Onset Of Indian Summer Monsoon Research Articles

Weakening of La Niña Impact on Negative Indian Ocean Dipole Under Global Warming

AbstractAs global warming intensifies, the coupling relationship between negative Indian Ocean Dipole (nIOD) and La Niña has substantially changed. However, the characteristics and mechanisms of these changes are not yet fully understood. Here, we find that the impact of La Niña on nIOD has considerably weakened since 1999, with the frequency of nIOD occurrences during La Niña years plummeting to a mere one‐third of the pre‐1999 levels. This is primarily attributed to the early onset of Indian summer monsoon and the decrease in La Niña intensity, while the effect of variations in Bjerknes feedback is relatively minor. Model simulations suggest that the influence of La Niña on nIOD will continue to weaken under future global warming through similar mechanisms as in the observations, increasing the complexity of air‐sea coupling in the Indian Ocean.

Enhanced impacts of the North Pacific Victoria mode on the Indian summer monsoon onset in recent decades

AbstractVictoria mode (VM), the second dominant mode of North Pacific sea surface temperature variability, has been identified as one of the important factors influencing the Indian summer monsoon (ISM) onset. The positive phase of the May VM delays the ISM onset by both tropical and extratropical pathways. Here, we found a significant interdecadal enhancement of their relationship since the early 1990s, which is mainly attributed to the structure changes and increased variance of the VM. In recent decades, the VM has shown more significant warm SST anomalies in the tropical central Pacific, which drive the large-scale divergent circulation more effectively. This enhanced divergent circulation leads to low-level divergence and reduced rainfall in the tropical Asian summer monsoon region. The reduced rainfall excites equatorial Rossby wave response and anomalous easterly winds in the northern Indian Ocean, delaying the ISM onset. Besides, the increased variance of the VM after 1992/1993 stimulates a stronger extratropical Rossby wave train. This stationary Rossby wave train induces a stronger cooling to the northwest of India, which weakens the land-sea thermal contrast and leads to the delayed ISM onset. This finding should be taken into account to improve short-term predictions of the monsoon onset.

Climatological Madden-Julian Oscillation during boreal spring leads to abrupt Australian monsoon retreat and Asian monsoon onsets

Abrupt monsoon onsets/retreats are indispensable targets for climate prediction and future projection, but the origins of their abruptness remain elusive. This study establishes the existence of three climatological Madden-Julian Oscillation (CMJO) episodes contributing to the rapid Australian summer monsoon retreat in mid-March, the South China Sea (or East Asian) summer monsoon onset in mid-May, and the Indian summer monsoon onset in early June. The CMJO displays a dynamically coherent convection-circulation structure resembling its transitionary counterpart, demonstrating its robustness as a convectively coupled circulation system and the tendency of the transient MJOs’ phase-lock to the annual cycle. The CMJO is inactive during the boreal winter due to destructive year-to-year modulations of El Niño-Southern Oscillation. We hypothesize that the interaction between atmospheric internal variability (MJO) and the insolation-forced slow annual cycle generates the sudden monsoon withdrawal/onset during the boreal spring. Understanding the factors determining the timing and location of the MJO’s phase-locking and its variability is vital for monsoon forecasting and climate projection.

Delayed Onset of Indian Summer Monsoon in Response to CO2 Removal

AbstractUnderstanding the response of the Indian summer monsoon (ISM) onset to CO2 forcing is of utmost importance for rain‐fed agriculture and water management. In this study, we utilized an idealized symmetric CO2 removal scenario from the sixth phase of the Coupled Model Intercomparison Project to analyze the reversibility of monsoon onset. The results show that ISM onset is reversible but exhibits strong asymmetry: it undergoes minimal changes during the ramp‐up phase, but experiences rapid postponement as the CO2 begins to decline; Eventually, it is delayed more than 1 week when the CO2 concentration is restored to the initial level. To investigate the possible underlying mechanisms, we decomposed the climate response to CO2 forcing into the fast and slow processes. Notably, it is the enhanced slow response, which is driven by long‐term sea surface temperature (SST) changes, that dominates the asymmetric response of ISM onset. This slow response delays the ISM onset by strengthening near‐surface poleward land‐sea moist static energy contrast, thereby weakening the lower‐tropospheric monsoonal circulation. Based on the atmospheric component model simulations, we found that both the uniform SST change and patterned SST changes in the slow response contribute to the delay of ISM onset, but the latter plays a dominant role. Our results emphasize the importance of thoroughly assessing regional hydrological cycle features when designing the CO2 removal pathways.

