Global Monsoon Research Articles

Interannual Covariation of the North American and African Summer Monsoons

ABSTRACTThe North American summer monsoon (NASM) and the North African summer monsoon (NAFSM) are two vital subsystems of the global monsoon. To date, the potential inter‐monsoon relationship between the NASM and NAFSM has not been fully understood. To fill this gap, we investigate the NASM–NAFSM relationship on the interannual timescale during the period of 1979–2022. Based on statistical methods (including correlation, empirical orthogonal function and cross wavelet analyses), we identify a noteworthy interannual covariation of the NASM and NAFSM. This observed NASM–NAFSM covariation can be explained by atmospheric circulation anomalies associated with sea surface temperature (SST) anomalies in the tropical central‐eastern Pacific and tropical Atlantic, suggesting the critical roles of tropical Pacific–Atlantic SST anomalies in shaping the NASM–NAFSM covariation. The results of Coupled Model Intercomparison Project Phase 6 (CMIP6) models indicate that a model's ability to simulate the NASM–NAFSM covariation tends to be related to its ability to reproduce the modulating effects of the tropical Pacific–Atlantic SST anomalies. These results have potential implications for seasonal forecasts of the NASM and NAFSM variations, suggesting that the NASM and NAFSM can be considered simultaneously in climate predictions.

Deciphering the Variations and Mechanisms of Global Land Monsoons during Marine Isotope Stage 3

Abstract Marine Isotope Stage 3 (MIS 3) is characterized by significant millennial-scale climatic oscillations between cold stadials and mild interstadials, which presents a valuable case for understanding hydrological response to abrupt climate change. Through a set of coupled model simulations, our results broadly show an antiphased interhemispheric change in land monsoonal precipitation during the present-day relative to MIS 3 interstadial and the stadial–interstadial transition, with a general decrease in the Northern Hemisphere but an increase in the Southern Hemisphere. The antiphased pattern is largely caused by the change in orbital insolation during the present-day relative to MIS 3 interstadial, whereas by the weakened Atlantic meridional overturning circulation during the interstadial–stadial transition. However, there are obvious discrepancies in precipitation response and underlying mechanisms among individual monsoon domains and across different periods. Based on the moisture budget analysis, we indicate that the dynamic factor mainly explains the decreased monsoonal rainfall in the Northern Hemisphere during the present-day relative to the MIS 3 interstadial, whereas the thermodynamic term is largely responsible for the increased precipitation in the Southern Hemisphere. In contrast, the dynamic factor plays an important role in the variation of precipitation over all the monsoon zones from the MIS 3 interstadial to stadial states, with the thermodynamic term mainly contributing to the decreased tropical monsoonal precipitation in the colder Northern Hemisphere. Our results help improve the understanding of global monsoon variations under intermediate glacial climate conditions and shed light on their behaviors under potentially rapid climate change in the future.

Understanding the Biases in Global Monsoon Simulations from the Perspective of Atmospheric Energy Transport

Abstract Understanding global monsoon (GM) variability and projecting its future changes rely heavily on climate models. However, climate models generally show pronounced biases in GM simulations, and the reasons for this remain unclear. Here, we evaluate the performance of 20 pairs of climate models that participated in both phase 5 of the Coupled Model Intercomparison Project (CMIP5) and phase 6 of CMIP (CMIP6) and identify the sources of their GM simulation biases from an energy transport perspective. The multimodel mean improvement in CMIP6 compared to CMIP5 is demonstrated by the increasing skill scores for various GM metrics from 0.20–0.79 to 0.48–0.83. More specifically, the dry biases in the Northern Hemisphere Summer Monsoon (NHSM) precipitation in CMIP5 [root-mean-square error (RMSE): 1.85 mm day−1] are reduced in CMIP6 (RMSE: 1.66 mm day−1). This higher simulation skill is associated with higher skill in simulating the precipitation-solstitial mode, monsoon intensity, and monsoon domains. The improvement in the NHSM precipitation simulation results from that in the meridional transport of atmospheric energy. Atmospheric energy budget analysis shows that the negative biases in downward surface longwave radiation and northward energy transport are smaller in CMIP6 than in CMIP5 in the boreal summer, resulting in a more realistic interhemispheric thermal contrast and meridional gradient of moist static energy. However, a major weakness of the CMIP6 models is found in the Southern Hemisphere Summer Monsoon precipitation simulation due to the positive bias in the top-of-the-atmosphere downward longwave radiation. This study shows that reasonably reproducing the meridional global atmospheric energy transportation is necessary for skillful GM simulation.

