Land surface processes and monsoon*Yongkang Xue1,2, W.-P. Li1,4, F. de Sales1, C. R. Mechoso2, H.-M. H. Juang3, C. Nobre51). Department of Geography, UCLA, 2). Department of Atmospheric & Oceanic Sciences,
UCLA, 3). NCEP, NOAA, 4). National Climate Center, China Meteorological Administration,
5). Centro de Previsão de Tempo e Estudos Climáticos (CPTEC/INPE/MCT)*Yongkang Xue, 1255 Bunche Hall, Department of Geography, UCLA, Los Angeles, CA 900951524, USA. email: yxue@geog.ucla.eduAbstract
This paper presents the possible impact of vegetation processes on East Asian and SouthAmerican monsoon development. Difficulty and challenge in study of land surface/monsoon
interactions are discussed. Case studies are presented to show how vegetation process affects
intraseasonal monsoon variability. We compare simulations by a NCEP GCM coupled with two
different land surface parameterizations, a two-layer soil model with no explicit vegetation
representation, and a more comprehensive biophysical model. It was found that vegetation-induced
heating produced large scale air flow tuning at the monsoon onset stage in East Asia, Africa, andSouth America, which affected the extent and intensity of the monsoon onset. It also found that in theEast Asian monsoon season, vegetation processes produce different spatial distributions of the Bowen
Ratio, in turn influencing the low-level temperature and pressure gradients, wind flow (throughgeostrophic balance), and moisture transport. This process creates the great east-west thermal gradient,which may contribute to the abrupt northward jump of the East Asian monsoon. In South America,
the seasonal varied sensible heating in Savanna and Shrub lands produced anticyclone system, which
prevented the early monsoon onset in October and early monsoon merge with ITCZ in December,which is the beginning of the monsoon mature stage. The different characteristics of land/monsoon
interaction in East Asian, African, and South American continents were also addressed. By and large,
the results show that feedbacks between vegetation and the atmosphere are crucially important for
proper simulation of intra-seasonal monsoon evolution, including its intensity, the spatial distributionof precipitation, and associated circulation at the continental scale, but not at planetary scale (after
excluding albedo and initial soil moisture).
Key words: Monsoon, vegetation, East Asia, South America, GCM

1. Introduction
Differential heating of the land andthe ocean has greatly influenced the strength,
duration, and spatial distribution of large-scalemonsoons. The land surface processes arecrucial in modulating the monsoon processes.
Although several GCM studies (i.e., Xue andShukla, 1994; Meehl, 1994; Lau and Bua
1998; Dirmeyer, 2000; Xue et al., 2004;
Koster, et al., 2004), which explore potential
role of land surface processes inhydrometeorologicalpredictability,
have
consistently shown that soil moisture andvegetation cover have the great impact on theclimate in monsoon regions, but role inintraseasonal monsoon variability is poorly
understood, as is the role of parameterizationsof land surface processes in seasonal
prediction. Furthermore, different models
exhibit very different sensitivity to the land
surface forcing (Koster et al., 2004). These
difference may be rooted in the ways that, forexample, they treat the surface water andenergy balances in their models, they
parameterize the vegetation/soil processes,
they couple with atmospheric model,
especially the PBL and cloud processes, or thedata that they use to specify the land surfacecondition.
Case studies are presented to showhow vegetation process affects intraseasonalmonsoon variability (Xue et al., 2004a, 2004b).
We compare simulations by using a NCEP
GCM coupled with two different land surface
parameterizations, a two-layer soil model withno explicit vegetation representation, and amore comprehensive biophysical model. The
results show that feedbacks between
vegetation and the atmosphere are crucially
important for proper simulations of intraseasonal monsoon evolution, including its
intensity, the spatial distribution ofprecipitation, and associated circulation at thecontinental scale.

2. East Asian study
Results of our general circulation
model investigation suggest that vegetation
processes
producedifferent
spatialdistributions of the Bowen Ratio (sensibleversus latent heating), in turn influencing the
low-level temperature and pressure gradients,wind flow (through geostrophic balance), andmoisture transport. This process causes thelatitudinal heating gradient that contributes tothe clockwise and counterclockwise turning of
the low-level wind on the East Asian, African,and American continents during the earlystages of the monsoon development. For
example, vegetation processes produced
relative lower pressure to the south andrelative higher pressure to the north in East
Asia. An anomalous eastward wind withvegetation processes would be produced while
Coriolis forcing balanced the pressure gradient
force based on geostrophic balance. This
turning would affect the extent and intensity ofthe monsoon onset. In the monsoon mature
stage, when the convective heating is
dominant, with and without vegetation heating
make no substantial differences.
In the East Asian study, the role ofland surface processes in the abrupt monsoon
northward jump was explored. The abrupt
northward jump of the East Asian monsoon
has been described by numerous studies but it
has not been simulated before and its cause isstill unclear. It was found that the simulation
with/without vegetation processes had large
differences in northward low-level moisturetransport. In the late part of June, the moist
region and meridional wind in the simulations
with vegetation processes had a dramaticnorthward expansion between 25°N and 40°N,
which was consistent with the timing of thenorthward jump of the precipitation band andprovided the necessary moisture. Withoutvegetation, on the other hand, the simulation
did not show such dramatic northward
expansion.
The maximum humidity andmeridional wind were confined to the south of
the Changjiang River.
Since northwardtransport of the water vapor was the main
moisture source of the East Asian summer
monsoon. The differences that discussed

