Stable isotope evidence of dual (Arabian Sea and Bay of Bengal) vapour sources in monsoonal...

tters 250 (2006) 511–521www.elsevier.com/locate/epsl

Earth and Planetary Science Le

Stable isotope evidence of dual (Arabian Sea and Bay of Bengal)vapour sources in monsoonal precipitation over north India

Saikat Sengupta, A. Sarkar ⁎

Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, India

Received 25 May 2006; received in revised form 12 August 2006; accepted 14 August 2006Available online 20 September 2006

Editor: M.L. Delaney

Abstract

High resolution time series data of hydrogen (δD) and oxygen (δ18O) isotope values of precipitation have been generated for thefirst time at Kolkata, eastern India where the summer monsoon clouds from Bay of Bengal (BOB) commence their journey overIndia. Use of a Rayleigh cum two component mixing model and comparison of Kolkata data with the International Atomic EnergyAgency (IAEA)–Global Network of Isotopes in Precipitation (GNIP) data base of New Delhi suggest that the precipitation at NewDelhi cannot be explained by simple continental effect of a BOB vapour source alone, traveling and raining successively alongKolkata–New Delhi route. It is necessary to invoke an admixture of ∼20% vapour originating from the Arabian sea with thevapour coming from BOB and finally causing summer monsoon rains at New Delhi. The findings have major implications to theregional water vapour budget over India.© 2006 Elsevier B.V. All rights reserved.

Keywords: Stable isotope; Monsoon; India; Vapour mixing

1. Introduction

Onset of monsoon, its propagation over the Indiansubcontinent and occasional failure are intimately con-nected to the agro-economy of the region. Spatial distri-butions of stable isotope compositions of oxygen (δ18O)and hydrogen (δD) in precipitation are powerful tracersfor identifying the sources of vapour generated duringthe monsoon. These can also be used to delineate theclimatic parameters controlling their latitudinal distri-bution [1–4]. Compared to Europe and North America,mapping of isotopes in precipitation over India has been

⁎ Corresponding author. Tel.: +91 3222 283392; fax: +91 3222282268.

E-mail address: anindya@gg.iitkgp.ernet.in (A. Sarkar).

0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2006.08.011

scanty [5–7]. This becomes further important due to thedual monsoon system [viz. summer monsoon (June–October) and winter monsoon (December–March)] overthe subcontinent having distinctly different origin anddynamics [8–10]. Long term (N10 yrs) InternationalAtomic Energy Agency (IAEA)–Global Network ofIsotopes in Precipitation (GNIP) time series data onisotopes in precipitation are available only for threestations in India viz. Mumbai, western India, NewDelhi, northern India, and Shillong, NE India [11].While the summer monsoon system in Mumbai isdominantly controlled by the vapour derived from theArabian Sea, the monsoonal precipitation in New Delhiis presumably linked with the vapours originated fromBay of Bengal [BOB; 4,12–15]. The BOB branch ofsummer monsoon enters into the Indian subcontinent

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through Kolkata (formerly Calcutta, eastern India). Itcovers a distance of more than 1400 kms before reachingNew Delhi (op.cit.). Yet neither exists any IAEA stationat Kolkata nor is there any time series record of isotopesin precipitation at this first entry point of summer mon-soon. Based on the isotopic compositions of shallowground waters (proxy for long term average precipita-tion) a systematic continental effect on the BOB vapoursource has earlier been suggested for the precipitationover the Kolkata–Delhi sector (op.cit.). However, thecomposition of rainfall can be substantially modified inthe aquifers by post-precipitation evaporation processes[16]. Also averaging out of summer and winter monsoonprecipitations can change the isotope values as they haveentirely different sources and compositions. The presentwork, for the first time, reports two year high resolutiontime series data of isotopes in precipitation at Kolkatawhere the monsoon clouds commence its journey overIndia. It is demonstrated that the precipitation over thenorthern part of India (viz. New Delhi) cannot be ex-plained by simple continental effect of a BOB vapoursource as suggested earlier. Also an attempt has beenmade to delineate the sources of vapour generating rainsduring both the summer and winter monsoons at

Fig. 1. Map showing the sampling station and tracks of cyclonic disturbances(dark arrow); note the southward shift in the origin (solid stars) and longer traconset (thick solid line) and withdrawal (thick dashed line) of summer monsooDelhi and Mumbai are also shown (circled cross). Surface wind stream direcwith arrows) indicate possible transport of Arabian sea vapour through west c(for details see text); note the turning of Arabian sea wind streams coming th

Kolkata. The data also have important implications forlarge scale vapour transport and regional climatemodeling.

2. Study area and methodology

Weekly composite precipitation samples have beencollected for nearly two complete years (June, 2004–October, 2005) at Barasat locality (22° 44.43′ N, 88°29.45′ E) of northeast Kolkata, ∼100 km north of theBOB coast (Fig. 1). In general at Kolkata the summermonsoon precipitation occurs between June and October.Kolkata does get rain fromwintermonsoon i.e. Novemberto March. However, compared to more northeastern in-land station like Shillong or SE coast of Tamilnadu andAndhra Pradesh, the intensity and frequency are far less.Extreme summer conditions (with temperature soaring to∼40 °C) prevail between April and May till summermonsoon onset takes place in the first week of June.Because of near-continuous rain weekly samples werecollected during the summer monsoon period whereasonly large interval sampling was possible during the post-summer monsoon and winter monsoon period. The sam-ples cover precipitation for two summermonsoon and one

generated in the BOB during July–August (light arrow) and Septemberk length of disturbances during September. Also shown are the dates ofn in different parts over India. Locations of IAEA GNIP stations Newtions (June–September, 2004; NCEP/NCAR reanalysis data; thin linesoast and mixing with BOB vapour over Gangetic plain near Allahabadrough Gujarat and Rajasthan towards the low pressure cell in Pakistan.

