The second rainy season onset in the Central Highlands of Vietnam
Ming-Cheng Yen1 , Hai Manh Bui1 , Chi-Ming Peng1 , Yen-Ta Fu1 , Tu Duc Dinh1 , and
Neng-Huei Lin1
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National Central University
November 26, 2022
Abstract
Posted on 26 Nov 2022 — CC-BY 4.0 — https://doi.org/10.1002/essoar.10506341.1 — This a preprint and has not been peer reviewed. Data may be preliminary.
Two distinct rainfall stages over the Central Highlands (CH) of Vietnam during the rainy season have been objectively defined using the high-resolution Vietnam Gridded Precipitation dataset for 1983–2010 (28 years): a second rainy season (SRS)
embedded in the conventional rainy season. Surprisingly, the pronounced interannual variation in the SRS onset date has led
to three apparent regimes: an early (late) SRS with a 1 month longer (shorter) rainfall period occurring in early July (until
mid-August) and a normal SRS starting in late July. Almost all the early SRS years occur during El Niño developing phases,
particularly during the Niño3.4 sea surface temperature (SST) increase from January through December. Water vapor budget
analyses reveal that the interannual variation in the divergent water vapor flux is in response to the warmer July tropical Pacific
SST anomalies, resulting in rainfall enhancement over the CH and eventually inducing early SRS onset.
Hosted file
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606375-the-second-rainy-season-onset-in-the-central-highlands-of-vietnam
1
manuscript submitted to Geophysical Research Letters
The second rainy season onset in the Central Highlands of Vietnam
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Bui-Manh Hai1, Chi-Ming Peng1,2, Yen-Ta Fu1, Duc-Tu Dinh1,3, Neng-Huei Lin1, and
Ming-Cheng Yen1
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Department of Atmospheric Sciences, National Central University, Chung-Li, Taiwan.
WeatherRisk Explore Inc., Taipei, Taiwan.
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Aero-Meteorological Observatory, Vietnam Meteorological and Hydrological Administration,
Hanoi, Vietnam.
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Corresponding author: Ming-Cheng Yen (tyenmc@atm.ncu.edu.tw)
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Key Points:
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A second rainy season (SRS) is embedded within the conventional rainy season
Early (late) SRS with a 1 month longer (shorter) rainfall period occurs in early July (until
mid-August); a normal SRS starts in late July
Almost all the early SRS years occur during El Niño developing phases
manuscript submitted to Geophysical Research Letters
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Abstract
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Two distinct rainfall stages over the Central Highlands (CH) of Vietnam during the rainy season
have been objectively defined using the high-resolution Vietnam Gridded Precipitation dataset
for 1983–2010 (28 years): a second rainy season (SRS) embedded in the conventional rainy
season. Surprisingly, the pronounced interannual variation in the SRS onset date has led to three
apparent regimes: an early (late) SRS with a 1 month longer (shorter) rainfall period occurring in
early July (until mid-August) and a normal SRS starting in late July. Almost all the early SRS
years occur during El Niño developing phases, particularly during the Niño3.4 sea surface
temperature (SST) increase from January through December. Water vapor budget analyses reveal
that the interannual variation in the divergent water vapor flux is in response to the warmer July
tropical Pacific SST anomalies, resulting in rainfall enhancement over the CH and eventually
inducing early SRS onset.
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Plain Language Summary
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The Central Highlands (CH) of Vietnam contribute up to 90% of the country’s total coffee
production and 25% of its total hydropower potential. A second rainy season (SRS) is observed
in late July in this region, which is distinct from the conventional rainy season that occurs in late
April–early May. Because the onset of the SRS has a strong impact on coffee yield and
hydropower potential in the CH, this study examines the climatology of and interannual variation
in the SRS onset date during 1983–2010. An early (late) SRS with a 1 month longer (shorter)
rainfall period occurs in early July (until mid-August). Almost all the early SRS years occur
during El Niño developing phases. An association between the early SRS onset years and
strengthening of the water vapor flux convergence induced by the warmer July tropical Pacific
SST anomalies is discovered.
