Monsoon instability raises food questions for India

A street in Calcutta floods during monsoon season. After some decades of increasing rainfall, climate change could bring drier monsoons, said Jacob Schewe from the Potsdam Institute for Climate Impact Research. Credit: Mark E Dyer/Flickr

Monsoon rains in India may fail more frequently as climate change proceeds into the 22^nd century, German researchers said this week. That danger could be critical for farming in what is set to become the world’s most highly populated country by 2030, and would follow an already expected wetter period. “Previous studies showed that Indian monsoon rainfall would increase more or less linearly with global warming over the next century,” said Jacob Schewe from the Potsdam Institute for Climate Impact Research. “The monsoon can respond to climate change in a more complicated way. We’ve seen that it matters to look further into the future.”

In South Asia, summer monsoon rains fall as winds blow from the southwest Indian Ocean over the continent between June and September. They end when the wind direction reverses in September or October. What Indian monsoon rain seasons will do as the world warms is an important and difficult question that many researchers are trying to answer. Though more rainfall has been predicted, recent years haven’t matched that expectation. While factors like pollution have an effect, changes climate scientists already know a major climate pattern plays a very important part in monsoons.

“There is a coupling between the El Niño Southern Oscillation and the monsoon that’s been observed for a long time,” Jacob told me. In years when El Niño occurs, an air movement pattern called the Walker circulation pattern gets shifted eastward. That brings high pressure over India and weakens the monsoon. While some changes in El Niño are already happening, the Walker circulation is expected to weaken, but not for some time yet. That could mean scientists’ climate models don’t pick up its effects. “People have looked at monsoon changes but not many studies have looked beyond 2100,” Jacob said. “You really have to consider longer timescales – beyond 2100 – to assess the full range of consequences for the monsoon.”

Unstable cycles

Jacob and Anders Levermann therefore looked at five monsoons simulations between the year 800 and 2200 made using one of the most detailed ‘Earth system’ models available. To start with they looked at how often dry and wet years happened from 800-2005, to try and understand these rainfall patterns. “From that period we’ve simply picked a couple of years, and analysed them in more detail,” Jacob explained. “If you compare things like pressure patterns over the region as they develop in spring and summer, in a particularly dry year with conditions in an average year, you can see differences. We tried to analyse different climate variables, come up with an idea of the mechanism behind them and test that.”

How fast air moves upwards and downwards, humidity and pressure were important variables, Jacob and Anders found, hinting at how monsoons sustain themselves. Warm air above land rises, sucking in cool wet air from the sea. When rain releases the moisture from the incoming air over land, it warms the air even further, strengthening the cycle. Air over land warmed by rain makes more rain more likely in the days after, while dry days reduce the chance of rain in later days. Competition between these wet and dry cycles can make the monsoon system unstable.

In the wet cycles, ‘latent heating’ due to rain (or precipitation, P) makes warm air rise, and brings in moist air from the sea (top cycles). A lack of rain and heating brings down dry air from above and lowers the humidity of the monsoon winds, preventing rain (bottom cycles). In Jacob and Anders’ simple model, the chance of a wet or dry day depends on the heat accumulated during previous periods. Credit: IOP Publishing

Spring’s role

“Our hypothesis is that this instability would be most vulnerable to external influences during the springtime, when the monsoon’s developing,” Jacob said. “Factors like the Walker circulation, sea surface temperatures in the region, or snow cover on the Tibetan plateau have all been linked in some way to monsoon strength. They might be important in the onset stage of the monsoons, preconditioning the monsoon towards a stronger or weaker season.” Writing in a paper published in the scientific journal Environmental Research Letters this week, Jacob and Anders devised a simple model based on this idea. That model created monsoon patterns similar to their original simulations for the period through to 2005.

In both their original simulations and simple models, the instability’s impact becomes suddenly more important during the 21^st century and beyond. “In a coupled climate model you can have a response that’s an increase in rainfall for a couple of decades, followed by a sharp decrease, where you end up below the pre-industrial average of rainfall,” Jacob said. As part of that shift, changes in the Walker circulation mean more high pressure over India on average, separately from El Niño. “Once the Walker circulation weakens under global warming, that tends to suppress the monsoon more. If you’re trying to gauge the implications of climate change for monsoon rainfall and agriculture, you might have to take into account if this finding is robust, and you have to plan your adaptation measures accordingly.”

Average of Indian summer monsoon (ISM) rainfall over a 10-year period across all of Jacob and Anders’ five detailed simulations (red line) and fifty runs of their simpler model (grey line), in millimetres per day.. Credit: IOP Publishing

Journal reference:
Schewe, J., & Levermann, A. (2012). A statistically predictive model for future monsoon failure in India Environmental Research Letters, 7 (4) DOI: 10.1088/1748-9326/7/4/044023