Observed impacts of the North Pacific Victoria Mode on Indian summer monsoon onset

The onset of the Indian summer monsoon (ISM) marks the arrival of the rainy season and affects billions of people. Therefore, exploring the factors influencing ISM onset is essential, especially sea surface temperature (SST) anomalies. In addition to the well-known El Niño-Southern Oscillation in the tropical Pacific, in this study, we find that the SST anomalies in the extratropical Pacific (Victoria mode, VM) can significantly impact ISM onset. The positive phase of the May VM causes monsoon onset delay, and two distinct mechanisms linking these phenomena are identified. The tropical pathway involves divergent circulation and equatorial Rossby waves. The VM-associated SST gradient causes low-level divergence and suppressed rainfall around the Indo-China Peninsula. Such suppressed rainfall induces anomalous anticyclone circulation by exciting the equatorial Rossby wave response, which prevents the establishment of monsoonal southwesterly winds and moisture transport, therefore delaying ISM onset. The extratropical pathway involves the Rossby wave train and land–sea thermal contrast. The VM triggers an eastward Rossby wave train via upper-level divergence around the Gulf of Alaska. This wave train propagates downstream and generates anomalous negative geopotential heights and low air temperatures to the northwest of India, which reduces the land–sea thermal contrast and delays ISM onset. Our result suggest that the ISM onset predictions should consider the role of May VM.

Paradigm shift in the onset phase of the Indian Summer Monsoon since 2000 and its potential connection to South Indian Ocean

The Indian Ocean basin has undergone significant changes over the past two decades, and this study examines major shifts in the onset characteristics of the Indian Summer Monsoon (ISM) in response to changes in the dynamical and thermodynamic conditions. A thermodynamic index shows that the post-2000 period witnesses an approximately 3-day earlier (May 27) onset of ISM than the pre-2000 period. In the absence of an abrupt and distinct change from the dry to the wet season, the post-2000 era can be distinguished by its early and longer onset process. The Subtropical Indian Ocean Dipole (SIOD) undergoes a phase change during its evolution, which weakens the Mascarene High (MH) and hence the pressure gradient. The weakening of the MH along with the heterogeneity in Sea Surface Temperature (SST) trends have led to a weaker meridional pressure gradient over the Indian monsoon domain, resulting in a weak onset of the ISM. Additionally, warming in the Arabian Sea starts earlier than expected. The prolonged duration of the Indian Summer Monsoon (ISM) onset process in the post-2000 era can be attributed to several factors. These include the broader ascending limb of the Hadley cell over the south equatorial Indian Ocean, reduced wind shear, and the early development of a warmer Arabian Sea prior to the onset of the monsoon.

Changes in the mechanism of the South-Asian summer monsoon onset propagation induced by the pre-monsoon aerosol dust storm