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1 Introduction
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An abrupt increase in rainfall during the monsoon season in Asia could have a strong impact on
many activities of two thirds of the world’s population, including agriculture, commerce,
forestry, and hydropower. Over Southeast Asia, rapid precipitation enhancement mainly occurs
during commencement of the Asian summer southwest monsoon, which signifies a transition
from the dry to the rainy season (Lau & Yang, 1997; Zhang et al., 2002; Nguyen-Le et al., 2015).
Therefore, research in past decades has increasingly focused on the summer monsoon onset date
(SMOD) or summer rainy season onset date (RSOD). For example, Zhang et al. (2002) used the
observed daily rainfall over the central Indochina Peninsula (ICP) to determine the mean SMOD
as being 9 May, with a standard deviation of 12 days. Applying the empirical orthogonal
function analysis on daily mean precipitation, Nguyen-Le et al. (2015) demonstrated that the
mean summer RSOD over the eastern ICP is 6 May, with a standard deviation of 13 days.
However, the immediate increase in rainfall over the eastern ICP is observed in early autumn
when the summer monsoon withdraws (Matsumoto, 1997; Yen et al., 2011; Chen et al., 2012;
Nguyen-Le et al., 2015).
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Chen and Yoon (2000) demonstrated that the more (less) Indochina monsoon rainfall during cold
(warm) summers is the result of global divergent water vapor flux following interannual
variation in the global divergent circulation in response to tropical Pacific sea surface
temperature (SST) anomalies. Numerous efforts have been made to explore the relationship
between the interannual variation in monsoon onset and the El Niño Southern Oscillation
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(ENSO). Years with cold (warm) SST anomalies in the equatorial central and eastern Pacific
Ocean in the preceding spring tend to have a stronger (weaker) monsoon circulation and an early
(late) SMOD and summer RSOD (Ju & Slingo, 1995; Zhang et al., 2002; Nguyen-Le et al.,
2015; Noska & Misra, 2016). As revealed by previous studies (e.g., Yen et al., 2011; Chen et al.,
2012), the maximum rainfall in coastal central Vietnam may undergo out-of-phase interannual
variation with the ΔSST(Niño3.4) index during October–November. However, Nguyen-Le et al.
(2015) illustrated that the mean autumn RSOD is 16 September, with a standard deviation of 12
days, and an early autumn RSOD was observed over the eastern ICP during the El Niño
development phase. Nevertheless, Nguyen et al. (2007) argued that the precipitation in a small
region of the Central Highlands (CH) of Vietnam is positively correlated with the equatorial
central to eastern Pacific SST from July to September but has no significant relationship with the
Indian Ocean SST.
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The main coffee growing region of Vietnam, the second largest coffee producer worldwide
(Amarasinghe et al., 2015), is located over the CH (green box in Figure 1a). Moreover, the
hydropower potential generated from this region accounts for 25% of the country’s total
hydropower potential (Dao & Bui, 2015). Because rainfall variation significantly influences
coffee production (Camargo, 2010) and hydropower potential, a thorough understanding of the
RSOD and SMOD over the CH is crucial to both Vietnam’s agriculture and economy. By using
the same daily rainfall observations from 10 meteorological stations over the CH, Ngo-Thanh et
al. (2018) found that the RSOD and SMOD are well differentiated from each other, with the
mean RSOD being 20 April and SMOD being 13 May, whereas Pham-Thanh et al. (2020)
demonstrated that the average RSOD was 28 April, with a standard deviation of 14 days, and that
this was approximately 3 weeks before the mean SMOD in some years. Apart from the differing
RSODs due to different determination criteria between them, both studies reported a strong
correlation between the RSOD and the ENSO, but Pham-Thanh et al. (2020) reported most
RSODs being later (earlier) during El Niño (La Niña) phases.
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As revealed from the 5-day-running-mean climatology of the rainfall index averaged over the
CH during May–October over the period 1983–2010 (Figure 1b), two distinct rainfall periods
emerge: the first period fluctuates along a rainfall of 7.8 mm day−1 and the second vacillates at
10.4 mm day−1. Hereafter, these two rainy periods are referred to as the first rainy season (FRS)
and second rainy season (SRS), respectively. Studies related to rainfall over the CH have mostly
focused on the RSOD (equivalent to the FRS onset date here: FRSOD; Ngo-Thanh et al., 2018;
Pham-Thanh et al., 2020). To the best of our knowledge, this unique SRS feature and its onset
date (SRSOD) have not previously been explored. Therefore, in this study, we objectively
determine the SRSOD over the CH for the climatology as well as for each individual year during
1983–2010. In addition, the possible mechanism underlying the pronounced interannual variation
in SRSOD is investigated.
