One of the nice things about a being a scientist is that you can sometimes go to workshops and meetings and actually hear something new now and again. At the well-timed Tropical Cyclones and Climate Workshop at Columbia a week or so ago, I got a chance to sit in and listen to Emanuel, Landsea, Knutson and Bell talking about their recent work, but I also got to hear some new voices (and issues) in the discussion. It was all pretty enlightening even if it didn’t end up in complete reconciliation of the different views.
Data quality: No-one was happy with the current state of the historical Tropical Cyclone (TC) databases, and some questions were even raised (by Landsea) about the quality of the satellite-era record (though no evidence was presented that demonstrated a real problem). However, no-one questioned the recent rise in hurricane activity in the Atlantic.
Modelling: Though it often seems as if the Knutson and Tuleya paper is the only relevant modelling study, there were quite a few new studies talked about that will be interesting – a new Japanese study that actually has high enough resolution to simulate (reasonable?) TCs, season-by-season studies of hurricanes using the GFDL hurricane model and boundary conditions from the reanalyses. Though it seems like a good idea, no-one has yet started using the IPCC AR4 model runs (with all the relevant 20th C forcings) to see if there is any support for a forced signal over the instrumental period…
Potential Intensity theory: This is the theoretical framework that most of this discussion falls into and a number of relevant points were made by Emanuel and others. I should point out that these points were simply new to me (because this isn’t my field) but are worth highlighting for the other interested amateurs out there.
- PI theory relates the maximum potential intensity that a hurricane could reach given the temperature gradient from surface to tropopause (the outflow) and the surface heat flux.
- The key ‘outflow’ temperature is not in the ‘mid-tropopshere’ but at the tropopause, and so the surface to mid-troposphere moist static stability (the lower tropospheric lapse rate) is not actually part of the theory and (apparently) hasn’t been shown to be a particularly important variable for intensity in observations.
- tropopause temperatures can be influenced by more than just greenhouse temperatures – volcanic eruptions and ozone depletion came up as additional factors
- the surface heat flux term is where the link to SST comes in, but depending on what is actually changing, there could be very different correlation between the heat flux and SST. For instance a change in radiative effects would produce a different SST-heatflux correlation than changes due to ocean circulation. Thus there is no unique theoretical relationship between SST and hurricane intensity, you really need to know what the real underlying change is. So looking at the results from imposed SST changes in an atmospheric model is not necessarily a useful analog for the intensity/SST link seen in the late 20th Century.
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.
Atlantic Multi-decadal Oscillation: The AMO, as an alternative reason to global warming for the changes in Atlantic SST, didn’t get much support here. Emanuel pointed out the evidence does not support the existence of a multidecadal pattern in the North Atlantic that is distinct from global SST trends. Bell, whose talk was heavily based on the notion that the Atlantic multi-decadal patterns are coherent and linked to SST variability didn’t particularly insist on the AMO link and appeared to concede that it could be a forced pattern.
NOAA’s ‘official’ position: This was highly criticised by many speakers, mainly on the grounds that it isn’t up to an agency to have an ‘official’ position on the science at all, let alone one that is not a consensus even within the agency. Thus the modified NOAA statement was welcomed. A number of NOAA employees were present, and Chris Landsea specifically criticised the agency’s previous position.
All in all, one ends up being just as confused as ever, but to paraphrase someone famous, I ended up being confused about maybe more fundamental questions than I was before the meeting. It’s not a reconcilliation, but it is progress of a sort…
As an aside, the three bloggers there (myself, RP Jr. and Chris Mooney) managed to find time for a pleasant glass of wine together – nice to finally meet them!