Abstract. The Intertropical Convergence Zone (ITCZ) is a narrow band of intense convection formed by the convergence of tropical trade winds, leading to intense convection and precipitation. Its seasonal and interannual variations have significant socio-economic impacts, particularly over the Indian subcontinent, where the shifts in ITCZ position influence monsoon rainfall. Using long-term precipitation and atmospheric reanalysis data, this study investigates the interannual variability of boreal summer ITCZ latitudes, quantified using multivariate probabilistic approach, over the Indian region and its associated circulation changes. Extreme northward ITCZ shift years are associated with enhanced convection, strengthened monsoon westerlies, and intensified ascent leading to increased precipitation over the monsoon core zone along with poleward-displaced regional Hadley circulation, while extreme southward shift exhibits opposite features. Large-scale climate modes such as the El Niño–Southern Oscillation modulate tropical heating and circulation, influencing ITCZ position. Additionally, a significant long-term northward shift of the ITCZ during 1940–2022 is identified, which is closely linked to changes in moist static energy transport. A corresponding poleward shift of the meridional energy transport indicates that this migration is driven by changes in atmospheric energy balance.

The large spatial and temporal variability of wet and dry spells of the Indian Summer Monsoon (ISM) poses the great challenge in understanding and predicting monsoonal rainfall. This challenge is further exacerbated over smaller regions, such as the southern tip of India, which receives the first spell of ISM rainfall. In this study, the characteristic features and possible precursors for wet and dry spells of rainfall over the southern tip of India are investigated. We also explore the variability in monsoon low-level jet (LLJ) in relation to wet and dry spells over a coastal station Thiruvananthapuram (8.48(degrees)N, 76.95(degrees)E) in southwest India using in situ observations and other ancillary datasets. The results show that a wet spell spanning 3-4 days contributes about 30% of seasonal rainfall. Wet spells are characterized by westerly wind anomaly in the southern tip of India and easterly wind anomaly in northern India, leading to anomalous cyclonic vorticity over the Indian subcontinent. The opposite happens during dry spells. These characteristics are prominent from 2 days prior to the initiation of the spells, suggesting they may be used as precursors for forecasting wet and dry spells over Thiruvananthapuram. Analysis of low- to mid-tropospheric (2 and 4 km) humidity reveals significant moistening (drying) during wet (dry) spells. Yet, both wet and dry spells experience humid (>80%) boundary layer. The differences in mid-level humidity and thermodynamical structures between wet and dry spells seem to contribute to distinct rainfall characteristics over the southern tip of India. These results indicate that the use of in situ observations along with large-scale reanalysis datasets may provide valuable information on the precursors for wet and dry spells over the southern tip of India, which can help both in regional- and city-level planning and management of water resources.

The summer (June through September) monsoon 2020 has been very erratic with episodes of heavy and devastating rains, landslides and catastrophic winds over South Asia (India, Pakistan, Nepal, Bangladesh), East Asia (China, Korea, and Japan), and Southeast Asia (Singapore, Thailand, Vietnam, Laos, Cambodia, Philippines, Indonesia). The withdrawal of the summer monsoon over India was delayed by 2 weeks. The monsoon season over East Asia has been the longest. China recorded a Dam burst in the twentieth century. Furthermore, the Korean Peninsula has experienced back-to-back severe tropical cyclones. Could the lockdown activities initiate to control the COVID-19 spread a possible cause for these major episodes? The strict enforcement of the lockdown regulations has led to a considerable reduction of air pollutants-dust and aerosols throughout the world. A recent study based on satellites and merged products has documented a statistically significant mean reduction of about 20, 8, and 50% in nitrogen dioxide, Aerosol Optical Depth (AOD) and PM2.5 concentrations, respectively over the megacities across the globe. Our analysis reveals a considerable reduction of about 20% in AOD over South as well as over East Asia, more-over East Asia than over South Asia. The reduced aerosols have impacted the strength of the incoming solar radiation as evidenced by enhanced warming, more-over the land than the oceans. The differential warming over the land and the ocean has resulted in the amplification of the meridional ocean-land thermal contrast and strengthening of the monsoon flow. These intense features have supported the surplus transport of moisture from the oceans towards the main lands. Some similarity between the anomalous rainfall pattern and the anomalous AOD pattern is discernable. In particular, the enhancement of rainfall, the reduction in AOD and the surface temperature warming match very well over two regions one over West-Central India and the other over the Yangzte River Valley. Results further reveal that the heavy rains over the Yangzte River Valley could be associated with the preceding reduced aerosols, while the heavy rains over West-Central India could be associated with reduced aerosols and also due to the surface temperature warming.