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Figure 1. (a) Topography; green box indicates the CH. (b) Daily climatology (5-day-running
mean) of the rainfall index averaged over the CH during May–October over the period 1983–
2010; the two blue lines indicate the mean rainfall during the FRS (9 May–25 July) and SRS
(26 July–20 October), respectively. Mean rainfall over central Vietnam during the 10 days (c)
before and (d) after the SRSOD, including the SRSOD, and (e) their differences. Shaded areas
in (e) indicate significant differences above 90% confidence level.
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2 Data and Methods
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2.1 Datasets
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To investigate the spatial and temporal characteristics of precipitation on a scale relevant to the
climate over the CH, two types of consistent, long-term, high-resolution gridded rainfall dataset
are acquired. The first is the Vietnam Gridded Precipitation (VnGP) dataset with a high
resolution of 0.1o × 0.1o and generated from 481 daily raingauge observations across all Vietnam
over the period 1980–2010 (Nguyen-Xuan et al., 2016). We use this dataset to construct the
rainfall indices and investigate the local rainfall variability over the CH. The second dataset used
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is the daily Precipitation Estimation from Remotely Sensed Information using Artificial Neural
Networks for Climate Data Record (PERSIANN-CDR), which has a resolution of 0.25o × 0.25o
and is generated from long-term multisatellite high-resolution observations spanning 1983–2020
(Ashouri et al., 2015). This dataset is used to examine the relationship between rainfall
variability in the CH and the surrounding activity embedded in the large-scale environment. To
depict the large-scale atmospheric circulation associated with the SRSOD, daily ERA-Interim
reanalysis on a 0.75o latitude–longitude grid (Dee et al., 2011) is employed. Finally, the
historical Oceanic Niño Index (ONI) provided by the Climate Prediction Center of National
Centers for Environmental Prediction is adopted as a measure of the ENSO. For consistency,
only the period 1983–2010 is covered in our analysis.
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2.2 Definition of the second rainy season onset date
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Because the SRS is a local feature in the CH and has not been observed in adjacent places,
determination of the SRSOD by using only the rainfall parameter is proposed. In addition to the
FRSOD and SRSOD, we designate RSRD and SRSRD as the rainy season retreat date and
second rainy season retreat date, respectively. The procedures for detecting these four
characteristic dates for the 28-year mean climatology are as follows:
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1. FRSOD: The daily rainfall amount is larger than the yearly mean precipitation (PYRM) for 5
consecutive days. And, there must be at least 10 days with daily rainfall amount larger than
the PYRM within 20 consecutive days after the FRSOD.
2. RSRD: The same constraint as for the FRSOD is used to determine the RSRD, but the
estimation is done backward from the year end.
3. SRSRD: The procedure is similar to that for the RSRD, but the average precipitation during
the period FRSOD–RSRD (PRSM: the average rainfall over the entire rainy season) is
considered instead of the PYRM and backward estimation is conducted starting from the date
calculated as the RSRD minus 10 days.
4. SRSOD: The same procedure as that for the FRSOD is used, but the PYRM is replaced by
the PRSM.
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The first two procedures are similar to those reported by Ngo-Thanh et al. (2018) except that the
PYRM over 28 years is considered instead of directly specifying 5 mm day−1 as a reference
level.
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First, both temporal variation and rainfall magnitude, as shown in Figure 1b, are greatly
consistent with the ground-truth rainfall index illustrated in Figure 2 of Pham-Thanh et al.
(2020), suggesting that the VnGP dataset is suitable for climate research related to the CH.
Consequently, two distinct rainy periods (Figure 1b and Table 1) are defined: the first fluctuates
along the average rainfall of 7.8 mm day−1 from 9 May to 25 July, whereas the second vacillates
at 10.4 mm day−1 from 26 July to 20 October. To further substantiate the clear temporal
development, the average rainfalls for both the 10 days before and after the SRSOD together
with their differences are depicted in Figures 1c–1e. The significant increase in precipitation
after the SRSOD implies that our procedure is capable of reasonably identifying the SRSOD and
capturing the two separate rainfall stages, the FRS and SRS, over the CH.
