Indian Summer Monsoon Characteristics Research Articles

Impact of air–sea coupling on the simulation of Indian summer monsoon using a high-resolution Regional Earth System Model over CORDEX-SA

A new high-resolution Regional Earth System Model, namely ROM, has been implemented over CORDEX-SA towards examining the impact of air–sea coupling on the Indian summer monsoon characteristics. ROM's simulated mean ISM rainfall and associated dynamical and thermodynamical processes, including the representation of northward and eastward propagating convention bands, are closer to observation than its standalone atmospheric model component (REMO), highlighting the advantage of air–sea coupling. However, the value addition of air–sea coupling varies spatially with more significant improvements over regions with large biases. Bay of Bengal and the eastern equatorial Indian Ocean are the most prominent region where the highest added value is observed with a significant reduction up to 50–500% precipitation bias. Most of the changes in precipitation over the ocean are associated with convective precipitation (CP) due to the suppression of convective activity caused by the negative feedback due to the inclusion of air–sea coupling. However, CP and large-scale precipitation (LP) improvements show east–west asymmetry over the Indian land region. The substantial LP bias reduction is noticed over the wet bias region of western central India due to its suppression, while enhanced CP over eastern central India contributed to the reduction of dry bias. An insignificant change is noticed over Tibetan Plateau, northern India, and Indo Gangetic plains. The weakening of moisture-laden low-level Somalia Jets causes the diminishing of moisture supply from the Arabian Sea (AS) towards Indian land regions resulting in suppressed precipitation, reducing wet bias, especially over western central India. The anomalous high kinetic energy over AS, wind shear, and tropospheric temperature gradient in REMO compared to observation is substantially reduced in the ROM, facilitating the favourable condition for suppressing moisture feeding and hence the wet bias over west-central India in ROM. The warmer midlatitude in ROM than REMO over eastern central India strengthens the convection, enhancing precipitation results in reducing the dry bias. Despite substantially improved ROM’performance, it still exhibits some systematic biases (wet/dry) partially associated with the persistent warm/cold SST bias and land–atmosphere interaction.

Influence of subtropical circulation systems on the changing El Niño-Indian summer monsoon relationship

Remote factors have a significant influence on the variability of Indian summer monsoon rainfall. The study investigates atmospheric conditions influenced by the El Niño episodes on the variable summer monsoon over India. Four composites of years were selected based on the occurrence / non-occurrence of El Niño and extreme monsoons over India. This study examines the upper tropospheric and lower tropospheric wind patterns, tropospheric temperature, and velocity potential for 1958 to 2019 using Japanese Reanalysis 55 (JRA 55) data. The regional meridional circulation established by the north-south pressure gradient is more susceptible to the monsoon condition over the Indian Subcontinent than the external factors. The spatial variability and intensity of Australian High in the southern hemisphere play a prominent role in the Indian summer monsoon characteristics. Subtropical circulations in both hemispheres influence monsoon conditions over India irrespective of El Niño condition. The tropical zonal circulation has appeared to be modulated by the El Niño Southern Oscillation (ENSO) condition over the tropical Pacific Ocean, thereby modify the monsoon circulation.

Elucidating intra-seasonal characteristics of Indian summer monsoon. Part-I: Viewed from remote sensing observations, reanalysis and model datasets

In this study, we examine the transitions in the monsoon phases (onset, active, break and the withdrawal) during an entire monsoon season. This makes use of a host of observational tools that come from GPM (Global Precipitation Measurement) and TRMM (Tropical Rainfall Measuring Mission) satellites for precipitation estimates, the vertical structure of rain, hydrometeors and cloud types from TRMM and CloudSat datasets. During onset, the mean moisture convergence, especially over west and south-west coast of India is 2 × 10−4 kg m−1 s−1; however, it carries much higher value of >4 × 10−4 kg m−1 s−1 during the active phase over central eastern India. Much lesser moisture convergence (<1 × 10−4 kg m−1 s−1) is noted over Western Ghats area during the break phase. However, there are northeasterly moisture fluxes present over southern part of India during withdrawal phase. The tall cumulonimbus clouds that extend out to 16 km are illustrate during onset, the active phase is dominated by alto stratus and nimbostratus type clouds that are somewhat shallower. In general, we noted an absence of such clouds during the break and the withdrawal phases. Those structures were consistent in a number of derived fields such as the moisture convergence, moisture fluxes, the energy conversions between the rotational and the divergent kinetic energy and the corresponding phases of the intra-seasonal oscillations.

