The Indian Summer Monsoon (ISM) is a large-scale circulation pattern that influences worldwide weather and climate. The interannual variation of the ISM rainfall (ISMR) has a direct effect on millions of people living in the Indian subcontinent. The teleconnections of this variation have changed significantly since the major climatic shift in the late 1970s. The second dominant mode of the interannual variability for the recent four and a half decades after the late-seventies of ISMR shows an intriguing north–south dipole pattern, with in-phase loading centred over east India (including the central and eastern Gangetic Plains) and out-of-phase loading centred over south-east peninsular India. This variability arises from the complex interactions among the anomalous cooling of the sea surface temperature over the Indo-Pacific warm-pool (IPWP) and the gradient of surface air temperature between Iran and the Arabian landmass. The gradient of heating of the (Iran minus Arab) surface air temperature is compatible with an area of dipole surface pressure with low-pressure at Iran and high-pressure at Arab. This lower-level dipole pressure is accompanied by the northwesterlies over Saudi Arabia, converging and pulling the climatological background cross-equatorial monsoonal flow northward, further downstream curving cyclonically towards eastern India, supplying moisture towards the Gangetic Plains, producing flooding. Moreover, the cooling of the warm-pool region corresponds to an anomalous widespread surface high-pressure, also pushing the cross-equatorial monsoonal flow northward. Contrarily, as the monsoonal flow is shifted northwards in either of the cases, it is supplying less moisture towards the southern peninsular India, resulting in depressed rainfall. Thus, the surface temperatures of the Middle-East and IPWP are playing a decisive role in setting up the dipole rainfall pattern between the east and south peninsular India. This teleconnection could further be examined in the climate forecast models.

High-altitude treelines in western Himalaya are highly sensitive to climate variability and atmospheric circulations, which influences regional temperature and moisture. We measured leaf-level ecophysiological responses-gas exchange, photosynthetic pigments (chla, chlb), chlorophyll fluorescence, photosynthetic rate (Pn), water use efficiency (WUE), vapor pressure deficit (VPD) between the co-occurring seedlings and trees in Rhododendron campanulatum D. Don and Quercus semecarpifolia Sm. growing at 3200–3450 m a.s.l. (treeline). Also, ERA5 climate datasets were analysed to understand how these tree species adjust to physiological stresses at treeline. Quercus semecarpifolia showed higher chla, chlb and chla+b content, specific leaf area (SLA), WUEi, Amass and Fv/Fm. Also, Fv/Fm values remained close to optimal threshold ( 0.80), indicating stable photosystem-II functioning, whereas lower values in R. campanulatum indicate greater susceptibility to photo-inhibition at treeline. Rhododendron campanulatum showed significantly higher VPD than Q. semecarpifolia, reflecting greater atmospheric dryness and evaporative demands. Principal component analysis (PCA) explained 37.2

This study investigates the relationship between the Azores High and Indian summer monsoon during June and September. An opposite correlation pattern is observed during June and September, with significant positive correlation over the Gangetic Plain and north Peninsular India during June, and significant negative correlation over Central India during September. A diagnosis based on monthly ERA5 reanalyzed circulation products archived on finer grids reveals that the sustenance of positive rainfall anomalies over the Gangetic Plain and north Peninsular India during June is supported by the strengthened Azores High near its mean position, zonal extension and northward shift of the Tibetan Anticyclone from its mean position, and strengthening of the Asian jet over the Eurasian region, whereas the sustenance of negative rainfall anomalies over Central India during September is supported by the Azores High, which is shifted to northeast of the North Atlantic, that is, away from its mean position, mid-latitude waves propagating at higher latitude, hence not affecting Indian summer monsoon rainfall due to their absence towards north India, weakened Tibetan Anticyclone, and weakening of the Asian jet over the Eurasian region.

