Introduction
The Himalayas are a 2900 km long, west–northwest to east–southeast trending
mountain range, which began to form between 40 and 50 Ma ago due to the
collision of two large landmasses, India and Eurasia. This immense mountain
range shapes the climate of southeast Asia and the Northern Hemisphere (e.g.
Clemens et al., 1991; Molnar et al., 2010). The regional climate of the
Himalayas is dominated by two distinct circulation regimes of the regional
atmosphere: the Indian summer monsoon (ISM) and the western disturbances (WD)
periods.
The ISM is one of the most energetic components of the Earth's climate
system, which develops in response to the movement of Intertropical
Convergence Zone (ITCZ) that separates the atmospheric circulation of the
northern and southern hemispheres (e.g. Gadgil, 2003; Allen and Armstrong,
2012). The warming of the Tibetan Plateau relative to the Indian Ocean, which
results in low pressure over Asia and higher pressure over the Indian Ocean
(Overpeck et al., 1996), pulls moisture from Southeast Asia and the Bay of
Bengal and transports it north-westward (Hren et al., 2009; Li et al., 2016).
There is an east–west gradient in monsoonal influence across the Himalayas
with the central Himalayas receiving up to 80 % of its annual
precipitation during the monsoon months according to some estimates
(Bookhagen and Burbank, 2010; Lang and Barros, 2004).
Western disturbances are eastward moving synoptic low-pressure systems
embedded in the lower to mid-tropospheric westerlies in the subtropics and
originate from the Mediterranean Sea or mid-West Atlantic Ocean (Dimri et
al., 2004, 2015; Maharana and Dimri, 2014; Madhura et al., 2015; Rao and
Srinivasan, 1969; Pisharoty and Desai, 1956). At times, secondary systems of
winds with embedded troughs develop over the Persian Gulf and the Black Sea
either directly or as a result of the arrival of low-pressure systems from
southwest Arabia (Dimri et al., 2004). Western disturbances cause heavy
precipitation (> 50 % of annual precipitation in winter) in
the western Himalayas (Lang and Barros, 2004) and northern India from
December to April (Pisharoty and Desai, 1956; Mooley, 1957; Agnihotri and
Singh, 1982). They are also found to be active during the summer months, but
with low frequency (Dhar et al., 1984; Dimri, 2006). When the troughs in the
mid-tropospheric westerlies extend southwards into lower latitudes, the WD
reach Afghanistan, Pakistan and India and are intensified by the moisture
drawn from the Arabian Sea (Chand and Singh, 2015). Cannon et al. (2015) have
shown that the heavy precipitation events occurring in the western and
central Himalayas due to WD are spatiotemporally independent. The strength
and frequency of WD over the last three decades (1979–2010) has increased in
western Himalayas and decreased in the central Himalayas. On the basis of the
variation in the deuterium-excess (d-excess) values of Ganges River water
at Rishikesh, it was suggested that a significant fraction of the snow-melt
and ice-melt components are derived from winter precipitation with a moisture
source from the mid-latitude westerlies (Maurya et al.,
2011).
The impact of climate change on the frequency and magnitude of precipitation
events in the tropics is still a matter of debate (Held and Soden, 2006;
Wentz et al., 2007; Allan and Soden, 2008). This may be partly due to an
inadequate understanding of the atmospheric water vapour dynamics (IPCC,
2013).
The flow of the Himalayan rivers, a key source of fresh water to more than a
billion people (Ives and Messerli, 1989), primarily depends upon the strength,
behaviour and duration of the WD and ISM. The rivers support one of the most
heavily irrigated regions in the world (Tiwari et al., 2009) and are also critical for hydropower
generation, the backbone of the region's economy (Karim and Veizer, 2002;
Archer et al., 2010; Jeelani et al., 2012). Abnormal precipitation brought by
WD and the ISM can lead to flooding or drought, which affects regional
economies. Therefore, it is important to study spatiotemporal variability of
the ISM and the WD in the Himalayas and to better characterize their causes and
consequences. This implies, among others, a better understanding of the sources
of atmospheric moisture forming precipitation in the region during the two
contrasting circulation regimes of the lower atmosphere.
The stable isotope composition of oxygen and hydrogen in water molecules is a
powerful tool for studies of the hydrological cycle, both with respect to its
present status and its past behaviour. In the modern environment, the
isotopic composition of precipitation serves as a conservative tracer for the
origin, phase transitions and transport pathways of water (e.g. Dansgaard,
1964; Rozanski et al., 1993; Gat, 1996; Araguas-Araguas et al., 2000). The
18O and 2H content in precipitation are controlled by the
following: (i) atmospheric parameters such as temperature, degree of rainout
of the moist air masses and the amount of rainfall (e.g. Dansgaard, 1964;
Yurtsever and Gat, 1981; Rozanski et al., 1982, 1993); and (ii) geographic
factors such as altitude, latitude, moisture sources and atmospheric
transport processes (e.g. Craig, 1961; Siegenthaler and Oeschger, 1980; Gat,
1996; Kendall and Coplen, 2001; Karim and Veizer, 2002). The
δ2H and δ18O values in precipitation are
tightly correlated and form a so-called global meteoric water line (GMWL) in
the δ2H–δ18O space, defined by the following
relationship: δ2H = 8⋅δ18O+10
(Craig, 1963). A secondary isotope parameter, deuterium excess (d=δ2H-8⋅δ18O; Dansgaard, 1964) defines
the position of data points in the δ2H–δ18O
space with respect to GMWL. The isotopic patterns of precipitation in the
tropics are expected to be different from the sub-tropics and temperate
regions due to large-scale convection systems, cyclonic storms and a
multitude of vapour sources (e.g. Midhun et al., 2013; Lekshmy et al., 2014,
2015). Consequently, the well-established isotope effects such as the amount
effect, the temperature effect and the altitude effect are not clearly
visible in the precipitation isotope data sets available for the Indian
subcontinent (Jeelani and Deshpande, 2017; Deshpande and Gupta, 2012;
Deshpande et al., 2010; Warrier et al., 2010; Yavada, 2007). However, at
local or watershed scales, stable water isotopes of precipitation showed a
good relationship with altitude and temperature (Jeelani et al., 2017a, 2015,
2013; Kumar et al., 2010). The abrupt change in stable water isotopic values
and the d-excess of precipitation during summer has been attributed to the
reversal/change in the source of precipitation from western disturbances to
southwest monsoons (Jeelani et al., 2017b; Breitenbach et al., 2010).
The present study was launched with three major objectives: (i) to assess the
seasonal variability of δ18O and δ2H in daily
precipitation along the southern foothills of the Himalayas, (ii) to identify
dominant moisture sources for precipitation in this region and (iii) to
demarcate the influence of the Indian summer monsoon and western disturbances in
the study area.
