Spring tropical cyclones modulate near-surface isotopic compositions of atmospheric water vapour in Kathmandu, Nepal
<p>While westerlies are recognized as a significant moisture transport in Nepal during the pre-monsoon season, precipitation is also attributed to moisture from cyclones originating in the Bay of Bengal (BoB) or the Arabian Sea (AS). Tropical cyclones exhibit negative isotopic values in both p...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2024-03-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/24/3279/2024/acp-24-3279-2024.pdf |
| Summary: | <p>While westerlies are recognized as a significant moisture transport in Nepal during the pre-monsoon season, precipitation is also attributed to moisture from cyclones originating in the Bay of Bengal (BoB) or the Arabian Sea (AS). Tropical cyclones exhibit negative isotopic values in both precipitation and atmospheric water vapour; however, the factors influencing isotopic fractionation during tropical cyclones remain poorly understood. We present the results of continuous measurements of the isotopic composition of atmospheric water vapour (<span class="inline-formula"><i>δ</i><sup>18</sup></span>O<span class="inline-formula"><sub>v</sub></span>, <span class="inline-formula"><i>δ</i></span>D<span class="inline-formula"><sub>v</sub></span>, and d-excess<span class="inline-formula"><sub>v</sub></span>) in Kathmandu from 7 May to 7 June 2021 during two pre-monsoon cyclones: cyclone Tauktae, formed over the Arabian Sea, and cyclone Yaas, formed over the Bay of Bengal. Our study reveals that tropical cyclones originating from the BoB and the AS during the pre-monsoon season modulate isotopic signals of near-surface atmospheric water vapour in Nepal. Comparing conditions before and after, we observed a significant depletion of <span class="inline-formula"><i>δ</i><sup>18</sup></span>O<span class="inline-formula"><sub>v</sub></span> and <span class="inline-formula"><i>δ</i></span>D<span class="inline-formula"><sub>v</sub></span> during both cyclones, attributed to changes in moisture sources (local vs. marine). Convective activity plays a pivotal role in the variability of <span class="inline-formula"><i>δ</i><sup>18</sup></span>O<span class="inline-formula"><sub>v</sub></span> and <span class="inline-formula"><i>δ</i></span>D<span class="inline-formula"><sub>v</sub></span> during both cyclones, confirmed by the spatial variations of outgoing longwave radiation (OLR) and regional precipitation during both cyclones. We also found a significant negative correlation between <span class="inline-formula"><i>δ</i><sup>18</sup></span>O<span class="inline-formula"><sub>v</sub></span> and/or <span class="inline-formula"><i>δ</i></span>D<span class="inline-formula"><sub>v</sub></span> and rainfall amount along the trajectories during cyclone Tauktae, probably resulting from integrated upstream processes linked to the earlier Rayleigh distillation of water vapour via rainfall rather than local rainfall. The decrease in <span class="inline-formula"><i>δ</i><sup>18</sup></span>O<span class="inline-formula"><sub>v</sub></span> and/or <span class="inline-formula"><i>δ</i></span>D<span class="inline-formula"><sub>v</sub></span> during cyclone Yaas is associated with the intensified convection and moisture convergence at the measurement site, while the lower cloud top temperatures (CTTs) and lower cloud top pressure (CTP) during intense convection contribute to higher d-excess<span class="inline-formula"><sub>v</sub></span> values during the final stage of cyclone Yaas. This characteristic is missing during cyclone Tauktae. Our results shed light on key processes governing the isotopic composition of atmospheric water vapour in Kathmandu with implications for the monsoon moisture transport and paleoclimate reconstructions of tropical cyclone activity.</p> |
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| ISSN: | 1680-7316 1680-7324 |