Sunday, June 05, 2011

Eli is an Evil Bunny

So ok, in the middle of one of the better tornados: weather or climate change? scrums Eli threw a curveball to John N-G, innocently asking

FWIW do the floods in Australia have anything to do with what people are doing to the atmosphere?
and got back a disputation

[Eli- The contemporaneous Reuters story does a decent job of summarizing the situation. (I say "decent" because I agree with it.) La Nina set up the weather pattern (there's no clear relationship between La Ninas and AGW), but the exceptionally warm oceanic temperatures in the area provided an extra dose of water vapor to the atmosphere, providing an add-on of 5%-7% to the flood-causing rainfall.

In a month or so I'll be in Melbourne attending an international earth science conference and I'll be keeping an eye (or ear) out for indigenous presentations on this topic.

We'll have an official answer in a year or so whether land use and urban planning contributed to the flood's impact to a significant extent. - John N-G]

Now, if Eli has taught the bunnies anything, it is to look at awkwardly put questions with a jaundiced eye. The sort of thing that Damon Runyon put best in a father's advice to a son going out into the world
Son, my father told me, there will come a time when you are out in the world and you will meet a man who says he can make a jack of hearts spit cider into your ear. Son, even if this man has a brand-new deck of cards wrapped in cellophane, do not bet that man, because if you do, you will have a mighty wet ear.
Observant bunnies note the use of the words "what people are doing to the atmosphere" and the answer (coming up) is a nice example for the next time Watts and Co. have a fit about using the term climate change instead of global warming, for indeed, we are doing more than simply taking a blow torch to the atmosphere, and, in broad outline, what we are doing can be understood in simple ways, although complex models provide needed detail.

Goes like this, springtime ozone depletion over Antarctica blows a hole in the belly of the ozone column. That means that there is a lot less ozone over the pole in the stratosphere. Eli has been at this game long enough to have heard every possible joke about sunburned penguins and more serious thoughts about this not doing good things for the phytoplankton upon which much of the oceans food chains depend, but it does something else that is obvious, it causes a strong, local (over the pole) cooling of the stratosphere, because the ozone is not there to absorb the sunlight which is flooding in as the sun rises in the spring. Such a cooling moves the tropopause upwards. Let us stress that this is both a local effect and and a short term one until the ozone hole heals in the late spring/summer.

Lagomorphs, and even pica everywhere ask, what does this have to do with weather (yes, yes, Eli knows) it rains in Brisbane, let alone Spain. To answer this question come Kang, Polvani, Fyfe and Sigmond in Science 332 (2011) 951 and to be honest a bunch of other people before them referenced in the "Impact of Polar Ozone Depletion on Subtropical Precipitation. They point out that there has been a strong movement of the Southern Hemisphere westerly jet towards the pole driven by the lifting of the tropopause associated with the Antarctic ozone depletion.

Kang and friends argue, supported by modeling that this effect is seen even as far as the tropics as the Hadley cells shift poleward, an observed trend.

Observations, cooling of the stratosphere due to the formation of the ozone hole, increased precipitation between 20 and 40 S latitude, shifting of the Southern Hemisphere westerly jet and the Hadley cell, poleward, and, oh yes, flooding in Australia can all be drawn together qualitatively as shown in the picture to the left taken from Feldstein's appreciation in the same volume of Science, and quantitatively with GCMs.


Michael Hauber said...

As a Brisbanite, I have to say that is quite scary. It shows that ozone depletion has contributed a significant moistening trend to this region, yet overall this region has gotten drier in the last 30 years. Implying that the drying trend would have been far worse without ozone depletion, and will be far worse in the future as we get our ozone back.

I do note that locally we get quite a lot of our rain from upper level troughs which are spun off from the jetstream, and I would expect that if the jetstream moved south this influence would lessen. I would also expect that that as the jetstream moves south then an upper level low that pinches off from the jetstream to this part of the world will be further away from the jetstream and so not pushed away to the east as hard. The Brisbane floods were caused by an upper level low which had separated from the jetstream and was close to stationary.

n-g said...

Thank you, evil bunny, for graciously calling this paper to my attention. There seem to be several questions raised.

For example, as Michael notes above, the past several years have been drought years for most of Australia, and the linear precipitation trend that the study seeks to understand covers only the period 1979-2000.

I found a good Bureau of Meteorology web site, and I looked at their trend map for summer precipitation:

This trend period is 1970-2010, so it's centered on the same period as the Science study, and should be a more robust measure of the transition before and after ozone hole formation. But it shows a negative trend, and the drying trend is strongest in the area of the Queensland floods!

Some further digging unearthed the fact that 1970 was the other major flood year for southeast Queensland, so using 1970 as the start of the period tilts things a bit, but any other starting date (even those that push 1970 into the latter half of the time series) also produces a negative precipitation trend for the area.

The existence of a long-term downward trend rather complicates things. The steady-state model runs in the paper should not be sensitive to particular years (indeed, various reference years for ozone were used in their simulations). So there's a downward trend, but without the ozone hole contribution it may have been even worse?

