ESDEarth System DynamicsESDEarth Syst. Dynam.2190-4987Copernicus PublicationsGöttingen, Germany10.5194/esd-8-313-2017Estimates of land and sea moisture contributions to the monsoonal rain over Kolkata, deduced based on isotopic analysis of rainwaterDarShaakir Shabirhttps://orcid.org/0000-0001-8739-269XGhoshProsenjitpghosh@ceas.iisc.ernet.inCentre for Earth Sciences, Indian Institute of Science, Bangalore, 560012, IndiaProsenjit Ghosh (pghosh@ceas.iisc.ernet.in)27April2017823133215December201614December20168March2017This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://esd.copernicus.org/articles/8/313/2017/esd-8-313-2017.htmlThe full text article is available as a PDF file from https://esd.copernicus.org/articles/8/313/2017/esd-8-313-2017.pdf
Moisture sources responsible for rains over Kolkata during the summer monsoon can
be traced using backward air-mass trajectory analysis. A summary of such
trajectories between June and September suggest that these moisture parcels
originate from the Arabian Sea and travel over the dry continental region and
over the Bay of Bengal (BoB) prior to their arrival at Kolkata. We use
monthly satellite and ground-based observations of the hydrometeorological
variables together with isotopic data of rainwater from Bangalore and
Kakinada to quantify the contributions of advected continental and oceanic
water vapour in the Kolkata rains. The vapour mass is
modified during its transit from its original isotopic value due to addition of evaporated
moisture from the BoB, and further modification occurs due to the process of
rainout during transport. The evaporated component is estimated using the Craig–Gordon equation. The rainout process is simulated using a Rayleigh
fractionation model. In this simulation we assume that the initial isotopic
composition of vapour originating from the continent is similar to the
rainwater composition measured at Bangalore. In order to explain the monthly
isotopic composition in southwest monsoon rainwater at Kolkata, we invoke
65–75 % moisture contribution from the BoB; the remaining moisture is
from the continental land mass.
Introduction
Indian land mass receives rainfall during the summer due to winds favouring
moisture transport from the region of the Intertropical Converge Zone (ITCZ)
located over the Indian Ocean. The process of moisture transport from the oceanic
region commences in June and continues until September. This period is
generally termed as the Indian Summer Monsoon (ISM) or the Southwest Monsoon
(SWM). Summer monsoon rain constitutes 50–90 % of the total annual
rainfall received by the entire country . The air
parcel during its northward journey picks up vapour from the surrounding
seas; namely the Arabian Sea and the Bay of Bengal (BoB). Studies have shown that the
composition of surface seawater surrounding the Indian land mass can be
isotopically distinguishable, i.e. heavier values for water in the Arabian Sea
located in the west and dominated by strong evaporative forcing, whereas the
BoB water in the east is characterized by lighter value due
to the influence of rainfall and river runoff (). Isotopic
composition measured in the shallow groundwater along the transect between
Kolkata and New Delhi were used in a simple box model to estimate the
percentage of moisture contribution in rain due to the process of
evapotranspiration (ET) . In this study
shallow groundwater over the Indo-Gangetic Plain was treated as the equivalent of
rainwater due to its short residence time and the estimated
presence of ∼ 40 % moisture due to recycling of shallow groundwater or rainwater. This was further verified in a recent study where the
average monthly (June–September) isotopic composition of rainwater measured
at Kolkata and New Delhi was explained by supplementing ∼ 20 %
moisture from the Arabian Sea to a parcel, modified due to Rayleigh-based
fractionation of the original moisture parcel originating from the BoB, in
addition to
∼ 45 % moisture being included as the ET component from recycling. In a
more recent observation on vapour isotopic composition from the northern
flood plain station located at the city of Roorkee, three moisture sources
for vapour were detected, namely BoB, western disturbances, and lake water
from the nearby regions . Satellite-based
observations and pan evaporation data indicate that the quantity of moisture
returned to the atmosphere by the process of evaporation is substantial and
plays a significant role in governing the regional water budget. According to
global estimates, the hydrological cycle involves an annual rate of
evaporation of about 500 000 km3 of water, around
86 % of which come from the oceans, with the remainder originating
from the continents . Using the Eulerian moisture
tracking method in a global study, showed that about
40 % of terrestrial precipitation originates from land evaporation.
