Indian Institute of Tropical Meteorology

Accurate prediction of extreme rainfall events during the Indian Summer Monsoon (ISM, June to September) is critical for disaster preparedness and mitigation. This study evaluates the performance of two operational numerical weather prediction models, a high-resolution version of Global Forecast System (GFS T1534) and the control member of the Met Office Global and Regional Ensemble Prediction System-Global (MOGREPS-G), in forecasting such events during the ISM from 2020 to 2023. The results demonstrate that, with respect to observations, both models tend to underestimate the mean and variability of rainfall; GFS-T1534 represents the mean and correlation better while MOGREPS-G represents the variability better over the Indian landmass. To assess the models’ performance for extreme rainfall prediction, we fix a rainfall threshold of 50 mm day−1, and the skill scores are computed including Probability of Detection, False Alarm Rate, Bias score and F1 score. Together, these scores indicate that both models show potential in short-range forecasting of extreme rainfall events, particularly within 24 h, but their skills remain limited at longer lead times. Specifically, the model biases vary over different geographical locations, often showing contrasting features. This underscores the need for model-specific post-processing and calibration techniques if these forecasts are to be used effectively for operational decision-making.

There is consensus that forcing due to Northern Hemispheric anthropogenic aerosols has played a significant role in the decline of South Asian monsoon precipitation since the mid-20th century. However, the future trajectory of regional aerosol emissions remains highly uncertain, particularly in light of potentially stricter air-quality regulations that could lead to reductions in aerosol loading across South and East Asia. Understanding how such changes may influence the near-term evolution of the monsoon is therefore critical. Here, we investigate the response of the South Asian summer monsoon to regional aerosol reductions using a suite of sensitivity experiments conducted with the IITM Earth System Model (IITM-ESMv2). Our simulations reveal a widespread intensification of monsoon precipitation over South and Southeast Asia following aerosol reductions. This response is driven by the combined effect of increasing greenhouse gas concentrations and declining absorbing aerosols over the subcontinent, which together enhance the land–sea thermal contrast. The strengthened thermal gradient promotes strengthened cross-equatorial low-level flow, leading to enhanced moisture transport and a sustained buildup of moisture across the monsoon region. The thermodynamic and dynamical changes favor widespread increases in precipitation. Our findings suggest that future air-pollution mitigation efforts across South and East Asia may play a critical role in shaping the near-future intensification of the monsoon, with important implications for regional hydroclimate over the coming decades.

Moist heat impairs the human body’s ability to cool through sweat-based evaporative cooling, posing a serious health risk. In India, this risk is especially acute, since the Indian summer monsoon (ISM) brings abundant moisture, and socio-economic conditions significantly increase the exposure and vulnerability to moist heat. However, there is a limited understanding of the characteristics and large-scale drivers of moist heatwaves during the ISM. This study uses the ERA5 reanalysis to analyse moist heatwaves and their relationship with active and break periods of the ISM during 1940–2023. An empirical orthogonal function analysis of daily maximum wet-bulb temperature (Tw) anomalies reveals that the first two principal components (PCs) explain key patterns of variability of moist heatwaves, with PC1 controlling their occurrence and PC2 controlling their spatial extent. Whilst breaks in the monsoon favour moist heatwaves in eastern and peninsular India, active rainfall events, corresponding to phases 5–7 of the Boreal Summer Intraseasonal Oscillation, favour moist heatwaves in northern and northwestern India. Specific humidity plays a larger role than dry-bulb temperature in controlling Tw variability in India. The results of this study reveal important characteristics of moist heatwaves during the ISM and offer potential for developing forecasting tools, which could ultimately benefit stakeholders in India.

Montane forests exhibit complex, heterogeneous microclimatic regimes that challenge the physiological plasticity of canopy and subcanopy species. In a Himalayan subalpine forest, we investigated how topographically mediated light environments shape the expression of photosynthetic traits in co-dominant trees — Quercus semecarpifolia (canopy) and Rhododendron arboreum (subcanopy)—across north- and south-facing slopes. Using leaf-level gas exchange measurements, we identified consistent patterns of light acclimation in Q. semecarpifolia and high-light-adapted R. arboreum. However, shade-acclimated R. arboreum individuals on north-facing slopes displayed idiosyncratic physiological signatures, including positive dark respiration rates (Rd) and negative light compensation points (LCP) — magnitudes theoretically implausible under standard C3 photosynthetic models.These anomalies suggest either (i) physiological re-fixation of respired CO2 under low light, (ii) non-linear error propagation in light response curve (LRC) fitting at extremely low PPFD, or (iii) extreme photoprotective plasticity unique to shade-adapted subcanopy species. Unlike Q. semecarpifolia and light-acclimated R. arboreum on south-facing slopes, these north-facing subcanopy individuals maintained high NPQ under minimal photon flux, indicative of disproportionate energy dissipation mechanisms.Our findings highlight how fine-scale microclimatic heterogeneity, especially in shaded montane niches, can generate unexpected and complex trait responses that deviate from established photosynthetic theory. These results necessitate refined methodological protocols and physiological models to interpret trait dynamics in low-light, high-humidity microclimates, particularly in the context of rising canopy temperatures and climate extremes. The study offers critical insights into species-specific limitations and compensations under montane microclimatic stress, with implications for predicting forest carbon cycling and resilience under future climate scenarios.

