Ocean interactions: Claudia Pasquero, following on from some of the comments that Emanuel made, showed the difference in modelled hurricanes as a function of including an interactive ocean component. One effect is to reduce the intensity over a case with fixed SST (because the hurricane cools the surface through enhanced vertical mixing),
but a more interesting effect is that if you change the heat content of the mixed layer (and hence the intial SST), the change in intensity is much greater than if you simply change the SST. This was a good demonstration of Emanuel's point - it matters why SST changes.http://www.realclimate.org/index.php/archives/2006/04/tropical-cyclones-workshop/#more-280We saw this last season. Warm "deep vortexes" were what allowed at least some of the cat-5 hurricanes to intensify. Vertical mixing is no longer acting as a brake on hurricane development. Instead of bringing cooler deep water to the surface, it's just more warm water.
What's interesting is, to date I haven't seen anybody studying the causes and/or implications of these deep vortexes, although I'm sure somebody out there must be studying it. Also interesting is that I get the sense that the recent studies of SST correlation with hurricane intensity do not take the depth of warm water into account. Just surface temp measurements. If I read these people correctly, that alone might conceivably explain their failure to account for the 80% increase in cat-4/5 storms.