Land Monsoon Research Articles

What drives the decadal variation of global land monsoon precipitation over the past 50 years?

Global land monsoon precipitation (GLMP) has shown quasi‐20‐year oscillations overlying a significant downwards trend over the past 50 years (1951–2004). However, what drives this decadal variation remains an open question. Based on Coupled Model Intercomparison Project Phase 5 (CMIP5) models, in this study we explored the possible drivers for the decadal variation of GLMP and quantified their contributions. A downwards trend of −0.30 mm/day (100 years) is observed in GLMP over 1951–2004 that explains 30% of total decadal variance, and it is primarily attributed to anthropogenic aerosols (AAs) external forcing. Further analyses showed that the downwards trend in GLMP mainly results from the widespread drying over Northern Hemisphere (NH) monsoon domain. Faster growth of AAs amount in NH than Southern Hemisphere (SH) over the past 50 years results in a reduced hemispheric thermal contrast (cooler NH–warmer SH) due to more AA‐forced cooling in NH. Reduced hemispheric thermal contrast facilitates a weakened NH summer monsoon circulation and finally suppresses monsoon precipitation. Besides the downwards trend, the remaining variation of GLMP exhibited quasi‐20‐year oscillations that explain 70% of total decadal variance. The decadal oscillations are primarily attributed to internal variability, in which the central tropical Pacific sea surface temperature anomaly and Arctic Oscillation together can explain 55% of its variation.

Reduced exposure to extreme precipitation from 0.5\u2009\xb0C less warming in global land monsoon regions

The Paris Agreement set a goal to keep global warming well below 2 °C and pursue efforts to limit it to 1.5 °C. Understanding how 0.5 °C less warming reduces impacts and risks is key for climate policies. Here, we show that both areal and population exposures to dangerous extreme precipitation events (e.g., once in 10- and 20-year events) would increase consistently with warming in the populous global land monsoon regions based on Coupled Model Intercomparison Project Phase 5 multimodel projections. The 0.5 °C less warming would reduce areal and population exposures to once-in-20-year extreme precipitation events by 25% (18–41%) and 36% (22–46%), respectively. The avoided impacts are more remarkable for more intense extremes. Among the monsoon subregions, South Africa is the most impacted, followed by South Asia and East Asia. Our results improve the understanding of future vulnerability to, and risk of, climate extremes, which is paramount for mitigation and adaptation activities for the global monsoon region where nearly two-thirds of the world’s population lives.

Mid-Pliocene global land monsoon from PlioMIP1 simulations

The mid-Piacenzian (Pliocene) warm period (approximately 3.3–3.0 Ma) monsoon not only provides us with a useful insight into the future monsoon under a warming scenario with high atmospheric CO2 concentration but is also hypothesized to play a key role in climate feedback through impact on the carbon cycle on a geological timescale. The monsoon evolution over time is one key to test this hypothesis. In this sense, we examine the mid-Piacenzian global land monsoon area (GLMA), precipitation (GLMP), and GLMP intensity (GLMPI) using all available model data from the Pliocene Model Intercomparison Project phase 1 (PlioMIP1). Relative to the pre-industrial period, both the GLMA and GLMP increase in the mid-Piacenzian and by an average of 7.3% and 5.8%, respectively. On a large scale, the increased GLMP comes mainly from northern Africa, Asia, and northern Australia. Quantitatively, it is the variation of GLMA, not of GLMPI, that makes a major contribution to the GLMP change. Further analysis reveals that anomalous inland water vapor transportation due to the reduced large-scale meridional thermal contrast in the mid-to-upper troposphere, together with the variation of vertical moisture advection and evaporation, explains most of the global land monsoon changes. A model–data comparison shows that the simulated monsoon precipitation variations are roughly consistent with the reconstructions in northern Africa, Asia, northern Australia, Central America, and eastern South America. Moreover, the intensified mid-Piacenzian Asian monsoon, which is supported by both simulations and reconstructions, or an overall intensified global land monsoon is hypothesized to contribute to the lowering of atmospheric CO2 concentration and the subsequent onset of the sustained major Northern Hemisphere glaciation.