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winter monsoon period (2004 and 2005). Rain watersamples were collected following the procedurementioned in IAEA guideline (www.isohis.iaea.org/userupdate%5Csampling.pdf ).

Water samples were equilibrated with CO2 and H2 at32 °C in a gas bench and cleaned gases were measuredin a Finnigan Delta Plus XP mass spectrometer for δ18Oand δD compositions respectively. The system wascalibrated by analyzing IAEA standards SLAP, GISPand SMOW. Analytical precision was monitored byrunning internal water standard NARM [prepared at

Table 1δD, δ18O, d-excess values and rainfall amounts at Kolkata and New Delhi

Month Date δ18O(‰)a

δD(‰)a

d-excess(‰)b

Rain(mm

Kolkataf

June, 2004 11/06/04–17/06/04 −6.78 −43.70 10.67 13518/06/04–24/06/04 −3.05 −13.99 59.725/06/04–01/07/04 −5.74 −34.63 48.7

July, 2004 02/07/04–09/07/04 −3.37 −18.37 6.56 52.910/07/04–17/07/04 −4.57 −25.53 34.724/07/04–30/07/04 −3.97 −31.95 38.8

August, 2004 31/07/04–06/08/04 −0.83 −5.26 4507/08/04–14/08/04 −6.03 −38.88 10.17 132.15/08/04–21/08/04 −9.32 −67.20 25.222/08/04–27/08/04 −7.54 −52.14 26.428/08/04–04/09/04 −6.12 −34.32 140.

September, 2004 05/09/04–11/09/04 −2.38 −8.25 11.05 72.212/09/04–18/09/04 −7.88 −52.09 176.19/09/04–25/09/04 −12.69 −90.11 22.726/09/04–02/10/04 −6.72 −41.96 35.3

October, 2004 03/10/04–09/10/04 −12.73 −90.90 10.91 249.December, 2004 13/12/04–20/12/04 −1.18 −2.52 9.06 Trac

21/12/04–27/12/04 0.08 11.86 TracMarch, 2005 15/03/05 −0.18 9.06 10.52 72June, 2005 07/06/05–13/06/05 0.72 17.93 10.98 Trac

14/06/05–20/06/05 −0.81 5.38 2228/06/05–01/07/05 0.71 17.83 87

6.98July, 2005 08/07/05–15/07/05 −5.43 −37.48 112

16/07/05–22/07/05 −6.64 −44.34 4023/07/05–30/07/05 −7.99 −56.11 52

August, 2005 31/07/05–07/08/05 −7.88 −49.91 8.00 9308/08/05–13/08/05 −7.17 −49.38 3214/08/05–20/08/05 −6.93 −45.61 2021/08/05–27/08/05 −2.17 −11.42 5128/08/05–03/09/05 −9.38 −62.49 43

11.16September, 2005 04/09/05–10/09/05 −9.42 −63.61 28

11/09/05–17/09/05 −10.01 −70.16 5418/09/05–24/09/05 −6.53 −41.79 3325/09/05–01/10/05 −6.47 −40.21 8.19 31

October, 2005 02/10/05–08/10/05 −10.10 −71.85 4509/10/05–15/10/05 −8.15 −57.82 44

aWeekly composite values; bmonthly weighted mean d-excess; ctotal weekly rawork; gIAEA GNIP data.

Physical Research Laboratory, Ahmedabad from Nar-mada river water: δ18O and δD (VSMOW) values−4.51‰ and −35.78‰ respectively] with each set ofsamples. A routine precision of ∼±0.1‰ and ±1‰have been obtained for δ18O and δD respectively. Allisotopic data are reported against VSMOW. For thepresent work daily precipitation (amount), temperatureand humidity data for the station DumDum (∼10 kmsouth of Barasat) have been collected from Indian Me-teorological Department (IMD) and used. Monsoonalwind speed data at a pressure level of 1000 mb are

fall.)c

Month Date δ18O(‰)d

δD(‰)d

d-excess(‰)d

Rainfall(mm.)e

New Delhig

January 1961–2001 −1.85 −0.9 12.3 19.7

February ‘’ 0.12 7.2 7.6 20.1

March ‘’ −0.38 2.7 5.1 15.2

5 April ‘’ 0.17 4.1 4 13.2

1 May ‘’ 0.85 10.3 0.1 22.7June ‘’ −0.39 3.9 1.5 74.8

6

July ‘’ −5.22 −32.4 9 217.41 August ‘’ −6.64 −44.9 8.3 256.7e September ‘’ −8.43 −57.5 8.5 122.1e

October ‘’ −6.91 −49 7.4 16.5e November ‘’ −6.06 −39.3 9.2 6.6

December ‘’ −1.08 −3.6 7.1 10.5

infall; dmonthly mean values; e∼40 yrs mean monthly rainfall; fpresent

Fig. 2. δ18O–δD variation in precipitation of Kolkata for 2004–2005.

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obtained from NCEP/NCAR re-analysis project atNOAA-CIRES Climate Diagnostic Center [17]. Windstream directions were constructed using Grid Analysisand Display System (GrADS) software.