Abstract. Tropospheric reactive iodine influences the oxidizing capacity of the atmosphere and serves as an important source of ultra-fine particles. However, the paucity of observations of gas-phase and aerosol iodine, combined with incomplete understanding and representation of iodine chemistry in models, leads to substantial uncertainties in understanding iodine abundance, speciation, and impacts. Motivated by known gaps in previous modeling studies, we introduced speciated aerosol iodine and aerosol iodide recycling to the global chemical transport model, GEOS-Chem. Modeled aerosol iodine is speciated into fine and coarse mode soluble organic iodine (SOI), iodate, and iodide. Aerosol iodide is recycled into the gas phase via heterogeneous chemistry involving halogen nitrates and hypohalous acids to form I2, ICl, and IBr, which represents an additional source of gas-phase iodine to the atmosphere. Iodide dehalogenation doubles the tropospheric burden of reactive iodine (Iy) while reducing model-measurement bias for IO and aerosol iodine. The rate of aerosol iodine conversion to Iy is more than twice as fast as the combined rates of inorganic ocean emissions and the photolysis of organic iodine gases, suggesting that aerosols are important in mediating the abundance and lifetime of tropospheric Iy. The incorporation of SOI and iodate into the model prevents iodide dehalogenation by partitioning iodide into less reactive reservoirs, which has a stabilizing effect for reactive iodine chemistry. These findings have implications for reactive halogen abundances and global oxidant budgets in the troposphere.

ABSTRACT The Southern Ocean (SO) plays a central role in regulating global heat and carbon exchange, yet the Indian sector remains one of its least observed regions despite spanning major oceanic fronts. Phytoplankton dynamics here are strongly influenced by light availability, nutrient supply and vertical mixing, all of which are being reshaped by rapid climate-driven changes. This study examines interannual variability in Subsurface Chlorophyll Maximum (SCM) characteristics and phytoplankton community structure during the austral summers of 2018 and 2020 across the Subtropical Front (STF), Subantarctic Front (SAF), Polar Front (PF) and the region south of the PF (SPF). Phytoplankton pigments were quantified using high-performance liquid chromatography (HPLC), and community composition was assessed via flow cytometry and microscopy. Modest interannual changes in upper-ocean structure resulted in pronounced differences in SCM depth, intensity and community composition. In 2018, shallow mixed layers (25–35 m) and deep euphotic zones (60–80 m) supported shallow, intense SCMs at the STF and SAF (Chlmax ≈ 1.4 mg m−3; Dmax 30–40 m), dominated by small flagellates and dinoflagellates. In 2020, deeper mixing (MLD 55–75 m) and reduced light penetration (Zeu 40–55 m) produced weaker, deeper SCMs at the STF (Chlmax 0.25–0.38 mg m−3), while the SAF developed a stronger, shallower SCM (Chlmax ≈ 1.6 mg m−3). Enhanced stratification at the PF and SPF in 2020 yielded stronger, shallower SCMs (Chlmax up to 0.74 mg m−3), accompanied by surface cooling and freshening indicative of melt water influence. Phytoplankton communities shifted accordingly: diatoms such as Fragilariopsis and Chaetoceros increased by 40–60% south of the PF in 2020, while picophytoplankton (Synechococcus, picoeukaryotes) peaked at the STF under warm, high-light conditions. Principal Component Analysis revealed a shift in dominant environmental controls—from density and salinity in 2018 to light and oxygen in 2020—indicating a transition from physically to radiatively regulated SCMs. These findings highlight the sensitivity of the Indian Sector of the Southern Ocean to small perturbations in stratification and light climate, with implications for regional carbon export, ecosystem functioning and future productivity under continued climate forcing.

<strong class="journal-contentHeaderColor">Abstract.</strong> Aerosols in the upper troposphere play an important role in Earth&rsquo;s radiative balance and atmospheric composition. Satellite observations have revealed a recurrent enhancement of aerosol extinction coefficient (AEC) in the upper troposphere and near the tropopause over the Asian summer monsoon (ASM) anticyclone (ASMA) region during July&ndash;August. However, substantial uncertainties remain regarding (i) the influence of ASM dynamics and climate variability on these aerosols, (ii) the extent to which the upper-tropospheric aerosol trends reflect changes in surface pollutant emissions, and (iii) the ability of global models to simulate aerosol amounts, variability, and key controlling processes in the upper-tropospheric ASMA region. Here, we present results from an AeroCom-coordinated global multi-model study addressing these issues. Using simulations from nine models for 2000&ndash;2018, we find large inter-model differences in non-volcanic AEC over the upper-tropospheric ASMA region, with coefficients of variation ranging from 64 % to 86 %. Approximately half of this spread is attributable to differences in transport and wet removal processes, as diagnosed using common tracers, with discrepancies in wet removal contributing about eight times more than those associated with transport. The multi-model ensemble indicates an overall increase in non-volcanic AEC over the past two decades, consistent with rising anthropogenic emissions in Asia, while interannual variability is linked to climate variability as represented by the Multivariate ENSO Index. Through comparison with satellite observations, we further identify persistent model deficiencies, particularly in the representation of volcanic aerosols, and highlight priorities for future coordinated model developments and evaluation.