Influence of Natural External Forcings on Interdecadal Variation of Global Land Monsoon over the Last Millennium in CESM-LME

Abstract Interdecadal variations of the global land monsoon have been previously attributed to internal fluctuations of the climate system, but the role of natural external forcings was underexplored. Here, we investigate this issue by using the Community Earth System Model ensemble simulations over the last millennium (LM) (AD 950–1850). Our analysis reveals that the surface temperature, with two dominant structures (global cooling/warming and longitudinal sea surface temperature gradient in the tropical Pacific, which affects the Walker circulation), predominantly shapes the leading forced mode of the global land monsoon. This mode, representing 19% of the total variance, manifests as consistent features across South Asia, the southern part of East Asia, North Australia, South America, and western South Africa, contrasting with other monsoon regions. Under global cooling conditions, the monsoon intensity is enhanced in the northern parts of the East Asian and eastern parts of the North and South African monsoons, but it decreases in the other monsoon regions. Under weak Walker circulation conditions, changes in atmospheric circulation in response to the sea surface temperature gradient in the tropical Pacific are associated with a substantial attenuation of almost all land monsoon regions. It was further shown that the global mean surface temperature and the tropical Pacific temperature gradient jointly account for 75% of the total variance in the leading mode of the global land monsoon, with 29% and 46% as their respective contributions. Furthermore, our results suggest that volcanic eruptions are the dominant external forcing for these variations. These findings provide valuable insights for future research on global monsoon dynamics. Significance Statement Our study using the Community Earth System Model reveals that surface temperature significantly impacts decadal global land monsoon (GLM) variations during the last millennium. Two key factors in relation to global-scale cooling/warming and changes in the tropical Pacific sea surface temperature gradient affect the GLM’s leading forced mode. The leading forced GLM features remain consistent across regions like South Asia, southern East Asia, North Australia, South America, and western South Africa. Under global cooling, monsoon intensities in most land monsoon regions are enhanced, while they are reduced in northern East Asian land monsoon regions. In contrast, weak tropical Pacific temperature gradient conditions cause a decrease in intensities in almost all land monsoon regions. Additionally, the global mean surface temperature and the tropical Pacific temperature gradient account for 75% of the total variance in the leading forced GLM variations, contributing 29% and 46%, respectively. Notably, volcanic eruptions emerge as the key external factor influencing these variations.

A moist‐thermal quasi‐geostrophic model for monsoon depressions

AbstractMonsoon depressions (MDs) are synoptic‐scale storms that occur during the summer phase of the global monsoon cycle and whose dynamical mechanisms remain incompletely understood. To gain insight into the dynamics governing the large‐scale structure of MDs, we formulate an idealised moist‐thermal quasi‐geostrophic model that includes distinct thermal and moisture fields in simple forms. A linear‐stability analysis of the model, with basic states corresponding to typical monsoon conditions, shows three distinct mode classifications: thermal‐Rossby modes, heavy precipitating modes, and a moist‐thermal mode. In the linearised model, the presence of a background precipitation gradient strengthens thermal‐Rossby modes by coupling the dynamics to latent heating. The separation of heavy precipitating modes from fast‐propagating thermal‐Rossby modes is further examined with numerical experiments of large‐amplitude MDs. Wind‐induced evaporation is found to amplify large‐amplitude MDs in conditions analogous to those over the northern Bay of Bengal. An energetic analysis shows the pathways by which the MDs derive energy from the background state. A further series of experiments through a continuum of meridional temperature gradients demonstrates the sensitivity of large‐scale MD dynamics to the background state and suggests a possible mechanism to explain variations in the propagation direction of MDs.

