The odd couple: hurricanes and coral bleaching

The Atlantic hurricane season is like the baseball season. Officially, it officially begins in the spring. Every year, the fans get all riled up for opening day. They then spend the subsequent months disappointed by the lack of drama and begin calling for the team to fire its manager (or hurricane forecaster). When the summer starts to wind down, the fans are reminded, once again, that all the big (ok, regular season) games tend to happen in late August and September.

With a trail of wreckage from Dean and Felix bearing down on Honduras and Belize (right), once again we're being reminded that hurricane activity often peaks right around now. Not in June, the time of the first peak in hurricane media coverage. Not in July, when everyone starts whining about the hurricane forecast. It happens in late August and September, when ocean temperatures in the tropical north Atlantic and the Caribbean tend to peak.

Coral bleaching events also happen during the annual peak in sea surface temperatures (SSTs). There are key differences, of course. Bleaching is driven largely by anomalously warm SSTs; UV light levels play a crucial role as well, although since the two variables tend to be correlated, temperature is a decent proxy. Hurricanes, on the other hand, are driven by a variety of factors, of which, the SSTs are only one.

You could say the ongoing furor over human-induced climate change in hurricane activity - the subject of Chris Mooney's recent book - is centered on this problem. The climate change --> warmer SSTs link is more or less straight-forward to test. The warmer SSTs --> more intense or more frequent storms is not. So, for example, the role of climate change in the 2005 Caribbean bleaching event, as we did in the paper published earlier this year, can be evaluated with more certainty than evaluating the role of climate change in the 2005 hurricane season.

While the SST link to hurricanes is more complicated than that to coral bleaching, it is not a complete surprise that strong hurricanes and coral bleaching events often occur in the same places each year. This year, there's been bleaching off Oman, near the region hit by Cyclone Gonu; in northern Madagascar, an area hit by a sequence of cyclones; in Okinawa, near the areas where typhoons have passed. So, the NOAA Coral Reef Watch data products like the Hotspot or DHW animations give a picture of the development of bleaching events and also a very rough idea of places that have experienced major hurricanes (try comparing the animations to this Google map of strong 2007 hurricanes). You can see that although thermal stress on corals is too low in much of the central and southern Caribbean where Dean and Felix have passed to cause coral bleaching, but temperatures are still above normal.

Like other odd couples, the combination of a coral bleaching event and a hurricane can be an utter disaster for the inhabitants of the area (e.g., corals weakened or killed by bleaching are then broken apart by the wave activity). A paper published earlier this year found an example of an opposite effect; cold water upwelling caused by the passage of hurricanes in 2005 reduced the thermal pressure on corals in the US Virgin Islands. Of course, the 2005 hurricanes and the bleaching were both driven by the warm water, so perhaps we would not have had one event without the other.