Effect of anthropogenic aerosol forcing on climate change in the North Pacific Ocean during the 20th Century

Abstract:

Reliable future projection by the climate or Earth system model is crucial for the issue on future climate change. For the reliable future projection, uncertainty of the aerosol effect on the climate change should be reduced, because the uncertainty has been large. Therefore, it is essential to understand the effect of anthropogenic aerosol forcing on climate change in the 20th century. In this study, we have assessed the effect by a comparison between the 20th century historical simulations (20C and piAero) with the aerosol forcing fluctuated realistically over time and fixed in the pre-industrial condition by MIROC-ESM. We focus on the climate change in the North Pacific Ocean (NPO) due to anthropogenic aerosol emitted from China in the late 20th century.

In the comparison between the two simulations, there has been little difference in the global mean surface temperature (SAT) from 1851 to 1900. Then the difference appears and reaches to about 0.2 deg. C in 1950’s. After 1960, the difference in SAT between the two experiments become large.

For SST change in the NPO, small positive trend is found after 1900 in the piAero, but not found in the 20C. Thus, the SST difference in the NPO between the two experiments is significant after 1900. While the positive SST trend in the NPO has been large in the piAero after 1960, SST in the Central NPO shows the negative trend in the 20C. These enlarge SST difference between the two experiments.

The negative SST trend in the Central NPO in the 20C is likely to be attributable to an increase of aerosol emission from China. The aerosol increase, which is also found in the NPO, makes solar insolation into the surface decrease mainly through the aerosol indirect effect. This effect decreases SST. Also, the effect is seen in the boreal spring and summer. However, the effect is not found in the piAero.

The Pacific Decadal Oscillation (PDO), which is the principal natural variability in the NPO, has been investigated. Linear trend of SST change after 1960 in the NPO enhances (weaken) SST anomaly features in the PDO positive (negative) phase. Response of the atmosphere to the PDO variability also has been varied since 1960. However, we cannot conclude that periodicity of the PDO is affected by the increase of aerosol.