4.2.2 The synoptic precipitation with typhoon impact
Compared with the existence of multiple types of isotopic variations in
normal precipitation, the time-series of typhoon-related precipitation
shows similar trend and can be divided into three main stages (Fig. 4).
The low humidity in the first stage is probably due to influence of the
frontal subtropical high pressure of the typhoon, which have a strong
re-evaporation effect (Fig. 8),
meanwhile the
δ18O was the highest due to it being in the initial
stage of precipitation (Xu et al., 2019). As the typhoon center
approached, the typhoon rain brands gradually controlled the
precipitation in the study area. The isotopic composition of water vapor
was depleted due to the high precipitation amount (Fudeyasu et al.,
2008), resulting in increase of atmospheric relative humidity and
decrease of δ18O value. When the typhoon rain brands
pass, the atmospheric relative humidity decreases again, and the
residual water vapor with high δ18O value becomes the
main source of moisture for precipitation, while evaporation also
gradually increases, resulting in the δ18O values of
precipitation increasing again. The results suggest that the moisture
source which affected by the precipitation and re-evaporation in
different stages of the typhoon are the main factor controlling the
variation of δ18O values during precipitation in the
study area.
[Insert Figure 8]
Although the time-series patterns of precipitation are the same, there
are large differences between the isotopic values of precipitation from
different typhoons, which may be influenced by the season of typhoon
occurrence, the source of moisture, and other factors (Table 1)
(Mullaugh et al., 2013; Xu et al., 2019; Zhang et al., 2015b). Typhoons
in the western Pacific Ocean occur mainly during the summer months when
the East Asian Summer Monsoon are prevalent (e.g., Typhoon Maria, TD,
and Typhoon Bailu in this study), but occasionally typhoons occur during
the withdraw of the summer monsoon period or even the beginning of the
winter monsoon period (e.g., Typhoon Yutu in this study), which may lead
to differences in the isotopic composition of precipitation between
them. The moisture of typhoon-related precipitation mainly originated
from oceanic air masses (Fudeyasu et al., 2008), but there are
significant differences in moisture sources due to the differences in
the origin, route, and intensity of different typhoons (Fig. 9). The
backward trajectory results show that the moisture source of different
typhoon-related precipitation varies greatly, resulting in different
values of δD, δ18O, and D-excess (Table 1, Fig. 9).
The moisture trajectory during Typhoon Maria, TD, and Typhoon Bailu
showed obvious characteristics of anticlockwise spiral (Fig. 9 a, b, and
d), especially the moisture at 2000 m heights, which was controlled by
the cyclone structure of typhoon. Compared with TD and Typhoon Bailu,
the moisture trajectories at 1000 m and 2000 m heights during Typhoon
Maria suggested moisture sources from local region, so the D-excess
value of Typhoon Maria related precipitation was significantly higher
than the others (Table 1). The moisture source of Typhoon Yutu related
precipitation was significantly different from the other three
typhoon-related precipitations (Fig. 9). The moisture at 2000 m height
mainly originated from the oceanic air mass (west Pacific), which
basically agreed with the path of typhoon, whereas the moisture at 500
and 1000 m heights originated from the continental air mass (Mongolian
region) and underwent long-distance transport, which may dominant by the
winter monsoon (Fig. 9 c). It is likely that moisture carried by typhoon
(2000 m height) should be much larger than that by general air transport
(500 and 1000 m heights) in the typhoon-related precipitation (Xie et
al., 2011). As a results, the D-excess value of the Typhoon Maria
related precipitation mainly shows a clear ocean-source character. At
the same time, the moisture of this Typhoon Yutu related precipitation
was influenced by the mixing of land-source moisture (500 and 1000m
heights), so the δD and δ18O value was relatively
higher than that of other typhoon-related precipitations.
[Insert Figure 9]
Previous studies have shown that the factors affecting the δD and
δ18O of atmospheric precipitation include two main
aspects. The first one is climatic background of the regional
environment, including the source and nature of moisture and all changes
in hydrogen and oxygen isotopes during water vapor production and
transport (Zwart et al., 2016; Quezadas et al., 2021). The other one is
local geographic factors, including various meteorological parameters
during precipitation, as well as local latitude and altitude (Langebroek
et al., 2011; Xie et al., 2011). The effects of regional climate on δD
and δ18O are synchronous and cause equilibrium
fractionation of isotopes in moisture, thus having a significant control
on the D-excess of atmospheric precipitation (Guo et al., 2021). The
correlation analysis results show that the D-excess of typhoon-related
precipitation is significant correlated with wind direction (Table 2),
which may be due to the influence of the cyclonic structure of the
typhoon on the transport of moisture for typhoon-related precipitation.
Local geographic factors mainly affect the dynamic fractionation of
isotopes during precipitation, such as re-evaporation, which leads to
higher δD and δ18O of precipitation and is influenced
by meteorological factors such as temperature, pressure and relative
humidity during precipitation. Thus, the δD and δ18O
of precipitation show significantly correlation with temperature,
relative humidity, and pressure in this study (Table 2, Fig. S1). There
is a significant negative correlation between δD and
δ18O of precipitation and temperature, showing an
inverse temperature effect, mainly related to the seasonal variation of
isotopic values, which is consistent with the results of monthly average
result (Fig. 6). It is noteworthy that such correlations are
strengthened in typhoon-related precipitation in the Pearson’s
correlation analysis (Table 2 c). However, the linear regression results
indicate that the correlation between δ18O of
typhoon-related precipitation and temperature is a spurious signal. When
the precipitation samples collected during Typhoon Yutu (which occurred
in November and had significantly lower temperatures than the rest of
the typhoon occurrences) are removed, there is no significant
correlation between δ18O value and temperature for the
remaining samples (Fig. S1 c). Therefore, the Pearson’s correlation
coefficient alone cannot be simply considered when analyzing the
relationship between δ18O value and meteorological
parameters. The lack of correlation between δ18O value
of typhoon-related precipitation and temperature may be due to the short
duration of typhoon-related precipitation and the small temperature
variation during the same precipitation event, as well as the fact that
typhoons mostly occur in summer and the temperature difference between
different typhoon-related precipitation events is small, which
eventually leads to no significant correlation between
δ18O value and temperature. Both Pearson’s correlation
coefficients and linear regression results indicate a significant
correlation between δ18O value of typhoon-related
precipitation and relative humidity and pressure (Table 2, Fig. S1),
which may be related to the significant re-evaporation effect in the
first and third stages of typhoon-related precipitation. Absolutely, due
to the limited sample size for typhoon-related precipitation, the
results of correlation analysis may have some deviations, but it can
still be a good reference for related studies. In the future work, we
will collect more samples and data for a more detailed and in-depth
study.
[Insert Table 2]