Equatorial refuge amid tropical warming

Abstract

Upwelling across the tropical Pacific Ocean is projected to weaken in accordance with a reduction of the atmospheric overturning circulation, enhancing the sea surface temperature (SST) increase relative to other regions in response to greenhouse forcing. In the central Pacific, home to one of the largest Marine Protected Areas (MPAs) and fishery regions in the global tropics, SSTs are projected to increase by 2.8°C by the end of this century. Of critical concern is that MPAs may not provide refuge from the anticipated rate of large–scale warming, which could exceed the evolutionary capacity of coral and their symbionts to adapt. Combining high–resolution satellite measurements, an ensemble of global climate models, and an eddy–resolving regional ocean circulation model, we show that warming and productivity decline around select Pacific islands will be mitigated by enhanced upwelling associated with a strengthening of the equatorial undercurrent (EUC). Enhanced topographic upwelling will act as a negative feedback, locally mitigating the surface warming. At the Gilbert Islands, the rate of warming will be reduced by 0.7 ± 0.3°C or 25 ± 9% per century, or an overall cooling effect comparable to the local anomaly for a typical El Niño by the end of this century. Since the EUC is dynamically constrained to the equator, only a handful of coral reefs stand to benefit from this equatorial island effect. Nevertheless, those that do face a lower rate of warming, conferring a significant advantage over neighboring reef systems. If realized, these predictions help to identify potential refuges for coral reef communities from anticipated climate changes of the 21st century.