The initiation of the Indian summer monsoon circulation during late May / early June arises through large-scale land-sea thermal contrast and setting up of negative pressure gradient between the Monsoon Trough over the Indo-Gangetic plains and the Mascarene High over the subtropical Indian Ocean. The meridional pressure gradient together with the Earth's rotation (Coriolis force) creates the summer monsoon cross-equatorial flow, while feedbacks between moisture-laden winds and latent heat release from precipitating systems maintain the monsoon circulation during the June-September (JJAS) rainy season (Krishnamurti and Surgi, 1987). This simplified view of the Indian monsoon is a useful starting point to draw insights into the variability of the large-scale monsoon circulation.

Assessments of impacts of climate change and future projections over the Indian region, have so far relied on a single regional climate model (RCM) - eg., the PRECIS RCM of the Hadley Centre, UK. While these assessments have provided inputs to various reports (e.g., INCCA 2010; NATCOMM2 2012), it is important to have an ensemble of climate projections drawn from multiple RCMs due to large uncertainties in regional-scale climate projections. Ensembles of multi-RCM projections driven under different perceivable socio-economic scenarios are required to capture the probable path of growth, and provide the behavior of future climate and impacts on various biophysical systems and economic sectors dependent on such systems. The Centre for Climate Change Research, Indian Institute of Tropical Meteorology (CCCR-IITM) has generated an ensemble of high resolution downscaled projections of regional climate and monsoon over South Asia until 2100 for the Intergovernmental Panel for Climate Change (IPCC)using a RCM (ICTP-RegCM4) at 50 km horizontal resolution, by driving the regional model with lateral and lower boundary conditions from multiple global atmosphere-ocean coupled models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). The future projections are based on three Representation Concentration Pathway (RCP) scenarios (viz., RCP2.6, RCP4.5, RCP8.5) of the IPCC.

Performance of twenty-eight state of art atmosphere general circulation models (AGCMs) in simulating regional characteristic features of summer monsoon rainfall and circulation over the south Asian and the western-north Pacific regions are examined. AGCMs depict good representation of climatological spatial distribution of monsoon rainfall over both the regions. Interestingly, the year-to-year variability of the south Asian (Indian) monsoon rainfall is well simulated by the models compared to the circulation features. Whereas, for the western-north Pacific region, the AGCMs depict poor representation of monsoon rainfall variability, but circulation features are simulated with better skill. Thus suggesting that, models with good performance of monsoon rainfall variability need not necessarily have better representation of monsoon circulation features. Further analysis indicate that most models with good skill in simulating monsoon variability have better representation of monsoon-SST teleconnections with the Indo-Pacific SSTs. For the south Asian region, better simulating models have good representation of monsoon-ENSO teleconnection whereas most models have poor representation of monsoon-IOD relationship, suggesting requirement for realistic representation of these teleconnections in the models. For the western-north Pacific region, most models have unrealistic relationship of monsoon rainfall with SSTs during preceding winter and concurrent monsoon season, indicating that the poor (better) skill in simulating rainfall (circulation) probably arise from the unrealistic (realistic) representation of monsoon-SST teleconnections. Thus, the importance of realistic representation of monsoon-SST teleconnections in the climate models have been brought out for better simulation of monsoon variability over the south Asian and the western-north Pacific regions.

During 2015, the southwest monsoon (SWM) rainfall over the country remained deficient with seasonal rainfall of about 86 1.1). Last year, the seasonal rainfall deficiency over the country as a whole was 12 years, with deficient monsoon, similar to 1904-05, 1965-66 and 1986-87 (www.imd.gov.in).