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Table 1. Various rainy season dates and related average rainfall statistics
rainfall unit: mm day-1
Category
Climate
Early
Late
Normal
FRSOD
9 May
5 May
11 May
9 May
RSRD
21 Nov
5 Nov
27 Nov
28 Nov
SRSOD
26 Jul
7 Jul
19 Aug
26 Jul
SRSRD
20 Oct
7 Oct
22 Oct
4 Nov
Yearly rainfall average
5.7
5.9
5.6
5.7
(PYRM)
Rainy season rainfall average
8.9
9.6
8.9
8.7
(PRSM)
FRS rainfall average
7.8
7.1
7.6
7.7
SRS rainfall average
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11.3
10.1
Onset date of the SRS over the CH from 1983 to 2010
Year
SRSOD
Year
SRSOD
Year
SRSOD
1983
3 Aug
1993
26 Jul
2003
20 Jul
1984
27 Jul
1994
6 Jul
2004
23 Jul
1985
6 Aug
1995
19 Aug
2005
23 Jul
1986
15 Jul
1996
19 Jul
2006
29 Jun
1987
13 Aug
1997
10 Jul
2007
29 Jun
1988
11 Sep
1998
15 Aug
2008
22 Jul
1989
18 Jul
1999
23 Jul
2009
12 Jul
1990
13 Aug
2000
18 Aug
2010
22 Jul
1991
14 Aug
2001
3 Aug
28 Jul
Average
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To reflect the apparent change in rainfall from the FRS to the SRS as well as the SRSOD
identification for each individual year, many sensitivity tests are performed to eventually obtain
the optimal criteria as follows. We first define some terms. P2SM denotes the average
precipitation during 9 May–20 October, a fixed period based on the climatological FRSOD–
SRSRD, for each individual year. PSA2 (PSB2) represents the average rainfall in the 20 days
after (before) the SRSOD including (excluding) the SRSOD. The SRSOD of the CH is then
determined by considering the first day after 27 June, just 1 month before 26 July of the
climatological SRSOD, which satisfied the following conditions:
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1. The 5-day-moving-averaged daily rainfall amount exceeds P2SM and persists for at least 5
consecutive days.
2. PSA2 is greater than PSB2 + 0.35 × P2SM.
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Consequently, the SRSODs for individual years during 1983–2010 are objectively determined
(Table 1), and they exhibit clear interannual variation. The average SRSOD for 28 years is 28
July, with a standard deviation of 17 days, whereas the earliest onset date is 29 June, occurred in
2006 and 2007, and the latest onset date is 11 September, 1998.
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3 Results
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Because of the wide distribution of SRSODs, investigating any specific characteristics among
them is noteworthy. By using a standard deviation of ±0.8 of the SRSOD over 28 years as a
measure, three discrete groups are selected and categorized as follows: early (1986, 1994, 1997,
2006, 2007, and 2009), late (1987, 1988, 1990, 1991, 1995, 1998, and 2000), and normal (other
years) SRS. Additionally, the first three procedures mentioned in Section 2.2 are applied to the
average rainfall index of each group to identify the FRSOD, RSRD, and SRSRD, whereas the
SRSOD is simply calculated as the average of each group’s onset dates. According to the
statistics shown in Table 1 and Figure 2a, the early SRS years not only start earlier in terms of
the FRSOD and SRSOD and end earlier in terms of the RSRD and SRSRD but also have a
rainfall period 1 month longer than the late SRS years, along with higher precipitation in almost
every epoch except the FRS. Coincidentally, both the FRSOD and SRSOD in normal years are
identical to those in the 28-year climatology, with fewer rainfall differences for each rainy spell,
which suggests that the 28-year climate mean might nearly reach the climatic norm with the
exception of the RSRD and SRSRD. Although the FRSOD in each category is in early May,
Pham-Thanh et al. (2020) stated that an RSOD over the CH between late April and early May
seems to be more reasonable, not to mention using different criteria and datasets.