However, significant variations from this average global value can occur
depending on position of station on the continent and net radiation influx,
along with influence of factors like land use and land cover
.
A quantitative understanding of the hydrological components involving
advected and land-recycled moisture is possible using stable isotope tracers
. Stable isotopes of oxygen and hydrogen in water
provide a method to determine contributions of land- and ocean-derived
moisture due to their distinct isotopic ratios. Isotopic ratios in rainwater
and water vapour mirror the isotopic composition of moisture sources modified
by the fractionation associated with the mechanism of precipitation during
its transport to a continental site
().
Attempts have been made for the quantitative
determination of the precipitation components using regional analytical
models (e.g.
) and global physical
models (e.g.
) at different timescales.
The Global Network of Isotopes in Precipitation (GNIP) (International Atomic
Energy Agency; IAEA) study over India offers several years of rainwater
isotopic values at monthly resolution for different stations along the travel
path. This database proves to be extremely useful to delineate the
contribution of different moisture sources to the regional precipitation.
In this study we use the rainwater isotopic data from the BoB and three
stations, namely Bangalore, Kakinada, and Kolkata. We have used simultaneous
satellite-based meteorological observations in a two-component mixing model
to deduce the moisture contribution from continental land mass due to
advection (also designated as land-based moisture) and supply from
evaporation of the BoB surface water during the year 2004.
Data and methods
The easterly winds during the SWM transport moisture that generates over
the Arabian Sea to continental destinations over southern India
. The global Lagrangian particle dispersion model,
which runs using ECMWF (European Centre for Medium-Range Weather Forecasts)
operational analysis for June, July, and August, reveals that the Arabian Sea and
the northern Indian Ocean act as sources of 95 % of the moisture
responsible for precipitation over the Indian land mass
. The isotopic composition of rainwater at Bangalore during the SWM of 2010 was explained using 70 %
moisture contribution from the Arabian Sea, while continental rainwater recycling
contributed the rest . The moisture parcels
originating from the Arabian Sea move over land under the prevailing easterly
winds and are modified in terms of composition due to participation of moisture
from land recycling. The measured isotopic composition of rain and/or vapour at
Bangalore, situated equidistant ∼ 300 km inland from the Arabian Sea in the west and the BoB in the
east, serves as an ideal representation of the isotopic signature of
continental moisture. Backward air-mass trajectories indicate that the
moisture parcel may travel further over land or over the BoB before
re-entering the Indo-Gangetic Plain (Fig. ). The SWM
monsoon enters the Indo-Gangetic Plain through a corridor over
the eastern coast near Kolkata.
The air masses were traced for -48, -72, and -96 h at 200, 500, 1000,
1500, 2000, and 2500 m elevations above the mean sea level at Bangalore and
Kolkata for all rainy days during the SWM of the year 2004 using the
meteorological input from Reanalysis 2 data. It was determined that 2004 was a normal monsoon
year. Indian Meteorological Department (IMD) defines a year as a normal
monsoon year if the rainfall is anywhere from half to less than 1.5 times the normal rainfall (over the land area). Hybrid
Single-Particle Lagrangian Integrated Trajectory (HYSPLIT)
analysis from the NOAA Air Research Laboratory
(http://www.arl.noaa.gov/ready/hysplit4.html) is used to track the
air parcel back in time. HYSPLIT is a complete system used for computing
simple air parcel trajectories as well as complex transport, dispersion,
chemical transformation, and deposition simulations .