Precipitation is an important component of the global water cycle, and the impacts of anthropogenic climate change on precipitation have significant implications on agricultural activities (Porter et al. 2014).

Variability of Indian summer monsoon droughts is investigated by computing all-India drought indices namely Percent of Normal Precipitation, Standardized Precipitation Index and percentage area of India under moderate and severe drought conditions. Observations for recent decades, post 1960, exhibit declining trend in monsoon rainfall with frequent occurrence and intensification of droughts along with an increase in percentage of area under moderate and severe drought conditions, in association with variations in sea surface temperature (SST). Historical simulations from CMIP5 models suggest that two models, ACCESS1.0 and INMCM4, could well simulate monsoon rainfall variability, particularly the frequent occurrence of droughts and spatial variability of rainfall during drought years in recent historical period (1961–2005). Future projections of all-India drought indices from these two models indicate frequent droughts during near and mid future (2010–2069) with respect to the recent historical period. Intensification of severe droughts for near and mid future are suggested to be more pronounced over north-central India. The reduction in rainfall in the near and mid future is dynamically consistent with a westward shift in large-scale monsoon circulation, particularly the monsoon trough over South Asia. Interestingly, future projections of monsoon teleconnections indicate a weakening (strengthening) of in-phase (out-of-phase) relationship of all-India drought intensity with the equatorial eastern Pacific and the Indian Ocean (western Pacific) SST. Whereas, a strengthening of in-phase relationship between percentage of area under drought conditions and the equatorial eastern Pacific SST is projected for near and mid future with respect to the recent historical period. These drought features are consistent in both the models.

An examination of the contrasting behaviour of the Indian summer monsoon during peak monsoon months of July and August 2002 in relation to the South China Sea (SCS) SST variations on intra-seasonal time scale have been carried out in this study. Monsoon rainfall over India was a deficit during July whereas enhanced rainfall activity was evident during August. Interestingly, SST over the SCS also exhibited similar contrasting behaviour with less variation during July and strong biweekly intra-seasonal (BWI) oscillation with 10–20 day periodicity during August. During August, the BWI oscillation in the SCS SST could induce anomalous cyclonic circulation and enhancement in rainfall over the SCS, suggestive of a strong air–sea interaction process. Moreover, a strong relationship between the Indian monsoon and SCS SST is evident during August, at 3-day lead time of SST. The SCS SST variations could enhance westward propagation of moisture flux from the SCS to the Indian subcontinent and thereby influences the Indian monsoon. The SST variations and air–sea interaction processes over the SCS, also westward propagation of BWI moisture flux were weak during July. Our analysis suggests that SST variations over the SCS could modulate the monsoon circulation as well as the moisture flux from the SCS to the Indian subcontinent and thereby influence the Indian monsoon, particularly on biweekly time scale at least 3 days in advance. The study indicates a potential role of the SCS SST in foreshadowing the biweekly intra-seasonal oscillation during the Indian summer monsoon period.

We demonstrate that a large-scale longitudinally symmetric global phenomenon in the Southern Hemisphere sub-polar region can transmit its influence over a remote local region of the Northern Hemisphere traveling more than 100° of latitudes (from ~70°S to ~40°N). This is illustrated by examining the relationship between the Southern Annular Mode (SAM) and the Korean Monsoon Rainfall (KMR) based on the data period 1983-2013. Results reveal that the May-June SAM (MJSAM) has a significant in-phase relationship with the subsequent KMR. A positive MJSAM is favorable for the summer monsoon rainfall over the Korean peninsula. The impact is relayed through the central Pacific Ocean. When a negative phase of MJSAM occurs, it gives rise to an anomalous meridional circulation in a longitudinally locked air-sea coupled system over the central Pacific that propagates from sub-polar to equatorial latitudes and is associated with the central Pacific warming. The ascending motion over the central Pacific descends over the Korean peninsula during peak-boreal summer resulting in weakening of monsoon rainfall. The opposite features prevail during a positive phase of SAM. Thus, the extreme modes of MJSAM could possibly serve as a predictor for ensuing Korean summer monsoon rainfall.

