South Asian Summer Monsoon Research Articles

Linkage between Southeast and South Asian Summer Monsoons: Relative Roles of the Pacific–Japan Pattern and El Niño–Southern Oscillation

Abstract The various regional summer monsoons over Asia have been known to be highly interactive, but their interrelationships are still under debate. The current study investigates the linkage between the South Asian summer monsoon (SASM) and the Southeast Asian summer monsoon (SEASM) and the involved physical mechanisms. A robust positive correlation is found between the first empirical orthogonal function (EOF) modes of SASM and SEASM precipitations. However, asymmetric features emerge between the mutually enhanced (ME) and mutually weakened (MW) SEASM–SASM cases. Apparent Pacific–Japan pattern occurs in the ME SEASM–SASM cases under a relatively weak sea surface temperature (SST) background, causing a southwest–northeast precipitation dipole with a positive center over the northern Bay of Bengal and a negative center over the eastern Arabian Sea and southern India induced by westerly anomalies with anticyclonic curvature associated with the anomalous heating over the tropical western North Pacific and cooling over the Maritime Continent. While in the MW SEASM–SASM cases, the El Niño–Southern Oscillation–related SST anomalies (i.e., a transition from the equatorial eastern Pacific warming in winter to cooling in summer) are substantial, which cause simultaneous weakening of the first modes of SEASM and SASM through anomalous western North Pacific anticyclone induced by the Pacific SST anomalies and the capacitor effects of other tropical oceans. In addition, intensified diabatic heating over the Maritime Continent driven by the anomalous zonal circulation related to the eastern Pacific cooling can enhance the precipitation over the eastern Arabian Sea and southern India via the westward-propagating Rossby wave. Significance Statement The South Asian summer monsoon (SASM) and the Southeast Asian summer monsoon (SEASM) can be independent from each other at one time but interactive with each other at another time. However, many features of their relationship remain unclear. This study is aimed at better understanding their linkage and the involved physical mechanisms. A robust positive correlation exists between the first modes of SASM and SEASM precipitations, but asymmetric features emerge between the mutually enhanced (ME) and mutually weakened (MW) SEASM–SASM cases. The Pacific–Japan pattern and El Niño–Southern Oscillation play a relatively important role in the ME and MW situations, respectively. A better understanding of the SEASM–SASM linkage can provide useful theoretical basis for improving seasonal prediction of Asian summer climate.

Possible Mechanisms behind Dominant Modes of Seasonal Cycle of Rainfall over India

Abstract The South Asian summer monsoon that occurs over 4 months from near summer solstice (June) to near autumnal equinox (September) evolves within the season due to interactions with other climate components. Since most climate patterns are seasonally phase-locked, their interaction and feedback with monsoon should also be season-dependent. Here, using rain gauge–based precipitation over India for 74 years, we show that the onset phase of the monsoon (June) is uncorrelated with the rest of the season (July–September). The propagation of the Madden–Julian oscillation (MJO) over the Indian Ocean triggers northward propagation of precipitable water that plays a key role during the onset phase. The MJO appears over the western equatorial Indian Ocean during both the early and late central Indian onset years. Normal and slow MJO phase propagation during early and late onset years, respectively, gives rise to interannual variations in June rainfall. After onset, till the end of the monsoon season, the interannual variability of the dominant seasonal mode of precipitation is regulated by the phase of El Niño–Southern Oscillation (ENSO) through anomalous moisture convergence. We show that, in June, ENSO does not play a key role because of the opposing impact on wind and moisture. However, from July to September, ENSO perturbs wind and moisture in a way that gives rise to positive feedback between monsoon, ENSO, and large-scale circulation. Such feedback of monsoon with ENSO results in a coherent impact of ENSO on monsoon precipitation in July–September. Our results suggest that the dominant mechanisms for interannual variations in Indian monsoon rainfall in June and the rest of the season are different. Significance Statement The variations in rainfall within the monsoon season in India are extremely important for agriculture and water resource management. For such purposes, prediction of the variations in rainfall within the season is more important than the prediction of the seasonal mean. However, very little study has been done to understand the generalized physical processes that govern the monthly variations in rainfall every year without assuming a particular intraseasonal feature. Here, we try to find the existence of the physical modes that are responsible for monthly variations in rainfall every year. We show that the dominant mechanisms behind the interannual variabilities of June and post-June rainfall are different. While June rainfall is governed by equatorial waves, post-June rainfall is a function of feedback between monsoon and ENSO. If the climate models capture these physical modes accurately, the prediction of the seasonal cycle of rainfall will improve substantially.