Maybe precipitation variability and my focus on a small area is masking the more robust long-term zonal-mean signal. But I'm hesitant to attribute zonal mean trends to particular locations in the absence of corroborating evidence at those locations. In my case, annual mean Texas precipitation is increasing while the expansion of the NH Hadley cell says it ought to be decreasing.

Here's an interesting quote from the abstract of a related paper by the same group (Polvani et al. 2011 J. Climate): "By specifying ozone and greenhouse gas forcings independently, and performing long, time-slice integrations, it is shown that the impacts of ozone depletion are roughly 2–3 times larger than those associated with increased greenhouse gases, for the Southern Hemisphere tropospheric summer circulation."

EliRabett said...

Yes, what is disturbing about this paper is that the effect is obvious on a qualitative scale once you think about it. The springtime cooling of the stratosphere will be massive and this moves the tropopause up which moves the various circulating systems poleward, so that the upward (drying) and downward (wetting) parts of the Hadley and other cells shift.

All this, on top of the stratospheric cooling from ozone destruction at mid latitudes by the homogeneous mechanism.

n-g said...

There's an ongoing controversy about whether tropical changes are affecting weather in the Antarctic or vice versa. I hope enough people have looked at the issue by now that the IPCC or somebody will be able to sort out the various bits of evidence and come to some reasonable conclusions.

Anonymous said...

The southern Hadley Cell has similarly been drifting southwards - the strongest expression of that is the decreased winter rainfall in the southwest of Western Australia. The Hadley Cell prevents cold fronts drifting north and across Western Australia, reducing our winter rainfall. If you look at a synoptic chart now for Australia it is a clear 'Summer' pattern with a high pressure system in the Great Australian Bight. In Winter this should over the centre of Australia.
If there was a significant 'climate change' in Australia that could be attributed to AGW, I would say that this decrease in rainfall since 1976 is it. We used to average about 880mm per year in Perth, now it is about 650mm.


Sou said...

I'm struggling to understand this. So far I gather that what is being said is that there is increased precipitation because of the ozone hole and its effect on the Hadley Cells pushing towards the poles down from the tropics. Is that right?

I can reiterate that while this past year was the wettest summer ever recorded down here in Victoria (south eastern Australia), it came after the longest hottest dry spell on record (several years of low rainfall and record high temperatures and lots of fires and little water anywhere). The wet we got down here this summer certainly came from the tropics. On the radar you could watch the rains from Cyclone Andrew and Cyclone Yasi moving down here to the south of Australia, and combined with wet coming up from the south west.

For those who don't know, most of Victoria usually has rains in autumn, winter and spring, with a mostly dry summer. Melbourne city and suburbs had several severe flash floods this summer, massive areas in western and northern Victoria were under water for several weeks, roads and bridges were washed away all over the state. Victoria is at the opposite end of Australia from Queensland (1500km south of Brisbane, 3000 km from the top of far north Queensland). Central Australia (inland) was inundated, as was some of the far north (Northern Territory), north west WA, parts of Tasmania - all record rain events.

If this ozone hole idea is for more rain down my way - it must only be when there's a strong La Nina like this past 12 months.

Or am I misunderstanding this completely?

cynthia said...

Texas is so parched and dusty that the people there are screaming out, begging God to deliver them a fat and juicy hurricane, which happens to be the tornado's closest and most watery cousins. And if God can't deliver on this, Neil Young will:

Recall that this is the guy who invented grunge rock without even trying. I suppose this is what being a natural is all about.

Anonymous said...

I haven't read it yet, but this paper (abstract) might shed some additional light on things.

Steve Bloom

Eric said...

I'd make a correction to n-g's comment about there being a 'controversy' about tropics --> Antarctic or vice versa. The answer of course is not one or the other, but both. I wouldn't characterize this as a 'controversy'.

The influence of ozone changes has been detected convincingly in spring/summer, and modeled convincingly (only) in summer. That's a big important effect and there is little question it has impact on the atmosphere far north of Antarctica. But it's just one or two seasons. In the other seasons (especially winter), there is no question that the tropics are influencing Antarctica, something that's been known and well understood for at least two decades. We added our little piece to this in our Nature Geoscience piece last week -- article here.

EliRabett said...

Another fine example of it not being one thing OR another but one thing AND another. OTOH very brave of you Eric. Eli is sure that Steve will be writing to you soon demanding you send him the entire tropical Pacific so he can audit it's temperatures.

fancyrat said...

Sou, Victoria (in Southern Australia for you Northern hemisphere people) is in the "Drier" section of Feldstein's diagram - 1500km South of Brisbane is about 13 degrees latitude. Melbourne is almost 38 degrees South.

The Southern Australian droughts of the last decade have been mostly associated with el Nino conditions, although the la Nina of 2007/2008 didn't seem to have much impact on the drought. We've been getting more el Nino and fewer La Nina events recently. I've been aware of the change in longitude of the cold fronts (it's had obvious effects on Melbourne's summer weather), but not aware of the cause. It looks like Victorian dryness has been due to both moved Hadley cells due to antarctic ozone depletion and more el Nino conditions due to greenhouse effects.

Tropical depressions (left over from cyclones) have a big rainfall impact in Victoria, especially on summer rain and floods.