Vishnu S Nair

@iisertvm.ac.in

Assistant Professor, School of Earth, Environmental and Sustainability Sciences
Indian Institute of Science Education and Research Thiruvananthapuram

Vishnu S Nair


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https://researchid.co/vishnus

RESEARCH, TEACHING, or OTHER INTERESTS

Atmospheric Science, Earth and Planetary Sciences, Oceanography, Computers in Earth Sciences
17

Scopus Publications

579

Scholar Citations

12

Scholar h-index

13

Scholar i10-index

Scopus Publications

  • Insights into Indian summer monsoon rainfall variability: early twentieth century warming vs. mid-twentieth century cooling
    Swagatika Chakra, S. Vishnu, Harsh Oza, Akash Ganguly, Amit Pandey, Virendra Padhya, R. D. Deshpande
    Climate Dynamics, 2025
  • Automated Operational Forecasting of Monsoon Low Pressure Systems
    D. L. Suhas, S. Vishnu, Salil Goyal, Sahadat Sarkar, Parthasarathi Mukhopadhyay, Paul A. Ullrich, William R. Boos
    Bulletin of the American Meteorological Society, 2024
    Monsoon low pressure systems (LPSs) are the dominant rain-bearing weather system of South Asia, often producing extreme precipitation and hydrological disasters in a region inhabited by nearly two billion people. Despite the importance of these storms, no operational system has automatically identified and tracked LPS in real time in numerical weather prediction model output; many commonly used vortex-tracking algorithms are ill suited for monsoon LPS because of the weak winds and cold cores of these systems. Here, we describe a new system that uses optimized algorithms to identify monsoon LPS in short- to medium-range forecasts from the U.S. Global Ensemble Forecast System (GEFS) and a version of the deterministic Global Forecast System (GFS) adapted and used operationally by the Indian Institute of Tropical Meteorology (IITM). We also assess the historical performance of these models in forecasting South Asian monsoon LPS, comparing this with the performance of the Integrated Forecasting System of the ECMWF. We assess the accuracy of model predictions of LPS genesis, position, intensity, and precipitation rates for forecast lead times of 1–5 days, yielding quantitative information on model biases to guide operational forecasters and disaster managers. The system we introduce here could be extended to other low-latitude regions affected by dynamically weak, heavily precipitating atmospheric vortices that are often not included in tropical cyclone inventories.
  • Historical and future trends in South Asian monsoon low pressure systems in a high-resolution model ensemble
    S. Vishnu, William R. Boos, William D. Collins
    Npj Climate and Atmospheric Science, 2023
    Historical trends in monsoon low pressure systems (LPS), the dominant rain-bearing weather system of South Asia, have been difficult to assess due to changes in the observing network. Future projections have also remained uncertain because prior studies concluded that many coarse-resolution climate models do not accurately simulate LPS. Here, we examine changes in South Asian monsoon LPS simulated by an ensemble of global models, including some with high spatial resolution, that we show skillfully represent LPS. In the ensemble mean, the number of strong LPS (monsoon depressions) decreased over the last 65 years (1950–2014) by about 15% while no trend was detected for weaker LPS (monsoon lows). The reduction in depression counts then moderated, yielding no trend in the periods 1980–2050 or 2015–2050. The ensemble mean projects a shift in genesis from ocean to land and an increase in LPS precipitation of at least 7% K−1, which together contribute to a projected increase in seasonal mean and extreme precipitation over central India.
  • Observed increase in the peak rain rates of monsoon depressions
    S. Vishnu, Mark D. Risser, Travis A. O’Brien, Paul A. Ullrich, William R. Boos
    Npj Climate and Atmospheric Science, 2023