Toward Predicting Changes in the Land Monsoon Rainfall a Decade in Advance

Predictions of changes of the land monsoon rainfall (LMR) in the coming decades are of vital importance for successful sustainable economic development. Current dynamic models, though, have shown little skill in the decadal prediction of the Northern Hemisphere (NH) LMR (NHLMR). The physical basis and predictability for such predictions remain largely unexplored. Decadal change of the NHLMR reflects changes in the total NH continental precipitation, tropical general circulation, and regional land monsoon rainfall over northern Africa, India, East Asia, and North America. Using observations from 1901 to 2014 and numerical experiments, it is shown that the decadal variability of the NHLMR is rooted primarily in (i) the north–south hemispheric thermal contrast in the Atlantic–Indian Ocean sector measured by the North Atlantic–south Indian Ocean dipole (NAID) sea surface temperature (SST) index and (ii) an east–west thermal contrast in the Pacific measured by an extended El Niño–Southern Oscillation (XEN) index. Results from a 500-yr preindustrial control experiment demonstrate that the leading mode of decadal NHLMR and the associated NAID and XEN SST anomalies may be largely an internal mode of Earth’s climate system, although possibly modified by natural and anthropogenic external forcing. A 51-yr, independent forward-rolling decadal hindcast was made with a hybrid dynamic conceptual model and using the NAID index predicted by a multiclimate model ensemble. The results demonstrate that the decadal changes in the NHLMR can be predicted approximately a decade in advance with significant skills, opening a promising way forward for decadal predictions of regional land monsoon rainfall worldwide.

Relative roles of anthropogenic aerosols and greenhouse gases in land and oceanic monsoon changes during past 156 years in CMIP5 models

AbstractRelative roles of anthropogenic aerosols (AAs) and greenhouse gases (GHGs) in land and oceanic monsoon changes during boreal summer over the period 1850–2005 in Coupled Model Intercomparison Project Phase 5 (CMIP5) models are explored. It is found that the GHG effect dominates rainfall trend over oceanic monsoon region. As a result, precipitation over western North Pacific (WNP) monsoon region and Intertropical Convergence Zone (ITCZ) over tropical eastern Pacific are strengthened through the so‐called “richest‐get‐richer” mechanism. Over land monsoon region, GHG and AA effects are different over India and East Asia (EA). The two effects tend to offset each other over India, but the AA effect dominates over EA and induces a drying trend. The weakened effect of GHGs on EA is attributed to the large offset of thermodynamic and dynamic effects associated with GHGs. While the former tends to strengthen EA rainfall through increased moisture, the latter tends to decrease EA rainfall due to the strengthened WNP monsoon impact.

The concept of global monsoon applied to the last glacial maximum: A multi-model analysis

The last glacial maximum (LGM, ca. 21,000 years ago) has been extensively investigated for better understanding of past glacial climates. Global-scale monsoon changes, however, have not yet been determined. In this study, we examine global monsoon area (GMA) and precipitation (GMP) as well as GMP intensity (GMPI) at the LGM using the experiments of 17 climate models chosen from the Paleoclimate Modelling Intercomparison Project (PMIP) according to their ability to reproduce the present global monsoon climate. Compared to the reference period (referring to the present day, ca. 1985, for three atmospheric plus two atm–slab ocean models and the pre-industrial period, ca. 1750, for 12 fully coupled atmosphere–ocean or atmosphere–ocean–vegetation models), the LGM monsoon area increased over land and decreased over the oceans. The boreal land monsoon areas generally shifted southward, while the northern boundary of land monsoon areas retreated southward over southern Africa and South America. Both the LGM GMP and GMPI decreased in most of the models. The GMP decrease mainly resulted from the reduced monsoon precipitation over the oceans, while the GMPI decrease was derived from the weakened intensity of monsoon precipitation over land and the boreal ocean. Quantitatively, the LGM GMP deficit was due to, first, the GMA reduction and, second, the GMPI weakening. In response to the LGM large ice sheets and lower greenhouse gas concentrations in the atmosphere, the global surface and tropospheric temperatures cooled, the boreal summer meridional temperature gradient increased, and the summer land–sea thermal contrast at 40°S – 70°N decreased. These are the underlying dynamic mechanisms for the LGM monsoon changes. Qualitatively, simulations agree with reconstructions in all land monsoon areas except in the western part of northern Australia where disagreements occur and in South America and the southern part of southern Africa where there is uncertainty in reconstructions. Simulations do not support an inter-hemispheric anti-phasing of monsoon intensity change as suggested by proxy data.