The back trajectories for Bangalore and Kolkata are displayed in
Fig. . The contribution of continental and BoB moisture to
the rain precipitated at Kolkata during the SWM is modelled using the isotopic
composition of rainwater collected at Bangalore and over the BoB. As the air
parcel travels towards Kolkata, its isotopic composition is modified due to
an interplay of processes like rainout and moisture addition from the BoB. To
model the isotopic composition, the transect between Bangalore and Kolkata is
divided into seven boxes of equal dimensions (Fig. ). It
was designed in such a way that the majority of air-mass trajectories pass
through it. This size of the boxes is chosen so that the total precipitable
water for each box remains fairly uniform. The monthly averaged (June, July, August, and September) isotopic
δ18O values for the SWM of different years (2004, 2008, 2010, and
2013) were extracted from IAEA data and other publications. The monthly
averaged δ18O value at Kolkata (2004) is modelled by adopting the
isotopic composition of monthly rainwater at Bangalore as an original value
(initial condition). While simulating the model with two-component mixing, we
assumed near-identical (similar to Bangalore) values for moisture originating
from the continent. The δ18O values measured in the rainwater at
Kakinada (2004), which is located within the modelling transect, are useful
for
validating our assumption.
Together with rainwater isotopic data, monthly averaged meteorological
data (2004) and isotopic composition of the BoB surface water have been used
in this study. Precipitation data from the Tropical Rainfall Measuring
Mission (TRMM) project (3B42 V7 derived); Goddard
Space Flight Center, Distributed Active Archive Center (GSFC DAAC)
(http://trmm.gsfc.nasa.gov/); and the Total Precipitable Water, Air Temperature,
Relative Humidity, and Wind Speed datasets from the Reanalysis 2
(http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis2.html) have
been used. Surface-water isotopic composition over the BoB was extracted from the
Global Seawater Oxygen-18 Database (V-1.21)
(http://data.giss.nasa.gov/o18data/). The monthly averaged oxygen isotopic
composition (δ18O) for the SWM of the year 2004 was retrieved from
the IAEA-GNIP
(http://www-naweb.iaea.org/napc/ih/IHS_resources_gnip.html) dataset for
the stations located at Bangalore, Kolkata, and Kakinada. The isotopic
composition of rainwater at Bangalore for the year 2008 was obtained from
Rangarajan et al. (2013), while for other years (2010 and 2013), it was obtained
from . Figure depicts the oxygen isotopic composition (δ18O) for
Bangalore, Kakinada, and Kolkata. Table shows the oxygen isotopic
composition (δ18O) of BoB rainfall for the year 2012 collected
during the CTCZ-2012 expedition. An average isotopic composition of rainfall
recorded over the BoB is used to obtain a representative isotopic value for
demonstrating the evaporated moisture over the BoB using the Craig–Gordon
equation.
Study area and modelling transect divided into boxes of equal
dimensions. The modelling transect was chosen such that the majority of the
HYSPLIT trajectories pass through it. Orange, green, and blue circles
represent the locations of the Kolkata, Kakinada, and Bangalore stations
respectively. The ocean layer represents the δ18O surface-water
isotopic composition . The isotopic composition
remains fairly constant over the BoB, with slightly depleted values near the
Ganges Delta (Box 7) due to freshwater mixing. Solid pink circles
represent the locations of rainfall collected for isotopic measurements over
the BoB.
Rainwater isotopic composition and collection locations over the BoB
collected during 2012 expedition.
