A team of scientists at NYU Abu Dhabi have published a paper arguing that monsoon winds that provide rain to the west coast of India may shift northward in the future, leaving southern areas of the country drier than they are now. The work was led by Ajaya Ravindran, senior scientist at the Center for Prototype Climate Modeling (CPCM), and was published in the journal Climate Dynamics.
The CPCM works to improve computer models that are used to forecast future climate. "In this particular case, we wanted to know how the regional climate is responding to climate change, specifically the monsoon," said Sandeep Sukumaran, an author on the paper and a postdoctoral associate at the CPCM. "We found that in the historical period and in our simulations of future climate, the monsoon circulation is gradually shifting northward."
As the monsoon winds move east across the Arabian Sea, they meet the Indian landmass. Clouds are forced to a higher elevation when they collide with the Western Ghats, a mountain range that runs along India's west coast. Rising, the clouds cool and condense at higher elevations, dropping rain on the windward slope.
"We've found that consistent with the shift of these strong winds to the north, the rainfall pattern is also shifting," Sukumaran said. "The pattern has been shifting for about the last 30 years. And by the end of the century, we expect that it would shift by 1.5 to 3 degrees north, or about 165 to 330 kilometers."
Interestingly, Sukumaran noted that the shift in their observational data is happening faster than it occurs in their model: "the model is underestimating the shift."
The evolving monsoon will impact the Arabian Sea differently than it will India. "There is strong upwelling in the Arabian Sea during the monsoons due to the winds, and the pattern of upwelling seems to be changing," Sukumaran noted. (Upwelling is a phenomenon caused by wind moving parallel to the coast. This wind action on the sea causes cooler, deeper water to rise to the surface near the coast in a current perpendicular to the wind direction.)
Upwelling is important for fisheries, as the cooler water brings with it to the surface nutrients that are depleted at higher ocean temperatures. "But we need to do more experiments to know exactly how to understand how this shift in winds is changing coastal upwelling," Sukumaran added.
In time, changes like these could have consequences for the fishing industry in the Arabian Sea, and the wider region.
Creating Climate Models
Models for forecasting future climate are extremely complex, as there are so many variables that can affect a climate model, including ocean and air temperature, sea and atmospheric currents, and also variables that are more difficult to track like cloud formation and precipitation. Indeed, "precipitation is the most difficult variable to predict," Sukumaran said, "because it is so hard to represent clouds in climate models."
This difficulty arises from the way clouds form, the structure of climate models, and limits on computing power.
"The typical size of a grid in a global climate model is 100 kilometers, which is actually much bigger than the size of a real cloud," Sukumaran said.
For example, the typical base of a thunderstorm cloud is about 10 kilometers. But researchers cannot do climate simulations on a scale that small because of computing limitations; dividing a global climate model into a grid of 10-kilometer squares is far beyond current resources, not just at NYUAD, but anywhere.
But these complexities do not mean that current climate models aren't rigorous. To determine the accuracy of a climate model, scientists take observational data from the past and plug that data into a model. They then run a climate projection up to another point in the past. Called a hindcast, this technique allows researchers to compare a model's projection to observational data. If the projection closely matches up with observations, it is a successful model.
The Human Factor
Climate change is natural up to a point, but emissions created by human activity also have a great impact. The Intergovernmental Panel on Climate Change (IPCC) provides estimates for climate forecasters based on three different emissions scenarios. One is a "business as usual" scenario, Sukumaran said, in which little is done by governments to curb carbon emissions. IPCC also provides data for scenarios in which emissions are greatly reduced and moderately reduced.
With their models, the researchers project for different levels of carbon emissions in the atmosphere: "According to our models, higher levels of emissions in the atmosphere will cause the monsoon to shift more rapidly to the north, and in the stricter scenario, the shift will be the least," Sukumaran said.
And changes in monsoon winds will, of course, have effects on the ground. The shift northward in India may provide less rain to the fertile and critical agricultural regions of India's southwest coast. "We are looking at how shifts in the monsoon will affect not only India, but also how these changes will impact the Middle East as well," Sukumaran added.