Mid-Holocene Ocean Feedback on Global Monsoon Area and Precipitation

AbstractMid-Holocene ocean feedback on global monsoon area, global monsoon precipitation (GMP), and GMP intensity (GMPI) was investigated by a set of numerical experiments performed with the Community Climate System Model version 4. Results showed that ocean feedback induced an increase in land monsoon area for northern Africa but a decrease for Asia and North and South America, and led to an increase in ocean monsoon area for the western Indian Ocean and southeastern Pacific but a decrease for the eastern Indian Ocean, tropical western Pacific, and tropical West Atlantic between the mid-Holocene and pre-industrial period. Dynamic ocean-induced changes in GMP and GMPI were 10.9×109 m3 d-1 and -0.11 mm d-1 between the two periods, respectively. Ocean feedback induced consistent change between the Northern and Southern Hemisphere land or ocean but opposite change between land and ocean in the Northern or Southern Hemisphere monsoon areas for monsoon precipitation. The mid-Holocene summer changes in meridion...

Improved Historical Analysis of Oceanic Total Precipitable Water

AbstractThe amount of water vapor in the atmosphere, total precipitable water (TPW), is an important part of the global water cycle, and a clearer understanding of ocean-area TPW is critical for understanding climate variations. This study uses satellite-period statistics and historical data to analyze monthly oceanic TPW beginning in the nineteenth century. Input data for analyzing the historical TPW includes outputs from an extended dynamic reanalysis and estimates of TPW based on historical sea surface temperature (SST). Methods are developed to optimally use the various inputs to produce an improved analysis. Cross-validation testing is used to guide analysis development. Some evaluation of the resulting analysis indicates several strong climate modes. A global mode indicates multidecadal increases in TPW since the nineteenth century, with strongest increases in the tropics and adjacent to land monsoon regions. Strongest multidecadal changes in the global mode are 1910–40 and since 1980. An ENSO mode for the extended period indicates a trend since the 1980s, opposite to the tendency in the global mode. There is no apparent multidecadal variation in the ENSO mode before 1980, suggesting that its multidecadal relationship with the global mode can change. Analysis of SST over the same period shows climate modes consistent with the TPW modes, and for the satellite period there are consistent variations in the satellite data, showing the strong link between SST and oceanic TPW.

Mid-Holocene global monsoon area and precipitation from PMIP simulations

Towards a better insight into orbital-scale changes in global monsoon, here we examine global monsoon area (GMA) and precipitation (GMP) as well as GMP intensity (GMPI) in the mid-Holocene, approximately 6,000 years ago, using all available numerical experiments from the Paleoclimate Modelling Intercomparison Project. Compared to the reference period, both the mid-Holocene GMA and GMP increased in the majority of the 35 models chosen for analysis according to their ability, averaging 5.5 and 4.2 %, respectively, which were mainly due to the increase in monsoon area and precipitation over the boreal land and austral ocean. The mid-Holocene GMPI decreased in most models and by an average of 1.2 %, mainly due to the decrease in monsoon precipitation intensity over the boreal ocean and austral land. The mid-Holocene GMA, GMP, and GMPI all showed opposite changes both between the land and ocean in the northern or southern hemisphere and between the boreal and austral land or ocean. Orbital-induced changes in large-scale meridional temperature gradient and land–sea thermal contrast are the underlying mechanisms, and the presence of an interactive ocean has an amplifying effect in the boreal land monsoon areas overall. Qualitatively, the model–data comparison indicates agreement in the boreal land monsoon areas and South America but disagreement in southern Africa and northern Australia.

An assessment of improvements in global monsoon precipitation simulation in FGOALS-s2