. Indian rainfall patterns and their teleconnections have exhibited significant changes following the major climatic shift observed in the late 1970s. The trend analyses of the Indian summer (June through September) monsoon rainfall have followed a statistically significant increasing/decreasing trend in western/eastern India after the late seventies. The increase in surface temperature over northeast Europe is a manifestation of Arctic amplification, warm temperature advection from the North Atlantic, Aincreased solar insolation, Aand drying of the region, which has led to increased subsidence and tropospheric pressure. The mid-tropospheric subsidence and surface warming over northeast Europe have made it an active centre-of-action for the emulation/propagation of the Rossby wave towards the Eurasian region, having a trough east over the Caspian Sea, followed by massive ridges over east Asia. The penetration of this trough towards the Indian landmass favours deep convection. The recent decades of warming of the tropical Indian Ocean have produced low pressure over the tropical western Indian Ocean and Somalia, which increases the cross-equatorial flow towards the Arabian Sea and decreases towards northern India. This increases moisture flow/convergence towards western India and decreases moisture towards northern India. The interaction of the moisture-embedded cross-equatorial flow with the upper-tropospheric deeply penetrated trough through the Indian landmass breaks out to heavy rainfall over western India, which causes a shift of the monsoon westward.

India gets maximum rain during the summer months of June through September, called Indian summer monsoon rainfall (ISMR). The increasing/decreasing trend of ISMR affects millions of agriculture-dependent people in India. The spatial deviation of long-term ISMR trends indicates the monsoon circulation shift and should be minutely observed. The current study has observed that the ISMR has statistically significantly increased/decreased in western/eastern India in the last four decades. The significance of these trends is checked by evaluating the incomplete beta function used as a p -Value calculator for the Student t -test. In the meantime, the Indian Ocean has become more active due to its recent warming. The warming of the equatorial Indian Ocean has increased the in-situ convection south of the equatorial Indian Ocean, whose subsidising Hadley’s branches has increased the upper-tropospheric geopotential height (GPH) in northern Europe and southern Africa. The north Europe GPH is associated with the Eurasian wave having massive ridges over eastern Europe and east Asia sandwiched with a trough east of the Caspian Sea. The penetration of this trough towards the Indian landmass has created favourable conditions for deep convection. Besides, the southern African GPH increases the cross-equatorial flow, which interacts with the mid-tropospheric trough over the Arabian Sea, increasing the moisture flow/convergence over western India. The interaction of the cross-equatorial flow with the upper-tropospheric penetrated trough through the Indian landmass increases the moist static energy, which results in heavy rainfall over west India and causes a shift of monsoon westward. This teleconnection could further be examined in the seasonal forecast models.

The variability of Indian summer monsoon rainfall (ISMR) has a socioeconomic impact on India. The profound relationship between ISMR and El Nino southern oscillation (ENSO) is getting weaker, due to which the impact of other climate modes has increased. Mid-latitude interaction with the monsoonal flow has increased in recent decades. Azores high, a high-pressure cell over the north Atlantic, modulates the mid-latitude wave pattern over the Eurasian region, consequently affecting Asian jet and Tibetan High. Accordingly, the repositioning of Tibetan High has shifted the ISMR band westward, causing above-normal rainfall in west and central India and below-normal rainfall in east and northeast India. The ISMR has significantly decreased over the Gangetic Plain, adversely affecting this region. This case study for the year 2022 summer monsoon has reflected one of the pieces of evidence of subdued rainfall over Gangetic Plain of India. The situation is unique because normal to above-normal rainfall was observed over the rest of the country. After analyzing various parameters, it is observed that the surface pressure anomaly over north India is against climatology, suggesting a rise in surface pressure and hence, weakening of the monsoon trough over the Gangetic Plain. This weak monsoon trough over the Gangetic Plain has reduced the monsoonal flow towards this region. Also, the strengthened Azore’s High impact through midlatitude waves reinforced the large deficit of ISMR over the Gangetic Plain during 2022.

The Indo-Pacific warm pool (IPWP), enclosed by a 28 degrees C isotherm, is vital in controlling atmospheric circulations affecting monsoonal flow. The warming trend of sea surface temperatures (SSTs) over the IPWP has expanded the IPWP region. This study examines the impact of the IPWP warming on the Indian summer monsoon rainfall (ISMR) patterns using ERA5 reanalysis and India Meteorological Department rainfall records based on station data from 1959 to 2021. Analyses based on correlation, regression and composite anomalies show the complex relationship between recent decades of IPWP expansion/warming and monsoon circulation. However, the effects of regional IPWP SST warming changes on the ISMR pattern remain unexplored. Here, we explore the changes in the monsoonal circulation owing to the warming and expansion of IPWP, by comparing two equal periods (1959-1989 and 1990-2021). The responses of monsoons to IPWP warming in these two periods revealed some interesting facts, but the complexity remained. Further, we examined the composite impacts of IPWP SST warming in three categories, that is, very cool, usual and extremely warm, on the dynamics of monsoon circulations. The very cool IPWP is associated with the dry monsoon, while the extremely warm IPWP produces copious rainfall over southern India and dryness over eastern north India. The study confirms the non-linear relationship between IPWP warming and ISMR, which has been investigated in detail.

