Global Study of Monsoons Finds Ocean Variations Have Driven Recent Shifts

NOAA A thunderhead rises during the monsoon season in Arizona.

[March 24, 11:10 p.m.: Apologies for the blog silence and slow comment moderation. I'm camped on a beach with Pace University students making a film about efforts to balance fishing with marine conservation. Back online Tuesday.]

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.

Associated Press A group of people hold onto a rope to help them cross a street as a monsoon hits Bombay, India.

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 Niño-Southern Oscillation, which has lately been in a mega-La Niña 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 intensified by mega-El Niño/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 intensification, 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 the21st century. The intensification is primarily attributed to a mega-El Niño/Southern Oscillation (a leading mode of interannual-to-interdecadal variation of global sea surface temperature) and the Atlantic Multidecadal Oscillation, and further influenced 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 Niño-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 frame