Fast response of global monsoon area and precipitation to regional carbonaceous aerosols

Monsoon exerts a significant social and economic impact on billions of people around the world. Changes in global monsoon (GM) rainfall under global warming have been well studied; however, the understanding of GM connection with regional aerosol emissions is limited. Using an atmospheric general circulation model (AGCM), we attempt to quantify the response of global monsoon area and precipitation to the removal of carbonaceous aerosols over North America (AM), North Africa (AF), Europe (EU), and Asia (AS). Response to the emission removal, the global average aerosol optical depths (AOD) are reduced by 2.21%, 1.8%, 1.16%, and 3.8% respectively. A southward shift in the GM domain is observed, responding to changes in magnitude and position of the Inter-Tropical Convergence Zone, when carbonaceous aerosols are removed over these regions in the Northern Hemisphere. The percentage departure in the area-averaged GM precipitation is found to be 4-fold in magnitude for the removal of emissions over EU when compared to that of AM and AF. A shrinkage in the GM area is also observed, which is maximum for EU (∼−30%) compared to AM and AF (∼−25%). The change in GM precipitation and area are found to be the least and opposite in nature for AS emissions. This study provides insight into the regional nature of carbonaceous aerosol effects on global precipitation.

Synchronous anti-phase variations between the East Asian summer monsoon and South America Summer Monsoon during Heinrich stadial 2

The interhemispheric anti-phase variation of monsoonal precipitation at the millennial timescale has been attributed to the latitudinal movement of the Intertropical Convergence Zone (ITCZ). However, a debate persists regarding the lead-lag or synchronous relationship between the Asian Summer Monsoon (ASM) and South American Summer Monsoon (SASM) on the onset and termination timings of the Heinrich stadials, hindering a deeper understanding of their underlying driving mechanisms. Here, we presented a new stalagmite δ18O record from Xianyun Cave, southeastern China. This record, with ∼5-year temporal resolution and an averaged uncertainty of ∼60 years on U–Th dates, spans Heinrich Stadial 2 (HS2) and represents East Asian Summer Monsoon (EASM) intensity variations. Based on our record, we further divided the evolution of the EASM during HS2 into three phases: an onset phase (HS2-I, 24,450–24,260 yr BP) and a termination phase (HS2-III, 23,950–23,630 yr BP), sandwiching a phase with relatively intensified monsoon (HS2-II, 24,260–23,950 yr BP). During the onset and termination of HS2, synchronous transition in monsoon records from both hemispheres was observed, driven by the large-scale meridional movement of the ITCZ. In HS2-I, a “Large excursion” observed in both tropical ISM and SASM records was also evidenced in the Xianyun record, suggesting an extension of the “Tropical atmospheric seesaw” to broader regions near tropical moisture sources. In phase II, our record precisely characterizes a centennial-scale strengthening of the EASM coinciding with a weakening SASM, while the Indian Summer Monsoon (ISM) showed no significant changes. The decoupling of the ISM and EASM may be related to the distinct responses of the regional ITCZ to El Niño-Southern Oscillation variabilities. Our study highlights the significant role of the tropical ocean-atmosphere coupling systems in the synchronized or divergent evolution of global monsoons within Heinrich stadials.

A Model‐Data Comparison of the Hydrological Response to Miocene Warmth: Leveraging the MioMIP1 Opportunistic Multi‐Model Ensemble

