Estimates of land and sea moisture contributions to the monsoonal rain over Kolkata, deduced based on isotopic analysis of rainwater

Abstract

Abstract. 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.

Variability of the Indian Monsoon in the Andaman Sea Across the Miocene‐Pliocene Transition

“…In contrast, tropical‐subtropical summer monsoon rainfall close to the Ganges/Brahmaputra Delta flood plains exhibits considerably higher δ 18 O values (for instance, summer monsoon rainfall in the year 2004 averaged −5.74‰; Dar & Ghosh, 2017). Further south, values of −5.76‰ at Bangalore (Dar & Ghosh, 2017) and −2.16‰ in Sri Lanka (Edirisinghe et al, 2017) are comparable or even higher, as they are less influenced by rainout depletion. As a result, δ 18 O sw variations in the Andaman area not only reflect changes in the local precipitation‐evaporation budget but also the seasonality of rainfall and the relative contribution of local precipitation versus river discharge of δ 18 O depleted freshwater from mountainous regions.…”

“…In contrast, tropical-subtropical summer monsoon rainfall close to the Ganges/Brahmaputra Delta flood plains exhibits considerably higher δ 18 O values (for instance, summer monsoon rainfall in the year 2004 averaged −5.74‰; Dar & Ghosh, 2017). Further south, values of −5.76‰ at Bangalore (Dar & Ghosh, 2017) and −2.16‰ in Sri Lanka (Edirisinghe et al, 2017) are comparable or even higher, as they are less influenced by rainout Table 3 Results of Cross-Spectral Analysis Between the Site U1448 Records and the 19, 23, and 41 kyr Orbital Components Over Selected Time Slices (5.55 to 4.91 Ma and 6.20 to 5.55 Note. A positive phase represents a lag, a negative phase a lead with respect to the orbital parameter; only data sets with a coherency higher than 90% (k > 0.48) are shown, except for δ 18 O sw (italic).…”

“…Summer (July-September, JJAS) rainfall δ 18 O over the southern Tibetan Plateau ranges from −18.26‰ in Lhasa to −16.62‰ in Nyalam (Gao et al, 2013). In contrast, tropical-subtropical summer monsoon rainfall close to the Ganges/Brahmaputra Delta flood plains exhibits considerably higher δ 18 O values (for instance, summer monsoon rainfall in the year 2004 averaged −5.74‰; Dar & Ghosh, 2017). Further south, values of −5.76‰ at Bangalore (Dar & Ghosh, 2017) and −2.16‰ in Sri Lanka (Edirisinghe et al, 2017) are comparable or even higher, as they are less influenced by rainout Table 3 Results of Cross-Spectral Analysis Between the Site U1448 Records and the 19, 23, and 41 kyr Orbital Components Over Selected Time Slices (5.55 to 4.91 Ma and 6.20 to 5.55 Note.…”

“…On longer timescales, the local freshwater δ 18 O strongly depends on the location of the initial moisture source for the local rainwater. Today, modeled backward airmass trajectories indicate a common transport of precipitation into the core area of the Indian monsoon from the Arabian Sea (Dar & Ghosh, 2017). On a local scale, moisture sources for precipitation depend on the prevailing wind directions over the Indian O sw (green) records with respect to 41 kyr obliquity maxima and 23 kyr precession minima for intervals (a) 5.55 to 4.91 Ma and (b) 6.20 to 5.55 Ma.…”

Modern rain-isotope data from Indian island and the mainland on the daily scale for the summer monsoon season

“…Due to the distinct isotopic characteristics, the rainwater samples from source and receiver regions provide a means to determine the contributions of land and ocean-derived moisture [4] .…”