A modeling effort is considered to understand the effect of dust aerosol storms in the Indian summer monsoon (ISM) onset propagation over the Indian subcontinent. The study found how the dust aerosol loading during the pre-monsoon period over the Indian subcontinent positively impacts the advancing ISM onset. The heavy loading of dust aerosol from middle east Asia to the northwest (NW) Indian region boosts the temperature gradient over the mid-troposphere. This sudden rise in the mid-tropospheric atmospheric temperature triggers the low-pressure belt over the monsoon trough. It amplifies the Elevated Heat Pump mechanism and loads enough dust for the dust storm over north India. The formation of the monsoon trough over the Indo-Gangetic Plain (IGP) pulls the moisture-loaded wind towards the Indian subcontinent and the IGP, followed by the onset of monsoon over Kerala within a week. Due to this storm, the intense heat gradient in the troposphere also induces the monsoon propagation and the monsoon advancement from Kerala to BoB within a short time. However, it makes up for the naturally delayed monsoon on time. Plain language summaryStudy shows how the pre-monsoon dust storm modulates the Indian summer monsoon (ISM) propagation over the Indian region. The onset of the monsoon was delayed in a particular year, and the pre-monsoon dust storm over northwest India changed the regular monsoon mechanism and generated an entirely different mechanism. The elevated heat pump theory applies that pushes the monsoon advancement faster than usual. A dust storm over north India causes a sudden rise in the mid-tropospheric temperature gradient and more rapid monsoon advancement than usual.

Effects of anthropogenic aerosols on the evolution of Indian summer monsoon

The NCAR Community Earth System Model is used to study the influences of anthropogenic aerosols on the Indian summer monsoon (ISM). We perform two sets of 30-year simulations subject to the prescribed perpetual SST annual cycle. One is triggered by the year 2000 climatology anthropogenic aerosol emissions data over the Indian Peninsula (referred to as AERO), and the other one is by the year 1850 (referred to as CTL). Only aerosol direct effects are included in the experiments. In our results, the transition of ISM in AERO relative to the CTL exhibits a similar ensemble-mean onset date with a larger spread, and more abrupt onset in late spring, and an earlier but more gradual withdrawal in early fall. The aerosols-induced circulation changes feature an upward motion over the northeastern Indian Peninsula and strengthened anticyclonic circulation over the Arabia Sea in the pre-monsoon season, and a northward shift of monsoon flow in the developed monsoon period along with strengthened local meridional circulation over northern India. The strengthened anticyclonic circulation over Arabia Sea caused a 16% increase in natural dust transport from the Middle East in the pre-monsoon season. The elevated aerosol heating over Tibet causes stronger ascending motion in the pre-monsoon period that leads to earlier and more abrupt ISM onset. The earlier monsoon withdrawal is attributed to the aerosol-induced anticyclonic flow within 10°–25°N and cyclonic flow within 0°–10°N over eastern India and Bay of Bengal that resemble the ISM seasonal transition in September.

Spatiotemporal Variabilities of the Recent Indian Summer Monsoon Activities in the Tibetan Plateau: A Reanalysis of Outgoing Longwave Radiation Datasets

Abstract Unstable hydrological cycles and water resource instability over and around the Tibetan Plateau (TP) are topics of wide concern. The Indian summer monsoon (ISM) is one of the TP’s most important moisture sources; as such, its behavior is key to any changes in precipitation and water-related environments. However, there have been relatively few thorough investigations into ISM activities. Here we primarily explore ISM activities using outgoing longwave radiation (OLR) datasets in TP, and precipitation isotopes recorded at Lhasa, for the period 1975–2020. Our major findings are that 1) the ISM onset (retreat date) is between ∼31 May and 19 July (∼8 August–27 September), with ISM duration of ∼40–110 days; 2) significant spatial inhomogeneous patterns are evident in ISM activities, i.e., the western part of our study area experiences earlier ISM onset, delayed retreat, longer duration, and greater intensity and strength, and the inverse is true in the eastern sector of the study area; 3) the ISM activities that dominate the 1975–98 period determine their general patterns over the entire 1975–2020 period, taking into account evident discrepancies in subperiods; and 4) the negative relations between precipitation δ18O and ISM intensity/strength at Lhasa confirm the ISM activities defined using OLR. These results will improve our understanding of hydrological cycles in TP and provide insights into hydrological studies in the “Asian Water Tower” region. Significance Statement Over the recent decades, the Tibetan Plateau (TP) (Asian Water Tower) has undergone dramatic environmental changes, evinced by the instabilities of hydrological cycles. As one of TP’s most important moisture sources, the Indian summer monsoon (ISM) is key to changes in precipitation and water-related environments. To get thorough investigations into ISM activities, we primarily explore ISM activities using outgoing longwave radiation (OLR) datasets in TP and precipitation isotopes at Lhasa. Significant spatial inhomogeneous patterns are evident in ISM activities: the western part experiences earlier ISM onset, delayed retreat, longer duration, and stronger intensity and strength, and the inverse occurs in the eastern sector. These results will improve our understanding of hydrology, meteorology, ecology, and paleoclimate reconstructions in the Asian Water Tower region.