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The intraseasonal oscillations (ISOs), including 10–20-day and 20–60-day modes, of the
observed rainfall in Vietnam have been well documented by Truong and Tuan (2018, 2019), but
these studies did not cover the CH. However, by using the VnGP dataset, Tuan (2019) found a
remarkable relationship between the rainfall submonthly scale ISO and heavy rainfall days in the
CH. Surprisingly, the SRSOD over the CH is synchronized with the developing phases of the
10–20-day and 20–60-day modes for each year in our study. Therefore, the original rainfall
indices and their respective filtered ISO modes plus the combined ISOs for all the selected years
in the aforementioned three groups are averaged with their center date coinciding with the
SRSOD and extending 30 days before and after, as illustrated in Figures 2b–2d. In general, the
precipitation clearly increases after the SRSOD in all three regimes, and the 10–20-day (20–60day) mode is more dominant in early (late) SRS years, whereas these two ISO modes are
compatible in the normal SRS years. These phenomena deserve extensive investigation in a
future study.
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To further substantiate the precipitous rainfall development over the CH during the transition
from the FRS to the SRS, the composite rainfall, 28-year climate means with their center date
coinciding with the SRSOD, evolution (Figures 2e–2i), and differences between two consecutive
pentads (Figures 2j–2n) of pentad-mean VnGP (Text S1) are closely examined around the
SRSOD. The prominent rainfall enhancement after the SRSOD is confirmed by the delineated
physical domain with doubled rainfall intensity in Figure 2l if compared to that by the climate
mean in Figure 1e.
manuscript submitted to Geophysical Research Letters
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Figure 2. (a)Daily composite rainfall indices for normal (grey histogram), early (blue line), and
late (red line) SRS years, respectively; two black, blue, and red horizontal lines indicate the
rainfall mean during the FRS and the SRS for normal, early, and late SRS years, respectively.
Daily evolution of composite P, 𝑃̃, 𝑃̂ , and 𝑃̃ +𝑃̂ for (b) normal, (c) early, and (d) late SRS
years; here 𝑃̃ and 𝑃̂ represent the 20–60-day and 10–20-day modes, respectively. (e)–(i)
Composite rainfall evolution and (j)–(n) differences between two consecutive pentads of
pentad-mean VnGP over central Vietnam centered on the SRSOD. Shaded areas in (j)–(n)
indicate a significant difference above 90% confidence level.
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Regarding the conspicuous interannual variation in the SRSOD, July should be focalized on
considering its critical role of water vapor budget (Text S2) in differentiating between the early
and late SRS years because early (late) SRSODs occur in early July (until mid-August). The July
composite charts of (𝜓𝑄 , 𝑄𝑅 , 𝑊 ) and (𝜒𝑄 , 𝑄𝐷 , 𝑃) for normal, early, and late SRS years are
displayed in Figures 3a–3f. In early SRS years, the abundant water vapor transported by strong
westerly winds over south-southeast Asia and the strengthened southeasterly wind over
northwestern Pacific (Text S3) is convergent toward the Philippine Sea west of 170°E and along
the trough in the south fringe of the Asian Monsoon Low, including the CH. Furthermore, the
significant increase in precipitation over these regions is accompanied and maintained by the
enhancement of convergent water vapor flux (Figure 3d). By contrast, the phenomenon in the
late SRS years associated with less vigorous water vapor transport and precipitation is similar to
that in the normal years. These arguments are illustrated further in Figures 3g–3l. A couple of
anomalous cyclonic cells of water vapor flux associated with the enhanced water vapor transport
stretch from north India toward the Philippine Sea west of 170°E, covering the CH (Figure 3g),
whereas the anomalous divergent water vapor flux ∆(𝜒𝑄 , 𝑄𝐷 , 𝑃) converges water vapor toward
these regions to maintain excessive rainfall (Figure 3h) in early SRS years. Evidently, the
interannual rainfall variation in the CH is further ascertained by the composite rainfall
differences (Figures 3i and 3l), confirming the decisive role of July composite charts in
differentiating between early and late SRS years.
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To explore the possible mechanism underlying the notable interannual variation in the SRSOD,
the time series of ONIs for each selected year in the early and late onset categories is displayed
in Figures 4a and 4b, respectively. Except for 2007, all early SRS years coincidently occur
during El Niño developing phases, particularly with the Niño3.4 SST increase from January
through December. Because of the developing effect of the tropical storm Toraji in early July,
the persistent rainfall in the CH meets the SRSOD criteria despite the La Niña developing phase
in 2007. For late SRS years, the large-scale environment appears to be considerably diverse and
to comprise two El Niño (1987 and 1991), one normal (1990), one La Niña (2000), and three La
Niña developing phases (1988, 1995, and 1998). This discrepancy in diversification warrants
further investigation in the future.
