Background
Writing that "while it is widely known that TCs [tropical cyclones] generate damaging waves that can cause lasting impacts on coral communities (Harmelin-Vivien, 1994)," the authors say there is evidence that TC-induced sea surface cooling in reef areas "can reduce the severity and duration of bleaching (Manzello et al., 2007)." They note, for example, that "meteorologists have long observed that TCs cool the upper ocean layer via entrainment and upwelling of cold subsurface water (Price, 1981)," and that "satellite and in situ observations of the pre- and post-storm SST field have revealed cool anomalies (up to 6°C) which can persist for days to weeks (Price et al., 2008) and extend 100s of kilometers from the TC center (Stramma et al., 1986)." But they add that this consortium of facts has been "largely overlooked as a mitigating factor in coral bleaching."

What was done
Carrigan and Puotinen, as they describe it, utilized "historic global TC track data in combination with reef locations and retrospective thermal stress data to determine how TC activity near reefs varies spatially and temporally, and whether TC cooling is likely to affect reefs when thermal stress is high," since it logically follows that "given the large extent of a cool wake (100s of km), a TC could benefit a much larger area of thermally stressed corals than it would damage, resulting in a 'net cooling benefit'."

What was learned
The two researchers say their results indicate that "TCs likely play a role in coral bleaching dynamics on a global scale," and that they "clearly demonstrate that the likelihood of beneficial TC cooling is widespread across the Caribbean, where the recent active period of TC activity coincided with an upswing in thermal stress levels." In addition, they say that this phenomenon "enabled appropriately timed TC cooling to lower thermal stress at specific reefs," adding that "it is plausible that a TC could also disrupt an SST warming cycle before thermal thresholds are exceeded, further adding to the potential for TC benefits to reefs."

What it means
In assessing the results of their analysis, Carrigan and Puotinen say "it is apparent that TCs play a role in bleaching dynamics at a global scale." And the flavor of their discussion indicates that that role is positive.

References
Harmelin-Vivien, M.L. 1994. The effects of storms and cyclones on coral reefs: A review. Journal of Coastal Research 12: 211-231.

Manzello, D.P., Brandt, M., Smith, T.B., Lirman, D., Hendee, J.C. and Nemeth, R.S. 2007. Hurricanes benefit bleached corals. Proceedings of the National Academy of Sciences USA 104: 12,035-12,039.

Price, J.F. 1981. Upper ocean response to a hurricane. Journal of Physical Oceanography 11: 153-175.

Price, J.F., Morzel, J. and Niiler, P.P. 2008. Warming of SST in the cool wake of a moving hurricane. Journal of Geophysical Research 113: 10.1029/2007JC004393.

Stramma, L., Cornillon, P. and Price, J.F. 1986. Satellite-observations of sea-surface cooling by hurricanes. Journal of Geophysical Research 91: 5031-5035.