3. Results

BOB vapours enter the east coast near Kolkata beforetraveling to the northern part of India. Hence examina-tion of the time series data of isotopic compositions andamount of rainfalls at Kolkata (Table 1, Fig. 2) is nec-essary to understand its vapour source related to specificmonsoon dynamics. Along with Kolkata, Table 1 alsoshows the δ18O, δD, d-excess values of precipitationand rainfall amounts of New Delhi (∼40 yrs average;GNIP IAEA data set [11]). Weekly δ18O and δD valuesof Kolkata precipitations (Fig. 2) show considerablescatter possibly due to the variations in small scalemicroclimatic parameters. Nevertheless the generaltrend in isotope data at Kolkata shows enrichmentduring November–June period in both δ18O and δDvalues while continuous depletion occurs during thesummer monsoon time. The δ18O peaks in summermonsoonal precipitation of 2004 and 2005 occur in Julyand June respectively. This is because the steady in-crease in monsoon intensity during 2004 started onlyfrom the month of July (slightly delayed) while for 2005it started from June itself. Monthly weighted mean

deuterium (d)-excess (d=δD–8δ18O; [3,5]) at Kolkatafor the period June–September, 2004 and 2005 areshown in Fig. 3a. The d-excess remains constant atnearly 10‰ level excepting a slightly lower valuearound July (∼7‰). The similarity in the trend of d-excess for two consecutive years indicates that origin ofthe vapour sources and other climatic parameters (e.g.temperature, humidity etc.) did not significantly varyduring these two summer monsoon periods. Hencemonthly weighted average of δ18O and δD can be con-sidered as reasonable representative of Kolkata precip-itation and thus clubbed together for explaining theannual variation. Fig. 3b shows the δD–δ18O scatterplot for all weekly precipitation samples. The best-fitline through all the data gives a relationship of δD=(7.88±0.1) δ18O+(8.93±0.9) (R2 =0.99) and representsthe local meteoric water line (LMWL) at Kolkata. Boththe slope and the intercept are close to those of pre-cipitation [δD=(8.2±0.1) δ18O+(10.6±0.6)] definingthe global meteoric water line (GMWL) reported byYurtsever and Gat [18]. The GMWL is drawn on thebasis of data collected from worldwide precipitationnetwork stations of IAEA. The similarity in slope andintercept between LMWL and GMWL indicates that theprecipitation at Kolkata did not suffer any kinetic effectduring and after the rainfall. Trends of rainfall distri-bution at Kolkata during two summer monsoon periods(2004 and 2005) are also very similar (Fig. 3c). For both

Fig. 4. (a) deuterium excess, (b) weighted δ18O and (c) mean rainfallvs. month of mean annual cycle for Kolkata and New Delhi. Meanannual cycle of Kolkata represents two years data.

Fig. 3. (a) Weighted d-excess vs. month for the years 2004 and 2005.(b) δD vs. δ18O scatter plot of all precipitation water defining theLMWL (c) rainfall amount vs. month for the years 2004 and 2005 atKolkata.

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years, peak rainfall was during the month of August.The average summer monsoonal precipitation duringthis period was ∼1100 mm, similar to the long termaverage summer monsoonal precipitation in Kolkatarecorded at the meteorological observatory [8]. Togetherthe above observations led us to conclude that theprecipitations of 2004 and 2005 represent normalmonsoon years. Hence their isotopic compositions canbe safely used for comparison with the long term isotopedata of IAEA stations at New Delhi or Mumbai.

4. Discussion

As mentioned earlier both Kolkata and New Delhifall in the same monsoon trough presumably connectedto the same vapour source. It is, therefore, important tocompare the annual variation in rainfall amount, isotopiccompositions and other parameters between these twostations for understanding the sources of moisture andphysical mechanisms of rain-out processes.

4.1. Annual variation of stable isotopes in precipitationat Kolkata and New Delhi: implication to moisturesources

Annual variations of weighted monthly mean d-excess, δ18O values and rainfall data of both Kolkataand New Delhi have been plotted in Fig. 4(a), (b) and(c). As mentioned earlier excepting July, d-excess valueremains nearly constant at 10‰ level at Kolkatathroughout the year. This value is close to the d-excessreported for measured surface water near the BOB coastof eastern India [4] as well as theoretically modeledvalues reported by Gupta and Deshpande [19]. Thissuggests that the precipitation at Kolkata too representsa single vapour source from BOB throughout the sum-mer monsoon. These vapours are driven to Kolkata byfrequent cyclonic disturbances (both storm and depres-sions) originated at BOB during this period (see laterdiscussion). The d-excess values at New Delhi alsoremain constant nearly at 8‰ level between July andSeptember (summer monsoon) and also indicate a fixedvapour source. However, unlike Kolkata, trend of d-

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excess at New Delhi significantly differs during winterand pre-summer monsoon months (Fig. 4a). Evapora-tion from the continental surface water bodies at lowhumidity can produce vapour mass with high d-excess[20]. This evaporated vapour mass when mixed with theatmospheric vapour can give rise to precipitation withhigh d-excess [21,22]. Initial high d-excess at NewDelhi (∼12‰ during January), therefore, indicatesrecycled continental vapour mass, a typical source ofrain during the winter monsoon, traveling from inlandAsia into India [8,23]. The relative humidity at NewDelhi continuously decreases from January to May withan accompanied increase in temperature [11]. Thecombined effect of humidity and temperature belowthe cloud base thus possibly causes kinetic fractionationof the falling raindrops and hence results in gradualdecrease in d-excess from January to May [3,24]. AtKolkata completely different situation prevails duringthe winter months with rains occurring due to sporadiccyclonic depressions again generated at BOB. The onlydifference these depressions have with those of summermonsoon time is that due to the BOB being hotter thanland a reverse wind trajectory sets in from northeasterlycontinental direction into the ocean. The resultant lowpressure zones and consequent cyclonic storms acquirelimited moisture from the BOB and rain over largestretch of east coast including Kolkata [10,23]. The d-excess between winter (precipitation was available onlyfor December and March, 2004–2005) and summermonsoon period exhibit near-constant values. Thisindicates that the sources of precipitation at Kolkataremain invariant throughout the year and unlike NewDelhi winter rain here has no appreciable component ofrecycled continental vapour.