Study area
Daily precipitation samples were collected at six stations located along
southern foothills of the Himalayas (Fig. 1 and Table 1). The stations cover
a distance of almost 2900 km, from Srinagar in the west (Kashmir state) to
Dibrugarh in the east (Assam state). In total, 548 daily precipitation
samples were collected and analysed. The largest numbers of samples were
available for Jorhat (242) and Srinagar (121) stations. The stations are open
to the Indian subcontinent from the south and shielded by the Himalayan massif
from the north. The elevation of the stations ranges from 99 (Jorhat) to
1872 m a.s.l. (Ranichauri). The mean annual temperature varies from 13.6
(Srinagar) to 24.2 ∘C (Jammu) and the mean annual precipitation
ranges from 693 mm at Srinagar to 2781 mm at Dibrugarh.
General characteristics of the stations collecting daily
precipitation samples for isotope analyses.
Station code
Station name
Latitude/Longitude
Altitude
Mean annual
Mean annual
Sampling period
Analysed
(m a.s.l.)
temperature
precipitation
rainfall events
(∘C)
(mm)
SGR
Srinagar
34∘04′59′′ N/
1595
13.6
693
April 2010 to
121
74∘47′50′′ E
September 2011
JMU
Jammu
32∘39′22′′ N/
267
24.2
1238
July 2009 to
98
74∘48′04′′ E
June 2011
PMR
Palampur
32∘06′01′′ N/
1275
19.1
2493
September 2008 to
31
76∘32′49′′ E
September 2010
RNC
Ranichauri
30∘18′50′′ N/
1872
15.1
1272
February 2009 to
31
78∘24′25′′ E
December 2010
JRH
Jorhat
26∘43′21′′ N/
99
24.0
2324
February 2010 to
242
94∘11′44′′ E
December 2011
DBR
Dibrugarh
27∘29′05′′ N/
111
23.2
2781
June 2009 to
25
95∘01′18′′ E
October 2010
Locations of the six sampling sites across the southern foothills of
the Himalayas: SGR – Srinagar, JMU – Jammu, PMR – Palampur, RNC –
Ranichauri, JRH – Jorhat and DBR – Dibrugarh. The position of the Kathmandu
station, Nepal (NPL), which is discussed in the text, is also marked in the
figure.
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Figure 2 shows long-term (1985–2014) monthly surface air temperature and
precipitation data for the six stations where daily precipitation samples were
collected. The data shown in Fig. 2 were grouped into two periods
(Table 2): (i) Indian summer monsoons and (ii) western disturbances.
The onset and duration of ISM periods were defined operationally on the basis
of the seasonal distribution of long-term monthly rainfall, and through the examination
of individual backward trajectories calculated for daily rainfall data
gathered in the framework of the present study (cf. Sect. 4.1).
The durations of ISMs varied from 3 months (July–September) for
the stations located at the western edge (Srinagar, Jammu, Palampur) to 5
months (May–September) for the stations located at the eastern edge of the
transect (Jorthat, Dibrugarh). For the central part of the transect (stations
Ranichauri and Kathmandu) the onset of the ISM was set at the beginning of June
and the termination at the end of September. For all stations except Srinagar station, the
onset of the ISM is marked by a distinct increase in the monthly rainfall amount (cf.
Fig. 2). For Srinagar station the rainfall imprint of the ISM onset and
duration was not present; however, it could still be defined on the basis of backward
trajectory analyses of the air masses associated with daily rainfall at this
site, as well as through the characteristic 18O and 2H
isotope signatures of this rainfall.
Long-term mean values of surface air temperature and cumulative precipitation,
calculated for ISM and WD periods, are reported in Table 2 for
stations where daily precipitation samples for isotope analyses were
collected in the framework of this study. Also, the peak-to-peak amplitude of
seasonal changes of monthly air temperature is reported. This amplitude
increases gradually from the eastern (Dibrugarh: 11.7 ∘C) to the
western edge of the transect (Srinagar: 23.1 ∘C), indicating
progressive transition from a maritime to a continental climate. The mean
temperatures for the ISM and WD periods also differ, the former being
significantly higher; the average difference is approximately 8 ∘C.
Long-term (1985–2014) characteristics of surface air temperature
and precipitation for Indian summer monsoon (ISM) and western disturbances
(WD) periods, for the stations collecting daily precipitation samples for
isotope analyses. Source of data: Srinagar, Jammu, Palampur, Jorhat and
Dibrugarh – https://pl.climate-data.org/ (last access: April 2016);
Ranichauri – Upadhyay et al. (2015).
Station
aDuration of ISM
bAT
ISM period
WD period
cR
(∘C)
T (∘C)
P (mm)
T (∘C)
P (mm)
Srinagar
July–September
23.1
23.0
150
10.5
543
0.28
Jammu
July–September
21.0
29.7
854
22.4
384
2.22
Palampur
July–September
17.2
23.2
1772
15.6
721
2.46
Ranichauri
June–September
14.4
20.5
842
12.4
431
1.95
Jorhat
May–September
12.3
28.1
1759
21.0
565
3.11
Dibrugarh
May–September
11.7
27.1
2151
20.5
630
3.41
a Onset and duration of ISM period defined
operationally on the basis of seasonal distribution of long-term monthly
rainfall (cf. Fig. 2) and through examination of individual backward
trajectories calculated for daily rainfall events collected by each station
in the framework of the present study.b Peak-to-peak amplitude
of long-term (1985–2014) seasonal changes of monthly surface air temperature
at the station.c The ratio of cumulative rainfall amount
collected at the given station during ISM and WD periods.
The stations not only differ with respect to annual amount of rainfall
(Table 1) but also with respect to cumulative rainfall amounts for ISM and WD
periods, the former being generally higher (Table 2). The striking exception
is Srinagar station – here the amount of rainfall during the WD period is
significantly higher (543 mm) than that during the ISM period (150 mm). The
ratio of cumulative rainfall amount during ISM and WD periods (parameter R
in Table 2) varies from 3.41 for Dibrugarh to 1.95 for Ranichauri. For
Srinagar, the value of this parameter is significantly lower than 1 (0.28).
Long-term (1985–2014) monthly surface air temperature and
precipitation data for six stations where daily sampling of rainfall for
isotope analyses was conducted. SGR – Srinagar, JMU – Jammu, PMR –
Palampur, RNC – Ranichauri, JRH – Jorhat and DBR – Dibrugarh. Source of
data: Srinagar, Jammu, Palampur, Jorhat and Dibrugarh –
https://pl.climate-data.org/; Ranichauri – Upadhyay et al. (2015).
