Studying Dynamics of the South West Monsoon in Indian Sub-Continent through Geospatial Correlation of Isotopes in Air Moisture

The isotopic data of atmospheric air moisture collected from Kakinada in coastal region (Andhra Pradesh), Sagar in Central India (Madhya Pradesh), Kanpur in Indo-Gangetic plains (Uttar Pradesh), Roorkee in foot hills of Shiwalik (Uttarakhand), Jammu in Sub-Himalayan region (Jammu & Kashmir) and Manali in Himalayan region (Himachal Pradesh) measured in Isotope Laboratory of NIH, Roorkee and has been analysed using Matlab to find out the local/ regional component of atmospheric moisture. The isotopic analysis confirms the continental and altitude effects which were earlier deciphered by the precipitation. Further, the isotopic data analysis validates the impact of local moisture on rainfall and help in resolving the moisture sources in different seasons. This analysis further concludes that the isotopic composition of air moisture can be applied for finding the onset of monsoon.


Introduction
Atmospheric moisture is an important component in the water cycle and its major flux on the Indian subcontinent comes through southwest monsoon that arrives through Arabian Sea and Bay of Bengal branches [1]. The process of tracking the journey of water molecules using stable isotopes of oxygen and hydrogen can lead to quantitative understanding of the associated physical processes because of the isotopic fractionation being proportional to the extent the process has advanced. Monsoon is a large scale wind circulation phenomenon that brings large amount of moisture on the Indian sub-continent. Earlier some studies [2][3][4] were carried out using conventional meteorological parameters on monsoon dynamics except timing of onset and withdrawal of monsoon due to the complexity between mixing of sub-regional moisture with continental scale moisture arising directly from Arabian Sea and/or Bay of Bengal and/ or Indian Ocean. Datta et al. [5], Bhattacharya et al. [6], and Peng et al. [7] characterized isotopes in precipitation which is an event based phenomenon and that too mainly during monsoon; the results of these works could not be used for modelling and prediction. The role of isotopes in air moisture was not mentioned in the above studies and observation of stable isotope variation of atmospheric water vapour, which is not same in all regions, has an important significance to indicate the moisture origins and its transport. Using the isotopic signatures of water molecules, it is possible to track the movement of air moisture and hence the southwest monsoon transect. This is due to fact that with the onward movement of the air moisture heavy isotopes get systematically depleted [8] and the recycling of moisture due to condensation and subsequent evaporation cause a difference in relative enrichment in deuterium and oxygen isotopic composition. A few case studies on this philosophy were examined in coastal China [9,10]. In India, the studies on isotopic composition of air moisture were carried out at National Institute of Hydrology, Roorkee (i) to identify the air moisture sources [11] ; to study the climatological conditions [12]; to find the correlations between isotopic composition of air moisture at Roorkee with Hyderabad [13], Sagar [14], Kanpur [15], Jammu [16] and Manali [17].

Study Area
The samples were collected at Kakinada, Sagar, Kanpur, Roorkee, Jammu and Manali and the detail of the meteorological parameters of these stations is given in Table 1 and Figure 1.

Methodology Sample collection
The condensed air moisture samples for isotopic analyses were collected on daily basis at all the stations from 9.30 am to 10.30 am by condensation method [18]. In this method, the air moisture sample is collected using the conical condensation device. After setting up the conical condensation device the date, time, temperature and relative humidity are recorded using thermo-hygrometer [18]. The samples at Manali could not be collected during the period December to April due to heavy snow fall. Data concerning daily meteorological variables, including surface air temperature (minimum temperature, maximum temperature and mean temperature) and relative humidity was also recorded at all the stations.

Sample analysis
Stable isotopes ( 2 H or D) in water were analysed using GV-Isoprime Dual Inlet Isotope Ratio Mass Spectrometer. For δ D analysis, 400 µl of the water sample is equilibrated with H 2 along with Pt catalyst at 40°C for 3 hrs, and then the equilibrated gas is introduced into the mass spectrometer. The measured values are reported as delta (δ) values [19]. The precision of measurement for δ D was within ± 1%.

Data analysis
Before carrying out the statistical analysis processing of isotopic values of air moisture obtained for various stations has been carried out. The period of collected samples of air moisture varies from 2 to 5 years and there is a variation of data collection dates in each year. Therefore, a programme is developed in Matlab to compute average values of the isotopic composition (δD) on different dates from the observed data. Further, the isotopic data was statistically analysed by computing the cumulative isotopic values (δD) of all the stations using Matlab software.

Results and Discussion
The cumulative isotopic composition (δD) of air moisture of all the stations (Manali, Sagar, Kakinada, Kanpur, Jammu and Roorkee) is shown in Figure 2. It is observed that the slope of all the stations changes with time except Kakinada station where it remains constant whole year due to the continuous air moisture supply received from the Bay of Bengal. On the basis of change in slope, the equations were developed for all the stations (Tables 2 and 3), which are different in case of all the stations due to the difference in air moisture sources.
The most depleted values for cumulative δD were observed at Roorkee which is followed by Kanpur, Jammu and Sagar which may be deciphered due to the Continental effect. In Manali, the depletion is found only in the monsoon season due to the air moisture [17]. However, no depletion was found in the pre (May-June) and post monsoon (October-December) seasons.
No change in slopes is observed in Kakinada (whole year) and pre & post monsoon months in Manali due to recycling of air moisture. On the basis of data and change in slope, the dates where the slope is changing can be recorded which is helpful in finding the onset and withdrawal dates of the monsoon for all the stations. The dates   SW Monsoon δD =-43x+7685 δD =-54x+7596 δD =-34x+3378 δD =-65x+7487 δD =-71x+8838 δD =-88x+11445 Local/ Regional δD =-5x-3267 δD =-42x+4380 δD =-34x+3378 δD =-38x+64.2 δD =-58x+4830 δD =-50x+1240 of air moisture originating from local/regional sources can be easily differentiated from the air moisture received through the SW monsoon. As per IMD, New Delhi the normal dates for onset of monsoon in Kanpur, Roorkee and Jammu are observed between 25 th June to 1 st week of July but as per the isotopic data the dates observed ranged from 26 th May to 3 rd July indicating that the air moisture arrives earlier than the actual precipitation, which is helpful in predicting the onset of monsoon.

Conclusions
The isotopic analysis confirms the continental and altitude effects which were earlier deciphered by the precipitation. Further, the isotopic data analysis validates the impact of local moisture on rainfall and help in resolving the moisture sources in different seasons. This analysis further concludes that the isotopic composition of air moisture can be applied for finding the onset of monsoon.

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