PNNL

The Science

Understanding global monsoons (GM) and projecting future changes relies heavily on climate models. However, climate models generally show pronounced biases in GM simulations, and the reasons for this remain unclear. In this study, researchers developed a diagnostic framework to identify the sources of the GM simulation biases from an energy transport perspective. This framework focuses on the observed processes of interhemispheric energy transport that is closely linked to summer precipitation in the Northern (NH) and Southern Hemisphere (SH) monsoon regions. To explore energy transport in monsoons, scientists compared data from Coupled Model Intercomparison Projects (CMIP) 5 and 6, a framework for climate model experiments. This study shows that negative biases in downward surface longwave radiation and northward energy transport are smaller in CMIP6 than in CMIP5 during the boreal (northern) summer, resulting in reduced dry biases in the NH summer monsoon in CMIP6. However, a positive bias in the top-of-atmosphere downward longwave radiation in CMIP6 degrades in simulations of the SH summer monsoon.

The Impact

Summer precipitation variations over the GM region have large impacts on freshwater resources, which support approximately two-thirds of the global population. This study evaluates the skill of climate models that participated in CMIP5 and CMIP6 using the above diagnostic framework. The multimodel mean improvement in CMIP6 compared to CMIP5 is demonstrated by the increasing skill scores for various GM metrics from 0.20~0.79 to 0.48~0.83. More specifically, the dry biases in the NH summer monsoon precipitation in CMIP5 are reduced by ~11% in CMIP6. In more than 41 percent of the NH monsoon region, a systematic improvement of the CMIP6 model skill is observed, leading to higher skill for monsoon precipitation intensity in the CMIP6 models. By demonstrating the connections between model biases in the monsoon and energy transport, this study shows that reproducing the meridional global atmospheric energy transportation—the movement of energy across the equator to maintain a local balance—is necessary for skillful GM simulation.

Summary

The GM system is closely linked to interhemispheric energy gradient and transport, which are driven by the seasonal cycle of solar incidence. It remains elusive whether scientists can link model biases of the monsoon to energy budgets or interhemispheric energy transport. In this study, researchers developed a diagnostic framework to understand and evaluate the performance of current climate models from the perspective of atmospheric energy transport. During the boreal summer, a pronounced interhemispheric thermal contrast promotes stronger southward (northward) moist static energy transport in the upper (lower) level, leading to a more vigorous monsoon circulation and increased precipitation in the NH. Conversely, during the austral summer, similar processes enhance monsoon activity in the SH. Interhemispheric energy transport is primarily driven by interhemispheric differences in the net energy flux to the atmosphere, which is associated with the downward longwave radiative flux from the top of atmosphere and the upward longwave radiative flux from the surface. The improvement in the GM simulation in CMIP6 results from the improvement in the northward-southward transport of atmospheric energy, which in turn results from improvement in simulating the downward surface longwave radiation. This study highlights that a reasonable reproduction of the meridional global atmospheric energy transport is necessary for a skillful GM simulation.

PNNL Contact

L. Ruby Leung, Pacific Northwest National Laboratory, ruby.leung@pnnl.gov

Funding

This research was supported by the Department of Energy, Office of Science, as part of research in the Regional and Global Model Analysis program area, Earth and Environmental System Modeling Program.