The interannual variation of the Indian summer monsoon (ISM) affects millions of people in India and the global weather and climate. The teleconnections that affect this variation are not stable. The recent four decades of the second dominant mode of ISM rainfall show a unique north–south tripole pattern, with above‐normal rainfall in the north and peninsular India sandwiching suppressed rainfall in central‐east India. The pattern relates to extending the Indo‐Pacific warm‐pool's warmer sea‐surface temperature (SST) towards the south of the equatorial eastern Indian Ocean. Most of the time, this warming and the extension of the warm‐pool's warmer SST are associated with La Niña events, which activate more in situ vigorous convection. The Rossby‐gyres generated west of the equatorial heating increase the tropospheric height over north India, shifting and strengthening the Tibetan High northwards, facilitating heavy rainfall in the north. Meanwhile, the more vigorous convection south of the equatorial eastern Indian Ocean produces compensatory subsidence over central‐east India, suppressing rainfall. The northern hemispheric Rossby‐gyres brings anomalous cyclonic circulation over peninsular India, producing excess rainfall. Also, the dipole pressure anomaly between the northwest Pacific and south tropical Indian Ocean generates anomalous lower‐level easterly winds over the Bay of Bengal. It supplies excess moisture to the north India convections. The co‐occurrence of the active Atlantic intertropical convergence zone supports this tripole rainfall pattern. This teleconnection could further be examined in climate models.

The Indian subcontinent, due to its enormous variety of geographical features, is associated with inhomogeneity. Hence, in the present study, we have classified the whole Indian subcontinent into four homogeneous regions, such as North India, Central India, South India, and Northeast India, based on the similarity in rainfall characteristics and associations with the regional/global circulation parameters. These four major homogeneous regions have quite different climatological distribution rainfall as well as the degree of variability, depicting the need of individual exploration of them. The rainfall indices are prepared for an extended period of 120 years from 1901 to 2020 to understand the interannual variability of it over these homogeneous regions. The changes in the temporal trend with time are explored by diving the whole time period into four periods of 30 years each, i.e., 1901–1930 (Period 1), 1931–1960 (Period 2), 1961–1990 (Period 3), and 1991–2020 (Period 4). During period 4, a declining trend is seen over North India while other three regions show an increasing trend. Also, the wavelet analysis is carried out to see the periodicity of rainfall pattern and found that the interannual variability is more during the postindustrial period due to the lack of any dominant periodicity. And, finally, we investigated the teleconnection patterns during these four defined periods for all the homogeneous regions with global sea surface temperature where we get the ENSO monsoon relation is weaker during the recent period for all the regions except north India, explaining the reason behind the drying trend over there.

In the current climate, a better understanding of the factors that cause variability in Indian summer monsoon rainfall over northeast India is critical because it is susceptible to climate change. The variability and remote teleconnections keep altering owing to this change in climate reign. Therefore, the variability and teleconnections should be studied with recent datasets. In the present study, the northeast India summer monsoon rainfall is elucidated, showing the vital role of Atlantic Nino during the recent four decades. The Atlantic Nino intensifies the intertropical convergence zone (ITCZ) across Atlantic and West Africa. Consequently creates a widespread upper-tropospheric divergence, inciting successive positive-negative geopotential height anomalies. These changes in upper-tropospheric geopotential height and mid-latitude wave originating from North Atlantic following the path passing over northeast India, creating a similar pattern as Pacific-North America Pattern manifests the linkage between Atlantic Nino and northeast India summer monsoon rainfall by increasing the upper-tropospheric positive geopotential height anomaly over there. A Pacific-Japan type pattern is also associated with the positive phase of Atlantic Nino, which restricts the cross-equatorial flow from transporting moisture towards the South China Sea and enhancing the moisture transport towards northeast India. And this triggers the convection process, thereby strengthening the monsoonal rainfall over there. These results are also confirmed by using the CFSv2 model sensitivity experiment and a case study of the year 2017.

