The seasonal rains called monsoons matter enormously to human affairs, from the Indian subcontinent to the American Southwest. Getting a better understanding of the forces that will shape these features of the climate system in coming decades is a big research priority, but also a very tough challenge given the many factors in play.
In a study published in this week?s Proceedings of the National Academy of Sciences, researchers analyzing monsoon patterns around the Northern Hemisphere since the 1970s conclude that there has been a substantial intensification of summer monsoon rainfall and circulation. The researchers say natural variations in the Pacific and Atlantic Oceans appear to be the main force behind the shift. Climate models have tended to project a different result.
I asked a variety of scientists working on these questions to evaluate the paper and related materials in an e-mail discussion including one of the authors, Peter Webster, a Georgia Institute of Technology climate scientist.
I distributed the abstract and a news release from the University of Hawaii, where the lead author, Bin Wang, is chairman of the department of meteorology.
Here?s an excerpt from the release:
Current theory predicts that the Northern Hemisphere summer monsoon circulation should weaken under anthropogenic global warming.
Wang and his colleagues, however, found that over the past 30 years, the summer monsoon circulation, as well as the Hadley and Walker circulations, have all substantially intensified. [Explore this Real Climate post to see how much this finding conflicts with what had been conventional wisdom.]
This intensification has resulted in significantly greater global summer monsoon rainfall in the Northern Hemisphere than predicted from greenhouse-gas-induced warming alone: namely a 9.5% increase, compared to the anthropogenic predicted contribution of 2.6% per degree of global warming.
Most of the recent intensification is attributable to a cooling of the eastern Pacific that began in 1998. This cooling is the result of natural long-term swings in ocean surface temperatures, particularly swings in the Interdecadal Pacific Oscillation or mega-El Ni?o-Southern Oscillation, which has lately been in a mega-La Ni?a or cool phase. Another natural climate swing, called the Atlantic Multidecadal Oscillation, also contributes to the intensification of monsoon rainfall.
Here?s a link to the paper and the abstract, followed by the discussion so far:
?Northern Hemisphere summer monsoon intensi?ed by mega-El Ni?o/southern oscillation and Atlantic multidecadal oscillation?
Bin Wang, Jian Liu, Hyung-Jin Kim, Peter J. Webster, So-Young Yim, and Baoqiang Xiang
Prediction of monsoon changes in the coming decades is important for infrastructure planning and sustainable economic development. The decadal prediction involves both natural decadal variability and anthropogenic forcing. Hitherto, the causes of the decadal variability of Northern Hemisphere summer monsoon (NHSM) are largely unknown because the monsoons over Asia, West Africa, and North America have been studied primarily on a regional basis, which is unable to identify coherent decadal changes and the overriding controls on planetary scales. Here, we show that, during the recent global warming of about 0.4?C since the late 1970s, a coherent decadal change of precipitation and circulation emerges in the entirety of the NHSM system. Surprisingly, the NHSM as well as the Hadley and Walker circulations have all shown substantial intensi?cation, with a striking increase of NHSM rainfall by 9.5% per degree of global warming. This is unexpected from recent theoretical prediction and model projections of the 21st century. The intensi?cation is primarily attributed to a mega-El Ni?o/Southern Oscillation (a leading mode of interannual-to-interdecadal variation of global sea surface temperature) and the Atlantic Multidecadal Oscillation, and further in?uenced by hemispherical asymmetric global warming.
These factors driving the present changes of the NHSM system are instrumental for understanding and predicting future decadal changes and determining the proportions of climate change that are attributable to anthropogenic effects and long-term internal variability in the complex climate system.
In my query to climate scientists, I noted that the work appeared to raise significant questions about the limits of climate models and pose a challenge for anyone arguing that recent shifts in monsoons are due to human-driven climate change. Here?s the discussion (I cleaned up some e-mail shorthand but the rest is as written; it is technical in spots):
Kevin Trenberth, Distinguished Senior Scientist, National Center for Atmospheric Research:
I do not find this result at all surprising, but some of the material is a bit misleading. I have not read the paper, however there continues to be confusion about changes in monsoons (in this case), or ENSO [the El Ni?o-Southern Oscillation], etc. and the effects of those changes in terms of precipitation and other effects. So the terminology makes a difference.
For instance please see Trenberth, K. E., 2011: ?Changes in precipitation with climate change.? Climate Research, 47, 123-138, doi:10.3354/cr00953
So while the monsoon winds might weaken the precipitation nonetheless increases (more bang for the buck) as a weaker circulation carries more water vapor (and latent energy). ENSO might weaken by some definitions but droughts and floods increase in magnitude. The way one frames the questions about the role of climate change matters. Decadal variability has been acknowledged in many other recent publications such as?
Dai, A., 2013: ?The influence of the Inter-decadal Pacific Oscillation on U.S. precipitation during 1923-2010.? Climate Dynamics, doi:10.1007/s00382-012-1446-5, in press.
?and extensively in climate models in the work of Clara Deser, see the paper in Nature Climate Change recently and articles such as this?
Deser, C., A. S. Phillips, V. Bourdette, and H. Teng, 2012: ?Uncertainty in climate change projections: The role of internal variability.? Climate Dyn., 38, 527-546, DOI 10.1007/s00382-010-0977-x.
