El Nino and the likelihood of mass coral bleaching

The seasonal forecasting system recently developed by NOAA Coral Reef Watch suggests that coral reefs in the Caribbean and in part of the central equatorial Pacific (the Line Islands, including Kiritimati or "Christmas Island") are at risk of coral bleaching in the coming months due to warm ocean temperatures. The forecast is due to the seeming return of El Nino in the Pacific.

The word from the NOAA Climate Prediction Center is that El Nino conditions are expected to prevail though the winter. Climate buffs can "see" the El Nino development in maps of sea surface temperature anomalies (warm water in the central and eastern Pacific) the thermocline (deepening in the eastern Pacific, meaning less upwelling off of South America) and sea surface height (the wind reversal on the equator means higher water in the central and eastern Pacific).

Not all El Nino events are created equal, and the models currently disagree on the current trajectory. This is especially important to remember when predicting the effect that El Nino conditions may have on other parts of the planet, what we luddites call "teleconnections". For example, a terrific paper by Kim et al. in Science demonstrated clear differences between the effect of "central pacific warming" and "eastern pacific warming" - which can both be classified as an El Nino event, depending what metric is applied - on ocean temperatures and hurricane tracks in the Caribbean.

One silver lining: For people living in the central Pacific, especially the parched islands of the southern Gilberts (Kiribati) Islands like Arorae, the development of El Nino conditions hopefully also means an end to the two-year drought that has claimed many of the coconut trees.

UPDATE: More on the Caribbean bleaching threat here