Climatic Impact of Vegetation Change in the Asian Tropical Region
Kazuo Mabuchi1, Yasuo Sato1, and Hideji Kida2
1: Meteorological Research Institute, 2: Kyoto UniversityAbstract
Using a global climate model which includes a realistic land surface model, several numericalsimulations were performed to investigate the impact of Asian tropical vegetation changes on theclimate. The morphological, physiological, and physical changes of the land surface vegetation in
the Asian tropical region certainly induce statistically significant climate changes in these and thesurrounding areas. That is, from the results of the bare soil and C4 grass experiments, the decreasein the roughness length, and from the results of the green-less experiment, the decrease of the latent
heat flux, exert strong influences on the horizontal and convective circulations of the atmosphere.
Consequently, the distribution of precipitation will undergo change. Other energy and waterbalances at the land surface are also influenced by the vegetation changes, and the induced changes
are generally statistically significant.
Keyword: Climate change, Land-atmosphere interaction, Vegetation change, Asian tropical region.

1. Introduction
Tropical vegetation controls the physical and biogeochemical interactions over climatically
influential areas on the Earth, and plays important roles in the formation of the regional and global
climates. Until recently, there have been numerous studies concerning the impact of vegetation
change in the tropical region. There have been, however, few numerical impact studies that target
the influence of vegetation change in the Asian tropical region. Using a global climate model which
includes a realistic land surface model, several numerical simulations were performed to investigate
the impact of Asian tropical vegetation changes on the climate. The control simulation, underconditions of the actual vegetation, and three vegetation change impact experiment simulations
were performed, with the results of the impact experiments compared with those of the control. The
horizontal resolution of the model used in these simulations was 1.875°, being finer than that of themodels used in previous vegetation change impact studies. As a result, it was determined that theeffects of vegetation changes in the Asian tropical region had spatially different features (Mabuchiet al. 2004a, Mabuchi et al. 2004b).
2. Experimental design
To investigate the impact of vegetation changes on climate in the Asian tropical region, threeexperimental areas were defined: the Indian subcontinent area (IND), the Indochina Peninsula area(ICP), and the maritime continent area (MTC). The IND, ICP, and MTC areas are mainly coveredby the grassland, tropical seasonal forest, and tropical rain forest types of vegetation, respectively.
In the control integration, the actual global vegetation and climatic SST values were used. Inorder to estimate the initial values of soil water content, including the ice content in the soil and soiltemperature, a 10-year spin-up calculation was carried out. Using the soil values obtained from thespin-up calculation, the control integration was continued for 20 years.
After the control integration, three vegetation change impact experiments were performed:
the bare soil experiment (BS), the C4 grass experiment (C4), and the green-less experiment (GR). In
the BS experiment, it was assumed that the vegetation on the ground was almost removed. In the C4experiment, while the morphological and physical parameters were set as C3 grass type, thephysiological parameters associated with the photosynthesis processes for C4 plants were used. In
the GR experiment, the types of vegetation in the experimental areas were same as those in thecontrol, but the greenness values of the vegetation in the areas were all set to zero. In the BS and C4
impact experiments, the actual vegetation types in the experimental areas (the IND, ICP, and MTCareas), were changed to a single vegetation type for each of the impact experiments. In each impact
experiment, a 10-year spin-up integration was first performed, starting from the soil conditions at
the end of the control run, and then the main experimental impact time integration was continued
for 20 years under the changed vegetation conditions. The results of these three 20-year impact time
integrations were compared with the results of the 20-year control integration.
3. Results
The morphological, physiological, and physical changes of the land surface vegetation in theAsian tropical region certainly induce statistically significant climate changes in these and thesurrounding areas. That is, from the results of the bare soil and C4 grass experiments, the decreasein the roughness length, and from the results of the green-less experiment, the decrease of the latent
heat flux, exert strong influences on the horizontal and convective circulations of the atmosphere.
Consequently, the distribution of precipitation will undergo change. Other energy and waterbalances at the land surface are also influenced by the vegetation changes, and the induced changes
are generally statistically significant.
The vegetation changes were implemented only in the Asian tropical region. There were,
however, possible influences of the vegetation change on the mid-latitude atmospheric circulation.
It was considered that the vegetation changes from the forest type to grassland or bare soil induced
modifications in the Hadley and Walker circulations. In particular, the divergence/convergenceanomaly pattern that appeared at the upper atmospheric level in DJF in the C4 grass experiment wasvery similar to that of an ENSO event. The height anomalies at the 500-hPa level were also similar
to those found in an ENSO event. The possibility exists that the deforestation of the Asian tropicalregion could induce similar teleconnections as those associated with ENSO events.
The influences of vegetation changes in the Asian tropical region were more complicated thanthose in the Amazon. One reason for this was that the Asian tropical region is strongly influenced by
the Asian monsoon circulation; another reason being that the land-sea distribution and thedistribution of vegetation in the Asian tropical region are not as simple as a tropical rain forest likethe Amazon.

References
Mabuchi, K., Y. Sato, H. Kida, 2004a: Climatic impact of vegetation change in the Asian tropicalregion. Part I: Case of the Northern Hemisphere Summer. J. Climate (in press).
Mabuchi, K., Y. Sato, H. Kida, 2004b: Climatic impact of vegetation change in the Asian tropicalregion. Part II: Case of the Northern Hemisphere Winter and Impact on the Extratropical
Circulation. J. Climate (in press).

Fig. 1. Comparison of the JJA mean results of the impact experiments with those of the control
integration (impact - control). The differences in the velocity potential (10 6 m 2 s –1) and thedivergence/convergence of wind vectors between C4 and CN are indicated. The upper panel
indicates the results at the surface, and the lower panel those at the 250-hPa level. The areas where
the Student’s t-test values indicate statistically significant differences (at the 95 % level) are shaded.

Fig. 2. Comparison of the DJF mean results of the impact experiments with those of the control
integration (impact - control). The differences in the velocity potential (10 6 m 2 s –1) and thedivergence/convergence of wind vectors between C4 and CN are indicated. The upper panel
indicates the results at the surface, and the lower panel those at the 250-hPa level. The areas where
the Student’s t-test values indicate statistically significant differences (at the 95 % level) are shaded.