S no.LatitudeLongitudeδ18O(‰)(decimal(decimaldegrees)degrees)118.9989.39-1.74218.9989.39-1.55318.9989.39-1.53419.0289.39-1.31519.0089.60-4.26719.0090.00-4.80819.0088.84-3.71919.0088.84-3.581019.0088.84-3.551119.0188.80-2.601219.0189.00-3.181319.0189.00-0.061419.0189.01-2.041519.0189.01-2.141719.0189.01-1.421819.0089.01-1.541919.0289.01-1.322019.0289.01-0.062119.0289.02-0.042219.0289.02-3.552319.0089.02-2.982419.0089.00-2.682519.0189.00-2.842619.0189.01-0.712719.0189.01-0.782819.0189.00-0.882919.0189.00-0.973019.0189.01-0.813119.0189.01-0.143218.0689.01-0.573316.6988.98-1.983414.5188.86-0.36Designing Rayleigh's model for rainout
Rayleigh's distillation equation is modified to include a vapour mixing
process. The isotopic composition of the continental and the BoB vapour
mixture has been numerically simulated using the improvised version of
the Rayleigh distillation model. Figure shows a schematic
representation of the numerical expression and procedure. The model is run
for the months covering the SWM period. The oxygen isotopic composition of
rainwater at the continental stations and the BoB, meteorological parameters,
BoB surface-water isotopic composition, and satellite-based precipitation data
are used as input parameters to actuate the model. The δ18O of
Kolkata rain is predicted after introducing modification in the isotopic
composition of residual vapour measured at Bangalore. The procedure involves
accounting for the rainout in the Rayleigh's distillation model and a two-component mixing formulation where advected vapour and the vapour supplied
from the BoB are mixed to generate an integrated vapour.
δ18O value in vapour over the Bangalore region is used as an original
isotopic value (initial condition) to start the model run. Isotopic
composition of the vapour is calculated from the measured isotopic values of
rainwater, assuming an equilibrium fractionation. As the moisture parcel
loses water by the process of rainout, the residual vapour isotopic
composition is given by
δVi=(δVi0+1)×fiαi-1-1,
where δVi is the isotopic composition of vapour after rainout in
the ith box, δVi0 is the initial isotopic composition of
vapour, αi is the fractionation factor
calculated for the dew point temperature at
850 mb pressure level for the ith box, and fi is the fraction of
vapour remaining in the air-mass, given by
fi=W1-∑PiWi,
where W is the total precipitable water over the box and P is
rainfall over each box. The subscript denotes the box number.
Upon leaving the continental land mass, the air parcels traverse the BoB
and pick up moisture along the way, leading to a modification of the original
vapour isotopic value. In order to account for this change, the modified
equation to calculate the fraction of vapour remaining in the ith box
is
fi=W1-(Pi-Ei-1)Wi.
E is the evaporation contribution from the BoB. Moisture in the air mass is replenished as it travels over the BoB.
The isotopic composition of the evaporation flux over each box is estimated
using the model:
δViBoB=δl-hδa-ϵ*-ϵ1-h,
where δViBoB is the isotopic composition of the evaporation flux
supplied by the BoB; δl is the BoB surface-water
isotopic composition; h is the relative humidity as a fraction of unity;
δa is the isotopic composition of the vapour over the BoB,
calculated assuming equilibrium relationship between rain and vapour over the
BoB; ϵ* is the equilibrium enrichment factor; and ϵ is the
kinetic enrichment factor given by , where
ϵ*=(α-1)α-103 and α is the equilibrium fractionation
factor.
The isotopic composition of the resultant vapour formed by mixing two
moisture sources depletes the heavy isotopes with progressive rainout. The
depleted vapour moves to the next box and is mixed with the moisture
generated by the BoB. The resulting vapour undergoes rainout and so on, until
the final value in the Kolkata rainwater is achieved.
Monthly averaged δ18O isotopic composition for the Bangalore
(blue circle) station. The bars represent the standard deviation from monthly
mean. Isotopic composition of Kakinada (green square) and Kolkata (red
diamond) for the year 2004.
Backward air-mass trajectories (-48, -72, and -96 h)
prior to a rainy day at 200, 500, 1000, 1500, 2000, and 2500 m above mean sea level
for a single year (2004) at Bangalore and Kolkata during all rainy days of
the SWM. The modelling transect is chosen such that the majority of the
trajectories pass through it.
Schematic of the modelling procedure involved.