Indeed regionally, interannual and decadal variability is still dominant in the climate record and will be for a long time. The whole idea of regional climate prediction that has come to the fore mainly because of need/demand is not based on sound science owing to fundamental predictability issues. How one does attribution is indeed an issue.
Andrew Turner, a climate scientist at the University of Reading in England (interviewed by Vikas Bajaj for the Green blog last year):
The paper?offers a useful framework for which decadal variations in the global (or northern hemisphere) may be explained via large scale modes of oceanic variability. This potentially could lead to added predictability for a certain proportion of monsoon rainfall variability.
However, this decadal predictability, ?necessary for infrastructure planning, energy policy, business development, and issues related to sustainability? (to quote the PNAS paper) is only useful to end-users (people on the ground) if rainfall in local regions can be better predicted. Basing local planning decisions on hemisphere-scale signals may be misguided, and so probably if any decadal predictions can be made for these hemisphere-scale portions of the monsoon then they need to be supplemented by information from other sources of predictability in order to target the local level. Whether this can be done is another matter.
A certain amount of care is needed in interpreting the results of this paper at a local or even regional monsoon level. For example the increasing trend in the coherent NHSM decadal precipitation shown in the paper (Figure S3B: the spatial pattern and associated principal component time series of the EOF) in fact suggest a weakening over recent decades in much of India and East Asia. This is in stark contrast to the headline result.
Correspondingly, in those regions I mention (which, by population, represent the most important regions of the global monsoon) much of the increasing precipitation signal is coming from oceanic regions: the eastern Arabian Sea, the Bay of Bengal, the South and East China Seas and Bay of Bengal. The declining signal over India shown by the GPCP decadal mode is broadly consistent with gauge measurements since the 1950s ? that several research groups including my own are trying to understand, perhaps relating to emissions of anthropogenic aerosol ? although there are discrepancies between these gauge-based data sets themselves (see our recent review in Nature Climate Change, for example).
We already know that (regional) monsoon variability on the scales for sub-seasonal to interannual are higher than the projected model trends of future mean monsoon rainfall (I?ve just seen that Kevin has mentioned this also). In some models, the decadal variability for monsoons such as the South Asian monsoon also outweighs the magnitude of the future trends, and in others it does not (the review above is one example showing this). We need to better characterise the decadal variability in the regional monsoons and understand why models show such different behavior.
Reacting to Trenberth, Peter J. Webster, an author of the new paper and climate scientist at Georgia Tech, wrote this, directed to Kevin Trenberth:
Kevin,
Thanks for your insight. But perhaps you might read the paper first before directing (misdirecting) the attention away from Andy?s questions.
To me, the issue is that there is a trend when one considers the global monsoon (and yes, Kevin has written about the global monsoon, too). I think that one of the problems that we have had in monsoon stuff is to consider the monsoons bit by bit. We received reviewer criticism from (I guess) Indian scientists who have spoken of a diminishing monsoon. Yet, India is a small place and their statistics are based on the average annual rainfall over India that doesn?t include rainfall from Pakistan, Nepal, Bangladesh or any countries in East Asia. I think that one has to be careful in drawing global conclusions from local or even regional trends or events (read Sandy!) What this paper does is show that if one carefully considers the larger scale aspects of the monsoon including the oceanic monsoon (see some of the papers by Winston Chao a decade or so ago: thoroughly recommend) then one can start to see trends on a global basis. I think that some times we get caught up in minitua or filligree of climate. So if we make a contribution in this paper it is that viewed holistically that the monsoons are changing and there is a supporting physical basis for this.
Webster added this note, directed to Andrew Turner (and me):
Dear Andys (two),
Andy T?s point is well taken but please read the previous note. We start by making the point that there is a trend in the global monsoon as we describe it. Yes, there are changes in the Indian monsoon but note that this a regional perspective. Let us say that we start at looking at the rainfall over India and then extrapolate that to the globe. One gets an opposite global trend that is completely misleading. And one is left trying to understand regional changes without a global context. But if one finds a global monsoon trend and if it turns out to be robust, then one has a handle to help understand local or regional changes. To me, the most important thing is finding large scale physics that help make sense regional changes.
Andy T is correct that the largest changes in the South Asian monsoon rainfall is occurring in the Bay of Bengal which, by the way, is the regional maximum in precipitation and has been for a long time. And, I believe that the Indian Ocean is the ocean of fastest sea-surface temperature increase. So I am hopeful that we can increase interdecadal prediction by understanding the links between global physics and regional response.
Years ago, Tim Palmer and I had a discussion about the impact of global forcing (e.g., CO2) on circulation patterns. It seemed to us that the impact would be on the ?normal modes? of natural variability. That is, one would expect impacts to be on the gross features of the climate system (PDO, AMO, ENSO?.). Whereas the magnitude and frequency of these phenomena may be forced to change, it is less likely that we would get new phenomena. We may even have written this up 20 years ago in some obscure journal. But I think that this point is pertinent to our paper.
I?ll add more as responses come in. If you have questions, post them and I?ll alert the group I?ve queried.
It?s important not to rely too much on a single study, of course. Here?s recent research, also from the University of Hawaii, that has a closer focus on southern Asia and draws different conclusions: ?Global Warming Shifts The Monsoon Circulation, Drying South Asia?. And looking further in the future, other researchers see greenhouse-driven warming becoming a big and harmful influence in that populous region: ?A statistically predictive model for future monsoon failure in India.?
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