A dominant mode of interannual variability of Indian summer monsoon rainfall shows west-east dipole pattern with above normal rainfall towards west and central India and subdued rainfall towards the east and northeast India, and is related to the vigorous Azores High. The vigorous Azores High is accompanied by enhanced subsidence resulting in well-built widespread upper-troposphere convergence. This forms the meridional vorticity dipole consisting of anomalous cyclonic and anti-cyclonic circulation at 30°N and 50°N, respectively, and boosts the Rossby wave source. The cascading down Rossby wave train imposes successive negative, positive and negative Geopotential height (GPH) anomalies over north Mediterranean, northwest and northeast of India, respectively. The negative GPH anomaly at the north Mediterranean increases the Asian jet towards the Caspian Sea, strengthening the monsoon circulation through the ‘silk-road’ pattern. While, the dipole GPH anomaly north of India shift the Tibetan High westwards, triggering monsoon activity towards the west.

The Indian rainfall and the Atlantic Niño have similar seasonality of summer time and have a positive relationship, especially with the North-Eastern India rainfall. The positive phase of the Atlantic Niño intensifies the Atlantic intertropical convergence zone, resulting in intense local convection and tropospheric warming, generating stronger widespread upper-troposphere divergence. Consequently, pushing the upper-troposphere geopotential height (GPH) poleward provokes meridional stationary wave. The zonally extended Tibetan High over Northern Africa is pushed northward. Thus, creating the consecutive anomalous negative, positive, and negative GPHs over the tropical east Atlantic, Mediterranean, and Northwest Europe, respectively. The negative GHP anomaly of North-West Europe imposes a positive GPH anomaly downstream at Central Asia by the Rossby wave dispersion. The positive GPH anomaly at Central Asia shifts the Tibetan High maximum GPH north-westward, consequently shifting the intense rainfall over the North-Eastern India.

The summer monsoon season contributes about 80% of annual rainfall in the highly populated region of north India. The teleconnections moderating the variation of summer monsoon rainfall in this region are not satisfactorily understood. A pathway of the equatorial central Pacific sea surface temperature (PSST) influences the north India summer rainfall is revealed from the high‐resolution, more reliable, and state‐of‐the‐art 41‐year (1979–2019) observational data analysis and numerical experiment. The rise in PSST intensifies convection over the equatorial central Pacific with compensatory subsidence over the western equatorial Pacific. Consequently, a much broader and intense anomalous convergence is developed at the upper‐troposphere, which subsequently intensifies the Asian subtropical westerly jet‐stream. The Asian jet intensification strengthens the tropospheric wind shear at north India, as the low‐level monsoonal winds are easterlies over there. The strong wind shear restricts the convective activities in north India. These results are also confirmed using the CFSv2 model sensitivity experiment and a case study of the year 2010.

Tree-ring width index chronologies of three different species (Ceckus deodara, Abies pindrow and Picea smithiana) from the western Himalaya have been carried out in relation to understand the climate fluctuations. The first principal component developed based on the above species showed significant positive relationship with Standardized Precipitation Index (SPI) and negative with Heat Index (III) during pre-monsoon month, but the highest correlation coefficients of tree growth with heat and SPI were observed during May, are -0.52 and 0.60 respectively, which are highly significant at 0.1% level. The results indicate that increasing heat index during May might lead to dry soil by accelerating the potential evapotranspiration which creates the moisture deficiency over the region. The significant positive relationship of tree growth with SPI indicates that tree growth of the western Himalaya is moisture sensitive, which enable us to extend SPI to monitor dry and wet periods before the instrumental period. The reconstructed SPI back to 1778 shows climate variability, on both inter-annual and inter-decadal time scale. The reconstruction showed increasing wetness in the twentieth century and recent few decades. Extreme and severe dry is noticed during 1921 (-2.4) and 1892 (-2.0) in the entire reconstruction has been associated with very low tree growth during the corresponding year over the western Himalaya.

A regional tree ring-width index chronology prepared from various tree core samples of the western Himalaya has been analyzed in relation to climate fluctuations. The correlation analysis of tree ring chronology shows significant positive correlations with regional rainfall and standardized precipitation evapotranspiration index (SPEI) and negative correlations with temperature and vapor pressure (VP) during the spring season. The correlation coefficients (CCs) of tree ring-width index chronology with rainfall, temperature, SPEI, and VP during 1901–1990 are 0.50, −0.49, 0.65, and −0.51, respectively. All CCs are significant at 0.1% level. The highly significant CCs between tree ring-width index chronology and SPEI indicate that tree growth over the western Himalaya is more sensitive to soil moisture availability than rainfall, whereas the rising VP is found to have a significant moisture stress condition to tree growth by accelerating the evapotranspiration, which is not conducive for the development of tree growth in the region. So, based on the strong association between tree ring-width index chronology and SPEI; the reconstructions of SPEI and VP are developed back to AD 1861, that show the long period of dryness during 1936–1963.