E / P ratio for each box of the modelling transect for the SWM months
of 2004.
Discussion and results
After spawning from the Arabian Sea, as the air mass enters the Indian
land mass through its western coast, the constitution of the moisture parcel
is modified due to the process of rainout and addition of continental vapour
or BoB vapour before re-entering the corridor of the Indo-Gangetic Plain
through the east coast. The isotopic composition of vapour over Bangalore is
taken as the representative of the continental vapour. The isotopic
composition of the air mass is modified as it moves towards Kolkata under the
prevailing wind direction during the SWM. The rainwater δ18O value
decreases consistently as the SWM period progresses and follows patterns
similar to each other for the sites at Kolkata and Bangalore. The
δ18O rainfall approaches a minimum for the month of October for both
locations (Fig. ). A consistent pattern recorded in the
isotopic values during the SWM period at both the sites suggests a common
source for moisture responsible for rain. This was confirmed from the
observation documenting the backward trajectories for both the stations.
However, there were situations during the SWM period where a large difference in
the monthly rainwater isotopic data of the two stations was noted. Conversely, such a lack
of consistent temporal patterns suggests involvement of
different sources. In the time series analysis of rainfall δ18O, a
monthly lag in registering the isotopic minima was noticeable, corresponding
to the timing of rainfall maxima at both the stations. This indirectly
implies participation of a second moisture source in the case of Kolkata
precipitation. Such a pattern can be consistently explained employing the Rayleigh
distillation model after taking into account the rainout process and mixing
of vapour generated from the BoB region.
The air parcel during the SWM period moves towards Kolkata from Bangalore
under the prevailing wind, where the original isotopic composition is modified
due to the interplay of (i) rainout, which follows Rayleigh-type distillation
and ii) mixing of the vapour generated over the BoB. For E=0, i.e.
assuming no moisture contribution from the BoB, for the whole season the
modelled derived δ18O value for rainfall at Kolkata is
-8.04 ± 0.96 ‰. This value is lower than the observed
value by 2.3 ‰.
Table shows fraction of vapour remaining in the air mass
(f) and the isotopic composition of vapour (δ18Ov)
calculated over each modelling box for the SWM months. The vapour isotopic
composition decreases as the monsoon progresses, with most depleted values
observed during the month of September. This depletion is indicative of the
gradual reduction in contribution of BoB moisture due to the saturated nature of
incoming air parcels laden with vapour originating from the Arabian Sea region.
Figure shows the E / P ratio for each of the boxes
(E=0 for the first box since the BoB contribution at Bangalore is
assumed to be zero). The mixing of the advected component and the BoB
component leads to modification of the vapour isotopic composition. The final
isotopic composition of the vapour is governed by the relative contributions
of both these sources. held the cyclonic
disturbances originating from the BoB responsible for the maximum drop in
isotopic values during the early phase of the SWM. There is a tendency of low-pressure zones to
develop and remain confined to ∼20∘ N in the BoB
during onset time. The position of such low-pressure zones shifts southward
to ∼15∘ N during the later phase of the SWM. This explains the isotopic
variability recorded in Kolkata rainfall
during the SWM period. It is worth mentioning that the significant
differences in the evaporation contribution arise from box numbers 5, 6, and
7.
The evaporation contribution from the first four boxes remains somewhat the
same for the whole period of the SWM. For the month of September, the
contribution from boxes 5, 6, and 7 is smaller than boxes 1–4. This can be
attributed to the decreasing strength of the monsoonal wind and more
contribution of moisture originating from the continent. The Rayleigh's
distillation model is used to track further changes in the isotopic
composition of the vapour (δ18O), as the vapour progressively loses water
during condensation. Figure depicts the
model results at monthly and the seasonal timescales. The isotopic
composition of vapour is calculated from the rainfall isotopic composition
calculated assuming equilibrium between vapour and the liquid phase. The
model run used three values as initial conditions (δ18O (δ18Omean+ SD), δ18Omean, and (δ18Omean- SD),
capturing the uncertainty or spread in the continental vapour isotopic value
measured at Bangalore. This includes monthly uncertainty in rainwater values
based on number of samples collected during a month. The model performance is
fair, within the uncertainty limits at a monthly timescale. However, the
model performance improved significantly when the same simulation was run with
average δ18O for the entire SWM period. The modelled value is
-6.05(±0.69) ‰ and the actual observed value is 5.76(±1.99) ‰.
