engleski

Observing Island-Trapped Waves around Lastovo Island, Adriatic Sea

Island-trapped waves (ITWs) are a special type of coastal-trapped waves (CTWs), a wellknown oceanographic phenomenon whereby the Earth’s rotation causes the trapping of surface and internal waves at coastal boundaries, resulting in wave propagation along the coast. Until recently, most research related to ITWs has focused on islands in the deep ocean, like Hawaii and Bermuda, while the phenomenon has only recently been recognised in a shallow and seasonally stratified water body like the Adriatic Sea. On the island of Lastovo, the link between those physical processes and ecosystem diversity and functioning has not yet been explored. Therefore, a pilot experiment was conducted during the stratification period in 2021, where a set of thermistors was deployed at nine equidistant depths of 5 to 45 metres at opposite sides of the island, on rocky cliffs facing north (islet Maslovnjak), south (Cape Struga), and at the control station on the island of Korčula (islet Vrhovnjak). Bottom-moored ADCP was also deployed to the west of Cape Struga, with measurements every 4 metres, between 11 and 83 metres. Diurnal internal waves were observed throughout the investigated period. Temperature oscillations at opposite sides of the island were out of phase, indicating the clockwise propagation of the internal waves around this northern hemisphere island. Several episodes with large diurnal thermocline oscillations were observed, with a range surpassing 20 metres. A ship-based survey was also organised in July 2021 on the southern side of the island. CTD measurements were performed twice a day, around 06:00 (UTC+2) and around 18:00 (UTC+2), when a deep thermocline and a shallow thermocline were expected, respectively. The CTD temperature measurements showed that the ITW range was declining away from the coast. Complementary to the CTD measurements, discreet water nutrients and plankton (bacterio-, phyto- and zooplankton) samples were taken for further analysis to test the hypothesis of ITWs as the main driver of primary production at Lastovo island archipelago.

Lastovo island ; island-trapped waves ; oceanography ; stratification ; thermocline

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