Predicting depth-varying thermal stress on corals in the tropical Pacific using ocean surface variables and in situ temperature observations

The health of coral reef ecosystems across the globe is threatened by thermal anomalies in nearshore environments. Prediction of this stressor can be limited as coral reef ecosystems extend to depths well beyond where sea surface temperature (SST) is an indicator for the local temperature field. Observations, both in situ and remotely sensed, of additional ocean surface variables such as sea surface height (SSH) can aid in the assessment of these stressors in regions of the tropical Pacific. This is an area where SSH acts as indicator of subsurface temperature variability. Here we will describe nearly two decades of in situ forereef temperature observations made between 2000-2018 in the island nation of Palau at depths of 2-90 m and the insights they bring to understanding the predictability of subsurface temperature, and consequently thermal stress on coral reef ecosystems. An empirical model of temperature versus depth was built base on these in situ temperatures, as well as concurrent SST products and SSH observations. Depth averaged R² values for temperature modeled vs observed were greater than 0.8, where SST predicted temperatures in the upper mixed layer and SSH predicted temperatures into the thermocline. Additionally, we use an ocean state estimate covering 2009-2017, which captured the interannual and season variability at both Palau and Pohnpei, an additional site of ocean temperature observations between 10 and 130 m, to test the viability of using SSH and SST to predict subsurface temperatures across Micronesia. The role of climate variability in the form of ENSO was assessed to understand the connection between large scale and nearshore variability over the time scales of the observations and ocean state estimate. These studies lend insight to the predictability of temperature fields from the surface down to the mesophotic zone, an area being debated as a refuge for coral communities in a changing climate.