The performance of Version 2 of the Flexible Global Ocean-Atmosphere-Land System model (FGOALS-s2) in simulating global monsoon precipitation (GMP) was evaluated. Compared with FGOALS-s1, higher skill in simulating the annual modes of climatological tropical precipitation and interannual variations of GMP are seen in FGOALS-s2. The simulated domains of the northwestern Pacific monsoon (NWPM) and North American monsoon are smaller than in FGOALS-s1. The main deficiency of FGOALS-s2 is that the NWPM has a weaker monsoon mode and stronger negative pattern in spring-fall asymmetric mode. The smaller NWPM domain in FGOALS-s2 is due to its simulated colder SST over the western Pacific warm pool. The relationship between ENSO and GMP is simulated reasonably by FGOALS-s2. However, the simulated precipitation anomaly over the South African monsoon region-South Indian Ocean during La Nina years is opposite to the observation. This results mainly from weaker warm SST anomaly over the maritime continent during La Nina years, leading to stronger upper-troposphere (lower-troposphere) divergence (convergence) over the Indian Ocean, and artificial vertical ascent (descent) over the Southwest Indian Ocean (South African monsoon region), inducing local excessive (deficient) rainfall. Comparison between the historical and pre-industrial simulations indicated that global land monsoon precipitation changes from 1901 to the 1970s were caused by internal variation of climate system. External forcing may have contributed to the increasing trend of the Australian monsoon since the 1980s. Finally, it shows that global warming could enhance GMP, especially over the northern hemispheric ocean monsoon and southern hemispheric land monsoon.

Future change of the global monsoon revealed from 19 CMIP5 models

The variability of global monsoon area (GMA), global monsoon precipitation (GMP), and global monsoon intensity (GMI) in the present climate (1979–2003) and the future warmer climate (2075–2099) under Representative Concentration Pathways 4.5 (RCP4.5) scenario was examined based on 19 Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations. In the present‐day simulations, the ensemble mean precipitation reproduces the observed GMA, GMP, and GMI, although the spread of individual models is large. In the RCP4.5 simulations, most (17 of 19) of the CMIP5 models project enhanced global monsoon activity, with the increases of GMA, GMP, and GMI by 1.9%, 3.2%, and 1.3%, respectively, per 1 K of surface warming. The diagnosis of a column‐integrated moisture budget indicates that the increase in GMP is primarily attributed to the increases of moisture convergence and surface evaporation, whereas horizontal moisture advection has little effect. A further separation of dynamic and thermodynamic factors shows that increase of the moisture convergence comes mainly from the increase of water vapor, but is partly offset by the convergence effect. The increase of the surface evaporation is caused by the increase of sea‐air specific humidity difference, while the change in surface wind speed plays a minor role. The GMP experiences a great year‐to‐year variation, and it is significantly negatively correlated with the Niño3.4 index averaged over a typical monsoon year (defined from May to the following April) in the pre‐industrial control and present‐day simulations, similar to observations. Under the RCP4.5 warming, such rainfall variability is intensified, and the relationship between monsoon and El Niño strengthens. A large proportion of intensification in the year‐to‐year monsoon rainfall variability arises from the land monsoon region.

Boreal summer continental monsoon rainfall and hydroclimate anomalies associated with the Asian-Pacific Oscillation

With the twentieth century analysis data (1901–2002) for atmospheric circulation, precipitation, Palmer drought severity index, and sea surface temperature (SST), we show that the Asian-Pacific Oscillation (APO) during boreal summer is a major mode of the earth climate variation linking to global atmospheric circulation and hydroclimate anomalies, especially the Northern Hemisphere (NH) summer land monsoon. Associated with a positive APO phase are the warm troposphere over the Eurasian land and the relatively cool troposphere over the North Pacific, the North Atlantic, and the Indian Ocean. Such an amplified land–ocean thermal contrast between the Eurasian land and its adjacent oceans signifies a stronger than normal NH summer monsoon, with the strengthened southerly or southwesterly monsoon prevailing over tropical Africa, South Asia, and East Asia. A positive APO implies an enhanced summer monsoon rainfall over all major NH land monsoon regions: West Africa, South Asia, East Asia, and Mexico. Thus, APO is a sensible measure of the NH land monsoon rainfall intensity. Meanwhile, reduced precipitation appears over the arid and semiarid regions of northern Africa, the Middle East, and West Asia, manifesting the monsoon-desert coupling. On the other hand, surrounded by the cool troposphere over the North Pacific and North Atlantic, the extratropical North America has weakened low-level continental low and upper-level ridge, hence a deficient summer rainfall. Corresponding to a high APO index, the African and South Asian monsoon regions are wet and cool, the East Asian monsoon region is wet and hot, and the extratropical North America is dry and hot. Wet and dry climates correspond to wet and dry soil conditions, respectively. The APO is also associated with significant variations of SST in the entire Pacific and the extratropical North Atlantic during boreal summer, which resembles the Interdecadal Pacific Oscillation in SST. Of note is that the Pacific SST anomalies are not present throughout the year, rather, mainly occur in late spring, peak at late summer, and are nearly absent during boreal winter. The season-dependent APO–SST relationship and the origin of the APO remain elusive.