Coupled Model Inter-comparison Project Phase 5 (CMIP5) model outputs of the South and East Asian summer monsoon variability and their tele-connections are investigated using historical simulations (1861-2005) and future projections under the RCP4.5 scenario (2006-2100). Detailed analyses are performed using nine models having better representation of the recent monsoon teleconnections for the interactive Asian monsoon sub-systems. However, these models underestimate rainfall mainly over South Asia and Korea-Japan sector, the regions of heavy rainfall, along with a bias in location of rainfall maxima. Indeed, the simulation biases, underestimations of monsoon variability and teleconnections suggest further improvements for better representation of Asian monsoon in the climate models. Interestingly, the performance of Australian Community Climate and Earth System Simulator version 1.0 (ACCESS1.0) in simulating the annual cycle, spatial pattern of rainfall and multi-decadal variations of summer monsoon rainfall over South and East Asia appears to more realistic. In spite of large spread among the CMIP5 models, historical simulations as well as future projections of summer monsoon rainfall indicate multi-decadal variability. These rainfall variations, displaying certain epochs of more rainfall over South Asia than over East Asia and vice versa, suggest an oscillatory behaviour. Teleconnections between South and East Asian monsoon rainfall also exhibit a multi-decadal variation with alternate epochs of strengthening and weakening relationship. Furthermore, large-scale circulation features such as South Asian monsoon trough and north Pacific subtropical high depict zonal oscillatory behaviour with east-west-east shifts. Periods with eastward or westward extension of the Mascarene High, intensification and expansion of the upper tropospheric South Asian High are also projected by the CMIP5 models.

The Indian summer (JJAS) shows high variability in both space and timescales. Changes in precipitation extremes play an important role on the regional scale due to their serious socio-economic consequences. This study, therefore, is mainly focused on understanding the variation of precipitation extremes during summer monsoon season in the presence of cyclonic disturbances forming over the Bay of Bengal (BOB), Arabian Sea, Land Area (LA) and Total. For this, several indices of observed precipitation extremes, in terms of frequencies, intensities and spell duration have been computed for the period 1951–2007 using daily APHRODITE data of 0.5° latitude × 0.5° longitude resolution. Correlation analysis reveals that a large part of the country exhibits positive relationship between the indices of precipitation extremes and frequency of cyclonic disturbances. Correlations with the indices of frequencies defined as seasonal count of days when rainfall exceeds 30, 20 and 10 mm show that spatial extent and strength of the positive relationship decreases with increase in threshold values. Disturbances forming over BOB play dominant role in precipitation during Indian summer monsoon.

The eastern Pacific Ocean received a record highest number of sub-tropical convective activities during boreal summer (June–September) of 2015, since last four decades. The associated rainfall distribution was also atypical with anomalously enhanced rainfall extending from equator to sub-tropical central-eastern Pacific. The present analysis reveals a pronounced meridional sea surface temperature (SST) gradient across central-eastern Pacific, with the mean SST exceeding 28 °C over sub-tropical north Pacific, setting up favorable conditions for these enhanced convective activities. It is found that these anomalous features promoted northward spanning of westerly anomalies and drastically modified the east–west circulation over sub-tropical north Pacific. This seems to induce large-scale subsidence over the off-equatorial monsoon regions of south and south-east Asia, thus constituting an east–west asymmetry over sub-tropical Indo-Pacific region. Based on our observational study, it can be concluded that the sub-tropical convective activities over east Pacific may play a pivotal role in mediating the Pacific-monsoon teleconnection through the unexplored meridional SST gradient across Pacific.

