Revisit the Regular Sea Water Temperature Drop in the Coral Reef Area off the Thermal Discharge outlet in Nanwan Bay

Abstract

Abstract The aim of this study is to revisit and examine the cooling mechanisms in the vicinity of the thermal discharge outlet in the western region of Nanwan Bay, in light of newly available data sets. These data sets consist of simultaneous in-situ measurements and remote sensing data collected during June-July 2016. The data sets include (a) in-situ data from ADCP, uCTD, and bottom temperature gauges, (b) remote sensing current fields from High Frequency Coastal radar, and (c) satellite SST from Himawari-8. The analysis focuses on the correlation between eddies and cold events, and explores the role of an alternative cooling water mechanism, namely internal wave shoaling. Cold events and cyclonic eddies were identified using indices derived from surface current vectors, including divergence, vorticity, strain, Okubo-Weiss, and IROS. Internal wave run-up events were identified when the propagation speed of the bore, which exhibited a sudden temperature drop, agreed with theoretical predictions. Over the 28-day measurement period, the study identified 16 cold events in the presence of cyclonic eddies and 9 events without eddies. Furthermore, the findings indicated that the phenomenon of internal wave run-up had been underestimated. In summary, the study suggests that eddies may explain roughly 55% of the likelihood of cooling events occurring in Nanwan Bay, while internal wave run-up accounts for approximately 31%. These results underscore the coexistence of both eddies and internal waves in this particular region.