The trends of the monthly weighted mean δ18Ovalues between New Delhi and Kolkata are similarduring summer monsoon (Fig. 4b). At Kolkata the δ18Ogradually decreases from June onwards reachingminimum in October displaying a total depletion of∼7‰. The weighted monthly mean rainfall data atKolkata display a corresponding increase during thisperiod reaching maximum in August (Fig. 4c). This canbe explained by the so called “amount effect” whenmore fraction of moisture successively condenses froma vapour mass of fixed isotopic composition [1,3].However, the rainfall–δ18O spectra shows two monthsphase lag between peak rainfall (August) and δ18Ominimum (October) at Kolkata (Fig. 4b, c) and cannotbe explained by the amount effect alone. Keeping initialsource vapour composition fixed and using availabledata on net moisture and rainfall flux at Kolkata (seelater discussion) we calculate a depletion of only

∼0.4‰ due to the amount effect for the summer mon-soon months. This suggests the contribution of otherfactors which play significant role for producing ∼7‰depletion in the precipitation during these months. Themost plausible explanation of this anomaly seems to bethe change in the locations of formation of the cyclonicdisturbances in the BOB during these months. Thecyclonic disturbances (both cyclones and depressions)collect huge amount of moisture from near surface to550 mb pressure level in the BOB during summermonsoon months. These disturbances then approachtowards coast following the surface wind trajectories.The maps of these disturbance tracks show that betweenJuly and August these are originated around 20° N andare both narrow and focused (Fig. 1; [8]). During thelater part of summer monsoon (September and possiblylater) the origin of these tracks are shifted much south-ward up to 15° N when they become more chaotichaving longer path lengths. It is tempting to speculatethat the October vapours probably traverse longer andmore chaotic path over the ocean compared to thosein August. As a result, in October more fraction ofmoisture is possibly rained out on the way (even withinthe ocean) and would supply a more depleted vapoursource to a coastal station (like Kolkata) than the other3 months. The depletion due to this possibly masks theamount effect at Kolkata. In case of New Delhi thisphase lag is just 1 month (Fig. 4b, c). Below we showthat the difference in phase lag between New Delhiand Kolkata is essentially a result of different dates ofwithdrawal of summer monsoon in these two places(Fig. 1).

During July to September, average weighted δ18Ovalues of Kolkata are slightly more than that of NewDelhi (Fig. 4b). This possibly indicates continental effectalong the BOB trajectory and results due to gradual rainout of same vapour source from coast (Kolkata) to inland(New Delhi; op. cit.; [12]). The summer monsoon rainfor the months of June and October at Kolkata are,however, depleted in δ18O compared to those in NewDelhi. The summer monsoon enters near the east coastand Kolkata during the first week of June while it takesabout ∼15 days to travel over India before it reachesNew Delhi almost during the third week of June (Fig. 1;[8]). The low d-excess (∼2‰ ) and high δ18O of NewDelhi in June, therefore, suggest the continued presenceof the continental vapour mass when at Kolkata thevapour regime from summer monsoon has already set in.Due to this difference in moisture source the effect ofcontinentality is not visible in New Delhi rain of June.Likewise withdrawal of summer monsoon over NewDelhi starts in the last week of September while at

Fig. 5. Results of Rayleigh cum two component mixing modelbetween Kolkata and New Delhi. Solid diamond: estimated δ18Ocompositions of rain between Kolkata and New Delhi using Rayleighmodel; solid square: estimated δ18O compositions of rain betweenAllahabad and New Delhi using two component mixing model; dashedline: measured δ18O compositions of dug well water by Krishnamurthyand Bhattacharya (1991; [13]); solid circle: estimated δ18O composi-tions of rain using dug well data and model (op.cit.); open circle:measured δ18O compositions of rain at Kolkata (present work) andNew Delhi (IAEA data); ET: evapotranspiration.

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Kolkata it continues raining up to the second week ofOctober (Fig. 1; [8]). This causes a change in the sourcefrom BOB to continental vapour mass (and hence theincrease in δ18O) at New Delhi during the October. AtKolkata, on the contrary, precipitation takes place from acontinued BOB vapour source producing 18O depletedrain at the same time.

The gradual increase in δ18O at New Delhi (withconstant d-excess) from October to December has ear-lier been explained by evapotranspiration process of thesummer monsoon rainfalls [24,25]. As rainfall contin-uously decreases during this period (Fig. 4c), vapourgets less fractionated and rains become gradually en-riched. Thus the October rain at Kolkata differs fromthat at New Delhi not only in terms of vapour-source butalso by the fractionation mechanism. The October rain,therefore, is excluded from the summer monsoon rain-fall period for further quantification.