    Most extreme precipitation in the densely populated region of central India is produced by atmospheric vortices called monsoon lows and monsoon depressions. Here we use satellite and gauge-based precipitation estimates with atmospheric reanalyses to assess 40-year trends in the rain rates of these storms, which have remained unknown. We show that rain rates increased in the rainiest quadrant of monsoon depressions, southwest of the vortex center; precipitation decreased in eastern quadrants, yielding no clear trend in precipitation averaged over the entire storm diameter. In an atmospheric reanalysis, ascent increased in the region of amplifying precipitation, but we could not detect trends in the intensity of rotational winds around the storm center. These storm changes occurred in a background environment where humidity increased rapidly over land while warming was more muted. Monsoon lows, which we show produce less precipitation than depressions, exhibit weaker trends that are less statistically robust.
  • Exploratory Precipitation Metrics: Spatiotemporal Characteristics, Process-Oriented, and Phenomena-Based
    L. Ruby Leung, William R. Boos, Jennifer L. Catto, Charlotte A. DeMott, Gill M. Martin, J. David Neelin, Travis A. O’Brien, Shaocheng Xie, Zhe Feng, Nicholas P. Klingaman, Yi-Hung Kuo, Robert W. Lee, Cristian Martinez-Villalobos, S. Vishnu, Matthew D. K. Priestley, Cheng Tao, Yang Zhou
    Journal of Climate, 2022
    Precipitation sustains life and supports human activities, making its prediction one of the most societally relevant challenges in weather and climate modeling. Limitations in modeling precipitation underscore the need for diagnostics and metrics to evaluate precipitation in simulations and predictions. While routine use of basic metrics is important for documenting model skill, more sophisticated diagnostics and metrics aimed at connecting model biases to their sources and revealing precipitation characteristics relevant to how model precipitation is used are critical for improving models and their uses. This paper illustrates examples of exploratory diagnostics and metrics including 1) spatiotemporal characteristics metrics such as diurnal variability, probability of extremes, duration of dry spells, spectral characteristics, and spatiotemporal coherence of precipitation; 2) process-oriented metrics based on the rainfall–moisture coupling and temperature–water vapor environments of precipitation; and 3) phenomena-based metrics focusing on precipitation associated with weather phenomena including low pressure systems, mesoscale convective systems, frontal systems, and atmospheric rivers. Together, these diagnostics and metrics delineate the multifaceted and multiscale nature of precipitation, its relations with the environments, and its generation mechanisms. The metrics are applied to historical simulations from phases 5 and 6 of the Coupled Model Intercomparison Project. Models exhibit diverse skill as measured by the suite of metrics, with very few models consistently ranked as top or bottom performers compared to other models in multiple metrics. Analysis of model skill across metrics and models suggests possible relationships among subsets of metrics, motivating the need for more systematic analysis to understand model biases for informing model development.
  • Why the droughts of the Indian summer monsoon are more severe than the floods
    S. Vishnu, A. Chakraborty, J. Srinivasan
    Climate Dynamics, 2022
  • Ocean state forecasting during VSCS Ockhi and a note on what we learned from its characteristics: A forecasting perspective
    R Harikumar, P Sirisha, Anuradha Modi, M S Girishkumar, S Vishnu, K Srinivas, Rakhi Kumari, G Yatin, P Dinesh Kumar, T M Balakrishnan Nair, M Mohapatra
    Journal of Earth System Science, 2022
  • Assessing Historical Variability of South Asian Monsoon Lows and Depressions With an Optimized Tracking Algorithm