AbstractThe Miocene (23.03–5.33 Ma) is recognized as a period with close to modern‐day paleogeography, yet a much warmer climate. With large uncertainties in future hydroclimate projections, Miocene conditions illustrate a potential future analog for the Earth system. A recent opportunistic Miocene Model Intercomparison Project 1 (MioMIP1) focused on synthesizing published Miocene climate simulations and comparing them with available temperature reconstructions. Here, we build on this effort by analyzing the hydrological cycle response to Miocene forcings across early‐to‐middle (E2MMIO; 20.03–11.6 Ma) and middle‐to‐late Miocene (M2LMIO; 11.5–5.33 Ma) simulations with CO2 concentrations ranging from 200 to 850 ppm and providing a model‐data comparison against available precipitation reconstructions. We find global precipitation increases by ∼2.1 and 2.3% per degree of warming for E2MMIO and M2LMIO simulations, respectively. Models generally agree on a wetter than modern‐day tropics; mid and high‐latitude, however, do not agree on the sign of subtropical precipitation changes with warming. Global monsoon analysis suggests most monsoon regions, except the North American Monsoon, experience higher precipitation rates under warmer conditions. Model‐data comparison shows that mean annual precipitation is underestimated by the models regardless of CO2 concentration, particularly in the mid‐ to high‐latitudes. This suggests that the models may not be (a) resolving key processes driving the hydrological cycle response to Miocene boundary conditions and/or (b) other boundary conditions or processes not considered here are critical to reproducing Miocene hydroclimate. This study highlights the challenges in modeling and reconstructing the Miocene hydrological cycle and serves as a baseline for future coordinated MioMIP efforts.

Characteristics of boundary layer height and its influencing factors in global monsoon regions

Monsoon precipitation changes has led to substantial changes in the spatial pattern of land surface energy in most monsoon regions of the world, which subsequently affect the structure and development of the atmospheric boundary layer (ABL) over land. Based on the ERA-Interim reanalysis data (1979–2018), the present study analyzes the long-term variability characteristics of Boundary layer height (BLH) and its influencing factors in the global land monsoon region. The results show that the long-term annual mean BLH averaged over all the monsoon regions across the globe is approximately 653.2 m, but there exist great differences between various regions. The highest BLH occurs in the Australian (AUS) region, followed by that in South Africa (SAF), North Africa (NAF), North America (NAM) and South America (SAM) regions, and the lowest BLH occurs in the Asian (ASN) monsoon region. The differences in BLH between various regions reflect significant regional differences in surface energy distribution. A larger sensible heat flux SHF usually corresponds to a higher BLH, while NAM with the largest latent heat flux LHF corresponds to not the lowest BLH, which is related to its smaller cloud radiation cooling. In the past 40 years, the spatial variation of BLH in ASN region is the most special among all monsoon regions, which is mainly manifested in the increasing trend in the east (east of 100°E) and the decreasing trend in the west (west of 100°E), partly due to the distribution of more precipitation in the east and less precipitation in the west in recent decades. The differences in BLH distribution in different monsoonal regions are mainly caused by SHF. Wind shearSand vertical sensible heat advectionHmv, as the main driving sources of ABL development, play a non-negligible role in the variances of BLH structure in different monsoon areas, although their contributions are much smaller than that of SHF.The role of S in promoting or inhibiting the development of ABL is not only related to S size, but also closely related to the atmospheric turbulence environment. Additionally, each monsoon region's Hmv influence on BLH is the most complex, and changes in near-surface temperature and annual precipitation can only partially explain the observed occurrences. The complicated process must also take into account the significance of local vertical motion brought on by weather variations and surface irregularities in various monsoon regions.

Global terrestrial monsoon area variations since Last Glacial Maximum based on TraCE21ka and PMIP4-CMIP6 simulations