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Methods
The Central Research Institute for Dryland Agriculture (CRIDA) and India
Meteorological Department (IMD) collected most of the precipitation samples
under the aegis of the “National Program on Isotope Fingerprinting of
Waters of India” (IWIN) (Deshpande and Gupta, 2008, 2012). The India Meteorological
Department, New Delhi, provided relevant meteorological
data for the stations from its sub-offices. Deuterium and 18O isotope composition
of the rainfall samples were analysed using the IWIN-IRMS facility at the Physical Research
Laboratory (PRL) Ahmadabad, following the standard equilibration method in which
water samples were equilibrated with CO2 (or H2) and the
equilibrated CO2 (or H2) gas was analysed by a Delta V Plus
isotope ratio mass spectrometer (IRMS) in continuous flow mode using a
Gasbench II preparation and introduction system (Maurya et al., 2009). The analytical uncertainty of the isotope
analyses (one sigma) was ±1.0 and ±0.1 ‰ for
δ2H and δ18O, respectively.
The reconstruction of backward trajectories of the air masses arriving at the
sampling stations was done through the framework of the Hybrid
Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT4, revision
February 2016 – Stein et al., 2015). The model was driven by the archived
Global Data Assimilation System (GDAS1 product, available at
ftp://arlftp.arlhq.noaa.gov/pub/archives/gdas1, last access: March
2016) meteorological data available for every 3 h at
1.0∘ × 1.0∘ horizontal resolution (corresponding to
approx. 100 km × 100 km), with 23 sigma pressure layers between
1000 and 20 hPa (Parrish and Derber, 1992). All trajectories were calculated
with a temporal resolution of 30 min. For each location, the trajectory
release point was set up at 500 m above the local ground level in order to
represent mean elevation of moist air masses.
Ten-day backward trajectories representing daily rainfall samples were
calculated as trajectory ensembles, each consisting of twenty seven ensemble
members released at 12:00 LT on the day of precipitation sample
collection. Ensembles were produced by varying the initial trajectory wind
speeds and pressures, according to the HYSPLIT ensemble algorithm, in order
to account for the uncertainties involved in the simulation of individual
backward trajectories. A slight modification of the default algorithm was
used, with the horizontal range of sampling of the initial values reduced from a 1
to a 0.5 grid cell width in the driving meteorological field. As no
information about exact timing of precipitation events (except of the date)
was available, the question of representativity of the backward trajectories
released at 12:00 LT was investigated in some detail. For selected events
multiple releases (every three hours) were realized for the given day. The
results did not reveal any significant changes in ensemble patterns, neither
in terms of trajectory source areas nor in the behaviour of the presented
meteorological variables. Apparently, the ensemble scheme largely captured
the variability of the transport patterns associated with generation of
rainfall events sampled at the stations.
For footprint analysis, individual 10-day backward trajectories starting from
12:00 LT were calculated for each station collecting daily rainfall and for
Kathmandu station, Nepal, which represented the central region of the transect. Daily
releases over the course of three consecutive years (2009–2011) were
simulated. The chosen period corresponds with the period of precipitation
sampling at the stations. Daily trajectories calculated over the three-year
period were then aggregated to produce the footprint maps. Footprints
representative of the ISM and WD seasons were calculated using the subsets of
available trajectories. The output was a 0.5∘ × 0.5∘
footprint signal, which was later smoothed spatially using the focal averaging
method.
Results and Discussion
Seasonality of isotope characteristics of daily rainfall
Figure 3 shows seasonal changes of δ18O and deuterium excess
for the two stations with the largest number of daily isotope data available
(Jorhat – 242 data points, and Srinagar – 121 data points). These two
stations are located on the western (Srinagar) and eastern (Jorhat) edges of
the study transect (cf. Fig. 1). They have been selected to illustrate the
seasonal evolution of the isotopic composition of daily precipitation at the
stations along the east–west study transect. Figure S1 in the Supplement
summarizes δ18O and deuterium excess records available for
other four stations. The 18O isotope composition of daily
precipitation at Jorhat and Srinagar stations shown in Fig. 3a reveals a
distinct seasonality, particularly well displayed in the data available for
Jorhat station. During development of the Indian summer monsoon, the
18O content in daily precipitation at this site gradually
decreases, from δ18O values fluctuating around 0 ‰ at
the onset of ISM, to very negative δ18O values (up to ca.
-20 ‰) recorded at its termination. During the WD period the
18O content progressively increases, reaching positive δ
values towards the end of the period. The seasonality of the 18O
signal is also well marked at other stations (Fig. S1a). As suggested by
Fig. 3a, the largest heavy isotope depletion in rainfall is expected at the
transition from the ISM to the WD period. Indeed, the three most negative
δ18O values of daily rainfall recorded during this study
(Ranichauri: -19.48 ‰, Jorhat: -22.79 ‰) were observed
in September and October. Conversely, the most positive δ18O
values (Jammu: +8.20 and +9.28 ‰) were observed towards the end
of the WD period (April, June). Although d-excess records shown in Fig. 3b
are rather noisy, it is apparent that the d-excess progressively decreases
at Srinagar site from high d-excess values recorded in January–February
(d > 20 ‰) to low d-excess values
(d < 10 ‰) recorded in September. At Jorhat, temporal
evolution of the d-excess values largely coincides with the trend observed
at Srinagar. However, during the WD period, the d-excess is much more
variable at
Jorhat and ranges from 0 to more than 40 ‰.
Seasonal variations of δ18O (a) and
d-excess (b) of daily rainfall collected at the Jorhat (JRH) and
Srinagar (SGR) stations. The Indian summer monsoon (ISM) period at Srinagar
(July–September) and Jorhat (May–September) is marked in blue and red
shading, respectively.
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Negative values of the d-excess were recorded from time to time at all
stations along the transect (35 events in total, cf. Figs. 3 and S1b). An
extreme value (-28.8 ‰) was recorded at Palampur station in
February 2010 and was associated with very light rain (< 1 mm). As
seen in Figs. 3 and S1b, the largest number of events characterized by
negative d-excess values was
recorded at Jammu station (19 in total). Such events predominantly
occurred at this site during the WD period (17 out of 19 registered cases).
Negative d-excess values were also observed during the western African
monsoon (Risi et al., 2008; Landais et al., 2010) and were generally
attributed to the re-evaporation of raindrops. Raindrops falling through
unsaturated atmosphere below the cloud-base level undergo partial
evaporation. This process was investigated in the laboratory by
Steward (1975) and was modelled (e.g. Bony et al., 2008). The relative
magnitude of 2H and 18O fractionation effects associated
with the evaporation process cause the evaporating water droplet to become
enriched in heavy isotopes and evolve in the
δ2H–δ18O space along the line with a slope
significantly lower than eight (e.g. Rozanski et al., 2001). Thus,
evaporating raindrops will move away from the local meteoric water line
(LMWL), eventually reaching negative d-excess values. The extent of partial
evaporation is mainly controlled by the relative humidity of the atmosphere
through which raindrops are falling as well as by the isotopic composition of
ambient moisture. In fact, the WD period at Jammu station is characterized by
an exceptionally low relative humidity (average value for WD period ca.
35 %) when compared to other stations. This may explain the large number
of rainfall events with negative d-excess values recorded at this station.
