Estimating high-resolution Red Sea surface temperature hotspots, using a low-rank semiparametric spatial model

TL;DR

This paper develops a Bayesian semiparametric spatial model to accurately estimate and project high-temperature hotspots in the Red Sea, aiding in the conservation of endangered coral reefs.

Contribution

It introduces a novel low-rank semiparametric spatial model with a Dirichlet process mixture for tail inference, improving hotspot detection over existing methods.

Findings

Outperforms parametric and semiparametric alternatives in SST hotspot estimation.

Projects future hotspots under climate change scenarios until 2100.

Identifies large high-temperature regions threatening coral reefs.

Abstract

In this work, we estimate extreme sea surface temperature (SST) hotspots, i.e., high threshold exceedance regions, for the Red Sea, a vital region of high biodiversity. We analyze high-resolution satellite-derived SST data comprising daily measurements at 16703 grid cells across the Red Sea over the period 1985-2015. We propose a semiparametric Bayesian spatial mixed-effects linear model with a flexible mean structure to capture spatially-varying trend and seasonality, while the residual spatial variability is modeled through a Dirichlet process mixture (DPM) of low-rank spatial Student-$t$ processes (LTPs). By specifying cluster-specific parameters for each LTP mixture component, the bulk of the SST residuals influence tail inference and hotspot estimation only moderately. Our proposed model has a nonstationary mean, covariance and tail dependence, and posterior inference can be drawn efficiently through Gibbs sampling. In our application, we show that the proposed method outperforms some natural parametric and semiparametric alternatives. Moreover, we show how hotspots can be identified and we estimate extreme SST hotspots for the whole Red Sea, projected until the year 2100, based on the Representative Concentration Pathways 4.5 and 8.5. The estimated 95\% credible region for joint high threshold exceedances include large areas covering major endangered coral reefs in the southern Red Sea.