Abstract
El Niño Southern Oscillation (ENSO) events modulate oceanographic processes that control temperature and productivity in tropical waters, yet potential interactions with low frequency climate variability, such as the Pacific Decadal Oscillation (PDO), are poorly understood. We show that ENSO and PDO together predicted (i) maximum sea-surface temperatures (SST), which were associated with coral bleaching and declines in coral cover, and (ii) maximum chlorophyll-a concentrations, which were associated with high densities of coral-predatory Acanthaster starfish, across the tropical north Pacific Ocean since 1980. Asynchrony between the positive PDO and negative ENSO (i.e., La Niña) was associated with peaks in annual SST. By contrast, synchrony between the positive PDO and positive ENSO (i.e., El Niño) was associated with peaks in chlorophyll-a. Both conditions led to ecological disturbances and significant loss of coral cover, however, spatial models revealed where impacts to reefs were expected under varying climate scenarios. The 2015/17 ENSO event was coupled with a positive PDO and resulted in high SST and Acanthaster abundances in eastern Micronesia, while positive coral growth occurred in western Micronesia. Our novel approach for forecasting coral growth into the future may be applicable to other oceanic regions with differing oceanographic modulators.
Highlights
Coral bleaching is a consequence of anomalous thermal-stress events that have led to coral mortality in extreme conditions and to changes in the composition of remaining coral assemblages[1,2]
The present study differs from both approaches and sought to predict changes in ocean temperatures across years to decades that were driven by climate oscillations associated with El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO)
ENSO events coupled with PDO variability accurately predicted changes on the coral reefs of Micronesia
Summary
Coral bleaching is a consequence of anomalous thermal-stress events that have led to coral mortality in extreme conditions and to changes in the composition of remaining coral assemblages[1,2]. The 1997/98 ENSO was attributed to extensive coral bleaching in western Micronesia[9,10], whereas the 2015/17 event impacted central and eastern Micronesia 2000 km farther eastward These disturbance patterns suggest that our ability to predict reef futures relies on a better understanding of how ENSO combines with secondary, low frequency climate variability to regulate ocean temperatures across the tropical Pacific Ocean[11]. Short-term predictive models can accurately forecast ocean temperatures 3–4 months into the future using autoregressive and linear inverse models that are extensions of recent climate observations[12] These types of models, for example, predict thermal stress events and form the basis for coral bleaching alerts that are derived from satellite SST data[13,14,15]. ENSO can cause dynamic shifts in coastal upwelling, currents, and regional rainfall[26]
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