Hidden heatwaves and severe coral bleaching linked to mesoscale eddies and thermocline dynamics

Marine heatwaves (MHWs) are significantly impacting marine ecosystems, causing species migration, community shifts, mass mortality, and biodiversity loss. The severity of MHWs is typically assessed using remotely sensed sea-surface temperatures (SSTs), often calculated as Degree Heating Weeks (DHW) or Degree Heating Months (DHM) above a bleaching threshold. While these metrics capture surface heating, they often fail to account for subsurface conditions, which are crucial for understanding the impact on organisms living at various depths, especially in vertically stratified environments like coral reefs. Coral reefs often extend from the surface to depths exceeding 60 meters, with significant temperature gradients across this range. The lack of continuous in-situ temperature data at various depths has hindered the understanding of subsurface MHWs and their impact on ecosystem health. This study addresses this gap by analyzing a rare dataset of long-term in-situ and satellite observations from Moorea, French Polynesia, a location with a rich history of concurrent ecological and oceanographic data. The study investigates the vertical structure of MHWs and their links to mesoscale eddy dynamics and the cooling effects of internal waves. The goal is to understand how subsurface heat affects coral bleaching severity and the overall resilience of coral reef ecosystems in the context of climate change.

Numerous studies have linked coral bleaching in shallow waters to anomalously high SSTs and accumulated DHW, particularly during El Niño events. However, these SST-based metrics often only partially explain the observed variation in bleaching. Some studies reported less bleaching than predicted by DHW, hypothesizing coral acclimatization or shifts to heat-tolerant species. Conversely, some studies reported more bleaching than expected, suggesting the limitations of SST in predicting bleaching. This discrepancy highlights the need to consider factors beyond surface temperatures, such as genetic variation, cryptic species, and subsurface temperature dynamics. The spatial and temporal scales of SST data also significantly impact MHW severity estimates. High-resolution daily data, analyzed at finer spatial scales, are more effective at capturing localized heating events than coarser, regionally averaged data. The study acknowledges this and utilizes a higher-resolution degree heating days (DHD) metric to better compare events. Previous research demonstrated the potential for subsurface attenuation of heating due to regional upwelling and local internal waves. Internal waves can periodically transport cooler water onto reef habitats, creating complex vertical temperature dynamics and modifying the effects of surface heating.

The study utilized a unique dataset collected near Moorea, French Polynesia over 15 years (2005-2019). The dataset includes: 1. **Sea-surface temperatures (SSTs):** Daily SST values were obtained from the NOAA Coral Reef Watch ‘CoralTemp’ product at both regional (2° × 2°) and local (0.1° × 0.1°) scales. The maximum monthly mean (MMM) SST was used to define the bleaching threshold (MMM + 1°C). 2. **In-situ temperatures:** High-resolution (2-min intervals) temperature data were collected at depths of 10, 20, 30, and 40 m using Seabird temperature recorders directly mounted on the reef. An error in instrument setup resulted in 2-hour sampling intervals for the 40m logger between August 21, 2015, and August 25, 2016. 3. **Sea level anomalies (SLAs):** SLAs were determined using both in-situ measurements from bottom pressure recorders at 10 m depth on the reef and satellite altimetry data from the Copernicus Marine Service. Satellite SLAs were averaged over a 0.5° latitude × 1.5° longitude area north of Moorea. 4. **Regional hydrography:** Data from NOAA World Ocean Database (WOD) and Argo floats provided vertical temperature profiles in a 2° × 2° box around Moorea. **Data analysis involved:** * **MHW quantification:** Degree Heating Days (DHD) were calculated using a 12-day moving window to capture high-resolution heat accumulation. A similar Degree Cooling Days (DCDIW) metric was developed to quantify cooling due to internal waves. * **Internal wave analysis:** A filtering approach was employed to separate low-frequency temperature variations (multi-day weather patterns and seasonal effects) from high-frequency variability (internal waves). This allowed for the estimation of non-internal wave (NIW) temperatures and the calculation of Internal Wave Cooling (IWC). * **Coral bleaching assessment:** The prevalence of coral bleaching was determined using photoquadrats from two permanent sites in the Moorea Coral Reef Long Term Ecological Research (LTER) program. Images were analyzed using CoralNet software to quantify coral cover and bleaching. * **Mesoscale eddy analysis:** Satellite-derived sea level anomalies and surface currents were used to identify mesoscale eddies and examine their influence on thermocline depth and internal wave activity.

The study revealed significant differences between surface and subsurface MHWs. Although peak SSTs were similar in 2016 and 2019, the duration above the bleaching threshold was much longer in 2019, resulting in higher DHD. Subsurface temperature patterns differed considerably. In 2016, internal waves resulted in significant cooling at depth, mitigating subsurface heating. In contrast, 2019 showed reduced internal-wave cooling (IWC), leading to prolonged and severe subsurface heating. This difference in IWC was linked to contrasting mesoscale eddy fields. In 2016, a cyclonic eddy north of Moorea caused depressed SLAs and enhanced IWC, while in 2019 an anticyclonic eddy caused elevated SLAs and reduced IWC. The 2019 event led to extensive and prolonged coral bleaching across depths (54 ± 8.6% bleaching at 10m depth, with 71-72% of Pocillopora colonies bleached), resulting in significant coral mortality (as high as 42% for Pocillopora spp.). This contrasted with the minor bleaching and negligible mortality observed in 2016. The analysis of longer-term data (since 2005) showed a strong correlation between SLA depressions and increased IWC, and conversely, elevated SLAs and reduced IWC. The study also identified other MHW events with either high or low IWC, demonstrating the variability of subsurface heating and its ecological impact. The magnitude of temperature fluctuations due to IWC was comparable to long-term ocean warming. Argo profile analysis further supports this, showing that in 2019 the thermocline was significantly deeper and stratification weaker than in 2016.

The findings highlight the crucial role of mesoscale eddies and thermocline dynamics in modulating subsurface MHWs and their effects on coral reefs. The reliance on SST data alone for predicting coral bleaching can be misleading. High-resolution in-situ data are essential for capturing the complex interactions between surface and subsurface processes. The 2019 event demonstrates how reduced IWC, linked to mesoscale eddies and elevated SLAs, can amplify subsurface heating, causing severe and widespread coral bleaching even when surface temperatures are not exceptionally high. The results emphasize the need for integrated monitoring of both surface and subsurface conditions, including internal waves and mesoscale eddies, for accurate assessment of MHW impacts on coral reefs. The observed trend of increasing SLAs near Moorea since 2014 might indicate future increases in thermocline depths and reduced IWC, increasing the vulnerability of coral reefs to even moderate surface warming. This underscores the importance of considering 'ocean weather,' including subsurface variability, when evaluating climate change impacts on coastal ecosystems. Future research should explore the potential role of internal waves in nutrient upwelling and their contribution to coral resilience.

This study demonstrates the crucial role of subsurface heatwaves, driven by mesoscale eddies and thermocline dynamics, in causing severe coral bleaching and mortality. The reliance on surface temperature data alone for assessing MHW impact is insufficient. High-resolution in-situ data are necessary for accurate prediction and management of coral reef health in the face of climate change. Future research should focus on the interaction of mesoscale eddies and internal waves under a changing climate and their combined impacts on coral reefs.

The study focused on a single location, Moorea, limiting the generalizability of the findings. The dataset, while unique, is not representative of all coral reef ecosystems. The internal wave filtering methodology assumes a specific type of non-linear wave interaction with the reef slope, which might not be universally applicable. While the study linked mesoscale eddies to IWC, more research is needed to fully understand this complex relationship.