Recent trends, variations and tele-connections between the two large regional sub-systems over the Asian domain, the South Asian and the East Asian monsoons are explored using data for the 1901–2014 period. Based on trend analysis a dipole-type configuration with north-drought and south-flood over South as well as East Asia is observed. Two regions over South Asia, one exhibiting a significant decreasing trend in summer monsoon rainfall over northeast India and the other significant increasing trend over the northern parts of the west coast of India are identified. Similarly two regions over East Asia, one over South Korea-southern parts of Japan and the other over South China are also identified both indicating a significant increasing trend in the summer monsoon rainfall. These trends are examined post 1970s. Possible factors associated with the recent trends are explored. Analysis of sea surface temperature (SST), mean sea level pressure and winds at lower troposphere indicates that the entire monsoon flow system appears to have shifted westwards, with the monsoon trough over South Asia indicating a westward shift by about 2–3° longitudes and the North Pacific Subtropical High over East Asia seems to have shifted by about 5–7° longitudes. These shifts are consistent with the recent rainfall trends. Furthermore, while the West Indian Ocean SSTs appear to be related with the summer monsoon rainfall over northern parts of India and over North China, the West Pacific SSTs appear to be related with the rainfall over southern parts of India and over South Korea- southern Japan sector.

Relationship between the Southern Annular Mode (SAM) and the India summer monsoon rainfall (ISMR) has been examined based on the data period 1949–2013. While the entire data period indicates a significant increasing trend in SAM, recent decades 1983–2013 indicate no trend. The relationship between the two strengthened considerably since 1983. Results reveal that the February–March SAM is significantly related with the subsequent ISMR. A positive (negative) SAM during February–March is favorable (unfavorable) for the ensuing summer monsoon rainfall over the Indian sub-continent. The delayed response is relayed through the central Pacific Ocean. We propose a hypothesis that states: when a negative (positive) phase of February–March SAM occurs, it gives rise to an anomalous meridional circulation in a longitudinally locked air–sea coupled system over the central Pacific that persists up to the subsequent boreal summer and propagates from the sub-polar latitudes to the equatorial latitudes inducing a warming (cooling) effect over the central equatorial Pacific region. In turn, this effect concomitantly weakens (strengthens) the monsoon rainfall over the Indian sub-continent. Thus, the February–March SAM could possibly serve as a new precursor to foreshadow the subsequent behavior of the Indian summer monsoon.

The Indian summer monsoon rainfall had three-decade long alternate dry and wet epochs during the 150 years from 1840 to 1989. The dry epochs had frequent drought monsoons affecting agriculture, power generation and the overall economy of the country. A high percentage of severe cyclones in the Bay of Bengal moved northwards during the dry epochs causing disasters in Bangladesh, Myanmar and the Indian states of Odisha and West Bengal. These dry epochs have been shown to be associated with the cold phase of the Atlantic multidecadal oscillation in sea-surface temperature. Using the available tropospheric temperature (re-analysis) data since 1948, the recent dry epoch during 1960-89 which had 10 monsoon drought years was found to have cold upper tropospheric temperature anomaly over Central Asia. This cold anomaly region has also experienced a long-term cooling trend. Extrapolating the naturally occurring epochal nature of the ocean-atmosphere system into the future, we fear that the epoch 2020-49 is likely to be another dry one, and the cooling trend over the Asian continent is likely to make it even more severe in its impact than 1960-89. This article presents details of an ocean-atmosphere instability that generates frequent drought monsoons during dry epochs which needs urgent research.

The study diagnoses the relative impacts of the four known tropical Indo-Pacific drivers, namely, El Niño Southern Oscillation (ENSO), ENSO Modoki, Indian Ocean Dipole (IOD), and Indian Ocean Basin-wide mode (IOBM) on African seasonal rainfall variability. The canonical El Niño and El Niño Modoki are in general associated with anomalous reduction (enhancement) of rainfall in southern (northern) hemispheric regions during March-May season. However, both the El Niño flavours anomalously reduce the northern hemispheric rainfall during June-September. Interestingly, during boreal spring and summer, in many regions, the Indian Ocean drivers have influences opposite to those from tropical Pacific drivers. On the other hand, during the October-December season, the canonical El Niño and/or positive IOD are associated with an anomalous enhancement of rainfall in the Eastern Africa, while the El Niño Modoki events are associated with an opposite impact. In addition to the Walker circulation changes, the Indo-Pacific drivers influence the African rainfall through modulating jet streams. During boreal summer, the El Niño Modoki and canonical El Niño (positive IOD) tend to weaken (strengthen) the tropical easterly jet, and result in strengthening (weakening) and southward shift of African easterly jet. This anomalously reduces (enhances) rainfall in the tropical north, including Sahelian Africa.