    S. Vishnu, W. R. Boos, P. A. Ullrich, T. A. O'Brien
    Journal of Geophysical Research Atmospheres, 2020
    Cyclonic low‐pressure systems (LPS) produce abundant rainfall in South Asia, where they are traditionally categorized as monsoon lows, monsoon depressions, and more intense cyclonic storms. The India Meteorological Department (IMD) has tracked monsoon depressions for over a century, finding a large decline in their number in recent decades, but their methods have changed over time and do not include monsoon lows. This study presents a fast, objective algorithm for identifying monsoon LPS and uses it to assess interannual variability and trends in reanalyses. Variables and thresholds used in the algorithm are selected to best match a subjectively analyzed LPS data set while minimizing disagreement between four reanalyses in a training period. The stream function of 850 hPa horizontal wind is found to be optimal in this sense; it is less noisy than vorticity and represents the complete nondivergent wind, even when flow is not geostrophic. Using this algorithm, LPS statistics are computed for five reanalyses, and none show a detectable trend in monsoon depression counts since 1979. Both the Japanese 55‐year Reanalysis (JRA‐55) and the IMD data set show a step‐like reduction in depression counts when they began using geostationary satellite data, in 1979 and 1982, respectively; the 1958–2018 linear trend in JRA‐55, however, is smaller than in the IMD data set, and its error bar includes 0. There are more LPS in seasons with above‐average monsoon rainfall and in La Niña years, but few other large‐scale modes of interannual variability are found to modulate LPS counts, lifetimes, or track length consistently across reanalyses.
  • Synoptic scale systems
    Savita Patwardhan, K. P. Sooraj, Hamza Varikoden, S. Vishnu, K. Koteswararao, M. V. S. Ramarao, D. R. Pattanaik
    Assessment of Climate Change Over the Indian Region A Report of the Ministry of Earth Sciences Moes Government of India, 2020
  • Assessment of climatological tropical cyclone activity over the north Indian Ocean in the CORDEX-South Asia regional climate models
    S. Vishnu, J. Sanjay, R. Krishnan
    Climate Dynamics, 2019
  • Quantifying Tropical Cyclone's Effect on the Biogeochemical Processes Using Profiling Float Observations in the Bay of Bengal
    M. S. Girishkumar, V. P. Thangaprakash, T. V. S. Udaya Bhaskar, K. Suprit, N. Sureshkumar, S. K. Baliarsingh, J. Jofia, Vimlesh Pant, S. Vishnu, G. George, K. R. Abhilash, S. Shivaprasad
    Journal of Geophysical Research Oceans, 2019
  • On the relationship between the Indian summer monsoon rainfall and the EQUINOO in the CFSv2
    S. Vishnu, P. A. Francis, S. S. V. S. Ramakrishna, S. S. C. Shenoi
    Climate Dynamics, 2019
  • On the relationship between the Pacific Decadal Oscillation and monsoon depressions over the Bay of Bengal
    Sasidharannair Vishnu, Pavanathara Augustine Francis, Satheesh Chandra Shenoi, Surireddi Satya Venkata Siva Ramakrishna
    Atmospheric Science Letters, 2018
  • Teleconnection between the North Indian Ocean high swell events and meteorological conditions over the Southern Indian Ocean
    P. G. Remya, S. Vishnu, B. Praveen Kumar, T. M. Balakrishnan Nair, B. Rohith
    Journal of Geophysical Research Oceans, 2016
  • On the decreasing trend of the number of monsoon depressions in the Bay of Bengal
    S Vishnu, P A Francis, S S C Shenoi, S S V S Ramakrishna
    Environmental Research Letters, 2016
  • The role of ENSO and MJO on rapid intensification of tropical cyclones in the Bay of Bengal during October–December
    M. S. Girishkumar, K. Suprit, S. Vishnu, V. P. Thanga Prakash, M. Ravichandran
    Theoretical and Applied Climatology, 2015
  • Evaluation of High-Resolution WRF Model Simulations of Surface Wind over the West Coast of India
    S. Vishnu, P. A. Francis
    Atmospheric and Oceanic Science Letters, 2014

