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Figure 9.12 Schematic
showing possible land-
atmosphere feedback
mechanisms associated with
fluctuations of the water and
energy cycles, in leading to
extreme floods or prolonged
drought events in monsoon
regions. (Adapted from Lau
and Bua
1998
)
Large scale remote forcing
Precipitation
Δ
P
>
0
Cloudiness
Δ
Cl
>
0
NF2
Δ
LW
sfc
<
0
Δ
SH
<
0
Soil
moisture
Δ
NF1
Ground temp
Δ
T
g
<
0
Moist convergence
(
P
-
E
) > 0
S
>
0
Hydrologic cycle
Energy cycle
Surface shortwave
Δ
Evaporation
Δ
E
>
0
SW
sfc
<
0
1996
;Lianget al.
2001
) indicating that downscaling approaches may be
needed to capture this unique feature of the East Asian monsoon.
9.3.5 Land-atmosphere feedback
Recent modeling studies have shown that land-atmosphere processes can affect
monsoon and monsoon-ENSO relationships by altering the energy and water
cycles within the AM regions, through surface heat fluxes and hydrologic
feedback mechanisms. Lau and Bua (
1998
) carried out a series of numerical
experiments using a NASA global climate model, similar to those described in
Section
9.3.3
. Their results suggest an atmosphere-ocean-land feedback
scenario as illustrated in Figure
9.12
. If the soil moisture content of the
Asiatic land mass is abnormally high during the start of a monsoon season,
land surface evaporation will be increased. This will lead to increased moisten-
ing of the atmospheric boundary layer,more unstable airmasses and hencemore
convection and rainfall, resulting in a positive feedback leading to further
moistening of the land region. However, the cloudy sky condition stemming
fromenhanced convectionwill shield off and reduce solar radiation reaching the
land surface, causing the land to cool. As the land mass cools off, the resulting
decreased land-sea thermal contrast can only support a weaker large-scale
monsoon circulation, with reduced monsoon rainfall, thus producing a negative
feedback, which halts further increase in soil moisture. These feedbackmechan-
isms are dependent not only on local processes but also on remote forcing such
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