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« Workshop Discussion Threads
Questions for Jenni Evans
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The first question is whether the net effect of all hurricanes has a cooling effect (albeit slight) on the tropical oceans in the present climate. If it does, then depending on how things change, this signal might also change.
In terms of current simulations of a warmer climate, if the sub-surface ocean cools and the surface warms, then the cooling at the surface due to this mixing from below will be larger (a bigger change in temperature).
What is my confidence? Low at this point, because we have to answer the question of whether this is a season-long net feedback and my inferences about the changes in SST and sub-surface temperatures from GCM would have to be right.
So why did I pose this idea? Because it has been discussed and debated quite widely and because I felt that thinking about hurricane impacts back *onto* the climate system is important. If hurricanes contribute to the net climate that we observe - and I think that they must - then we need to think about their interactions in both directions.
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Theories about the Hadley cell and the midlatitude storm track tie their strength to the magnitude of the difference between the temperatures in the tropics and those near the poles.
If the poles warm more than the tropics these circulations could reduce in strength, since less heat has to be transported out of the tropics to make the temperature difference smaller.
One signature of the Hadley cell in the tropics is in the amount of deep convection (the thunderstorms we need for developing hurricanes). The stronger the Hadley cell, the more thunderstorms we need to help with the heat transport. So, a weaker Hadley cell likely means less convection in the tropics - or less efficient convection (more smaller clouds and fewer really intense ones).
This is the basic idea for why reducing the equator to pole temperature gradient could cause a reduction in some aspects of hurricanes (most likely their frequency or the average lifetime - number of days - for an individual storm).
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Past simulations have tried to address the problem of a large climate shift by taking into account the different patterns of ocean currents resulting from different continental configurations. This won't get you the whole way with hurricanes but, coupled with information about the other factors we talked about (deep thunderstorm activity, monsoons, wind changes with height) it will give you a sense of what might happen.
However, if the climate crosses over a bifurcation point such as you describe, I think that all bets are off...
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I went back to check myself on the ocean sub-surface cooling. I realized that it had come from observations from the recent climate - see chapter 5 of the IPCC report. I'm attaching a couple of graphics taken out of that report.
Fig 5.2 shows the spatial distribution of ocean heat content (per unit area), showing decreasing trends in the tropical Indian Ocean and western tropical Pacific.
I will post an accompanying IPCC Fig 5.3 in a following message - I can't figure out how to attach two files now :(
Fig 5.3 shows T(lat,z) for each ocean basin. From this figure, the decrease in heat content comes from a sub-surface decrease in zonally averaged temperature. (I attached a copy cut out of the report since the IPCC copy on their website had a white out region not in the report copy).
The explanation given for this sub-surface cooling is a shallowing of the thermocline due to more active ENSO in the past 20 years or so. If we take the current model projections of a more ENSO-like mean climate, this cooling *may* persist. This projection is based on a shallower thermocline resulting from different tropical surface wind stresses due to the change in the Walker cell in this scenario.
Does this make sense? Certainly this cooling is not a confident prediction, but it is consistent with recent observations and current explanations for those observations.
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See the previous post for discussion of this figure and how I see it fitting into projections of sub-surface cooling.
Attachments:IPCC WG1 Fig 5.3 (Acrobat (PDF) 84kB Oct21 08)
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