RECENT SCHOLAR PUBLICATIONS

  • Contribution of Low Pressure System Rainfall to Interannual Variability of the Indian Summer Monsoon
    S Vishnu
    Journal of Climate 39 (11), 2891-2905 , 2026
    2026
  • Drivers and Variability of Marine Heatwaves in the North Indian Ocean and their Impacts on South Asian Monsoon Rainfall
    L JOSEPH, N Skliris, D Dey, R Marsh
    Frontiers in Climate 8, 1801667 , 2026
    2026
  • Insights into Indian summer monsoon rainfall variability: early twentieth century warming vs. mid-twentieth century cooling
    S Chakra, S Vishnu, H Oza, A Ganguly, A Pandey, V Padhya, ...
    Climate Dynamics 63 (10), 1-16 , 2025
    2025
  • Drivers and Variability of Marine Heatwaves in the North Indian Ocean and their Impacts on South Asian Monsoon Rainfall
    L Joseph, N Skliris, S Vishnu, D Dey, R Marsh
    EGUsphere 2025, 1-24 , 2025
    2025
    Citations: 1
  • Automated operational forecasting of monsoon low pressure systems
    DL Suhas, S Vishnu, S Goyal, S Sarkar, P Mukhopadhyay, PA Ullrich, ...
    Bulletin of the American Meteorological Society 105 (12), E2444-E2460 , 2024
    2024
    Citations: 3
  • Past and future trends in South Asian monsoon depressions and their extreme rainfall
    WR Boos, V Sasidharan Nair, WD Collins, MD Risser, TA O'Brien, ...
    AGU Fall Meeting Abstracts 2023, A14J-01 , 2023
    2023
  • Historical and future trends in South Asian monsoon low pressure systems in a high-resolution model ensemble
    S Vishnu, WR Boos, WD Collins
    npj Climate and Atmospheric Science 6 (1), 182 , 2023
    2023
    Citations: 9
  • Observed increase in the peak rain rates of monsoon depressions
    S Vishnu, MD Risser, TA O’Brien, PA Ullrich, WR Boos
    npj Climate and Atmospheric Science 6 (1), 111 , 2023
    2023
    Citations: 12
  • Rise in Rainfall of South Asian Monsoon Low-Pressure Systems
    VS Nair, WR Boos, MD Risser, TA O’Brien, PA Ullrich, WD Collins
    EGU23 , 2023
    2023
  • Historical and future trends in South Asian monsoon low pressure systems in a high-resolution model ensemble. npj Climate Atmos. Sci., 6, 182
    S Vishnu, WR Boos, WD Collins
    2023
    Citations: 5
  • A Review of Recent Progress In Understanding Mechanisms And Trends Of Monsoon Depressions (Invited Presentation)
    WR Boos, DL Suhas, S Vishnu, M Diaz
    American Meteorological Society Meeting Abstracts 103, 1.1 , 2023
    2023
  • Exploratory precipitation metrics: Spatiotemporal characteristics, process-oriented, and phenomena-based
    LR Leung, WR Boos, JL Catto, C A. DeMott, GM Martin, JD Neelin, ...
    Journal of Climate 35 (12), 3659-3686 , 2022
    2022
    Citations: 42
  • Ocean state forecasting during VSCS Ockhi and a note on what we learned from its characteristics: A forecasting perspective
    R Harikumar, P Sirisha, A Modi, MS Girishkumar, S Vishnu, K Srinivas, ...
    Journal of Earth System Science 131 (2), 92 , 2022
    2022
    Citations: 6
  • Why the droughts of the Indian summer monsoon are more severe than the floods
    S Vishnu, A Chakraborty, J Srinivasan
    Climate Dynamics 58 (11), 3497-3512 , 2022
    2022
    Citations: 20
  • Assessing historical variability of South Asian monsoon lows and depressions with an optimized tracking algorithm
    S Vishnu, WR Boos, PA Ullrich, TA O'brien
    Journal of Geophysical Research: Atmospheres 125 (15), e2020JD032977 , 2020
    2020
    Citations: 80
  • Synoptic scale systems
    S Patwardhan, KP Sooraj, H Varikoden, S Vishnu, K Koteswararao, ...
    Assessment of climate change over the Indian region: A report of the … , 2020
    2020
    Citations: 11
  • Global track dataset of monsoon low pressure systems
    S Vishnu, B WR, U P. A, OB T. A
    Zenodo https://doi.org/10.5281/zenodo.3890646 , 2020
    2020
    Citations: 9
  • Feature Tracking in TempestExtremes: Automated Detection and Characterization of Extreme Weather
    PA Ullrich, CM Zarzycki, M Pinheiro, EE McClenny, KA Reed, ...
    AGU Fall Meeting Abstracts 2019, A33K-2999 , 2019
    2019
  • Automated identification of South Asian monsoon low pressure systems: Historical variations across reanalysis products
    V Sasidharan Nair, WR Boos, PA Ullrich, TA O'Brien
    AGU Fall Meeting Abstracts 2019, A23H-3009 , 2019
    2019
  • Assessment of climatological tropical cyclone activity over the north Indian Ocean in the CORDEX-South Asia regional climate models
    S Vishnu, J Sanjay, R Krishnan
    Climate Dynamics 53 (7-8), 5101-5118 , 2019
    2019
    Citations: 19

