Climate scientist, Dr Michael Notaro, and his colleagues at the Nelson Institute Center for Climatic Research, University of Wisconsin–Madison, study climate variability and change, including how the weather, including monsoons, can be influenced by variations in vegetation in a given region of the world.

He and his colleagues used a fully coupled atmosphere–ocean–ice– land computer model that allows the atmosphere to interact with dynamic vegetation changes. They ran their initial calculations where the total vegetation cover fraction across the six global monsoon regions was reduced and the climatic response was assessed. Responses among the regions that were consistent included reductions in LAI, moisture and heat fluxes from the land to the atmosphere, and atmospheric moisture; enhanced atmospheric subsidence; and increases in ground and surface air temperature. The most distinct changes in vertical motion, precipitable water, and precipitation occurred along the edges of the monsoon season, with small changes in mid-monsoon rainfall. Unique responses to lower levels of vegetation cover were noted among the monsoon regions and some surprising results jumped out at the scientists. While the monsoon is delayed and weaker over northern Australia owing to diminished leaf area, it actually occurs earlier over China. The study suggested that the subtropical monsoon regions – cooler areas further from the equator – are characterised by a larger response in sensible heat than latent heat flux, while the opposite is true for tropical monsoon regions closer to the equator. Sensible heat describes the heat that’s absorbed or released by any substance while it changes temperature, while latent heat is the heat absorbed or released by a substance changing phase, e.g. solid to liquid or liquid to gas. Northern Australia experienced the most substantial decline in both moisture flux convergence – the concentration of moisture that precedes convection events like thunderstorms – and precipitation in response to reduced vegetation abundance.

What We Do to Our Vegetation Affects Our Climate

Dr Notaro concludes from this research that vegetation dynamics are most definitely important elements of the climate system, in particular over northern Australia. He agrees that climate models need to include better representations of observed vegetation and to be evaluated in terms of their simulated land-atmosphere interactions compared to observations. It is apparent that heat stress could reduce vegetation cover in certain regions, which could enhance surface warming. In the case of northern Australia, drought events can lead to loss of vegetation, which could amplify the drought intensity through positive vegetation–precipitation feedbacks. Ongoing farming practices in China and India and grazing in North America and northern Australia likely reduce the total leaf area and could lead to surface heating and a dampened water cycle. He and his colleagues are closing in on some of the specific answers to this phenomenon.

Dr Notaro investigated the hypothesis that subtropical and tropical monsoon regions showed unique responses to vegetation anomalies. He and his co-workers used a coarse global climate model, the Community Climate System Model (CCSM), and saw that reduced vegetation cover led to an earlier subtropical Chinese monsoon and delayed, weaker tropical Australian monsoon. However, significant climate and LAI biases in the coarse global climate model clouded the reliability of these findings. To fix this problem, the team used observed boundary conditions and applied them across China and Australia in the high-resolution Regional Climate Model Version Four (RegCM4). The model matched the observed dominance of crops, grass, and evergreen trees in southern China and grass and shrubs in northern Australia.

The team then ran a regional climate model for the period of 1960 to 2013. The Australian and Chinese monsoon regions’ LAI was modified in such a way as to look at contrasting vegetation effects between tropical and subtropical regions. He found that a greater leaf area supports reductions in albedo, air temperature, wind speed, atmospheric boundary layer height, and ascending motion and increases in diurnal temperature range, wind stress, evapotranspiration, and specific humidity. This leads to enhanced rainfall during Australia’s pre-mid monsoon season, but this was not found to be the case for China. Modified leaf area was found to cause dramatic changes in the temporal distribution and intensity of Australian rainfall events. Enhanced North Australian LAI supports a reduction in the frequency of dry periods, a greater frequency of drizzle or light-moderate rainfall periods, and decline in occurrence of extremely wet periods. In other words, the increase in LAI in northern Australia spreads the rainfall out over time so there are fewer dry spells and fewer heavy rain intervals. Inconsistencies between China’s monsoon response in RegCM4 and the one that Dr Notaro published a few years ago using CCSM are attributed to CCSM’s excessive forest cover and leaf area, exaggerated roughness mechanism, and deficient evaporation and transpiration response. Basically, that model ignored crops, so China was unrealistically represented as being covered with trees. Essentially though, the more forest you have, the greater effect you can have on even a significant climate phenomenon such as a monsoon. The implications are pretty impressive. Certainly, Dr Notaro aims to continue this line of investigation. But it’s more than just scientific curiosity that motivates him. Why is All of This Important? The timing and intensity of the summer monsoons has critical effects on, for example, the flooding in the Yangtze River Basin of China, drought in northern China and associated Yellow River flow and crop production both in China and Australia. It can also affect grazing stock and cattle productivity in Australia, along with the duration of fire-season across the Australian savannahs. A better understanding of the monsoon’s controls could help with seasonal predictions and agricultural planning. Moreover, validation of the computer model applied by Dr Notaro over the tropical Australian monsoon and subtropical Chinese monsoon will allow its successful application to other regions. This will potentially lead to the development of powerful climate prediction tools that can save lives and money. And perhaps we can try to plant appropriate trees and vegetation to actually help make a difference.

Why is All of This Important?

The timing and intensity of the summer monsoons has critical effects on, for example, the flooding in the Yangtze River Basin of China, drought in northern China and associated Yellow River flow and crop production both in China and Australia. It can also affect grazing stock and cattle productivity in Australia, along with the duration of fire-season across the Australian savannahs. A better understanding of the monsoon’s controls could help with seasonal predictions and agricultural planning. Moreover, validation of the computer model applied by Dr Notaro over the tropical Australian monsoon and subtropical Chinese monsoon will allow its successful application to other regions. This will potentially lead to the development of powerful climate prediction tools that can save lives and money. And perhaps we can try to plant appropriate trees and vegetation to actually help make a difference.