Effect of spring Bering Sea ice on the Indian summer monsoon onset process

Effect of spring Bering Sea ice on the Indian summer monsoon onset process

Triggering effect of an unusual northwestward-moving tropical cyclone over the Bay of Bengal on the extremely early Indian summer monsoon onset

Triggering effect of an unusual northwestward-moving tropical cyclone over the Bay of Bengal on the extremely early Indian summer monsoon onset

Projection of the Indian Summer Monsoon onset using a regionally coupled atmosphere–ocean model

Projection of the Indian Summer Monsoon onset using a regionally coupled atmosphere–ocean model

Revisiting the different responses of the following Indian summer monsoon rainfall to the diversity of El Niño events

There is evidence that the interannual relationship between El Niño events and the following Indian summer monsoon rainfall (ISMR) has weakened with the more frequent occurrence of central Pacific (CP) El Niño events. We revisited the following ISMR responses to the two different types of El Niño events using observations and reanalysis datasets. Our results show that the ISMR anomalies associated with eastern Pacific (EP) and CP El Niño events are different, with decreased (increased) rainfall in early summer (June–July) following EP (CP) El Niño events. This is primarily attributed to the different responses to anomalous warming of the sea surface temperature (SST) in the northern Indian Ocean (NIO), which is characterized by double peaks in the warming SST during EP El Niño events, but only one peak during CP El Niño events. For EP El Niño events, the second SST warming peak in early summer contributes to the lower level antisymmetric wind pattern over the tropical Indian Ocean (TIO), which delays the onset of the Indian summer monsoon (ISM) and decreases the supply of moisture to India, implying a decrease in the ISMR. By contrast, for CP El Niño events, the cooling SST over the western TIO directly induces a significantly positive meridional SST gradient and drives the lower level southwesterly wind anomalies, resulting in an eastward shift in the decreased antisymmetric winds over TIO and the early onset of ISM. These circulation features are associated with anomalous upper-level divergence over TIO and sinking over India, jointly leading to the excess ISMR in early summer. These results suggest that, in addition to the key role of the warming of the NIO SST, cooling of the SST over the western TIO during CP El Niño events should be considered carefully in understanding the El Niño–ISMR relationship.

Atmospheric turbulence characteristics in the troposphere and lower stratosphere of core monsoon zone

The geographical location of Central India (CI) is often under the influence of a variety of weather phenomena, viz., the Indian Summer Monsoon (ISM) and its associated low-pressure systems, heat waves, cold waves, and western disturbances. The present study investigates the Vertical Structure of atmospheric Turbulence (VST) in terms of the refractive index structure parameter (Cn2), in the troposphere and lower stratosphere over the core monsoon zone in the CI. Ten years (2011−2020) of GPS-radiosonde (GPS-RS) measurements and reanalysis datasets are employed to bring out the turbulence characteristics over the study region. The strong turbulence values below 12 km during the monsoon season are due to the frequent convective instabilities and higher vertical velocities, whereas contrasting VST is noted during non-monsoon seasons with significant turbulence intensities reaching up to the upper levels. The frequent convective instabilities and updrafts make the monsoon season's VST more homogeneous isotropic turbulence in character. Observed seasonal variability in VST is attributed mainly to moisture availability, thermal structure, presence of tri-model clouds, and a flip-flop pattern of mid-tropospheric wind circulation conditions. The early signature of the ISM onset with the higher cold cloud fraction is responsible for the observed weak turbulence zone centered at 14 km altitude. This striking weak Cn2 < 10−17 m−2/3 over the monsoon core zone VST is one of the most intriguing features observed in the present study. The signatures of various weather conditions such as heat waves, depressions, and cold waves are reflected in the observed variations of the VST during summer, monsoon, and winter, respectively. The variations in the pre/post VST and during specific weather events are attributed to the remarkable difference in relative humidity and temperature conditions. The present results provide a quantitative assessment of VST, including its seasonal evolution over Bhopal, using GPS-RS observations for the first time. The impact of better temporal/height sampling of GPS-RS data is evidenced by implementing the Thorpe method to estimate the outer scale of turbulence. The present analysis will also assist in designing experimental specifications for the proposed wind profiler in CI's Atmospheric Research Testbed for all-weather scale applications.

