Onset Of South Asian Summer Monsoon Research Articles

Comparative analysis of indices in capturing the onset and withdrawal of the South Asian Summer Monsoon

The South Asian summer monsoon (SASM) exerts a profound influence on South Asia and the southern Tibetan Plateau. The timing of its onset and withdrawal significantly impacts regional rainfall, a critical water source for agriculture and the economy. Various SASM indices, employing different variables, have been employed to gauge monsoon onset and withdrawal, each demonstrating distinct characteristics. This study provides a comprehensive comparison of prominent SASM indices related to onset and withdrawal, revealing similar timing but varying magnitudes. Across nearly all indices, a consistent pattern emerges, indicating a trend towards earlier onset and delayed retreat during 1979–2018, marked by pronounced interdecadal variability, with a notable turning point around 1997. The earlier onset and later retreat are likely to enhance rainfall and potential water resources for South Asia and the Tibetan Plateau. Delving into the mechanisms revealed a delayed connection between the SASM onset to the large-scale sea surface temperature (SST) anomalies characterized by the Oceanic Niño Index (ONI) in the preceding spring, but a simultaneous connection between SASM withdrawl and ONI in autumn. Current index definitions, relying on single or dual variables, may fall short in accurately depicting monsoon onset and withdrawal. To address this, we introduce a novel monsoon index derived from multiple large-scale circulation variables, offering improved trend capture and enhanced representation of interannual variability in SASM onset and withdrawal. This study advances our understanding of SASM indices and their correlation with monsoon rainfall, providing insights into the dynamics of SASM onset and withdrawal.

Mesoscale Convective Systems Dominate the Energetics of the South Asian Summer Monsoon Onset

AbstractThe rapid onset of the South Asian summer monsoon (SASM) is featured as an abrupt increase of rainfall and atmospheric column‐integrated moist static energy (MSE). Using convection‐permitting simulations, a novel cloud tracking technique, and an MSE budget, the relative roles of mesoscale convective systems (MCSs) and non‐MCS deep convection in the SASM onset energetics are studied. The analysis shows MCSs greatly contribute to the sudden increases of rainfall, diabatic MSE sources, and the subsequent enhanced export of column‐integrated MSE during the rapid onset phase. A “diabatic sources‐energy export” plane analysis delineates the different roles of MCSs and non‐MCS deep convection. Although deep convection always leads to a net energy export, mesoscale organization of deep convection dominates the convection‐associated positive feedbacks that accelerate the energy cycle of the SASM onset. Sensitivity experiment shows that the results are insensitive to the model resolutions.

A Moist Entropy Budget View of the South Asian Summer Monsoon Onset

AbstractUsing a high‐resolution global model with explicit representation of convection, the physical processes involved in the abrupt onset of South Asian summer monsoon are investigated within a moist entropy budget framework. The monsoon onset is a two‐stage transition. During the first stage, which starts around 2 months before onset, the source terms of column‐integrated moist entropy gradually increase, while the export by the large‐scale circulation slowly strengthens. The second stage is marked by a sudden increase of radiative heating and surface latent heat flux 10 days prior to the onset, followed by abrupt strengthening of large‐scale export of moist entropy. When either cloud‐radiative or wind‐evaporation feedback is disabled in numerical experiments, the monsoon experiences much smoother and weaker onset. The evolution of the system in a gross moist stability plane demonstrates that these positive feedbacks destabilize the system and are responsible for the abruptness of the transition.

MJO Affects the Monsoon Onset Timing Over the Indian Region

AbstractThis study examines the linkage between the Madden‐Julian oscillation (MJO) and the South Asian summer monsoon onset through spatiotemporal wave‐filtering and composite analyses of observational data sets during 1979–2016. We identify two major factors in determining the summer monsoon onset timing: the background conditions associated with seasonal transitions and the intraseasonal variations associated with an active MJO. The background conditions undergo sharp seasonal transition over the western Indian Ocean two to three pentads before the onset dates, while a typical monsoon onset is often associated with the arrival of the wet phase of the tropical MJO over the Indian Ocean, likely due to the promotion by the MJO‐initiated eastward propagating westerly wind anomaly. Conversely, the atmospheric circulation associated with the leading dry phase of a strong MJO during the climatological mean onset dates may lead to a delayed monsoon onset.