Global monsoon over land (GM_L) have significant variations in their dominated area (GMA_L). The quantitative assessment of changes in GMA_L is essential for understanding spatial environment impacts and the underlying driving mechanism of GM_L evolution. Simulation results of a suite of transient climate evolution simulation over the last 21,000 years (TraCE21ka) and five-model ensemble outputs of the Paleoclimate Modelling Intercomparison Project in their latest phase (fourth, PMIP4-CMIP6) are analyzed to study the changes in global, hemispheric and six sub-monsoon areas over the past 21 ka over land. Both datasets can reproduce large-scale characteristics of GMA_L variability, although there are differences in the magnitudes of their changes. Compared to 0 ka, TraCE21ka simulations reveal that GMA_L reduced by about 4.89 × 106 km2 (decreased by 10.89%) during the Last Glacial Maximum (LGM, 21–18 ka), and reached a maximum of 46.61 × 106 km2 (increased by 3.83%) during the middle Holocene (MH, 9–6 ka). In comparison, the results of PMIP4-CMIP6 yield a smaller change of GMA_L during the LGM (decreased by 3.72%) and a larger magnitude of GMA_L during the MH (increased by 4.73%). The analysis of GMA_L over the past 21 ka reveals the varying influences of external forcing, with insolation playing a dominant role. Rising atmospheric greenhouse gas concentrations (GHG) and ice sheet melting have induced a stepwise expansion in GMA_L during the last deglaciation, superimposed with several abrupt millennial-scale weakened monsoon events under meltwater fluxes (MWF) forcing. After that, insolation caused a gradually declining trend in GMA_L during the Holocene. The position of the Intertropical Convergence Zone (ITCZ) under orbital forcing, leading to an anti-phase reaction between Northern Hemisphere monsoon areas (NHMA_L) and Southern Hemisphere monsoon areas (SHMA_L), particularly after 9 ka. Among the six sub-monsoons, they show diverse variations and regional characteristics under different forcings. Although regional monsoons like Indonesia-Australia monsoon (IAM) exhibit nonlinear behavior due to internal variabilities, the overall response of global monsoons to external forcing is predominantly linear. In addition, changes in the hydrological processes can indicate the formation and expansion of monsoon domains. Moisture budget shows that thermodynamic effect has always dominated monsoon hydrological cycles over the past 21 ka. The differences in regional monsoon hydrological processes arise from the dynamic effect. Moreover, δ18O records have been used to test these numerical simulation results. Which, together with diverse thermodynamic and dynamic component configurations of terrestrial paleo-monsoon variations, would help to better understand the underlying mechanism of global monsoon and its influences in the future.

Nonlinear Response of Global Monsoon Precipitation to Atlantic Overturning Strength Variations During Marine Isotope Stage 3

AbstractMonsoon rainfall proxy records show clear millennial variations corresponding to abrupt climate events in Greenland ice cores during Marine Isotope Stage 3 (MIS3). The occurrence of these abrupt climate changes is associated with Atlantic Meridional Overturning Circulation (AMOC) strength variations which greatly impact the global oceanic energy transport. Hence, the AMOC most likely plays a key role in modulating the global monsoon rainfall at millennial time scale. No modeling work has hitherto investigated the global monsoon system response to AMOC changes under a MIS3 background climate. Using the coupled climate model CCSM3, we simulated MIS3 climate using full 38 ka before present boundary conditions and performed a set of freshwater hosing/extraction experiments. We show not only agreement between modeling results and proxies of monsoon rainfall within the global monsoon domain but also highlight a nonlinear relationship between AMOC strength and annual mean global monsoon precipitation related to oceanic heat transport constraints. During MIS3, a weakened AMOC induces a decrease in annual mean global monsoon rainfall dominated by the northern hemisphere, whereas southern hemisphere monsoon rainfall increases. Above about 16 Sverdrups a further strengthening of the AMOC has no significant impact on hemispheric and global monsoon domain annual mean rainfall. The seasonal monsoon rainfall shows the same nonlinear response like annual mean both hemispherically and globally.

Emergence of the modern global monsoon from the Pangaea megamonsoon set by palaeogeography

Emergence of the modern global monsoon from the Pangaea megamonsoon set by palaeogeography

Long term trends and variabilities of rainfall of the global monsoon systems during boreal and austral summer seasons

Long term trends and variabilities of rainfall of the global monsoon systems during boreal and austral summer seasons

Mechanical and Thermal Forcings of Asian Large-Scale Orography on Spring Cloud Amount and Atmospheric Radiation Budget over East Asia