Future changes in Indian summer monsoon characteristics under 1.5 and 2 °C specific warming levels

The rainfall during Indian summer monsoon (ISM) is very important for the population living in the Indian sub-continent. The recent Paris climate agreement determined to keep the global mean temperature rise well below 2 °C and pursue efforts to limit it within 1.5 °C. This global temperature rise would influence the ISM mechanism over the Indian sub-continent. This study examines the possible changes in the ISM characteristics at 1.5 and 2 °C specific warming levels (SWLs) with respect to the historical period. This analysis uses a set of 12 regional climate simulations under Coordinated Regional Climate Downscaling Experiments-South Asia (CORDEX-SA). The land as well as the oceans shows a warming signature due to the effect of anthropogenic forcings with much higher warming over land. The increase of global temperature to 1.5 °C (2 °C) SWL leads to an earlier onset of ISM over India by 7 (11) days in the model ensemble. The increasing land-sea temperature contrast gradually increases the strength of the Findlater jet (by 0.5–0.9 m/s), which transports more moisture towards land and causes higher rainfall (increase by 2–10%) over India. The study reveals that under a higher SWL of 2 °C the mean rainfall is augmented as compared to that under 1.5 °C. However, there exists uncertainty in the findings due to inter-model differences especially for the duration of ISM activity and the spatial distribution of rainfall at different SWLs.

Indian Summer Monsoon and El Niño Southern Oscillation in CMIP5 Models: A Few Areas of Agreement and Disagreement

Using the CMIP5 model outputs, a few characteristics of Indian Summer Monsoon (ISM) rainfall and Niño 3.4 temperature are analysed during June–July–August–September (JJAS). Focusing on specified regions around central-northeast India, some general characteristic features of ISM precipitation are studied, which are shown to be varying among models. The trend of decreasing rainfall in that region as noticed in observations suggests an inconsistency among models. The ENSO also shows variation, and its phasing indicates disagreement. Unlike other models, FGOALS-g2 is identified as not suggesting any trend in Niño 3.4 temperature and needs attention for model evaluation purposes. ISM and ENSO correlation in either historical or the RCP 8.5 scenario confirm a negative signature, agreeing with the usual ISM, ENSO connection. Precipitation over the globe shows a rising trend in an ensemble of CMIP5 model outputs for the RCP 8.5 scenario, though no consensus is reached for the Indian region. Precipitation time series around the Indian subcontinent vary widely among models. Analyses with various future scenarios indicate that the Indian subcontinent shows much larger uncertainty, in terms of precipitation, compared to that from the whole world. This study identifies a few areas where CMIP 5 models are in agreement or disagreement with each other. Such an analysis could be useful for understanding various processes in CMIP 5 models that involve ISM precipitation and can lead to improving the representation of processes in models.

Possible changes in the characteristics of Indian Summer Monsoon under warmer climate

The Indian Summer Monsoon (ISM) spans four months starting from June and ending in September and produced wide spread rainfall over Indian continents mainly due to land–sea heating contrast between Indian Ocean and large Asian land mass. ISM is controlled by semi permanent features such as heat low over northwest sector of India, cross-equatorial flow and the low level westerly jet over the Arabian Sea at 850hPa, the tropical easterly jet over the Indian Ocean at 200hPa, Mascarene High, and anti-cyclone over the Tibet. Any fluctuation in Indian Summer Monsoon Rainfall (ISMR) during ISM on intra seasonal to inter annual is manifestation of change in wind circulation and temperature distribution. Therefore, in order to understand the change in magnitude and pattern of ISMR under warmer climate, it is necessary to qualitatively and quantitatively assess the change in associated monsoon wind circulation and temperature distribution. The current study examines the changes in magnitude and spatial distribution of ISMR and associated change in wind circulation and temperature distribution under forced scenarios in selected climate models contributed to International Panel of Climate Change (IPCC) 4th Assessment Report (AR4). It is found that under A2, B1 and A1B emission scenarios, future projected change in spatial distribution of ISMR shows deficit and excess of over the lower part of western and eastern coast of India in simulation of HadGEM1, ECHAM5, and MIROC (Hires) model which seems to be manifestation of anomalous anticyclonic flow at 850hPa in Arabian Sea and anomalous westerly flow at 200hPa