Browsing by Author "T.P. Sabin"

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Linkage of water vapor distribution in the lower stratosphere to organized Asian summer monsoon convection
(Springer Science and Business Media Deutschland GmbH, 2021) Bhupendra Bahadur Singh; Raghavan Krishnan; D.C. Ayantika; Ramesh K. Vellore; T.P. Sabin; K. Ravi Kumar; Simone Brunamonti; Sreeharsha Hanumanthu; Teresa Jorge; Peter Oelsner; Sunil Sonbawne; Manish Naja; Suvarna Fadnavis; Thomas Peter; Manoj K. Srivastava
Accumulation of water vapor in the upper troposphere/lower stratosphere (UT/LS) over the Asian continent is a recognized feature during the boreal summer monsoon. While there has been a debate on the role of monsoon convective intensities on the UT/LS water vapor accumulations, there are ambiguities with regard to the effects of organized monsoon convection on the spatial distribution of water vapor. We provide insights into this aspect using high precision balloon measurements of water vapor from a high-elevation site Nainital (29.4° N, 79.5° E), India, located in the Himalayan foothills and satellite retrievals of water vapor from the Microwave Limb Sounder (MLS). We also use precipitation estimates from the Tropical Rainfall Measuring Mission (TRMM) satellite (i.e., merged product 3B42 and precipitation radar 3A25 estimates of rain rate and rain type viz convective/stratiform), reanalysis circulation data, as well as numerical model simulations. We first evaluate the MLS estimates of water vapor mixing ratios with in situ high precision hygrometer balloon observations over Nainital. It is seen from our analyses of the MLS data that the LS water vapor distribution is closely linked to the organization of the South Asian monsoon convection and its influence on the UT/LS circulation. This link between LS water vapor distribution and organized monsoon convection is also captured in the in situ observations on 3 August 2016. It is evidenced that periods of organized summer monsoon convective activity over the Indian subcontinent and Bay of Bengal promote divergence of water vapor flux in the UT/LS; additionally the Tibetan anticyclonic circulation causes widespread distribution of the UT/LS water vapor. In addition to the effects of Asian monsoon convection, we also note that global climate drivers such as El Niño-Southern Oscillation (ENSO), Brewer–Dobson circulation (BDC), and Quasi-Biennial Oscillation (QBO) can contribute to nearly 38% of the UT/LS water vapor variability over the Asian monsoon region. The main result of our study indicates that widespread spatial distribution and accumulation of water vapor in the LS (about 80% of total accumulation between May and August months) tend to co-occur with organized monsoon convection, intensified divergence of water vapor flux in the UT/LS and intensified Tibetan anticyclone. On the other hand, the circulation response and LS water vapor distribution to pre-monsoon localized deep convection tend to have a limited spatial scale confined to Southeast Asia. Results from model experiments suggest that the UT/LS circulation pattern to organized monsoon convection has resemblance to stationary Rossby waves forced by organized latent heating, with the westward extending response larger by about 15° longitudes as compared to that of the pre-monsoon localized deep convection. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.
Upper tropospheric moistening during the Asian summer monsoon in a changing climate
(Springer Science and Business Media Deutschland GmbH, 2024) Bhupendra Bahadur Singh; R. Krishnan; T.P. Sabin; Ramesh K. Vellore; Naresh Ganeshi; Manoj K. Srivastava
This study investigates the projected changes in the upper troposphere and lower stratosphere (UTLS) water vapor over the Asian summer monsoon (ASM) region based on satellite records, numerical simulations using variable-resolution global climate model focused over south Asia (HIST-natural and anthropogenic forcing in the historical period, and FUT-following RCP4.5 in future), and Coupled Model Intercomparison Project Phase 5 (CMIP5) datasets. The simulations generally reproduced the seasonal cycle in the UTLS water vapor and regional water vapor maximum. With progressive warming in future, excessive upper tropospheric moistening is noted over the ASM region in far-future (2070–2095) climate against the HIST climate (1980–2005) with water vapor mixing ratio increasing to ~ 7.5 ppmv relative to ~ 5 ppmv noted in the HIST. It is further noted that projected changes in water vapor are linked to anomalous warming (~ 1–4 K) in the upper tropospheric layers juxtaposed with zonally elongated ASM anticyclone and enhanced water vapor flux divergence by amplifications in rotational winds. Further, the simulations indicate robust increase in ASM upper tropospheric water vapor as compared to those at mid- and lower- troposphere in accordance with the Clausius–Clapeyron temperature dependence of moisture response to warming and amplified troposphere warming with altitude. A simple comparison between the ASM and the entire globe indicates that upper tropospheric water vapor-temperature relationship has a similar response, however, the projected variability in temperature and moisture is significantly larger (about twice) over the ASM region highlighting strong regional influence. Nonetheless, the projections indicate that ASM is a potential regional source in modulating UTLS water vapor budget in a warming climate. © 2023, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.