Influences of North Pacific anomalies on Indian summer monsoon onset

AbstractThis study finds a significantly decreasing (increasing) trend (1979–2018) of sea level pressure (SLP) over coastal East Asia (far North Pacific) during May and June, which resembles the negative (positive) Pacific Decadal Oscillation (PDO) (North Pacific Oscillation, NPO)‐like pattern. Thus, we examine May composite differences between early and late Indian summer monsoon (ISM) onset years. Sea surface temperature (SST) differences show that early (late) ISM onset is accompanied by a negative (positive) PDO‐like state. Whereas, in SLP, an intense low pressure over northwestern India, the Arabian Sea to Southeast Asia (far North Pacific, East Asia) is associated with the early (late) onset. During the early onset, warm surface air temperature (SAT) anomalies over southeast Russia propagate towards Central Asia to the Middle East and the northwest Indian subcontinent, strengthening the land‐sea thermal contrast, which subsequently strengthens the monsoon low. We thus show how the positive NPO, negative PDO, and high spring Bering Sea ice link with the SAT anomalies. In the early onset, more wave packets generated in response to these North Pacific anomalies propagate towards the Atlantic and European region, reaching and converging over the northwest Indian subcontinent. The warm surface anomalies possibly draw eastward‐propagating waves towards northwest India, thus intensifying the monsoon low there. Meanwhile, in the CAM5 PDO simulation, we observe a similar pattern of atmospheric responses, where warm SAT anomalies associated with stationary Rossby wave trains propagate from the North Pacific towards the North Atlantic to Central Asia and the northwest Indian subcontinent, strengthening the monsoon low. Terefore, our study highlights the crucial role of the North Pacific anomalies in modulating the ISM onset processes via atmospheric pathways.

Seasonal prediction of Indian summer monsoon onset with echo state networks

Although the prediction of the Indian Summer Monsoon (ISM) onset is of crucial importance for water-resource management and agricultural planning on the Indian sub-continent, the long-term predictability—especially at seasonal time scales—is little explored and remains challenging. We propose a method based on artificial neural networks that provides skilful long-term forecasts (beyond 3 months) of the ISM onset, although only trained on short and noisy data. It is shown that the meridional tropospheric temperature gradient in the boreal winter season already contains the signals needed for predicting the ISM onset in the subsequent summer season. Our study demonstrates that machine-learning-based approaches can be simultaneously helpful for both data-driven prediction and enhancing the process understanding of climate phenomena.

Impact of Aeolus horizontal line of sight wind observations in a global NWP system

This study evaluates the impact of Aeolus horizontal line-of-sight (HLOS) wind observations in the National Centre for Medium Range Weather Forecasting (NCMRWF) Unified Model (NCUM) assimilation and forecast system from 20 May to 22 June 2020. The quality of both Rayleigh clear and Mie cloudy Aeolus HLOS winds are validated during the study period and found to be useful for assimilation. Observing System Experiment (OSE) and adjoint-based Forecast Sensitivity to Observation Impact (FSOI) methods are used to assess the impact of the Aeolus HLOS wind observations in the global NCUM system. The Indian Summer Monsoon onset and the life cycle of a severe cyclonic storm Nisarga formed over the eastern Arabian Sea are investigated. The FSOI study clearly shows that HLOS wind observations significantly impact the model forecast, especially over the Tropics and Southern Hemisphere extra-tropics. The impact of HLOS winds in the upper troposphere and above is more marked. Forecasts with the assimilation of the HLOS winds enhanced the circulation during the onset of the Indian summer monsoon and weakened the Monsoon Low Level Jet (MLLJ) when the Nisarga cyclone formed over the Arabian Sea. The high-resolution wind observations from Aeolus reduced the position error of the tropical cyclone Nisarga in the NCUM analysis.

