Hadley Cell configuration and switch to Greenhouse Climate

[jonthed]

Hello.

I remember reading about this more in depth a while ago and see it crop up in various discussions every now and again on this forum, but was hoping somebody could point me in the right direction for the current state of the science on this.

I'm referring to the fact that at certain conditions the Hadley cells rearrange to a single cell (per hemisphere) system, leading to a 'Greenhouse' Climate with much more equable conditions from the equator to the poles.

I have tried to find what I can but have not found very much.

I found this paper:

http://www.fields.utoronto.ca/programs/scientific/10-11/biomathstat/Langford_W.pdf

Which seems to suggest it is the temperature gradient between the equator and the poles that governs this switch, and that the system exhibits hysteresis between the two states.

I also found this one which had quite a bit on the switch between the states:

http://www.seas.harvard.edu/climate/eli/research/equable/hadley.html

There were a few other papers I found but none seemed to give me the kind of answers I was hoping to find.

Basically, is there science now supporting the fact that as the temperature gradient decreases the Hadley Cell system will switch states? And if so, have they got an estimated range for that value? Then obviously I would like to know how far away from that point we are now!

If the Hadley Cell does flip to a single cell 'Greenhouse' Climate (which it seems the hemispheres can do independently) this will have massive implications for the changes we can expect, especially to the cryoshpere. The GIS will surely disappear much faster than currently predicted, as will the permafrost. Possibly even the clathrates.

So does anyone have any up to date information on this aspect of the science? Are the jet stream and Hadley cells exhibiting behaviour that should be getting us worried?

Thanks for any contribution you may have to offer.

(If there is an existing thread for this please let me know, I couldn't find one)

[icefest]

Here are some posts discussing the topic, but I think one unified thread is an improvement.

http://forum.arctic-sea-ice.net/index.php/topic,317.msg22333.html#msg22333
http://forum.arctic-sea-ice.net/index.php/topic,682.msg22393.html#msg22393
http://forum.arctic-sea-ice.net/index.php/topic,130.msg14973.html#msg14973 << I like this one.

[AbruptSLR]

jonthed,

My two-bit input to this topic is that I believe:
(a) The chances of switching to an equable atmospheric pattern before about 2080 to 2090 are minimal at best, as there simply is not enough GHG in the atmosphere before then, even in the RCP 8.5 scenario; however,
(b) If we follow a BAU GHG emissions scenario to about 2080 and if the Earth System Sensitivity, ESS, by that time is 4.5 degrees C, or more, then I think that it is possible that the atmosphere could possibly start a decades long process to flip into an equable mode; which in my opinion would be a terrifying period filled with Super Cyclones, and untold other climate change disasters; which would be so bad that they would induce governments to try geoengineering to try to stop such an occurrence.  As to whether, we stay on a BAU pathway, and whether the ESS could get this high this century (let alone whether geoengineering would help or possibly make matters worse), remains to be seen.

Best,
ASLR

[colding]

I hope I'm not making a fool of myself by repeating common knowledge, but I stumbled upon this from 2010:

http://www.meteo.psu.edu/~sbf1/papers/EQUABLE.pdf

Cheers,
  jules

[DoomInTheUK]

My understanding is that, just like sea ice, it will initially be a seasonal issue.

It might well be a century or more before the climate settles on a permanent single cell system in the Northern hemisphere and maybe a few more centuries for the Southern.

In the run up to that, the system will switch back and forth with periods of 3, 2 or 1 cells. Due to 2 cells being an unstable state, it will fairly quickly flip between 1 or 3. We may have periods of days or weeks with no jet stream, only for it to re-emerge. Heaven knows what weather patterns will occur during these spells.

You'll certainly need to see an ice free Arctic before it happens - that's a pre-requisite to decrease the temperature differential between poles and tropics. My WAG would be another 20 years and the process may be getting started.

If you think we've been having some wierd weather lately, just wait. Lot's more 'interesting times' ahead.

[ccgwebmaster]

(a) The chances of switching to an equable atmospheric pattern before about 2080 to 2090 are minimal at best, as there simply is not enough GHG in the atmosphere before then, even in the RCP 8.5 scenario; however,

What levels of GHG would be needed for an equable atmospheric pattern, at minimum? The flaw of even RCP 8.5 is surely that it isn't taking into account potential natural feedbacks (magnitude and rate an unknown quantity here of course).

[AbruptSLR]

ccgwebmaster,

Sorry for the slow response, but per the attached RCP 8.5 scenario, I am implying that the CO₂ levels will need to be around 800ppm (note that the RCP 8.5 includes other GHGs like methane, so I am only talking about the CO₂ portion of the GHG mix) before switching to an equable climate.

Best,
ASLR