δ2H–δ18O relationship for daily
isotope data available for Jorhat (JRH) and Srinagar (SGR) stations. Local
meteoric water lines were calculated separately for Indian summer monsoon
(ISM) and western disturbances (WD) periods.
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Figure 4 shows daily δ2H and δ18O data
available for Srinagar
and Jorhat stations, grouped into ISM and WD periods
and plotted on a δ2H–δ18O graph. Analogous
plots for other stations are presented in the Supplement (Fig. S2). During
the monsoon season the linear relationship between δ2H and
δ18O is generally better defined, pointing to moisture sources
of a similar nature and similar rainfall formation conditions. At the
Srinagar site, there is a striking difference between local meteoric water
lines representing the ISM and WD periods. The intercept of the LMWL
representing the ISM is more than two times lower than the LMWL representing
the WD period, with both lines displaying similar slopes. A similar situation
is observed for Ranichauri station. For Jorhat and Dibrugarh the seasonal
differences in the LMWL are less pronounced. Local meteoric water lines
representing the WD period for Jammu and Palampur stations have significantly
lower slopes (5.8 and 5.5, respectively) pointing to the importance of the
re-evaporation of raindrops at these sites outside of the monsoon season
(Fig. S2).
Mean isotope characteristics of daily precipitation collected at the six
stations along southern foothills of the Himalayas, calculated for the Indian
summer monsoon (ISM) and western disturbances (WD) periods. Slopes of the
best fit lines approximating the relationship between surface air temperature
(Δδ18O / ΔT) and precipitation amount
(Δδ18O / ΔP), based on daily data available
for each period, are reported in the last two columns of the table.
Station
aPeriod
δ18O (‰)
δ2H (‰)
d-excess (‰)
bN
c(Δδ18O / ΔT)
c(Δδ18O / ΔP)
Srinagar
ISM
-5.81±0.79
-42.6±5.8
3.9±1.6
31
+
±
WD
-4.19±0.48
-17.2±3.7
16.3±0.8
90
±
-
Jammu
ISM
-5.34±0.63
-30.9±4.6
11.5±1.1
51
+
±
WD
1.94±0.58
17.3±3.6
1.8±1.8
47
+
±
Palampur
ISM
-8.70±2.51
-60.9±17.6
8.7±2.5
12
±
±
WD
-1.24±0.28
-0.7±0.2
11.3±1.4
19
±
±
Ranichauri
ISM
-8.96±2.11
-60.8±14.3
10.8±2.5
18
±
±
WD
-2.16±0.58
-4.1±1.1
13.2±3.5
13
+
±
Jorhat
ISM
-6.88±0.52
-43.3±3.3
11.7±0.9
172
±
±
WD
-1.99±0.24
-2.2±0.3
13.8±1.6
70
-
±
Dibrugarh
ISM
-6.64±1.65
-40.2±10.1
12.9±3.2
16
-
n.d.
WD
-1.92±0.64
-4.5±1.5
10.9±3.6
9
±
n.d.
a Onset and duration of the Indian summer monsoon (ISM)
period defined operationally on the basis of the seasonal distribution of
long-term monthly rainfall (cf. Fig. 2) and thorough examination of individual
backward trajectories calculated for daily rainfall data collected by each
station in the framework of the present study. Duration of the ISM for Srinagar,
Jammu and Palampur: July–September; for Ranichauri: June–September; for
Jorhat and
Dibrugarh: May–September. The rest of the year is defined as western disturbances (WD).b Number of daily precipitation samples analysed.c “+” signifies a positive, significant (R≥0.4)
correlation between δ18O and daily temperature or
precipitation. “-” signifies a negative, significant (R≥0.4)
correlation between δ18O and daily temperature or
precipitation. “±” signifies the lack of a significant (R≥0.4)
correlation between δ18O and daily temperature or
precipitation. “n.d.” signifies “not determined”.
The summary isotope statistics of daily rainfall data available for all six
stations, grouped into ISM and WD periods, are presented in Table 3.
Arithmetic averages of three isotope parameters characterizing daily rainfall
(δ2H, δ18O, d-excess) with their respective
uncertainties were calculated for both periods. Arithmetic averaging was
chosen in order to better reflect average conditions at moisture sources and
in the regional atmosphere during the considered periods. The
δ18O values of daily precipitation were correlated with the daily
surface air temperature and rainfall amount available for each station.
Linear correlations were calculated separately for ISM and WD periods. The
last two columns of Table 3 summarize those calculations. Box and whisker
plots of daily δ18O and d-excess data available for each
station and season are summarized in Fig. 5.
Box and whisker plots for δ18O and d-excess values
of daily rainfall at the six stations (Srinagar – SGR; Jammu – JMU;
Palampur – PMR; Ranichauri – RNC; Jorhat – JRH; and Dibrugarh – DBR)
collecting daily rainfall samples along southern foothills of the Himalayas
(cf. Fig. 1). The data are grouped into Indian summer monsoon (ISM) and
western disturbances (WD) periods.
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As seen in Table 3 and Fig. 5, ISM and WD periods are characterized by
distinct isotope signatures of precipitation collected along the southern
foothills of the Himalayas. Average δ2H and
δ18O values for the ISM period are significantly lower than those
recorded for the WD period at the given station. The largest difference (ca.
7.5 ‰ for δ18O and 60 ‰ for
δ2H) was observed at Palampur station. Rainfall collected
during ISM and WD periods also differs with regards to the mean d-excess values. During
ISM, d values are generally lower than those observed during the WD period. In
one case (Jammu station) this regularity is broken (very low mean d-excess
for WD period). This anomalously low mean d-excess stems from the fact that a large
proportion of rainfall events exhibit negative d-excess values (cf.
discussion above). There is a lack of distinct spatial trends in the mean
isotope characteristics of daily rainfall along the study transect during
the two seasons (cf. Fig. 5). As far as the link between δ18O
and local surface air temperature is concerned, positive, significant
correlations were found for three stations: Srinagar (ISM period), Jammu (both
periods) and Ranichauri (WD period). Significant negative correlations were
observed for Jorhat and Dibrugarh stations (ISM period).
(a) Upper diagram: 10-day backward trajectories arriving at
Jorhat station on 22 May 2010, 12:00 LT
and representing the Indian summer monsoon (ISM) period. (b) Upper diagram: 10-day backward
trajectories arriving at Jorhat station on 17 February 2011, 12:00 LT and
representing the western disturbances (WD) period. Twenty seven ensembles (grey
lines in the background) and the mean trajectory (heavy line) are shown.
The colours of the mean trajectory indicate elevation above ground level in
metres. Empty white dots on the mean trajectory indicate 24 h intervals.
Lower diagrams of (a) and (b) show the evolution of the six
selected parameters of the air parcel along the trajectory. Evolution of the mean
value of each parameter is marked with a heavy black line and the associated
uncertainty in grey. Dashed vertical lines mark the part of the mean 10-day
trajectory visible in the upper diagram. International boundaries are only
indicative and as provided by the software.