Prediction of Indian summer monsoon onset with high resolution model: a case study

India is an agro-based country, receives nearly 80% of the annual rainfall during the summer monsoon season, and has a vital socioeconomic security link. The start of the Indian rainy season is observed over the country's southern tip (Kerala) and is referred to as the monsoon onset. The Indian Summer Monsoon (ISM) onset is one of the key aspects and unique for each year. This paper aims the prediction of ISM onset for different years by using the operational numerical weather forecasts at medium-range time scales by using different atmospheric variables from UKMO, and NCMRWF deterministic (NCUM) and ensemble model (NEPS) weather forecasts. For predicting the 2018 onset, we have adopted objective methods like the rainfall criteria, daily circulation index, potential temperature, moisture transport etc. The results for an early onset year (2018) show that the models could predict the onset date agreeing with the observed India Meteorological Department (IMD) onset date of 29 May 2018 from both UKMO 5-day and NCUM 10-day forecasts. This study also emphasizes on the utilization of the medium-range numerical weather forecasts, providing enough time to plan the farming activities. We also present the models’ performance/ skill for assessing the onset dates for the delayed onset (2019) and timely onset (2020) monsoons with NCUM 10-days lead time, and the results agree with IMD dates. From the UKMO, NCUM, NEPS models’ rainfall forecast show that the onset date on 29 May 2018 as IMD. Similarly, the analysis from circulation index, potential temperature, vertically integrated moisture transport analysis, outgoing long wave radiation, tropospheric temperature index clearly shows the onset date agreeing with IMD observations. Similarly, for the late onset (2019) and timely onset (2020) years, the daily circulation index agrees with IMD observed onset dates. Finally, these results from the deterministic and probabilistic forecasts are consistent with the observed onset dates, and these medium range forecasts are highly helpful to compute the monsoon onset date with higher lead times (10 days) within the error of one day.

Correction to: The modulation of Indian summer monsoon onset processes during ENSO through equatorward migration of the subtropical jet stream

In this study, we demonstrate a new mechanism, on how the warm phase of El Nino and Southern Oscillation (ENSO) delays the Indian Summer Monsoon onset through coupled ocean-atmospheric processes. Strong basin-wide warming is prominent over North Indian Ocean (NIO) during the El Nino years. The warming is intense over the South West Indian Ocean (SWIO) due to the westward propagation of the equatorial Rossby waves from the Pacific Ocean. It increases the convective activities over the southern tropical Indian Ocean (0–10° S), adjacent to the SWIO region. The warming over the SWIO and the NIO strengthens the divergent wind from the Indian Ocean to the sub-tropics via a wind-evaporation-SST feedback mechanism, which causes more upper-level convergence within 30° N latitudes. Besides, a warmer Indian Ocean enhances the upper-level diabatic heating over the southern Arabian Peninsula and Eastern Indian landmass. These factors strengthen but shift the local Hadley circulation over the Indian Ocean more southward, with an ascending branch centered over the SWIO region. The shifting of the local Hadley circulation during the El Nino years causes the Sub Tropical Jet (STJ) to migrate more southward and centered more over the Indian subcontinent. This southward movement of the STJ over the Indian subcontinent in response to the El Nino condition inhibits the establishment and propagation of the tropical easterly jet during the monsoon season, which subsequently hinders the monsoon circulation, thus delays its onset.

The modulation of Indian summer monsoon onset processes during ENSO through equatorward migration of the subtropical jet stream

The modulation of Indian summer monsoon onset processes during ENSO through equatorward migration of the subtropical jet stream