The highly populated north central India receives 90% of annual rainfall during June to September. The interannual variation of summer monsoon rainfall is less studied compared to central and western India, due to its weak signal with the El-Niño-Southern Oscillation (ENSO). Previous studies have reported a marked decadal variation in the ENSO influences on north India rainfall, but the teleconnections of this variation are not satisfactorily understood. A pathway of the changing ENSO influences on north central India rainfall is revealed from observational data analysis and numerical experiments. While La Niña-like conditions produce anomalous northeasterly wind over India and reduce the tropospheric wind shear, the emergence of the Atlantic Niño appears to overtake this ENSO influence. The Atlantic Niño intensifies the meridional stationary wave affecting pressure anomaly over northwest Europe. This excites the Eurasian Rossby wave train along the mid-latitude producing upper-troposphere high pressure anomaly, subsequently affecting north India. Future work should examine the extent to which these teleconnections are represented in climate forecast models to aid the seasonal prediction of north central India rainfall.

Tree-ring chronologies from different sites of western Himalaya have been used in this study to examine the climate variability/change over the region. The 1st principal component (PC1) which has been computed by multi species tree ring chronologies of western Himalaya is negatively correlated with heat index (HT), temperature (TM) and potential evapotranspiration (PET) but positively with the rainfall of the region during spring season. However, HT and PET showed the stronger influence on tree growth than temperature which has been used to reconstruct the spring season PET back to A.D. 1779. The extended periods of low PET have been found at ending phase of Little Ice Age (LIA) during 1827–1845. The reconstructed PET showed that the advance of the glaciers over the western Himalaya might have influence on reduction of downward shortwave radiation on the earth's surface; which may in turn cause low temperature and low PET over the region. The result indicates that longer tree ring chronologies from the western Himalaya are very useful to get valuable information on PET and glacier fluctuation during the last few centuries. The highest PET in the entire reconstruction was noticed during the year 1921 in the 20th century, which has been associated with lowest trees growth in the entire western Himalaya.

The northwestern part of India occupies a vast landmass which roughly lies in the area bounded by 70.5 degrees E-80.5 degrees E longitudes and 27 degrees N-37 degrees N latitudes of South Asia. This is an important region of food-grain production in the country. The summer season (June to September) contributes about 75% of annual precipitation and the winter season from December to March 15-20%. These precipitations are very important for the crops and maintaining the western Himalayas Glaciers. The interannual variability of summer and winter precipitation are examined using observed and reanalysis datasets for the period of 1948-2015. The analysis shows changes in teleconnection pattern around the late-1970s, when the major 'climate shift' was observed in the Indo-Pacific Oceans. The summer precipitation teleconnection change is related to the change in the shape and position of the equatorial Pacific warming. And, the winter precipitation is mostly influenced by the two major weather phenomenon Arctic Oscillation/North Atlantic Oscillation (AO/NAO) and El-Nino-Southern Oscillation (ENSO), which exert strong control on the weather/climate of the Northern Hemisphere particularly in the boreal winter. The AO/NAO phenomenon were more influencing in the earlier decades, while the ENSO in the recent decades.

Tree ring chronologies from different sites of hill forest in the western Himalaya of India have been carried out in relation to natural climate variability/change. The first principal component (PC1) prepared by using a multiple site tree ring-width chronologies of the western Himalaya is strongly negatively correlated with vapor pressure and positively with wet-day frequency. The correlation coefficients of PC1 with boreal spring season vapor pressure and wet-day frequency are − 0.61 and 0.42, respectively, indicating significant level at 0.1%. The relationship indicated that wet-day frequency and vapor pressure of the western Himalaya have significant role in modulating tree growth patterns during boreal spring season (March to May). The results indicate that increasing vapor pressure during boreal spring season may cause high transpiration and evaporation, which results in moisture stress condition over the region and has adverse impact on trees growth.