4.2. Modelling continental effect between Kolkata andNew Delhi: assessing dual moisture source (ArabianSea and BOB) in north Indian precipitation

As the slope of LMWL at Kolkata is very close tothat of the GMWL (see previous discussion) it is as-sumed that the precipitation and its co-existing vapourmaintained isotopic equilibrium during fractionation.Hence a Rayleigh distillation model [13,26] has beenapplied on the δ18O data of precipitation to assess the

continental effect along the Kolkata–New Delhi sum-mer monsoon track. For this the entire track betweenKolkata and New Delhi has been divided into tenrectangular boxes (box 1 being on the BOB coast andbox 10 over New Delhi), each having equal dimensioncovering the width of monsoon trough. Moisture ladenvapour, after entering box-1, partly condenses as rainand enters box-2 and so on. To calculate the remainingfraction of vapour over each box two parameters areconsidered: (a) net moisture flux entering into the firstbox and (b) rainfall flux over each box. For simplifi-cation only the vapour flux has been varied neglectingall other climatic parameters. Based on the availablewind speed (5 m/s) normal to the sides of the boxes andhumidity mixing ratio (12.3 gm/kg), the net moistureflux was calculated over the first box [27,28]. Rainfallfluxes were calculated using the rainfall data of variousstations along the track [8,29]. Fraction of moistureremaining in vapour over ith box (fi) is calculated as:

fi ¼ ðM−Xi

1

RiÞ=M ð1Þ

Where,M=net moisture flux over the first boxRi=rainfallflux over the ith Box.

In present work the standard Rayleigh equation isslightly modified for calculating the isotope composi-tion of rain of each box [3] and is given below:

d18OðiÞ;r ¼ d18O0;v þ 1000ðai−1Þð1þ lnfiÞ ð2Þ

Where δ18O0,v=stable isotope value of vapour en-tering first box, δ18O(i),r=stable isotope value of rain inith box, fi fraction of moisture remaining in vapour overthe ith box, αi=fractionation factor at ith box calculatedat 880 mb cloud base. Fractionation factor is calculatedfrom average surface temperatures of available stationsalong the trajectory [8,29] and corrected to 880 mbcloud base using the lapse rate of 6.4 °C/km.

The weighted average of the precipitation data fromJune to September at Kolkata (−5.4‰) has been used asthe starting rain in the first box and δ18O0,V is calculatedusing Eq. (2). The same value of δ18O0,V is used tocalculate the isotope composition of rain of the otherboxes as well. The October rain at New Delhi does nothave summer monsoon source (see previous discus-sion). Hence we have excluded the October rain atKolkata for better comparison with New Delhi. Cal-culated compositions of the rain in each box are plottedin Fig. 5 which shows that the actual precipitation inNew Delhi, as measured by IAEA [11], is ∼2.7‰enriched compared to our model estimate. Based on the

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analysis of shallow dug well samples (presumed to berepresentative of average long term precipitation)Krishnamurthy and Bhattacharya [13] obtained anenrichment of ∼1.17‰ compared to their modelledprecipitation of New Delhi. They explained this by∼40% evapotranspiration effect which enriched theprecipitation by returning unfractionated moisture tothe atmosphere [op. cit., 30]. On the contrary the actualprecipitation at Kolkata, measured by us, is depleted (by∼1‰; Fig. 5) than the dug well samples of Krishna-murthy and Bhattacharya [13]. This indicates that thedug well data suffered substantial post-precipitationevaporation and might give erroneous estimate of thecontinental effect between Kolkata and New Delhi. Ifevapotranspiration is the only causative factor for en-riching the New Delhi rain, an abnormally high (N80%)evapotranspiration effect (calculated from Gat andMatsui [21]) is then required to explain the ∼2.7‰enrichment observed by us. Very high evapotranspira-tion effect enriching the δ18O of rainfall has been ob-served over the tropical rain forests like Amazon andCameroon [30–32]. We discount this possibility due tothe fact that a) even tropical rainforests are incapable ofreturning ≥80% moisture to the atmosphere and b) nodense forest occurs within the BOB monsoon troughalong Kolkata–New Delhi sector. The difference bet-ween the observed and modelled values, therefore, de-mands a second source of enriched vapour at New Delhiduring the summer monsoon period that was added tothe vapour coming from the BOB.

Satellite based NCEP/NCAR reanalysis data [17] ofaverage surface wind stream directions show that, inaddition to BOB, New Delhi possibly receives vapourfrom Arabian sea as well (Fig. 1). These wind streamdirections match quite well with the long term surfacewind vector diagram for the month of July obtainedfrom the balloon based measurements by the IMD [8].Arabian Sea, being significantly more (by ∼1‰)enriched in δ18O compared to BOB [33,34], can poten-tially supply enriched vapour mass to New Delhi. Con-tribution of Arabian Sea vapour in New Delhi rain hasbeen speculated earlier [14]. The wind stream directionsover India (Fig. 1) show that the Arabian Sea vapour canenter into the west coast of India through two mainroutes. One route is through the Gujarat coast of NWIndia traveling over the state of Rajasthan and finallyreaching New Delhi. The other one enters through theNarmada and Tapti gap (defined by the valleys of twomajor west bound rivers Narmada and Tapti). It thentravels over the states of Maharashtra and MadhyaPradesh before finally mixing with BOB branch over theGangetic plain ([23,35] ; http://www.britannica.com/eb/

article-46384). The vapour contribution to New Delhifrom the Gujarat–Rajasthan route is virtually negligibleon two accounts: a) the states of Gujarat and Rajasthanremain essentially dry, arid to semi-arid and have lowerhumidity and higher temperature compared to the othernorth Indian states during the summer monsoon months.This leads to a higher moisture carrying capacity of airand a lower possibility of rainfall along this route (aver-age annual rainfall in Rajasthan ∼200 mm compared to∼600 mm in New Delhi; [8]), b) wind stream directionsshow that a major part of the Arabian Sea branch en-tering through this route turns north-westerly towardsthe low pressure zone of Pakistan before reaching NewDelhi (Fig. 1). We, therefore, think that significantamount of vapour from the Arabian Sea actually entersthrough the Narmada–Tapti gap and eventually mixeswith the BOB vapour source after traveling over thestates of Maharashtra and Madhya Pradesh. The windstream directions also suggest that the mixing of thesetwo branches possibly takes place somewhere aroundthe city of Allahabad (Fig. 1) although it is difficult topinpoint the exact location.

