Hyper East Asia Monsoons for the Foreseeable Future?

EGU 2011: Effects of aerosols on the East Asia summer monsoon

By Physics Today on April 6, 2011 10:53 AM | No TrackBacks

As East Asia's economy grows, so do its emissions of carbon and sulfur. The associated aerosols scatter and absorb solar radiation and modify the radiative properties of clouds. Those modifications, in turn, cause shifts in the monsoon patterns that affect the agriculture of one of the world's leading producers of rice and wheat.

Whether aerosols by themselves are responsible for the change in EASM behavior was one of the topics discussed at this year’s general assembly of the European Geosciences Union, which was held this week in Vienna, Austria.

The EASM is a seasonal change in atmospheric circulation and precipitation associated with asymmetric heating of the land and the sea. As the air over East Asia warms, it rises and creates a low-pressure area, which draws cool moist ocean air toward it. This inward movement generally leads to a predictable precipitation increase in July and August.

For the last three decades, however, the summer rain belt has shifted toward the south. Northern China has seen a drought in recent years while southern China has seen floods. Speaking at the EGU meeting, Jianping Li of the Institute of Atmospheric Physics, Chinese Academy of Sciences, pointed out that “the primary response of the EASM to global warming may be a southward shift of the rain belt, instead of an intensity change.”



“The maximum surface warming [increase up to 0.7°C over the past 30 years] occurs over high- to mid-latitudes [45°N–60°N],” explained Li. In the past, the highest temperatures occurred at lower latitudes. The warming rate that Li refers to is a change in temperature with time experienced in the north but not in the south. Because temperatures in the north are increasing while those in the south have remained relatively constant, there is a reduction in the pressure gradient, with the result that the moisture-laden sea air does not appear to be getting as far north as has been observed in the past. So rain now falls farther south although the average precipitation over the entire monsoon region remains the same.

Li uses models developed by the Intergovernmental Panel on Climate Change to investigate how surface temperature and the resulting monsoon patterns respond to anthropogenic and natural forcing. He tests model robustness by matching mean values, signal variations, and natural oscillations with data sets from the National Center for Atmospheric Research (NCAR) and the European Centre for Medium-Range Weather Forecasts.

“The largest uncertainty [of radiative forcing components] is aerosols,” Li said. While a main cause of planetary warming is carbon dioxide gas trapping long-wave radiation from Earth, aerosol particle effects are much more complex. Li added that “we should consider natural forcing too,” which includes varying solar input, volcanic emissions, and the Arctic oscillation.

Aerosol size and composition

Xiahong Liu, climate scientist from Pacific Northwest National Laboratory in Richland, Washington, also spoke at the EGU meeting. He pointed out that China's sulfur emissions have increased nearly fivefold between 1950 and 2011. At the same time, surface energy flux over East Asia has decreased by 5 W/m2 a decade. Sulfur emissions are converted to sulfate aerosols by oxidation in the atmosphere.



Liu takes the NCAR Community Atmosphere Model (CAM5) and applies it in a novel way to investigate the role of specific aerosol properties in the weakening of the EASM. Rather than looking at collective aerosol mass as has been done before, he examines how the size distribution and composition of different aerosols can bring about changes and be used to predict future effects. Sulfate and organic carbon (OC) generally have a cooling effect because they scatter incoming solar radiation back to space. Black carbon (BC)—also emitted by diesel combustion and biomass burning but with different chemical structure to organic carbon—traps radiation, which causes a warming effect.

In their particulate form, the combination of sulfate, OC, and BC can also modify cloud properties: A larger number of particles corresponds to a larger number of nuclei on which a fixed amount of water can condense in a lifted parcel of air. The increase in nuclei leads to a larger number of smaller cloud droplets and a more uniform cloud droplet size distribution than would occur without the extra particles. The resulting clouds produce less rain because rain drops form only when cloud droplets coalesce, which requires a broad particle size distribution.

Liu's experiments with CAM5 examine the effects of combinations of BC, OC, and sulfate emissions on atmospheric structure. Although the model is benchmarked against satellite measurements of aerosol absorption for 200 sites worldwide, including 10 in East Asia, there is still no satisfactory method of checking the composition of the absorbing aerosols. “We're trying to use Chinese data sets to get more information about composition, but sulfate emission is not reported to the government by very small factories,” added Liu. Actual aerosol effects may be stronger than those predicted by the model.

Weaker monsoons

Overall, the influence of aerosols on radiation and on clouds generates a net surface cooling over China of 1 to 2 kelvin. Aerosols also reduce the solar flux reaching the land. Since 1950, the total reduction in surface flux in China is thought to total 30 W/m2, with the biggest contribution occurring after 1980. Lower land temperatures flatten the land–sea temperature gradient, thus weakening the monsoon effect.

According to Li, because aerosols promote surface cooling, they cannot be responsible for the warming that causes the southerly precipitation shift. But according to Liu, aerosols do indeed shift the precipitation pattern. “Precipitation is loss of water vapor, and the latent heat capacity of the ocean balances this,” he explained. The latent heat of the ocean has a greater effect than that of the land, which drives the ocean–land temperature gradient and also the monsoon.

Liu agreed with Li, however, that the average amount of precipitation over the entire monsoon region remains the same. When water vapor is trapped in the aerosol-laden atmosphere, the amount of rain that falls in light showers decreases, but short periods of heavy preciptation intensify.

It might not be folks just in the southern part of East Asia that should be getting out their umbrellas, though. Long-range transport of pollution from East Asia to North America is a new cause for concern. As East Asia's economy continues to grow, so will its aerosol emissions. The effect of pollutant outflow as well as monsoon effects on agriculture might not be confined to East Asia alone.

Rachel Berkowitz