Abstract Asian large-scale orography profoundly influences circulation in the North Hemisphere. Considerable spring top-of-the-atmosphere (TOA) radiative cooling over Southeast China (SEC) is very likely related to upstream orography forcing. Here we investigate the mechanical and thermal forcings of Asian large-scale orography, particularly the Tibetan Plateau (TP), on downstream East Asian cloud amount and atmospheric radiation budget during March–April using the Global Monsoons Model Intercomparison Project simulations. The thermal forcing drives significant surface heating and a low-level cyclone over the TP, pumping low-level air to the middle troposphere. Ascent and water vapor convergence triggered by the thermal forcing favor air condensation, low–middle clouds, and resultant strong spring cloud radiative cooling over SEC. Moreover, the thermal forcing moves the position of cloud radiative cooling westward toward the TP. The TP’s blocking role weakens low-level westerlies over SEC, but its deflecting role increases downstream high-level westerlies, dynamically favoring cloud formation over SEC and the eastward ocean. In addition, the TP can force ascent and increase cloud amounts over the western and central TP. The thermal forcing contributes to 57.1% of total cloud amount and 47.6% of TOA cloud radiative cooling induced by the combined orography forcing over SEC while the mechanical one accounts for 79.4% and 95.8% of the counterparts over the ocean to the east of SEC. Our results indicate that Asian large-scale orography shapes the contemporary geographical distribution of spring East Asian cloud amount and atmospheric radiation budget to a large extent. Significance Statement Clouds tied to large-scale topography and circulation exhibit some remarkable geographical distributions. The global strongest cloud radiative cooling, with an intensity of up to −90 W m−2, occurs over Southeast China (SEC) during March–April. The primary purpose of this study is to understand the influences of Asian large-scale orography, particularly the Tibetan Plateau (TP), on this unique climatic phenomenon using the latest climate model simulations. Our results show that Asian large-scale orography forcing significantly increases ascent, low–middle cloud formation, and resultant strong spring cloud radiative cooling over SEC and downstream ocean. The sensible-heat-driven air pump induced by the TP’s thermal forcing maintains strong cloud radiative cooling over SEC. This study provides valuable insights that link Asian large-scale orography forcing to downstream cloud–radiation characteristics.

Freshwater input variability in a west coastal area of Korea and its links to the global monsoon and ITCZ shifts during the period 8500–7800 cal yr BP

In East Asia, freshwater inputs in coastal areas are mainly controlled by rainfall occurring during the East Asian summer monsoon (EASM), which is linked to seasonal latitudinal shifts in the intertropical convergence zone (ITCZ). To investigate the centennial-time-scale relationship between those inputs and the seasonal shifts in the ITCZ, we examined the behaviors of terrestrial input proxies in coastal sediments of western Korea, including grain size, elemental ratios from XRF core scanning (e.g., Zr/Ti and Si/Al), and C/N ratios. Correlation tests linked these proxies to past freshwater input variability, suggesting that elemental ratios can serve as a proxy for past EASM changes. During the period 8500–7800 cal yr BP, the stronger EASMs described by high-resolution peaks of Zr/Ti ratios corresponded to periods of intensified Indian summer monsoons (ISMs) and weakened South American summer monsoon (SASMs), supporting the existence of synchronous global monsoon (GM) events induced by a northward ITCZ shift. This study also shows that the 8.2-ka cooling event recorded in Greenland ice cores synchronously influenced the GM through a southward shift in the ITCZ, resulting in a weakened EASM and ISM in the Northern Hemisphere and an intensified SASM in the Southern Hemisphere. Our study thus demonstrates that the elemental ratios in coastal sediments measured using an XRF core scanner can be utilized to trace high-resolution (at least centennial-time-scale) variability in the EASM in terms of GM-ITCZ coupling.

Influence of Dynamic and Thermal Effects of Asian Topography on Tropical Cyclone Activity as Simulated in a Global Climate Model

Asian topography plays a significant role in regional and global weather and climate change. Based on the dataset of climate system model named CAS FGOALS-f3 participated in Global monsoons Model Inter-comparison (GMMIP), the MIP endorsement of Coupled Model Intercomparison Project Phase 6 (CMIP6), the role of Asian topography to the formation and movement of tropical cyclones (TCs) are discussed in this study. This study provides the first comparative analysis of the dynamic and thermal effects of Asian topography on the regional and global activity of TCs. The results indicate that the Asian topography promotes the generation and development of TCs, especially in the Northwest Pacific (WNP). The contribution of the Asian topography to the number of TCs reached about 50% in WNP. It is worth noting that there are still positive biases of TC track density in the experiment named “AMIP-NS,” which means the thermal effect of Asian topography is also essential for TC formation and development in WNP, which has not received much attention before. Besides, the possible reasons for the modulation of TC activity are given from two aspects: (1) The existence of Asian topography has changed the large-scale factors related to TC activities such as warm core, sea-level pressure, genesis potential index (GPI), which are beneficial to the generation and movement of TC. (2) Asian topography promotes the spread of Madden–Julian oscillation (MJO), which is also beneficial to the generation and movement of TC. It is worthwhile to investigate further the mechanisms by which Asian topography affects the activity of TCs.