Fraction of vapour remaining over each box and the
modelled isotopic composition of vapour over each box for the year 2004. The
values in brackets are the standard deviation from the calculated mean
values.
Dark blue represents the mean modelled δ18O (‰)
isotopic composition of rain over each box as calculated from Rayleigh's
distillation Eq. (5a–d) for the individual SWM months and e for the
whole period of the SWM. The bars represent the standard deviation. Green
and red solid circles represent the mean observed isotopic composition of
rain at Kakinada and Kolkata respectively.
Model validation
Figure depicts the mean modelled isotopic value for
rain at each box, with error bars representing the standard deviation from the
mean isotopic value obtained from (δ18O (δ18Omean+ SD), δ18Omean, and (δ18Omean- SD)
uncertainty in the initial vapour isotopic value. To validate our model
prediction, the results are compared with the isotopic composition of rain at
Kakinada, which lies in the transport pathway. The station lies in Box 4
of the transect (Fig. ), and the δ18O of rain
observed at Kakinada during the SWM is -3.8(±2.23) ‰. The observed
value is very close to the model-predicted value of
-4.16(±1.27) ‰. The model simulation yields a varying monthly
contribution of vapour from land and oceanic sources. The BoB acts as an
active source of moisture at the beginning of the SWM and its contribution to
vapour as simulated by the model over Kolkata (Box 7) given as a percentage of
the total precipitable water is 92 ± 8 % in June, 73 ± 16 % in
July, 62 ± 17 % in August, and 47 ± 17 % in September. The BoB
vapour supply diminishes as the monsoon gradually becomes weaker, and wind
patterns reverse upon onset of north easterlies.
Conclusions
In this study we quantified the source of moisture precipitating as rain at
Kolkata. The BoB is a major moisture contributor to precipitation at Kolkata,
supplying overall 65–75 % of the total precipitation during the entire
SWM and the continental contribution varies from 25 to 35 %. The
contribution of the BoB as the source of moisture at Kolkata attains a
maximum
at the commencement of the SWM during June but as strength of the monsoon
decreases, the moisture contribution from BoB diminishes, while the role of
continental vapour becomes important. The performance of the model is limited
at a monthly timescale but performs well for the whole period of the SWM
within the limit of uncertainty. The limitations at a monthly scale may arise
due to the model assumption where the isotopic composition of the BoB surface
water and the isotopic composition of rain over the BoB were held constant
over the entire duration of the SWM. This assumption was made due to the
unavailability of high-resolution datasets over the BoB. The performance of
the model can be improved taking into consideration the monthly variation in
the vapour and surface-water isotopic composition of rainfall over the BoB.
This is the first estimate of such a kind where variable contribution of
continental moisture in rain over Kolkata is invoked to explain the
observation in the δ18O of rainwater measured at Kolkata. The
findings have major implications for the regional water vapour budget in the
context of past and future climatic scenarios. The role of phenomena like the El Niño Southern Oscillation or Indian Ocean Dipole on the relative
contributions of continental and oceanic sources during the SWM can be
investigated with simultaneous observation.
All the data used in this study are available in online public data banks and published papers.
The authors declare that they have no conflict of interest.
Acknowledgements
We would like to thank the reviewers and SK Bhattacharya for their
help in improving the quality of the paper and Surajit Mondal for the BoB rain isotope data.
Edited by: S. M. Vicente Serrano
Reviewed by: R. J. van der Ent and one anonymous referee
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