![]()
Distinct seasonality seen in the isotope characteristics of daily rainfall at
the southern foothills of the Himalayas should be viewed in the context of
large-scale seasonal changes in the circulation regime of the regional
atmosphere. These changes can be best illustrated through the reconstruction
of back trajectories of moist air masses generating rainfall events at
selected sites of the east–west transect. Backward trajectory modelling was
performed for all daily precipitation events analysed in the framework of
this study (548 events). Figures 6 and 7 show examples of typical backward
trajectories (ensembles) representing ISM and WD periods, reconstructed for
Jorhat and Srinagar stations, and representative of the eastern and western
edges of the transect, respectively. The lower panels of Figs. 6 and 7 show
the evolution of selected parameters of the air parcels transported along the
trajectories: (i) elevation above the local ground (m), (ii) terrain height
(m a.s.l.), (iii) velocity of the air parcel (m s-1), (iv) temperature
of the air parcel (∘C), (v) precipitation rate (mm per 6 h) and
(vi) H2O mixing ratio (g kg-1). The ISM period at Jorhat
station (Fig. 6a) is dominated by the low-level transport of moist air masses
(water vapour content of ca. 18 g kg-1) from the equatorial Indian
Ocean and the Bay of Bengal. The calculated ensembles have relatively low
spread, reflecting the large-scale, uniform movement of the moist air masses
driven by monsoon circulation. The air masses reach the station from the
south, without any noticeable contribution from other directions. Rainfall is
generated when warm, moist air masses are lifted up over the continent and
cool down from ca. 28 to 20 ∘C. As seen in Fig. 7a, the monsoonal
air masses may reach also the Srinagar station located on the western edge of
the transect. However, the ensembles shown in Fig. 7a suggest that the more
direct route of moist monsoonal air masses (water vapour content of ca.
15 g kg-1), starting in the Arabian Sea and crossing the Indian
continent, is a more important source of rainfall for western Himalayas
during this season than trajectories passing over the Bay of Bengal and
travelling along the southern foothills of the Himalayas in the northwest
direction. During WD period, the overwhelming majority of air masses arrive
at rainfall collection stations from the west and northwest (Figs. 6b and
7b). Typically, they travel at high elevations (4000–6000 m a.g.l.),
passing the Black Sea and Caspian Sea regions, and further descend over
Afghanistan and Pakistan, towards the rainfall collection stations located at
the western edge of the transect (Srinagar, Jammu, Palampur). These dry air
masses (water vapour content around 1 g kg-1) pick up moisture of
continental origin from relative proximity to the collection sites and their
vapour content rises to approx. 3.5 g kg-1 (Fig. 7b – lower panel).
For stations located at the eastern edge of the transect (Jorhat, Dibrugarh)
more western routes of air masses prevail (Fig. 6b). Air masses travel east
within the latitude band of 20 to 30∘ N passing the Arabian
Peninsula, the Persian Gulf, the northern reaches of the Arabian Sea and
northern India, gradually warming up and losing elevation. On their way east
they gradually absorb moisture of both marine (Arabian Sea) and continental
origin. As a result of this process, their water vapour content rises from
approximately 3 g kg-1 at the eastern coast of the Arabian Peninsula
to ca. 8 g kg-1 in the proximity of the eastern edge of the transect.
(a) Upper diagram: 10-day backward trajectories arriving at
Srinagar station on 10 August 2010, 12:00 LT and representing the Indian
summer monsoon (ISM) period. (b) Upper diagram: 10-day backward
trajectories arriving at Srinagar station on 2 April 2011, 12:00 LT and
representing the western disturbances (WD) period. Twenty seven ensembles
(grey lines in the background) and the mean trajectory (heavy line) are
shown. The colours of the mean trajectory indicate the elevation above ground
level in metres. Empty white dots on the mean trajectory indicate 24 hour
intervals. The lower panels of (a) and (b) show the
evolution of the six selected parameters of the air parcel along the
trajectory. The evolution of the mean value of each parameter is marked by a
heavy black line and the associated uncertainty in grey. Dashed vertical
lines represent the part of the mean 10-day trajectory visible in the upper
diagram. International boundaries are only indicative and as provided by the
software.
![]()
Evolution of <inline-formula><mml:math id="M264" display="inline"><mml:mrow class="chem"><mml:msup><mml:mi mathvariant="italic">δ</mml:mi><mml:mn mathvariant="normal">18</mml:mn></mml:msup><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math></inline-formula> during ISM
period
The gradual, heavy isotope depletion of rainfall in the course of the development
and recession of the Indian summer monsoon, as apparent from Figs. 3a and S1, is not a
local phenomenon only restricted to the study transect. Such evolution of
δ18O in precipitation is observed over the entire region
influenced by the ISM. For instance, δ18O of monthly rainfall at
New Delhi decreases from ca. 0 ‰ in June to around -9 ‰
in September (Araguas et al., 1998; Battacharya et al., 2003). The
IWIN programme stations (Deshpande and Gupta, 2008, 2012) also show a similar
trend with the gradual decrease of δ18O values of rainfall towards
the end of the ISM season in September, e.g. Ahmedabad station in
western India (Deshpande et al., 2010). A reduction of the 18O
content of similar magnitude was observed for individual rainfall events
collected during the 2012 and 2013 monsoon period on Andaman Island, in the Bay of
Bengal (Chakraborty et al., 2016).
The extent of heavy isotope depletion of daily rainfall in the course of the
ISM evolution, observed in this study, is large (> 10 ‰
in δ18O). Regional thermal gradients are virtually
non-existent during ISM; hence, they cannot be used to explain the observed
gradual reduction of δ18O, with sea surface temperatures in
the Bay of Bengal fluctuating between 28 and 29 ∘C (e.g. Midhun et
al., 2013) and mean surface air temperatures at three low-elevation stations
of the study transect (Jammu, Jorhat, Dibrugarth) around 28 ∘C (cf.
Table 2). This gradual reduction of heavy isotope content in precipitation is
apparently a regional phenomenon inherently linked to the evolution of the
Indian summer monsoon and cannot be explained by local effects. Local effects
such as enhanced convective activity in the local atmosphere and the
associated higher degree of rainout of moist air masses (local amount effect)
or partial evaporation of raindrops, might explain the short-term
fluctuations of δ18O visible in Figs. 3a and S1a; however,
local effects will not explain the regional evolution of δ18O
and δ2H values over the course of the ISM. Clearly, another
explanation should be considered here. In this context, it is noteworthy that
Deshpande et al. (2015) have shown that major moisture uptake locations for
precipitation at Ahmedabad gradually change from over the Arabian Sea to
central Indian continental areas in the later part of the ISM.