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An atypical Indian drought occurs during July 2002 with frequent advection of dry air from over
the deserts instead of marine moist air from the southern Indian Ocean (Bhat, 2006) despite 2002
being one of the El Niño developing phase years. Consequently, the drier water vapor transport
associated with divergence of water vapor flux replaces the moist large-scale environment over
the CH to hinder the expected early SRS occurrence. On the basis of the distinct result shown in
Figure 4a, the 5-year (except 2007) July composite chart of Δ(𝑄𝐷 , SST) is constructed to support
the interannual rainfall variation outcomes depicted in Figures 3g–3i. From Figure 4c, we can
infer that the interannual variation in the divergent water vapor flux occurs in response to the
warmer July tropical Pacific SST anomalies to enhance the rainfall over the CH and eventually
induce early SRS onset.
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Figure 3. July composite charts of 𝜓𝑄 (contour), 𝑄𝑅 (vector), and W (shaded) for (a) normal, (c)
early, and (e) late SRS years, respectively. (b), (d), (f) Same as (a), (c), and (e) except for Q
(contour), 𝑄𝐷 (vector), and P (shaded). July composite charts of Δ(𝜓𝑄 , 𝑄𝑅 ,W) for (g) early and
(j) late SRS years. (h), (k) Same as (g) and (j) except for Δ(Q, 𝑄𝐷 , 𝑃); shaded areas and black
vectors in (g), (h), (j), and (k) denote a significant difference above 90% confidence level. (i),
(l) Same as (h) and (k) except for ΔVnGP; the areas that show a significant difference above
90% confidence level in (i) and (l) are encircled by solid-black contour.
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Figure 4. (a) Monthly SST of the ENSO index for six early SRS years and average of five El
Niño years. (b) Same as (a) except for seven late onset years and average of four La Niña
years. (c) July composite chart of Δ(𝑄𝐷 , SST) in five El Niño years. Shaded areas and black
vectors in (c) denote a significant difference above 90% confidence level.
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4 Conclusions
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The CH is a climatological subregion in Vietnam, and the region is the major production for
second largest coffee producer worldwide while also having a quarter of the total hydropower
potential of the country. Therefore, rainfall variation may affect coffee production and
hydropower potential, thereby directly affecting the agricultural and economic gross in the CH
and the entire country. The findings of this study are summarized as follows: By using the VnGP
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dataset for the 1983–2010 period, two distinct rainy periods in the CH are identified; the first
fluctuates along the average rainfall of 7.8 mm day−1 from 9 May (FRSOD) to 25 July, whereas
the second vacillates at 10.4 mm day−1 from 26 July (SRSOD) to 20 October (SRSRD).
However, the prominent year-to-year variation in SRSOD leads to three separate regimes: an
early (late) SRS occurring in early July (until mid-August) and a normal SRS starting in late
July. The early SRS years are characterized by higher precipitation with a 1 month longer rainfall
period than the late SRS years. Except for two unusual years (2002 and 2007) during 1983–2010,
all the early SRS years occur during El Niño developing phases, particularly with the Niño3.4
SST increase from January through December. The possible mechanism underlying the
pronounced interannual variation in the SRSOD is inferred from water vapor budget analyses;
the interannual variation in the divergent water vapor flux is in response to the warmer July
tropical Pacific SST anomalies, resulting in rainfall intensification over the CH and eventually
inducing early SRS onset.
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Acknowledgments and Data
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This study was supported by the Ministry of Science and Technology, Taiwan, under the grant
MOST-108-2111-M-008-027. Data can be accessed online
(VnGP:http://search.diasjp.net/en/dataset/VnGP_010; PERSIANNCDR:https://www.ncei.noaa.gov/data/precipitation-persiann/access/; ERAInterim:https://apps.ecmwf.int/datasets/data/interim-full-daily/levtype=sfc; ONI
indices:https://origin.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ONI_v5.php).
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