Time-Varying Upper Ocean Circulation and Control of Coral Bleaching in the Western Tropical Pacific

The western tropical Pacific Ocean (WTPO) features complicated ocean circulation systems and has the warmest world open-ocean waters. Small upper ocean temperature change there can exert significant impact on the regional coral reef ecosystems. In the past three decades, moderate to severe coral bleaching events have been observed in the WTPO surrounding Palau in 1998, 2010, 2016, 2017, and 2020. Reflecting the diversity of El Nino-Southern Oscillation (ENSO) variability, the observed coral bleaching severity does not correspond simply to the amplitude of an ENSO index, such as Nino-3.4. By conducting an upper ocean temperature budget, we found the time-varying upper ocean circulation advection acted to damp the anomalous surface heat flux forcing and played critical roles in controlling the surface ocean thermal conditions around Palau. This happened either directly via the advective temperature flux convergence, or indirectly through the pre-conditioning of upper ocean thermal structures.