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Linkage of water vapor distribution in the lower stratosphere to organized Asian summer monsoon convection

Abstract

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.

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Acknowledgments and data statement

The authors acknowledge the funding support from Ministry of Earth Sciences (MoES) India and the European Community’s Seventh Framework Programme (FP7/2007–2013) in the framework of the StratoClim project under grant agreement number 603557. The authors thank the Director, IITM and Director, ARIES for providing necessary facilities to carry out this research, and appreciate the help from Mr. Deepak Chausali in the balloon soundings. We are also thankful to the Editor and the two anonymous reviewers. The MLS water vapor data were obtained from http://mls.jpl.nasa.gov/products/h2o_product.php and the gridded OLR data from http://www.esrl.noaa.gov/psd/. The ERA-Interim data were obtained from http://apps.ecmwf.int/datasets/data/interim-full-daily/. The TRMM dataset were obtained from http://disc.sci.gsfc.nasa.gov/.

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Appendix A: Statistics of distribution and interpretation of bimodal-like appearance

Appendix A: Statistics of distribution and interpretation of bimodal-like appearance

The statistics of the distribution and robustness of separation between the two peaks are quantified in this section using statistical tests and finite mixture modeling as a tool. We perform a ‘dip test’ through Hartigans' dip test which is a standard test for multimodality and describes the departure of any sample from unimodality (Hartigan and Hartigan 1985). Further, we use Gaussian finite mixture modeling for estimation of mixing density and it is found that the mixing probabilities of mode A and mode B are 0.54 and 0.46, respectively. The bimodality separation is found to be 0.65 (see also Zhang et al. 2003). In addition, we also check Ashman’s D statistic which provides a robust measure of differentiation between the two distributions (Ashman et al. 1994). For a given dataset, if the Ashman’s D statistic is above ~ 2, it is understood to have good separation. The Ashman’s D statistic for the present analysis is noted to be 2.3 which indicate significant separation between the two peaks. However, it should be noted that the bimodal-like appearance discussed here is associated with the systematic transition in convective activity between pre-monsoon to monsoon season and it is equivalent to temporal separation between the two peaks. Therefore the observed statistical separation is a result on appearance and it is almost equivalent to separation of pre-monsoon and monsoon cases. Statistical summary of the frequency distribution is given in the following Table 2.

Table 2 Statistics of bimodal-like appearance of LS water vapor over the Asian summer monsoon region

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Singh, B.B., Krishnan, R., Ayantika, D.C. et al. Linkage of water vapor distribution in the lower stratosphere to organized Asian summer monsoon convection. Clim Dyn 57, 1709–1731 (2021). https://doi.org/10.1007/s00382-021-05772-2

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