above would have great impact on monsoon
development. The surface energy balanceswere analyzed to understand the mechanismsthat produced these differences. It was foundthat the differences in radiative heating andlatent heat flux were not large between thesimulations with/without vegetation.
Themajor differences were in the sensible heat
flux. Based on the geostrophic balance in
mid-latitude, the northward wind should beproduced by the east-west pressure gradient.
Among surface upward heating components,
only sensible heat fluxes in the simulation
with vegetation processes exhibited a clear
east-west gradient. Based on these analyses
and previous theoretical studies, we suggestthat the greater east-west thermal gradient,which produced strong northward transport ofmoisture and a cyclone condition, may
contribute to the abrupt northward jump of themonsoon.

3. South American study
In the South American case, theimpact of land surface processes on the SouthAmerican monsoon system (SAMS) onset andevolution are also investigated. SAMS is thesecond largest monsoon in the world. The
evolution of the SAMS starts during theaustral spring as intense convection around theequator. This then moves southeastward
towards the Amazon, where the monsoon
regime establishes itself in the austral summer.
The location of maximum precipitation isfurthest southeast from September toDecember and then moves back northeastward
from the subtropics and eventually returns to
the tropics and merge with the ITCZ in themonsoon mature stage. The dry and wet
seasons were present in the simulations
without vegetation processes, but some
important features of the SAMS evolution
were missed.
First, the southeastward
evolution of the precipitation from September
to December was diffuse. The monthly meanvalues showed a brief southward movement in
September followed by a uniform northwestsoutheast precipitation band that extended
from 5˚N to 25˚S in October and then a strong
northward movement, which caused an earlymerge with ITCZ in December, producing avery wet monsoon season. The dry area near
the equator did not appear in this period. On
the other hand, simulations with vegetation

processes correctly simulated the SAMS
southward movement in the austral spring.
The maximum monthly mean precipitation inOctober was correctly located in the Amazonand then gradually moved southeastward.
Furthermore, the northward movement started
after December and merged with ITCZ in theSAMS mature stage, consistent with the major
observed features of the SAMS evolution
processes.
To understand the mechanisms,
which cause these differences, surface energy
budget has been analyzed. It was found thatthe most substantial differences were in thepartitioning between latent and sensible heat
fluxes such that simulations with vegetation
processes persistently produced lower latent
heat flux and higher sensible heat flux.
Furthermore, these differences exhibited hightemporal and spatial variability, withmaximum changes in sensible heat flux being
occurred during October - January. The
analysis revealed that the seasonality ofsavanna in southeast Brazil and shrubs innortheast Brazil played crucial role in
reduction in evaporation and increase insensible heat flux in simulations. These
produced an anticyclone system anddivergence, which in turn prevented earlymonsoon development and early merge ofSAMS with ITCZ.

4. Discussion and summary
The results discussed above show
very different mechanisms in land/atmosphereinteractions in different continents. In East
Asia, the differences in latent heat fluxes
betweenthesimulations
with/without
vegetation were much smaller than sensible
heat fluxes. The differences in precipitation
generally correlated with the changes inmoisture flux. The influence of surface
processes on monsoon was mainly manifested
in its impact on the large-scale circulation. InAfrica, in June during the monsoon onset,
although the evaporation reduction in someareas in Africa may have contributed to theprecipitation decrease, the major rainfallchange in the Sahel was consistent with thechanges in moisture flux. In August, themonsoon mature stage, the effect ofevaporation prevailed and contributed to thelarge rainfall changes in the Sahel, consistentwith Xue (1997). Compared with the East
Asia, land-surface evaporation played a more
important role in the variation of West African
monsoon. In South America, the evaporation
was very dominant. It provided almost all themoisture sources for the precipitationdifferences between the simulations with andwithout vegetation processes. But theprecipitation anomalies were generally
consistent with the moisture flux convergence,
which is closely associated with sensible heatflux.
Our modeling results show that under
unstable atmospheric conditions, the evolutionof the monsoon's intensity, the spatial
distribution of precipitation, and associatedcontinental-scale circulation are affected by
the perturbation processes of vegetation, notjust by low frequency mean land surface
forcings such as monthly mean albedo. We
coupled a general circulation model with two
different land-surface parameterizations, withandwithout
explicit
vegetation
representations. A number of simulations
used these two models but with the same
initial soil moisture and monthly mean surface
albedo. Thus, the main differences between
these two models are short-term vegetation
forcings, such as radiative flux/canopy
interaction and transpiration. By comparing
the results from these two models, we canidentify the effect of vegetation andmechanisms of land/atmosphere interaction.
However, these studies are based on asingle GCM model and a specific year.
Recent studies with different models show thatdifferent GCM coupled with different landmodels exhibit very different sensitivity to theland surface forcing (Koster et al., 2004).
Propersimulations
ofland/monsooninteraction are challenge and difficulty. Thefactors and issues related to modeling
land/atmosphere interactions need be furtherexplored. In addition, the effects of different
land processes, such as land cover change andsoil moisture, in intra-seasonal variability alsoneed to be further explored.

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