Over the course of the ISM period, water availability from surface (lakes,
reservoirs, streams, wetlands) and sub-surface (soil, vadose zone)
environments and in the lower atmosphere over large continental areas of
India progressively increases to a substantial extent. For instance, in the
state of Assam 9.7 % of the region is under wetlands including rivers,
streams and riverine wetlands during this period. The open pan annual
evaporation (2.36 mm day-1) and annual potential evapotranspiration
(3 mm day-1) at Jorhat is the lowest in the country (Rao et al.,
2012). The relative humidity is generally higher during the summer months
from June to November. The low values of open pan and potential
evapotranspiration and high relative humidity in Assam suggest that the
atmosphere remains continuously loaded with locally generated moisture during
the summer months. As the monsoon progresses in India, enhanced soil moisture
and vegetation cover lead to increased evapotranspiration and recycled
precipitation. The recycling ratio, which is the ratio of recycled
precipitation to total precipitation, is highest (around 25 %) in
northeast India, where dense vegetation cover leads to high
evapotranspiration. A high precipitation recycling ratio was found at the end
of the monsoon in the month of September (Pathak et al., 2014).
The increasing amount of moisture in the lower atmosphere over the course of the ISM
makes the air column unstable and prone to convective activities. In should be
noted here that in the absence of horizontal thermal gradients between source
regions of atmospheric moisture and the continent, the principal mechanisms
which can generate rainfall are vertical uplift and the cooling of moist air
masses, which are associated with convective systems. If the horizontal and vertical
extent of convective systems increase as the monsoon progresses, they could
generate the observed gradual depletion of heavy isotopes in daily rainfall
over the course of the ISM evolution.
Looking from a broader perspective, a large-scale, regional recycling of
moisture of oceanic origin can also contribute to the observed evolution of
δ18O and δ2H during the ISM period. Northward
movement of the ITCZ pulls maritime moisture from Southeast Asia and the Bay of
Bengal and transports it north-westward. Moist air masses are lifted by
large-scale convection, lose part of their moisture content and return
towards the equator as the upper branch of Hadley cell circulation. Then,
they descend and mix with the low-level moist air masses of oceanic origin (cf.
Li et al., 2016 – Fig. 24). Descending air masses contain moisture depleted
of heavy isotopes, which is then incorporated in the moist air masses of
oceanic origin transported north-westward. A regional recycling loop such as
this, operating in the course of ISM evolution, may provide the required mechanism
for a gradual, large-scale reduction of the 18O and 2H
content in regional atmospheric moisture and precipitation during this
period. A rough assessment of this effect was made assuming a Rayleigh-type
rainout of moist air masses of oceanic origin (RH = 80 %,
T = 28 ∘C), with the initial δ18O value of
moisture equal to -10 ‰. It was assumed that rainout induced by
large-scale convection continues down to 15 % of the initial water
content. Dry air masses containing vapour depleted in heavy isotopes
return as an upper branch of the recycling loop and mix with the moist air
masses of oceanic origin. Five to six such recycling loops would be required
to reduce the initial 18O content of maritime moisture and the
rainfall by approximately 10 ‰ at the end of the monsoon period, in
accordance with observations.
It is likely that both of the mechanisms underlined above act together to
produce the observed characteristic evolution of δ18O and
δ2H in daily rainfall during the ISM period. Model runs of
isotope general circulation models available for Indian continent (e.g. Hoffman and Heimann, 1997;
Midhun and Ramesh, 2016) suggest that the models tend to underestimate the
amplitude of seasonal changes of δ18O, particularly in
northern India. A more comprehensive isotope modelling of monsoon circulation
would be needed to quantify the above-outlined mechanisms of moisture
recycling and their impact on the measured stable isotope composition of
precipitation in the region.
Finally, worth commenting on, are the large seasonal changes in the isotopic
composition of regional atmospheric moisture reservoir in response to the
contrasting circulation patterns of the regional atmosphere and moisture
recycling mechanisms discussed above. When the operation of the monsoon
circulation engine is terminated in September, the regional atmosphere is
still loaded with moisture heavily depleted in 2H and
18O. This remarkable heavy isotope depletion of the regional
atmospheric moisture reservoir survives for several weeks. In fact, the most
negative δ18O value (-22.79 ‰) was measured in
rainfall collected at Jorhat station on 11 October 2010. In the course of the WD
period, maritime moisture depleted in heavy isotopes is gradually replaced by
moisture of continental origin characterized by elevated concentration of
2H and 18O (cf. discussion below). This, in turn, is
reflected in rising δ values of rainfall over the course of the WD period.
Positive <inline-formula><mml:math id="M300" display="inline"><mml:mrow class="chem"><mml:msup><mml:mi mathvariant="italic">δ</mml:mi><mml:mn mathvariant="normal">18</mml:mn></mml:msup><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math></inline-formula> and
<inline-formula><mml:math id="M301" display="inline"><mml:mrow class="chem"><mml:msup><mml:mi mathvariant="italic">δ</mml:mi><mml:mn mathvariant="normal">2</mml:mn></mml:msup><mml:mi mathvariant="normal">H</mml:mi></mml:mrow></mml:math></inline-formula> values of daily rainfall
A striking feature of the isotope data generated in the framework of this study
is the relatively frequent appearance of positive δ18O and
δ2H values in the isotope records available for six stations
collecting daily rainfall. Positive δ values range from 0.17 to
9.28 ‰ for δ18O and from 5.3 to 56.6 ‰ for
δ2H. They constitute ca. 16 % of the collected and
analysed data. Positive δ18O and δ2H values
were mostly recorded during the WD period (ca. 25 % of all data available for
this period, compared to 5.5 % recorded during the ISM period). The station
where positive δ18O and δ2H values were
recorded most frequently (70 % of the data available for the WD period) was
Jammu. To better characterize rainfall events showing positive δ
values, d-excess values calculated for such events were plotted as a
function of (positive) δ18O values and the relative humidity
of the local atmosphere (daily means). The resulting plots are shown in
Fig. 8. As seen in Fig. 8, d-excess values decrease with increasing
δ18O values (R2=0.349) and increase with rising relative
humidity (RH) of the local atmosphere (R2=0.147). In a comprehensive
study of the western African monsoon precipitation near Niamey (Niger) (Landais et
al., 2010), a significantly higher slope of d-excess–RH correlation was found
(0.38) compared to that characterizing data points shown in the lower
panel of Fig. 8 (0.22±0.05). Also the variables were much better
correlated (R2=0.68, Fig. 7b in Landais et al., 2010). However, this is
not surprising, keeping in mind that the data reported by Landais et
al. (2010) originated from one station (Banbizoumbou) and were
restricted to the monsoon season (June–September). Conversely, the data
shown in Fig. 8 covered both seasons (ISM and WD) and represented six stations
distributed along a 2900 km transect of the southern foothills of the
Himalayas. Moreover, relative humidity data in the Landais et al. (2010) study
were reconstructed mean RH values for the lower troposphere
(200–1000 m a.g.l), whereas this study used daily means of RH values
measured near ground level.