Excessive equatorial light rain causes modeling dry bias of Indian summer monsoon rainfall

Simulating accurately the South Asian summer monsoon is crucial for food security of several South Asian countries yet challenging for global climate models (GCMs). The GCMs suffer from some systematic biases including dry bias in mean monsoon rainfall over the India subcontinent and excessive equatorial light rain between which the relationship was rarely discussed. Numerical experiments are conducted for one month during active monsoon with global quasi-uniform resolution of 60 km (U60 km) and 3 km (U3 km) separately. Evaluation with observations shows that U3 km reduces the dry bias over northern India and excessive light rain over the equatorial Indian Ocean (EIO) that are both prominent in U60 km. Excessive light rain in U60 km contributes critically to stronger rainfall and latent heating over the EIO. A Hadley-type anomalous circulation is thus induced, whose subsidence branch suppresses updrafts and reduces moisture transport into northern India, contributing to the dry bias. The findings highlight the importance of constraining excessive light rain for regional climate projection in GCMs.

South Asian Summer Monsoon under stratospheric aerosol intervention

The South Asian summer monsoon (SAM) bears significant importance for agriculture, water resources, economy, and environmental aspects of the region for nearly 2 billion people. To minimize the adverse impacts of global warming, Stratospheric Aerosol Intervention (SAI) has been proposed to lower surface temperatures by reflecting a portion of solar radiation back into space. However, the effects of SAI on SAM are still very uncertain. Our study identifies the main drivers leading to a reduction in the mean and extreme summer monsoon precipitation under SAI. These include SAI-induced lower stratospheric warming and the associated weakening of the northern hemispheric subtropical jet, changes in the upper-tropospheric wave activities, geopotential height anomalies, a reduction in the strength of the Asian Summer Monsoon Anticyclone, and, to some degree, local dust changes. As the interest in SAI research grows, our results demonstrate the urgent need to further understand SAM variability under different SAI scenarios.

Correction: Maritime Continent salinity as a new predictor for south Asian summer monsoon rainfall

Correction: Maritime Continent salinity as a new predictor for south Asian summer monsoon rainfall

Correction: Relationship between south Asian summer monsoon intensity and north Indian ocean tropical cyclone activity

Correction: Relationship between south Asian summer monsoon intensity and north Indian ocean tropical cyclone activity

Runoff Parameterization Enhances Regional Climate Model Accuracy for the South Asian Summer Monsoon

Runoff Parameterization Enhances Regional Climate Model Accuracy for the South Asian Summer Monsoon

Optimal Atmospheric Heat Sources for the Interannual Variability of South Asian Summer Monsoon

AbstractUsing a Green's function‐like approach, this study identifies optimal atmospheric heat sources for the two leading modes of South Asian Summer Monsoon (SASM) interannual variability. Optimal heating for the first mode, characterized by a lower‐level anomalous anticyclone over northern Bay of Bengal (BOB), is distributed over the Arabian Sea and tropical eastern Indian Ocean (EIO)‐Maritime Continent, with cooling over the BOB‐western North Pacific. In contrast, heating over the tropical southwestern Indian Ocean and equatorial Atlantic, along with cooling over the tropical EIO‐western Pacific, optimally drives the second mode, featuring an anomalous anticyclone over central‐northern India. El Niño/Southern Oscillation indirectly influences SASM by triggering heat sources resembling these optimal patterns. Other sea surface temperatures (SSTs), like those over equatorial Atlantic, can also generate similar heating structures, causing corresponding SASM anomalies. This suggests that the impact of SST modes on SASM depends on the similarity of induced heat sources to optimal patterns.

Greening of India and revival of the South Asian summer monsoon in a warmer world

While it is accepted that the tropical hydrological cycle has intensified during past interglacial periods due to changes in insolation, greenhouse gases and ice volume, their respective influences are uncertain. Here we present a pollen record from Bengal Bay to reconstruct vegetation changes in India’s core monsoon zone during two warm periods, the current and last interglacial, comparing the data with numerical model simulations to assess the influence of different forcing mechanisms. Results show tropical forest expansion between 11.7-5 ka and 127-120 ka, defining two Indian humid periods, with the last interglacial showing the strongest monsoon activity, consistent with salinity reconstructions. Model-data comparison highlights boreal summer insolation as the primary driver of vegetation dynamics and monsoon intensity during interglacial periods, with CO2 and ice-sheets having a limited effect. Vegetation remains unaffected by pre-industrial CO2 variations above 250 ppmv, a threshold value that characterizes most interglacials of the last million years.