Climatology of the global summer monsoon rainy seasons: Revisited from a high-resolution satellite climate data record

Reliable information on the monsoon rainfall at finer spatiotemporal scales has always been crucial for applications in hydrometeorology, water resource management, disaster prevention, and assessment of numerical model outputs. Here, the spatial-temporal characteristics of the summer monsoon rainy seasons worldwide are revisited using a global, high-resolution, bias-corrected satellite rainfall climate data record. The Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks–Climate Data Record (PERSIANN-CDR) provides 0.25° rainfall estimates for the latitude band 60°S–60°N from 1983 to the near present. The global monsoon regimes are defined based on the annual rainfall range and the summer rainfall ratio. The onset, peak, and withdrawal of the rainy season are then determined using a concise yet pertinent rainfall parameter with universal objective criteria.Overall, six major summer monsoon regimes of the globe, the Asian Monsoon, Indonesian–Australian monsoon, North African monsoon, South African monsoon, North American and South American monsoon, are all reasonably represented by PERSIANN-CDR. The progression patterns of climatological onset, peak, and withdrawal phases of these monsoons are consistent with those reported by previous global and regional monsoon studies. Furthermore, compared with other coarser resolution satellite rainfall estimates such as the Climate Prediction Center Merged Analysis of Precipitation (CMAP), the present result is clearly in better agreement with that obtained from ground-based observations since it allows the determination of more detailed information associated with local-scale factors such as the sharp land-sea contrast, local surface heating, and topography effects. However, some “spots”, particularly over complex topography lacking ground truth validations, are revealed to have rainy season characteristics that differ substantially from those over the surrounding areas and have not been fully described by published literature. These distinct features need to be confirmed by further regional monsoon studies.

Global monsoon: Concept and dynamic response to anthropogenic warming

Monsoon has been studied for centuries, yet only recently have regional monsoons been recognized as a global system. This paper begins with a review of the concept of Global Monsoon and related debating issues. We argue that GM drives annual cycles of Hadley circulation, Intertropical Convergence Zone, and subtropical high and dry climate regions. Land monsoon rainfall (LMR) provides water resources for about 70% of the world’s population. Here we review the climate sensitivity of global and regional LMR to anthropogenic warming projected by models participating in phase six of the Coupled Model Intercomparison Project (CMIP6), focusing on critical physical processes responsible for projected changes. In theory, regional mean LMR changes can be approximated by the changes in the product of the mid-tropospheric ascent and 850-hPa specific humidity, plus moderate contribution from evaporation. The greenhouse gases (GHGs) forcing increases moisture content but stabilizes the atmosphere; the two thermodynamic effects offset each other, resulting in a moderate thermodynamic impact on LMR. The GHGs-induced horizontally differential warming results in robust ‘‘northern hemisphere (NH)-warmer than- southern hemisphere (SH)’’, ‘‘land-warmer-than-ocean’’, and an El Nino–like warming pattern. The enhanced NH–SH thermal contrast will increase NH monsoon rainfall and reduce SH monsoon rainfall. The enhanced land–ocean thermal contrast will increase monsoon rainfall over the Asian–northern African monsoon regions. The projected eastern Pacific warming will reduce the North American monsoon. The Inter-model spread analysis suggests that the GHGs-induced circulation changes (dynamic effects) are primarily responsible for the regional differences. The last section discusses conceivable ways forward.