We used a two component mixing model to calculatethe contribution of Arabian sea vapour between Allaha-bad and New Delhi with the vapour remaining after thenormal rain out by Rayleigh process between Kolkata andAllahabad. We have, however, assumed addition of equalamount of moisture with same isotopic composition in allthe boxes between Allahabad and New Delhi (boxnumber 8, 9, 10; see previous discussion). To estimatethe δ18O composition of vapour derived from ArabianSea entering through the Narmada Tapti gap it is nec-essary to know the δ18O composition of precipitation inthis part of west and west central India. Although noprecipitation data exists, limited δ18O data of river water(possible proxy for long term precipitation) availablealong this stretch suggest uniform composition (∼−2‰)stretching from Mumbai to inland Maharashtra andMadhya Pradesh [4]. We have, therefore, used δ18Ovalue of∼−2‰ as the representative composition of raincondensed from theArabian Sea vapour. Entering throughthe Narmada Tapti gap, this vapour mixes with the BOBvapour between Allahabad and NewDelhi. It is importantto mention that the long term average δ18O of summermonsoonal rain in Mumbai is ∼−1.4‰ [11] and is closeto the river water values obtained along this route. Weused wind speed of 7 m/s (as against ∼5 m/s used forKolkata–Allahabad sector; [8]) and∼64 km width of theNarmada Tapti gap (approximate latitudinal distance bet-ween the river Narmada and Tapti on the west coast) forcalculating net moisture flux entering through the westcoast. The calculated net moisture flux eventually mixing

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with the BOB vapour is∼0.27×1012 tons/session (June–September). Assuming this moisture flux to be equallydistributed over the three boxes between Allahabad andNew Delhi (∼0.09×1012 tons/session/box) and using asimple two component mixing after the rain out in eachboxwe estimate the rain at NewDelhi to be of∼−6.26‰.Even this value is∼0.42‰ depleted than the precipitationrecorded at IAEA station inNewDelhi [11]. The onlywaythe ∼0.42‰ enrichment in New Delhi rain can beexplained is to introduce reasonable amount of evapo-transpiration effect over Allahabad–New Delhi stretch.We calculate (following Gat and Matsui [21]) ∼45%evapotranspiration effect for the remaining enrichment of∼0.42‰ in NewDelhi rain. This value is quite consistentwith the evapotranspiration amount estimated by Krish-namurthy and Bhattacharya [13] along the samemonsoontrough. Our modelled estimate indicate that ∼20%mixing of Arabian sea vapour with that of BOB origincoupled with ∼45% evapotranspiration can explain theδ18O composition of New Delhi precipitation.

In reality, however, these values might be somewhatdifferent. Exact estimation of either the Arabian Seacontribution or evapotranspiration effect requires iso-tope data of precipitation at number of stations betweenKolkata and Allahabad or along the Narmada–Taptigap–Allahabad route. These isotope data, however, donot exist today. It would be interesting to know theevapotranspiration amount throughout the Kolkata–New Delhi monsoon track against the ∼45% effectobserved at New Delhi. Less than a year GNIP pre-cipitation δ18O data at Allahabad do exist but are notadequate to assess this amount. Likewise, near constantδ18O value of river water along the Narmada–Tapti–Allahabad route, used in the present work, is queer. Thisis because substantial rain out does take place in this partof west and west central India which should change bothprecipitation and surface water δ18O during thepropagation of Arabian sea vapour mass. Precipitationisotope data at least in few stations in this route only canhelp in exact estimation of the Arabian Sea contribution.Other factor that can affect our estimation is the pos-sibility of Ganges estuary, rather than the BOB, as thelocal source of vapour producing 18O depleted rain atKolkata. As discussed earlier the cyclonic disturbancescausing rain in Kolkata are very large in size when seenin satellite images; also Kolkata is not a valley (unlikeMumbai) which is isolated but extensive plains allowingfor transfer across the whole width of the disturbancesand thus mixing. These disturbances, having definitetracks, also force the cloud masses to move from withinocean towards coast causing strong Rayleigh fraction-ation of BOB vapours en route and is the reason of

depleted values at Kolkata. The depleted δ18O values ofGanges water are, therefore, the effect of this rainfalland not vice versa. A simple mass balance calculationalso shows that even the evaporation of entire monsoon-al discharge of Ganges [36] can not supply the amountof moisture entering through the coast near Kolkata. We,therefore, consider that it is only the BOB which cansupply such large amount of moisture to be precipitatedthroughout the Kolkata–New Delhi track. The effect ofGanges estuary on the isotopic compositions of Kolkatarainfall, if any, would be very small.

5. Summary and conclusion

High resolution analysis of δD and δ18O of precip-itation at Kolkata, the first entry point of the BOBsummer monsoon clouds over India, indicates that thelocal meteoric water line closely follows the global me-teoric water line suggesting equilibrium rain-out process.Calculated d-excess values show that the BOB remainsthe source of vapour throughout the year causing bothsummer and winter monsoon rains at Kolkata. On thecontrary at New Delhi the winter precipitation has adominant source of recycled continental vapour mass inagreement with the earlier observation. Comparison ofd-excess, δ18O and rainfall spectra between New Delhiand Kolkata reveals that the effect of different dates ofmonsoon onset and withdrawal over India, along withshift in locales of cyclonic disturbances, masks theamount effect in δ18O. This is important because theatmospheric general circulation model (ECHAM-4version fitted with δ18O of precipitation; [37]) shows asignificant correlation between vertical shear (propor-tional to the latent heat released during monsoon preci-pitation) and δ18O. This means the later is more depletedduring intense monsoon and vice versa. Our data showthat this amount effect is in fact substantially influencedby the change in vapour source and circulation patternsover India and need to be taken into account in futuremodeling studies.

