Diurnal coastal upwelling was previously observed when sea breezes were exceptionally strong, or when the process occurred close to critical latitudes (30°N, 30°S) where local inertial oscillations may be resonantly excited. Our data collected in the Adriatic show that the pronounced diurnal upwelling is also possible under milder wind‐forcing and outside critical latitudes. It is found that the thermocline recorded in the summer of 2006 at the south coast of the island of Lastovo was subject to diurnal variability with a maximum range of about 30 m, and that the corresponding currents measured off the west coast of the island pointed to internal waves propagating around the island in a clockwise direction. We suggest that the summertime stratification occasionally promotes coastal waves that revolve daily around the island, creating the conditions needed for resonant excitation by sea breezes. Numerical modeling reveals that the 24‐h waves are trapped around the island due to the influence of both the Coriolis force and bottom slope, and that the 12‐h waves radiate away from the island. The biogeochemical data show that the diurnal upwelling may stimulate primary production in the area but may also adversely affect benthic organisms.
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