Maritime Continent salinity as a new predictor for South Asian summer monsoon rainfall

Maritime Continent salinity as a new predictor for South Asian summer monsoon rainfall

Relationship between south asian summer monsoon and ENSO primarily modulated by ENSO intensity based on two super large ensembles

Relationship between south asian summer monsoon and ENSO primarily modulated by ENSO intensity based on two super large ensembles

Relationship between south Asian summer monsoon intensity and north Indian ocean tropical cyclone activity

Relationship between south Asian summer monsoon intensity and north Indian ocean tropical cyclone activity

Distinct response of Asian summer monsoon circulation and precipitation to orbital forcing during six Heinrich events

Distinct response of Asian summer monsoon circulation and precipitation to orbital forcing during six Heinrich events

South Asian summer monsoon enhanced by the uplift of the Iranian Plateau in Middle Miocene

Abstract. The South Asian summer monsoon (SASM) significantly intensified during the Middle Miocene (17–12 Ma), but the driver of this change remains an open question. The uplift of the Himalaya (HM) and the Iranian Plateau (IP) and global CO2 variation are prominent factors among suggested drivers. Particularly, the impact of high CO2 levels on the Miocene SASM has been little studied, despite the wide range of reconstructed CO2 values around this period. Here we investigate their effects on the SASM using the fully coupled Ocean–Atmosphere Global Climate Model, CESM1.2, through a series of 12 sensitivity experiments. Our simulations show that the IP uplift plays a dominant role in the intensification of the SASM, mainly in the region around northwestern India. The effect of the HM uplift is confined to the range of the HM and its vicinity, producing orographic precipitation change. The topography forcing overall out-competes CO2 variation in driving the intensification of the SASM. In the case of extremely strong CO2 variation, the effects of these two factors are comparable in the core SASM region, while in the western region, the topographic forcing is still the dominant driver. We propose a thermodynamical process linking the uplift of the IP and the enhanced SASM through the release of latent heat. When compared with reconstructions, the simulated response of SASM to the IP uplift is in good agreement with observed precipitation and wind field, while the effects of the HM uplift and CO2 variation are inadequate to interpret the proxies.

Mean flow and eddy summer moisture transport over East Asia in reanalysis data and a regional climate simulation

Understanding the impact of atmospheric variability on climatological mean moisture transport is crucial because moisture transport determines continental water availability as well as convective organization and resulting precipitation. Here, we analyze the mean flow and eddy components of summer moisture transport in the downwind of the Tibetan Plateau (TP), a region that is characterized by interactions between monsoon systems, extratropical circulation, and mountainous weather systems. Using 40 years of ERA5 reanalysis data and a regional WRF simulation, we determine the absolute and relative contributions of mean flow and eddy moisture transport from multi-daily to sub-daily scales. We also link these components to large-scale circulation indices, precipitation, evaporation, and mesoscale convective systems (MCSs). The results show that the largest contributions of eddies to the climatological mean moisture transport are found in the immediate downwind region of the TP. Half of the total eddy transport downwind of the TP is due to multi-daily eddy transport and the other half is due to daily to sub-daily eddy transport. Regional precipitation anomalies are dominated by the mean flow component of southerly moisture influxes which in turn are positively correlated with different South Asian summer monsoon indices and negatively correlated with the West Northern Pacific monsoon index. The eddy transport from the south is positively correlated with a lower jet latitude but does not show any significant correlations with precipitation or MCS activity, likely due to the dominant role of the mean flow moisture transport. While the relative contributions of eddies to the climatological mean moisture transport are similar in ERA5 and WRF, the correlations between moisture transport components and large-scale circulation indices are generally weaker in WRF. This suggests that the dynamical downscaling does not significantly change the role of eddy moisture transport averaged for the region, but it resolves processes that decouple the moisture transport from its large-scale forcing.

Observed changes in the climate and snow dynamics of the Third Pole

The Third Pole (TP) is the world’s largest highland and has one of the biggest reservoirs of glacier ice mass and snow cover on the Earth. Three major Asian rivers (the Indus, Ganga and Brahmaputra) are nourished by the melting of glaciers and snow in Central Himalaya, which are inevitable for the socioeconomic sustainability and water security of South Asia. Here, we investigate the long-term (1980–2020) changes in snow depth and precipitation in TP, where major precipitation occurs in the form of rainfall in summer, and snowfall in winter and spring. The seasonal mean snow depth is deep (≥1 m) in winter and shallow (≤0.2 m) in summer. The average snowmelt and snow water equivalent are higher in the central and western Himalaya and Karakoram ranges in spring, which are the regions with most glaciers in TP. There is a significant positive trend in total precipitation, about 0.01–0.03 mm d−1 yr−1 in the central and eastern TP during the South Asian Summer Monsoon for the 1980–2020 period. Snowmelt is also increasing (>0.5 × 10−3 mm yr−1) in the western Himalaya during spring, which is consistent with the temperature rise (0.04–0.06 °C yr−1) there. In addition, there is a notable increase in the annual mean glacier melt (here, the water equivalent thickness) in TP (−1 to −5 cm w.e. yr−1), with its highest values in the eastern and central Himalaya (−3 to −5 cm w.e. yr−1), as estimated for the period 2003–2020. On top of these, by the end of the 21st century, the Coupled Model Intercomparison Project Phase 6 (CMIP6) projections show that there would be a significant decrease in snow depth and an increase in temperature of TP in all shared socioeconomic pathways (SSPs). Henceforth, the increasing trend in temperature and melting of snow/glaciers in TP would be a serious threat to the regional climate, water security and livelihood of the people of South Asia.

