Publication Date:
2018-09-11
Description:
This study analyzes the influence of local and regional
climatic factors on the stable isotopic composition
of rainfall in the Vietnamese Mekong Delta (VMD) as part
of the Asian monsoon region. It is based on 1.5 years of
weekly rainfall samples. In the first step, the isotopic composition
of the samples is analyzed by local meteoric water
lines (LMWLs) and single-factor linear correlations. Additionally,
the contribution of several regional and local factors
is quantified by multiple linear regression (MLR) of all possible
factor combinations and by relative importance analysis.
This approach is novel for the interpretation of isotopic
records and enables an objective quantification of the explained
variance in isotopic records for individual factors. In
this study, the local factors are extracted from local climate
records, while the regional factors are derived from atmospheric
backward trajectories of water particles. The regional
factors, i.e., precipitation, temperature, relative humidity and
the length of backward trajectories, are combined with equivalent
local climatic parameters to explain the response variables
d18O, d2H, and d-excess of precipitation at the station
of measurement.
The results indicate that (i) MLR can better explain the
isotopic variation in precipitation (R2 D0.8) compared to
single-factor linear regression (R2 D0.3); (ii) the isotopic
variation in precipitation is controlled dominantly by regional
moisture regimes (ca 70 %) compared to local climatic
conditions (ca 30 %); (iii) the most important climatic parameter
during the rainy season is the precipitation amount
along the trajectories of air mass movement; (iv) the influence
of local precipitation amount and temperature is not significant
during the rainy season, unlike the regional precipitation
amount effect; (v) secondary fractionation processes
(e.g., sub-cloud evaporation) can be identified through the
d-excess and take place mainly in the dry season, either locally
for d18O and d2H, or along the air mass trajectories for
d-excess. The analysis shows that regional and local factors
vary in importance over the seasons and that the source regions
and transport pathways, and particularly the climatic
conditions along the pathways, have a large influence on the
isotopic composition of rainfall. Although the general results
have been reported qualitatively in previous studies (proving
the validity of the approach), the proposed method provides
quantitative estimates of the controlling factors, both for the
whole data set and for distinct seasons. Therefore, it is argued
that the approach constitutes an advancement in the statistical
analysis of isotopic records in rainfall that can supplement or
precede more complex studies utilizing atmospheric models.
Due to its relative simplicity, the method can be easily transferred
to other regions, or extended with other factors.
The results illustrate that the interpretation of the isotopic
composition of precipitation as a recorder of local climatic
conditions, as for example performed for paleorecords of water
isotopes, may not be adequate in the southern part of the
Indochinese Peninsula, and likely neither in other regions
affected by monsoon processes. However, the presented approach
could open a pathway towards better and seasonally
differentiated reconstruction
Repository Name:
EPIC Alfred Wegener Institut
Type:
Article
,
isiRev
Format:
application/pdf
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