Assessing Marine Heatwave Variability in the Luzon Strait

The Luzon Strait, a 350-km wide channel located between Taiwan and the Philippines, connects the West Philippine Sea and the north Pacific Ocean. Multiple factors affect the circulation in the Luzon Strait, such as the Kuroshio Current, monsoon, and the West Philippine Sea circulation. Discrete periods of extreme ocean warming events, also known as marine heatwaves (MHWs), have been occurring longer and more frequently across the globe. Anomalous temperature events can cause drastic changes in the biogeochemical processes and trigger adverse effects on marine ecology in the surrounding areas. This study aims to understand the variation in MHWs in the study area (16-24°N, 115-126°E), focusing on the Luzon Strait, using a daily global 5-km sea surface temperature (SST) product from 1985 to 2022. Four points of known coral reef areas were also chosen to further assess the MHWs and their possible effects on marine ecology.  Six MHW indices were utilized to describe the frequency, duration, and intensity of MHW events. The highest frequency of 17 MHWs in a year occurred in 1998, while the longest duration per event of 144 days and the total duration in a year of 308 days were recorded in 2020 and 2021, respectively. The highest values for all three intensity parameters were recorded in 2021, with mean, maximum, and cumulative intensities reaching 2.62°C, 3.86°C, and 227.42°C-days, respectively. The spatial distribution of monthly SST and ocean current profile showed thermal areas and helped identify high-risk areas. Climate variations, such as El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO), were also explored as physical drivers of MHW in the study area. It has been observed that most of the years featuring MHW events at the four coral reef points occurred during the La Niña phase of ENSO, in conjunction with the negative phase of PDO, including 1998, 2010, and from 2020 onwards. Additionally, from 2016 to 2019, MHWs were observed at the same points during the positive phase of PDO, in conjunction with El Niño, La Niña, or Neutral phases of ENSO.