(a) The relationship between deuterium excess and positive
δ18O values measured in daily precipitation samples collected
at six stations distributed along the southern foothills of the Himalayas
(cf. Fig. 1). (b) The relationship between deuterium excess and
relative humidity of the local, near-ground atmosphere (daily means)
calculated for precipitation events exhibiting positive δ18O
values.
![]()
The 18O isotope composition of maritime moisture, collected onboard
a ship (mast top, ca. 25 m a.s.l.) cruising the Bay of Bengal during the
ISM period (from 13 July to 3 August 2012), varied between ca. -10 and
-14 ‰ (Midhun et al., 2013). If one adopts -10 ‰ as a
representative δ18O value for unaltered oceanic moisture from
which monsoon precipitation is formed, and further assumes that this
moisture is transported towards the southern foothills of the Himalayas
without any noticeable rainout effect, the expected δ18O value
of the first condensate would be around -1.0 ‰. However, it is highly
unlikely that unaltered maritime moisture can reach remote continental
sites such as Jammu station, where positive δ values are most common.
Hence, the “first condensate” scenario cannot fully explain the positive
δ18O and δ2H values recorded at the stations
along the transect, even if the partial evaporation of raindrops on their way to
the ground is considered.
As the majority of the positive δ values was recorded during the WD period,
the explanation of positive δ18O and δ2H values
should involve sources of moisture other than oceanic ones. One can
distinguish three components of the backward flux of water into the regional
atmosphere over the continental areas, each characterized by distinct isotope
signature: (i) water transpired by plant cover, (ii) water evaporated from
bare soil and (iii) water evaporated from surface water bodies. It is a
well-established fact that in the course of the transpiration process, leaf water
becomes progressively enriched in heavy stable isotopes, quickly reaching
hydrologic and isotopic steady state (e.g. Dongmann et al., 1974; Flanagan et
al., 1991). Under such conditions, the isotopic composition of water vapour
released into the atmosphere is isotopically identical to the source water
utilized by plants. In our case the water utilized by plants predominantly
originates from the rainy (monsoon) season. The amount-weighted mean
δ18O of ISM precipitation for three low-altitude stations
(Jammu, Dibrugarh, Jorhat) is -6.5 ‰. First condensate produced
from such water vapour (assumed condensation temperature of
+10 ∘C) will be characterized by δ18O values close
to +4.2 ‰, which fall within the range of positive δ
values of daily rainfall collected at the stations. Soil water evaporation
may also produce water vapour, the isotopic composition of which is identical
to that of the source (soil) water. However, due to the much larger size of the soil
water reservoir compared to leaf water, establishing a steady-state
isotope evaporation profile in the soil column requires much longer periods
of time than is the case for the leaf water reservoir (weeks instead of hours).
This is only possible under arid or semi-arid conditions, where periods
between consecutive rain events are long enough (e.g. Zimmerman et al., 1966;
Barnes et al., 1983) which is generally not the case for the study area.
Evaporation from bare soil in this case will resemble evaporation from open
water bodies. The isotopic composition of evaporating surface water bodies evolve
in the δ2H–δ18O space along the so-called local
evaporation line with the slope significantly lower than eight (e.g. Gat, 1996).
The mass balance considerations require that water vapour being released into
the local atmosphere in the course of such process is located on the local
evaporation line, to the left-hand side of the local meteoric water line
(LMWL). This water vapour has a somewhat reduced heavy isotope content when
compared to the source water subject to evaporation and is characterized by
high deuterium excess (e.g. Rozanski et al., 2001).
All three processes outlined above are most probably acting together under
climatic conditions characteristic of the study region. These processes are also apparently
capable of delivering sufficient amounts of moisture to the regional
atmosphere to produce rainfall characterized by positive δ values,
even at locations which are far away from oceanic sources of water. It is
likely that generally higher and more variable d-excess values of rainfall
events recorded in the course of the WD period (cf. Figs. 3a and S1a),
reflect the varying contribution of those three processes to the backward flux of
moisture into the regional atmosphere, generated by the Indian subcontinent
during that time of the year. The fact that d-excess values are inversely
correlated with δ18O and increase with the rising relative
humidity of the near-ground atmosphere (cf. Fig. 8), point to the partial evaporation
of raindrops as an additional mechanism contributing to the observed range of
positive δ18O and δ2H values.
Significance of elevated <inline-formula><mml:math id="M351" display="inline"><mml:mi>d</mml:mi></mml:math></inline-formula>-excess values
A higher than the global average d-excess value (ca. 10 ‰) in
meteoric waters originating from the Himalayas and Tibetan Plateau was often
used to infer Mediterranean or more generally westerly derived vapour (Tian
et al., 2005; Hren et al., 2009; Jeelani et al., 2010; Bershaw et al., 2012).
The observed high d-excess in rainfall was generally related to the higher
d-excess (ca. 20 ‰) found in the vapour generated over eastern
Mediterranean Sea (Gat and Carmi, 1970). Here we argue that these high
d-excess values recorded in the Himalayas do not necessarily originate from
the Mediterranean Sea. There were 38 rainfall events with
d > 20 ‰ observed in this study (ca. 6.9 % of all
events analysed). Higher d-excess values mostly occurred during the WD
period (31 out of 38 cases). However, the highest d values were recorded
during the ISM period (Jammu, 34.0 and 39.1 ‰; Jorhat, 40.7 and
44.0 ‰).
The largest number of events characterized by high d-excess values
was
recorded
at Srinagar station during the WD period (21 out of 90 analysed for this
period). The only station without elevated d-excess values was Dibrugarh.
Closer examination of backward trajectory ensembles calculated for days with
high d-excess values reveal that trajectories associated with daily
rainfall samples characterized by high d-excess values arrive at Srinagar
from the northwest, west or southwest. However, as discussed above, these air
masses are generally very dry and only pick-up moisture of continental origin
in relative proximity to rainfall collection stations (cf. Fig. 7b).
Surprisingly, high d-excess values recorded at Jammu station are almost
exclusively associated with characteristic monsoon-type circulation
(Fig. S3). In one case, recorded during the WD period (31 December 2010), the
air masses were circling around over the Indian subcontinent and interacting
strongly with the surface.
A common feature of almost all trajectories at all the precipitation sites with
high d-excess is their long residence time in relative proximity of the
sampling site, as illustrated in Fig. S4 for Jorhat station. This long
residence time leaves enough time for their prolonged interaction with the surface, during which
evaporation of surface water bodies (lakes, swamps, etc.) and/or
non-steady-state evaporation of soil moisture serve as important sources of
water vapour characterized by high d-excess values. Rainfall produced from
such vapour will retain this characteristic isotope signature in the form of
a high d-excess value. Some impact of atmospheric moisture with high
d-excess arriving from the eastern Mediterranean in the Himalayan region is
certainly possible, although in our opinion it is rather unlikely that this is
an important source of rainfall in the region. Low-level eastward moving,
turbulent transport of moisture from the eastern Mediterranean towards the
Himalayas will inevitably be associated with strong interaction with the
surface on the way (rainfall, backward moisture fluxes) which will blur the
original isotope signature of the moisture of marine origin.