Using the precipitation isotope data of Kolkata andIAEAGNIP data base of NewDelhi a Rayleigh cum twocomponent mixing model has been applied for assessingthe source of vapour in New Delhi rains. This suggeststhat the precipitation in New Delhi cannot be explainedby a simple continental effect of BOB vapour source. It isnecessary to invoke ∼20% contribution from theArabian Sea vapour source that enters India throughthe west coast and ∼45% evapotranspiration effect forexplaining the New Delhi rain during the summer mon-soon period. That the sufficient transport of moisturedoes take place from the Arabian sea to the BOB and

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across the Indian subcontinent has earlier been suggestedbased on the δ18O analysis of ground water from west,central and southwest Indian peninsular region [4]. Theo-retical calculations of monsoonal heat budget over Indianocean also support this [38]. Our study shows that such atransport possibly affects, albeit in a lesser quantity(∼20%), the total moisture budget even in the northernpart of India. Analysis of wind field data and a three-layerprecipitable water dataset indeed show that a net con-vergence of water vapour flux occurs over the northernIndia during the active phases of monsoon [39]. If truethen it has important implications in terms of the sourceand actual budget of the water vapour flux in Indianmonsoon which are still being actively debated [40–42].

Acknowledgement

All data have been generated in the new NationalStable Isotope Facility of IIT, Kharagpur. We sincerelythank Department of Science and Technology, NewDelhi for funding the mass spectrometer laboratoryunder this facility. This work forms part of the PhDthesis of SS who thanks IIT, Kharagpur for a fellowship.We thank Prof. S.K. Bhattacharya, Physical ResearchLaboratory, Ahmedabad for valuable comments andsuggestions during this work and Indian MeteorologicalDepartment for providing rain fall data. We thank Dr. P.Sanyal, M.K. Bera and D. Das for help in the laboratory.We also thank Prof. S.K. Tandon, Dr. K. R. Gupta andlate Prof. A. Chakrabarty for continuous encouragementfor establishing the national stable isotope facility.

References

[1] K. Rozanski, L. Araguás-Araguás, R. Gonfiantini, Isotopepattern in modern global precipitation, in: P.K. Swart, K.C.Longmann, J. Mckenzie, S. Savin (Eds.), Continental IsotopeIndicators of Climate, AGU Geophysical Monograph, AmericanGeophysical Union, Washington DC, 1993, pp. 1–36.

[2] J.R. Gat, Oxygen and hydrogen isotope in hydrologic cycle,Annu. Rev. Earth Planet. Sci. 24 (1996) 225–262.

[3] I.D. Clark, P. Fritz, Environmental Isotopes in Hydrogeology,Lewis Publishers, Boca Raton, 1997.

[4] S.K. Gupta, R.D. Deshpande, S.K. Bhattacharya, R.A. Jani,Groundwater δ18O and δD from central Indian Peninsula: influenceof the Arabian Sea and the BOB branches of the summer monsoon,J. Hydrol. 303 (2005) 38–55.

[5] W. Dansgaard, Stable isotope in precipitation, Tellus 16 (1964)436–467.

[6] K. Rozanski, L. Araguás-Araguás, R. Gonfiantini, Relationbetween long-term trends of oxygen-18 isotope composition ofprecipitation and climate, Science 258 (1992) 981–985.

[7] S.K. Gupta, R.D. Deshpande, The need and potential applica-tions of a network for monitoring of isotopes in waters of India,Curr. Sci. 88 (2005) 107–118.

[8] Y.P. Rao, Southwest Monsoon, Meteorological Monograph,Synoptic Meteorology, Indian Meteorological DepartmentMonograph, vol. 1/1976, 1976.

[9] D.A. Mooley, J. Shukla, Variability and forecasting of thesummer monsoon rainfall over India, in: P.C. Chang, T.N.Krishnamurti (Eds.), Monsoon Meteorology, Oxford Universitypress, New York, 1987, pp. 26–59.

[10] P.A.Menon,Ways of theWeather, National Book Trust, India, 1995.[11] GNIP/ISOHIS, Isotope Hydrology Information System of

International Atomic Energy Agency, (IAEA), Vienna, Austria.The ISOHIS Database. (2001) URL: http://isohis.iaea.org.

[12] S.K. Bhattacharya, S.K. Gupta, R.V. Krishnamurthy, Oxygen andhydrogen isotope ratios in groundwaters and rivers from India,Proc. Indian Acad. Sci., Earth Planet. Sci. 94 (1985) 283–295.

[13] R.V. Krishnamurthy, S.K. Bhattacharya, Stable oxygen andhydrogen isotope ratios in shallow ground waters from India anda study of role of evapotranspiration in the Indian Monsoon, in:H.P. Taylor Jr., J. O'neil, I.R. Kaplan (Eds.), Stable IsotopeGeochemistry: A Tribute to Samuel Epstein, GeochemicalSociety Special Publication, vol. 3, 1991, pp. 187–193.

[14] P.S. Datta, S.K. Tyagi, H. Chandrasekharan, Factors controllingstable isotope composition of rainfall in New Delhi, India,J. Hydrol. 128 (1991) 223–236.