Global monsoon systems and their modulation by the equatorial Quasi-Biennial Oscillation

Monthly-mean data of ERA-Interim reanalysis, precipitation, outgoing longwave radiation (OLR) and sea surface temperature(SST) are investigated for 40 years (1979-2018) to reveal the modulation of the global monsoon systems by the equatorial quasi-biennial oscillation (QBO), focusing only on the neutral El Niño-Southern Oscillation (ENSO) periods (in total 374 months). First, the climatology of the global monsoon systems is viewed with longitude-latitude plots of the precipitation, its proxies and lower tropospheric circulations for the annual mean and two solstice seasons, together with the composite differences between the two seasons. In addition to seasonal variations of Intertropical Convergence Zones (ITCZs), several regional monsoon systems are well identified with an anti-phase of the annual cycle between the two hemispheres. Precipitation-related quantities (OLR and specific humidity), surface conditions [i.e., mean sea level pressure (MSLP) and SST] and circulation fields related to moist convection systems show fundamental features of the global monsoon systems. After introducing eight QBO phases based on the leading two principal components of the zonal-mean zonal wind variations in the equatorial lower-stratosphere, the statistical significance of the composite difference in the precipitation and tropospheric circulation is evaluated for the opposite QBO phases. The composite differences show significant modulations in some regional monsoon systems, dominated by zonally asymmetric components, with the largest magnitudes for specific QBO-phases corresponding to traditional indices of the equatorial zonal-mean zonal wind at 20 and 50 hPa. Along the equator, significant QBO influence is characterized by the modulation of the Walker circulation over the western Pacific. In middle latitudes during boreal summer, for a specific QBO-phase, statistically significant modulation of low-pressure cyclonic perturbation is obtained over the Northern-Hemisphere western Pacific Ocean associated with statistically significant features of heavier precipitation over the eastern side of the cyclonic perturbation and the opposite lighter precipitation over the western side. During boreal winter, similar significant low-pressure cyclonic perturbations were found over the Northern-Hemisphere eastern Pacific and Atlantic Oceans for specific phases.

Global Lagrangian Tracking of Continental Precipitation Recycling, Footprints, and Cascades

Abstract This study addresses how moisture in continental precipitation is recycled globally from a moisture-tracking perspective. Using a state-of-the-art three-dimensional Lagrangian model and optimized water accounting diagnostics, we complete a 40-yr (1971–2010) tracking of moisture from continental precipitation in the present-day climate. Climatologically, we conclude that 62% of continental precipitation stems from evapotranspiration through Lagrangian tracking—a measure is known as the global “continental precipitation recycling (CPR)” ratio. The result bridges the long-standing gap between the explicit (i.e., moisture trajectory–based) and implicit (i.e., water budget–based) estimates of the global CPR ratio in the literature. On the 1° grid scale, nonlocal terrestrial sources dominate precipitation in almost 70% of the land areas, most prominent in the continental interior. The CPR ratio consistently exhibits a contrasting seasonality between the mid-to-high latitudes and monsoon regions worldwide, from which two kinds of moisture source–regulated hydroclimate are generalized. Given the importance of CPR, we identify terrestrial source hotspots for continental precipitation that deserve careful governance. Using the backward “WaterSip” and a newly proposed forward “WaterDrip” diagnostics, we conceptualize and quantify two important types of cascading moisture recycling (CMR), whereby the moisture from an earlier source precipitates over an intermediate source and then re-evaporates to sustain precipitation farther downwind. The watershed-scale CMR metrics uncover the interdependence among the regional water cycles. Findings here advance the scientific understanding of the atmospheric water cycle as seen through Lagrangian tracking, and offer insights into transboundary watershed management and land-use planning to improve freshwater sustainability. Significance Statement Water is crucial for human civilization. There has been a century-long discussion on the moisture sources of continental precipitation. Using Lagrangian tracking, we show that 62% of continental precipitation stems from evapotranspiration, closing the gap with the budget-based estimate. Terrestrial sources dominate precipitation in 70% of Earth’s surface, especially in the interior of South America, Africa, and Eurasia. Global monsoon regions and the mid-to-high latitudes share a completely different source-regulated hydroclimate. Terrestrial source hotspots for continental precipitation that deserve conservation are identified. Two types of processes that formulate a cascade of regional water cycles are proposed and evaluated. Findings here advance the understanding of the origin of continental precipitation, offering insights into water and land management for freshwater sustainability.