastal upwelling but do not confirm the underlying process, because current and salinity observations were unavailable.