Footprint analysis
To better characterize the contribution of different air masses arriving in
the course of ISM and WD seasons, at the six stations collecting daily rainfall
along southern foothills of the Himalayas, footprint analysis was performed.
Footprint maps were calculated for the 2009–2011 period, based on daily
simulations of 10-day long backward trajectories, starting at each of the
locations at noon local time. Footprint maps were prepared for three stations
(Jammu, Kathmandu and Jorhat) representing the western, central and eastern
parts of the study transect, respectively. Separate maps were constructed for ISM
and WD periods and are presented in Fig. 9. The maps shown in Fig. 9 provide
valuable insight into the great seasonal contrast in the circulation patterns
of the regional atmosphere, which in turn control the rainfall regime in the
region (amount, seasonal distribution and stable isotope composition).
Footprint maps of air masses arriving at three stations: Jammu
(JMU), Kathmandu (NPL) and Jorhat (JRH) representing the respective western,
central and eastern parts of the east–west study transect along southern
foothills of the Himalayas (a, b, c). Separate maps were prepared
for the Indian summer monsoon (ISM) and western disturbances (WD) periods.
Daily trajectories for the period 2009–2011 were reconstructed using the
HYSPLIT modelling framework (see text for details). International boundaries
are only indicative and as provided by the software. The colour scale
indicates the focally averaged number of trajectories passing through a grid
cell.
![]()
The footprint map representing the ISM period at Jorhat station clearly
demonstrates the overwhelming dominance of the monsoon circulation bringing
moisture-loaded air masses from the tropical Indian Ocean and the Bay of Bengal
towards the eastern region of the Himalayas. There is a very small
contribution (in the order of few percent) from the air masses arriving from
the west and northwest. The dominating influence of monsoon air masses is also seen
in the central portion of the transect (Kathmandu site), although the
presence of air masses originating in the Arabian Sea and crossing Indian
subcontinent in the northeast direction is also noticeable. The footprint map
for Jammu station representing the western Himalayas clearly shows three major
types of air masses arriving at this site during the ISM period: (i) maritime
monsoonal air masses originating in the Bay of Bengal and travelling along
southern foothills of the Himalayas, (ii) continental air masses coming from
the northwest and (iii) air masses originating in the Arabian Sea
and travelling along the India–Pakistan border towards eastern Himalayas;
the first of these being the dominating component.
During the WD period, the circulation patterns of the regional atmosphere change
radically. The Jorhat station receives air masses predominantly from
northern India and Pakistan, with a noticeable contribution from the Bay of
Bengal. The footprint map is generally more diffuse, indicating the presence of
continental air masses with an origin in central Asia as well as the Black Sea
and Caspian Sea regions. A similar occurrence is observed at Kathmandu station,
with majority of air masses coming from northern India and Pakistan. The
impact of maritime air masses (Bay of Bengal) is reduced, although it is still
visible. The western part of the Himalayas (Jammu station) is under the
overwhelming influence of air masses coming from the west (Iran, Iraq,
Afghanistan and Pakistan). While the small contribution of oceanic air masses
coming from the eastern Arabian Sea is still visible, air masses coming from the
Bay of Bengal are practically absent.
Conclusions
Isotope analyses of daily precipitation samples collected at six stations
located along the southern foothills of the Himalayas allowed a deeper
insight into the mechanisms controlling the isotopic composition of precipitation
in this important region of the Indian subcontinent. The analysis of the 2H and
18O isotope composition of daily precipitation, combined with
extensive backward trajectory modelling of the air masses associated with
rainfall in the study region, allowed several important conclusions to be
drawn with respect to origin of atmospheric moisture and precipitation in two
contrasting seasons (Indian summer monsoon and western disturbances).
It is suggested that the gradual reduction in the 2H and 18O
content of precipitation in the region, progressing from positive
δ18O values, down to less than -10 ‰ in the course
of ISM evolution, stems from convective activities in the regional atmosphere
and large-scale recycling of moisture of oceanic origin, driven by monsoonal
circulation. Superimposed on this general trend are short-term fluctuations
of the isotopic composition of rainfall which may stem from local
effects such as locally enhanced convective activity and the associated higher
degree of rainout of moist air masses (local amount effect), partial
evaporation of raindrops, or the impact of isotopically heavy moisture generated
in evapotranspiration processes taking place in the vicinity of rainfall
sampling sites. Seasonal footprint maps constructed for three stations
representing the western, central and eastern portions of the Himalayan region
indicate that the influence of monsoonal circulation reaches the western edges of
the Himalayan region. While the characteristic imprint of monsoon air masses
(increase of monthly rainfall amount) can be completely absent in the eastern
Himalayas, the onset of the ISM period is still clearly visible in the isotopic
composition of individual precipitation events.
The most characteristic feature of daily precipitation collected in the
study region during the WD period is its relatively high 2H and
18O content when compared to the ISM period, and the presence of a large
number of daily rainfall samples exhibiting positive δ18O and
δ2H values. These peculiar isotope characteristics can only be
explained when a continental origin of the source moisture for
this precipitation is postulated. Water stored in the soil during the ISM period
is returned to the regional atmosphere during the WD period through the
evapotranspiration processes. Backward trajectory modelling has shown that
long-range transport of air masses from the west and northwest,
characteristic of the WD period, occurs at high elevations and cannot bring
sufficient amounts of moisture to significantly contribute to precipitation
in the study area during this period. Instead, the major supply of moisture
for rainfall during the WD period is mainly of local (regional) origin, stemming
from the transpiration of plant cover, soil water evaporation and the evaporation of
surface water bodies. All of these processes deliver water vapour which is
significantly enriched in heavy isotopes compared to unaltered vapour of
oceanic origin. This enrichment is then reflected in the isotopic composition
of rainfall produced from such vapour. Isotope characteristics of rainfall
during the WD period are consistent with this scenario. Seasonal footprint maps
show that during this period eastward moving air masses may reach the eastern
edges of the Himalayas. Footprint maps also suggest the presence of air
masses coming from the Bay of Bengal, which bring moisture of oceanic origin
to the study area, cannot be excluded.
It appears that high d-excess values of daily rainfall collected along
southern foothills of the Himalayas can be associated with air masses of very
different origins. However, the common feature of almost all air masses is
their relatively long interaction with the continental surface, which
provides a chance to accommodate enough moisture of continental origin; this
moisture is characterized by elevated d-excess values, which then is transferred to the
local rainfall.