South Asian Summer Monsoon Precipitation Is Sensitive to Southern Hemisphere Subtropical Radiation Changes

AbstractWe study the sensitivity of South Asian Summer Monsoon (SASM) precipitation to Southern Hemisphere (SH) subtropical Absorbed Solar Radiation (ASR) changes using Community Earth System Model 2 simulations. Reducing positive ASR biases over the SH subtropics impacts SASM, and is sensitive to the ocean basin where changes are imposed. Radiation changes over the SH subtropical Indian Ocean (IO) shifts rainfall over the equatorial IO northward causing 1–2 mm/day drying south of equator, changes over the SH subtropical Pacific increases precipitation over northern continental regions by 1–2 mm/day, and changes over the SH subtropical Atlantic have little effect on SASM precipitation. Radiation changes over the subtropical Pacific impacts the SASM through zonal circulation changes, while changes over the IO modify meridional circulation to bring about changes in precipitation over northern IO. Our findings suggest that reducing SH subtropical radiation biases in climate models may also reduce SASM precipitation biases.

Future changes in South Asian summer monsoon circulation under global warming: role of the Tibetan Plateau latent heating

The South Asian summer monsoon (SASM) is a significant monsoon system that exerts a profound impact on climate and human livelihoods. According to 38 models from the Coupled Model Intercomparison Project Phase 6, the SASM circulation is projected to weaken significantly under global warming as seen in the weakened low-level westerly wind over the northern tropical Indian Ocean; however, the associated climate dynamics is still under debate. Here, we identify that the weakened low-level westerly wind is closely related to the enhanced latent heating over the Tibetan Plateau (TP), which corresponds with increased summer precipitation in the future. The intensified TP latent heating triggers an anomalous meridional circulation with ascending motions over the plateau and descending motions to the south, leading to an anomalous low-level anticyclone over the northern tropical Indian Ocean. This anticyclone greatly weakens the prevailing low-level westerlies of the SASM through easterly anomalies at the anticyclone’s southern flank. Moisture budget analysis further reveals that increased atmospheric water vapor, rather than the vertical dynamic component, makes the largest contribution to the increased precipitation over the TP. This result confirms that the enhanced TP latent heating is a driver of atmospheric circulation change and contributes to weakening the SASM circulation.

Multi-century flow reconstruction of the Lhasa River, China

Multi-century flow reconstruction of the Lhasa River, China

Quantifying the dynamical and radiative processes of the drastically weak South Asian summer monsoon circulation in 2015

The South Asian summer monsoon (SASM) circulation in 2015 is the weakest since 2000s, which results in severe drought over broad regions of the Indian peninsula. The 2015 SASM is closely related to the weakened summer meridional thermal contrast between southern Eurasia (SE) and the tropical Indian Ocean (TIO) at the mid–upper troposphere. Based on an updated climate feedback-response analysis method, this study conducts a quantitative attribution analysis of the thermal contrast anomalies associated with the 2015 SASM to multiple dynamical and radiative processes, particular for aerosol process. Result shows that the 2015 weak SASM is mainly attributed to the effect of water vapor (58%), followed by the effects of atmospheric dynamics (18%), clouds (15%), and aerosols (15%), respectively. These positive effects are partially offset by the negative contribution from surface dynamic process (-14%). As the most pronounced factor, the water vapor process weakens the SASM circulation via inducing SE cooling and TIO warming, which is closely linked to the decreased (increased) specific humidity over SE (TIO). Further analysis indicates that the total effect of aerosols is dominated by the changes in black carbon and sea salt. As two important components, the SE cooling and TIO warming separately account for about 51% and 49% to the 2015 SASM. The former is mainly attributed to the cooling effect of clouds, while the latter is mainly induced by the warming effect of atmospheric dynamics. Our result provides a new insight into the 2015 weak SASM from a quantitative perspective.