Abstract
<title>Abstract</title> <p>Climate change is reshaping coastal harmful algal blooms (HABs), yet how climate processes shape them remains poorly understood. We constructed a 1990–2025 country–year panel of coastal countries integrating country-level coastal exposure to ENSO, marine heatwaves (MHWs), HAB events and 13 environmental variables. Poisson pseudo-maximum-likelihood models showed that ENSO exposure had a modest linear relationship with HAB event counts, while MHW frequency exhibited a U-shaped nonlinear relationship. Probit models showed nonlinear associations of both climate variables with HAB occurrence probability, which rose increasingly rapidly at high MHW frequencies. The MHW–HAB relationship varied with the concurrent ENSO phase, strengthening during El Niño but weakening or reversing during La Niña. Spatial heterogeneity was strongest at the basin level, with El Niño affecting more basins than La Niña. At high latitudes, warm and cold ENSO phases had opposing effects, with a particularly strong positive La Niña effect. Sea surface height and iron strengthened MHW effects, while salinity and sea surface height moderated El Niño and La Niña effects, respectively. These findings establish a multiscale framework in which ENSO provides the large-scale climatic background for HAB events, whereas MHW frequency captures the key thermal stress of repeated heat extremes rather than transient local warming.</p>