Transport of short-lived climate forcers/pollutants (SLCF/P) to the Himalayas during the South Asian summer monsoon onset

Over the course of six years (2006–2011), equivalent black carbon (eqBC), coarse aerosol mass (PM1–10), and surface ozone (O3), observed during the monsoon onset period at the Nepal Climate Observatory–Pyramid WMO/GAW Global Station (NCO-P, 5079 m a.s.l.), were analyzed to investigate events characterized by a significant increase in these short-lived climate forcers/pollutants (SLCF/P). These events occurred during periods characterized by low (or nearly absent) rain precipitation in the central Himalayas, and they appeared to be related to weakening stages (or ‘breaking’) of the South Asian summer monsoon system. As revealed by the combined analysis of atmospheric circulation, air-mass three-dimensional back trajectories, and satellite measurements of atmospheric aerosol loading, surface open fire, and tropospheric NOx, the large amount of SLCF/P reaching the NCO-P appeared to be related to natural (mineral dust) and anthropogenic emissions occurring within the PBL of central Pakistan (i.e., Thar Desert), the Northwestern Indo-Gangetic plain, and the Himalayan foothills. The systematic occurrence of these events appeared to represent the most important source of SLCF/P inputs into the central Himalayas during the summer monsoon onset period, with possible important implications for the regional climate and for hydrological cycles.

Decadal-Scale Variation of South Asian Summer Monsoon Onset and Its Relationship with the Pacific Decadal Oscillation

The variation of the summer monsoon onset over South Asia was investigated by using long-term data of the onset over Kerala, India, during the 64-yr period from 1948 to 2011. It was found that the onset over Kerala shows variation on a multidecadal scale. In early-onset years, the sea surface temperature (SST) anomaly over the northern Pacific Ocean was very similar to the negative Pacific decadal oscillation (PDO). The stationary wave train related to the negative PDO reaches into central Asia and generates a warm anomaly, thereby intensifying the land–sea thermal contrast, which promotes summer monsoon onset over South and Southeast Asia. The correlation between the onset date over Kerala and the PDO has strengthened since 1976. Analysis of zonal wind in the upper-level troposphere for the period 1958–2002 indicates that the change in the correlation is related to the change in the wave train path. The wave train propagating from the northern Pacific Ocean to western Russia could propagate eastward more easily in 1976–2002 than in 1958–75.

Relationship between the Onset of the South China Sea Summer Monsoon and the Structure of the Asian Subtropical Anticyclone

The US National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data are employed to study the relationship between the variation in the structure of subtropical anticyclone and the summer monsoon onset over the South China Sea (SCS). The ridge surface of the subtropical anticyclone is defined by the boundary between westerly to the north and easterly to the south [or westerly-easterly boundary (WEB) in brief]. The WEB well represents the three-dimensional structure of the subtropical anticyclone. Under the thermal wind constraint, the WEB usually tilts toward the warmer zone in the vertical. In the Asian monsoon area, the ridgelines of the subtropical anticyclone at various levels in the troposphere in winter are continuous and exhibit a relatively zonal structure. The WEB tilts southward with increasing height and the mean position of the WEB is closer to the equator. In summer, the ridgelines of the subtropical anticyclone in the middle and lower (upper) troposphere are discontinuous (continuous) and the WEB either tilts northward or is vertical, with the mean position of the WEB being located at 25-30°N. May is the period when the structure of the subtropical anticyclone varies most significantly. During the seasonal transition, the tilt of the WEB changes from southward to northward. When the WEB becomes perpendicular to the earth's surface or tilts northward, the meridional temperature gradient vanishes or becomes positive, implying the replacement of the winter monsoon by the summer monsoon. Defining the “seasonal transition axis” (STA) as the vertical ridge axis that results from the switching of the tilt of the WEB from southward to northward, it is found that the STA first establishes over the eastern Bay of Bengal and Indochina Peninsula during the first pentad of May. The onset of the Asian summer monsoon circulation is therefore closely associated with the location of the STA. When the STA reaches the northeastern SCS in the fourth pentad of May, the SCS summer monsoon occurs. The south Asian summer monsoon onset corresponds to the establishment of the STA over central India in the first two pentads of June. Based on the close relationship between the STA and monsoon onset, the area-averaged meridional mean upper tropospheric (200-500 hPa) temperature gradient in the vicinity of the WEB is suggested to be an index to define the SCS summer monsoon onset. In comparison with the onset dates as determined by the area-averaged zonal wind at 850 hPa and OLR, the dates determined by these three indices are consistent in a large number of individual years, which indicates that the establishment of the STA, or the reversal of the meridional temperature gradient, captures the essential feature associated with the SCS summer monsoon onset.