MOST CITED SCHOLAR PUBLICATIONS

  • On the decreasing trend of the number of monsoon depressions in the Bay of Bengal
    S Vishnu, PA Francis, SSC Shenoi, S Ramakrishna
    Environmental Research Letters 11 (1), 014011 , 2016
    2016
    Citations: 98
  • Assessing historical variability of South Asian monsoon lows and depressions with an optimized tracking algorithm
    S Vishnu, WR Boos, PA Ullrich, TA O'brien
    Journal of Geophysical Research: Atmospheres 125 (15), e2020JD032977 , 2020
    2020
    Citations: 80
  • The role of ENSO and MJO on rapid intensification of tropical cyclones in the Bay of Bengal during October–December
    MS Girishkumar, K Suprit, S Vishnu, VPT Prakash, M Ravichandran
    Theoretical and Applied Climatology 120 (3), 797-810 , 2015
    2015
    Citations: 77
  • Teleconnection between the N orth I ndian O cean high swell events and meteorological conditions over the S outhern I ndian O cean
    PG Remya, S Vishnu, B Praveen Kumar, TM Balakrishnan Nair, B Rohith
    Journal of Geophysical Research: Oceans 121 (10), 7476-7494 , 2016
    2016
    Citations: 65
  • Quantifying tropical cyclone's effect on the biogeochemical processes using profiling float observations in the Bay of Bengal
    MS Girishkumar, VP Thangaprakash, TVS Udaya Bhaskar, K Suprit, ...
    Journal of Geophysical Research: Oceans 124 (3), 1945-1963 , 2019
    2019
    Citations: 46
  • Exploratory precipitation metrics: Spatiotemporal characteristics, process-oriented, and phenomena-based
    LR Leung, WR Boos, JL Catto, C A. DeMott, GM Martin, JD Neelin, ...
    Journal of Climate 35 (12), 3659-3686 , 2022
    2022
    Citations: 42
  • On the relationship between the Pacific Decadal Oscillation and monsoon depressions over the Bay of Bengal
    S Vishnu, PA Francis, SC Shenoi, SSVS Ramakrishna
    Atmospheric Science Letters 19 (7), e825 , 2018
    2018
    Citations: 35
  • Evaluation of High-Resolution WRF Model Simulations of Surface Wind over the West Coast of India
    S Vishnu, PA Francis
    AOSL 7 (5), 458-463 , 2014
    2014
    Citations: 22
  • Why the droughts of the Indian summer monsoon are more severe than the floods
    S Vishnu, A Chakraborty, J Srinivasan
    Climate Dynamics 58 (11), 3497-3512 , 2022
    2022
    Citations: 20
  • Assessment of climatological tropical cyclone activity over the north Indian Ocean in the CORDEX-South Asia regional climate models
    S Vishnu, J Sanjay, R Krishnan
    Climate Dynamics 53 (7-8), 5101-5118 , 2019
    2019
    Citations: 19
  • On the relationship between the Indian summer monsoon rainfall and the EQUINOO in the CFSv2
    S Vishnu, PA Francis, S Ramakrishna, SSC Shenoi
    Climate Dynamics 52 (1), 1263-1281 , 2019
    2019
    Citations: 19
  • Observed increase in the peak rain rates of monsoon depressions
    S Vishnu, MD Risser, TA O’Brien, PA Ullrich, WR Boos
    npj Climate and Atmospheric Science 6 (1), 111 , 2023
    2023
    Citations: 12
  • Synoptic scale systems
    S Patwardhan, KP Sooraj, H Varikoden, S Vishnu, K Koteswararao, ...
    Assessment of climate change over the Indian region: A report of the … , 2020
    2020
    Citations: 11
  • Historical and future trends in South Asian monsoon low pressure systems in a high-resolution model ensemble
    S Vishnu, WR Boos, WD Collins
    npj Climate and Atmospheric Science 6 (1), 182 , 2023
    2023
    Citations: 9
  • Global track dataset of monsoon low pressure systems
    S Vishnu, B WR, U P. A, OB T. A
    Zenodo https://doi.org/10.5281/zenodo.3890646 , 2020
    2020
    Citations: 9
  • Ocean state forecasting during VSCS Ockhi and a note on what we learned from its characteristics: A forecasting perspective
    R Harikumar, P Sirisha, A Modi, MS Girishkumar, S Vishnu, K Srinivas, ...
    Journal of Earth System Science 131 (2), 92 , 2022
    2022
    Citations: 6
  • Historical and future trends in South Asian monsoon low pressure systems in a high-resolution model ensemble. npj Climate Atmos. Sci., 6, 182
    S Vishnu, WR Boos, WD Collins
    2023
    Citations: 5
  • Automated operational forecasting of monsoon low pressure systems
    DL Suhas, S Vishnu, S Goyal, S Sarkar, P Mukhopadhyay, PA Ullrich, ...
    Bulletin of the American Meteorological Society 105 (12), E2444-E2460 , 2024
    2024
    Citations: 3
  • Drivers and Variability of Marine Heatwaves in the North Indian Ocean and their Impacts on South Asian Monsoon Rainfall
    L Joseph, N Skliris, S Vishnu, D Dey, R Marsh
    EGUsphere 2025, 1-24 , 2025
    2025
    Citations: 1
  • Contribution of Low Pressure System Rainfall to Interannual Variability of the Indian Summer Monsoon
    S Vishnu
    Journal of Climate 39 (11), 2891-2905 , 2026
    2026