[15] R.D. Deshpande, S.K. Bhattacharya, R.A. Jani, S.K. Gupta,Distribution of oxygen and hydrogen isotopes in shallowgroundwaters from Southern India: influence of a dual monsoonsystem, J. Hydrol. 271 (2003) 226–239.

[16] U. Zimmermann, K.O. Munnich, W. Roether, Downward move-ment of the soil moisture traced by means of hydrogen isotopes,Isotope Techniques in the Hydrologic Cycle, GeophysicalMonograph Series, vol. 11, American Geophysical Union, 1967.

[17] NCEP/NCAR reanalysis project, NOAA-CIRES climate diagnos-tics center (2005) URL: http://www.cdc.noaa.gov/cdc/reanalysis/reanalysis.shtml.

[18] Y. Yurtsever, J. Gat, Atmospheric waters, in: J.R. Gat, R.Gonfiantini (Eds.), Stable Isotope Hydrology: Deuterium andOxygen-18 in the Water Cycle, Tech. Rep. Ser., vol. 210,International Atomic Energy Agency, Vienna, 1981, pp. 103–142.

[19] S.K. Gupta, R.D.Deshpande, Synoptic hydrology of India from thedata of isotopes in precipitation, Curr. Sci. 85 (2003) 1591–1595.

[20] J.R. Gat, I. Carmi, Evolution of the isotopic composition ofatmospheric waters in the Mediterranean Sea area, J. Geophys.Res. 75 (1970) 3039–3048.

[21] J.R. Gat, E. Matsui, Atmospheric water balance in the AmazonBasin: an isotopic evapotranspiration model, J. Geophys. Res. 96(1991) 13179–13188.

[22] N.L. Ingraham, R.A. Matthews, Fog drip as a source ofgroundwater recharge in northern Kenya, Water Resour. Res.24 (1988) 1406–1410.

[23] D.S. Lal, Climatology, Sharada Pustak Bhawan, Allahabad, 2002.[24] S.K. Bhattacharya, K. Froehlich, P.K. Aggarwal, K.M. Kulkarni,

Isotopic variation in Indian monsoon precipitation: records fromBombay and New Delhi, Geophys. Res. Lett. 30 (2003) 2285.

[25] L. Araguás-Araguás, K. Froehlich, K. Rozanski, Stable isotopecomposition of precipitation over Southeast Asia, J. Geophys.Res. 103 (1998) 28721–28742.

[26] K. Rozanski, C. Sonntag, K.O. Munnich, Factors controllingstable isotope composition of modern European precipitation,Tellus 34 (1982) 142–150.

[27] B.N. Desai, S.K. Subramanian, Air masses up to 500 mb levelover the Indian summer monsoon trough area, Indian J. Meteorol.Hydrol. Geophys. 29 (1978) 54–60.

521S. Sengupta, A. Sarkar / Earth and Planetary Science Letters 250 (2006) 511–521

[28] S. Rangarajan, S.Mani, Total precipitablewater in the atmosphereover India, Proc. Indian Acad. Sci. 91 (1982) 189–207.

[29] HongKong Observatory dataset (2003) URL: http://www.hko.gov.hk/wxinfo/climat/world/eng/asia/india/india_e.htm.

[30] E. Salati, A. Dall'Olio, J. Gat, E. Matsui, Recycling of water inthe Amazon basin: an isotope study, Water Resour. Res. 15(1979) 1250–1258.

[31] B.A. Monteny, A. Casenave, The forest contribution to thehydrological budget in tropical west Africa, Ann. Geophys. 7(1989) 427–436.

[32] R. Njitchoua, L. Sigha-Nkamdjou, L. Dever, C. Marlin, D.Sighomnou, P. Nia, Variations of the stable isotopic compositionsof rainfall events from the Cameroon rain forest, Central Africa,J. Hydrol. 223 (1999) 17–26.

[33] J.C. Duplessy, Glacial to interglacial contrasts in the northernIndian Ocean, Nature 295 (1982) 494–498.

[34] G. Delaygue, E. Bard, C. Rollion, J. Jouzel, M. Stievenard, J.C.Duplessy, G. Ganseen, Oxygen isotope/salinity relationship in thenorthern Indian Ocean, J. Geophys. Res. 106 (2001) 4565–4574.

[35] P.K. Das, The Monsoons, National Book Trust, India, NewDelhi, 1995.

[36] E.M. Latrubesse, J.C. Stevaux, R. Sinha, Tropical rivers,Geomorphology 70 (2005) 187–206.

[37] M. Vuille, M. Werner, R.S. Bradley, F. Keimig, Stable isotopes inprecipitation in the Asian monsoon region, J. Geophys. Res. 110(2005) D23108.

[38] S. Hastenrath, L. Greischar, The monsoon heat budget of thehydrosphere-atmosphere system in the Indian Ocean Sector,J. Geophys. Res. 98 (1993) 6869–6881.

[39] D.L. Cadet, S. Greco, Water vapor transport over the Indian Oceanduring the 1979 summermonsoon. Part 1:water vapor fluxes,Mon.Weather Rev. 115 (1987) 653–663.

[40] D.A. Mooley, Some aspects of Indian monsoon depressions andthe associated rainfall, Mon. Weather Rev. 101 (1973) 271–280.

[41] K. Saha, F. Sanders, J. Shukla, Westward propagating pre-decessors of monsoon depressions, Mon. Weather Rev. 109(1981) 330–343.

[42] J.H. Yoon, T.C. Chen, Water vapor budget of the Indian monsoondepression, Tellus 57A (2005) 770–782.