Simulation of a Heavy Rainfall Event on 14 September 2004 over Dhaka, Bangladesh Using MM5 Model

An attempt has been made to simulate a heavy rainfall event on 14 September 2004 over Dhaka, Bangladesh using the fifth-generation PSU/NCAR Mesoscale model (MM5). This was an extraordinary rainfall event and recorded 341 mm rainfall in 24-h which was the highest ever recorded. The MM5 model was run on triple-nested domains at 45, 15, 5 km horizontal resolutions using Anthes-Kuo cumulus scheme. The model performance was evaluated by examining the different predicted parameters like mean sea level pressure, upper and lower level circulations, moisture, windshear, vorticity, convergence and rainfall. The model derived rainfall was compared with TRMM rainfall. The present results indicate that the MM5 model with the right combination of the nesting domain, horizontal resolution and cumulus scheme was able to simulate the heavy rainfall event, and associated dynamical and thermo-dynamical features reasonably well. The MM5 model suggested that the highly localized heavy rain over Dhaka was the result of an interaction of the monsoon land depression with southwest summer monsoon weather systems. The analysis shows that the depression almost remains stationary over southwest Bangladesh and zone of heavy rain was laid over Dhaka, and required moisture have been supplied from the Bay of Bengal.Keywords: Depression; Heavy rainfall; TRMM; MM5 model; High resolution.© 2011 JSR Publications. ISSN: 2070-0237 (Print); 2070-0245 (Online). All rights reserved.doi:10.3329/jsr.v3i2.6656 J. Sci. Res. 3 (2), 261-270 (2011)

Trends in global monsoon area and precipitation over the past 30 years

[1] The analysis of the GPCP and CMAP datasets during the past 30 years (1979–2008) indicates that there are consistent increasing trends in both the global monsoon area (GMA) and the global monsoon total precipitation (GMP). This positive monsoon rainfall trend differs from previous studies that assumed a fixed global monsoon domain. Due to the increasing trends in both the GMA and GMP, a global monsoon intensity (GMI) index, which measures the global monsoon precipitation amount per unit area, is introduced. The GMI measures the strength of the global monsoon. Our calculations with both the GPCP and CMAP datasets show a consistent downward trend in the GMI over the past 30 years. This decreasing trend is primarily attributed to a greater percentage increase in the GMA than in the GMP. A further diagnosis reveals that the decrease of the GMI is primarily attributed to the land monsoon in the GPCP, but to the oceanic monsoon in the CMAP.

An assessment of monsoon precipitation changes during 1901–2001

Changes of global land monsoon precipitation are assessed by using three sets of rain-gauge precipitation data for the period of 1901–2002 compiled by GPCC, CRU and Dai-dataset, respectively. The three datasets show consistent long-term changes of precipitation over the monsoon region with slightly different amplitudes. During 1901–2001, global land monsoon precipitation (GMI) exhibits multi-decadal variations, with an overall increasing trend from 1901 to 1955, followed by a decreasing trend up to 2001. The upward trends of global and Northern hemispheric land monsoon precipitation during 1901–1955 are mainly resulted from the increased precipitation over the North African, Indian and East Asian monsoon domains. For the whole period of 1901–2001, precipitation averaged over the Northern Hemisphere and global land monsoon areas both exhibit a decreasing trend although it is only statistically significant at the 5% level for the Northern Hemisphere. The robust decreasing trend of Northern hemispheric land monsoon precipitations during the twentieth century mainly comes from the downward trend of North African and eastern part of Indian monsoon precipitation and occurs mainly after the 1950s. The first leading mode of Empirical orthogonal function (EOF) analysis of precipitation annual range features a coherent change of North African, South Asian, Northeast China, southern South African, eastern Australian and western American monsoon, and a coherent change over the equatorial South African monsoon and eastern American monsoon. The corresponding principal component time series also indicate that the majority of global land monsoon precipitation has experienced an increasing tendency from 1901 to 1955 and a decreasing trend since the 1950s. Examination on the impact of station number change indicates a negligible influence on the results, especially after 1905.

Status and Distribution of Four Species of Hornbills from North and Central Western Ghat-A Report

The Western Ghats is one of the globally recognized Hot Spots for biodiversity in India (Mayers, 1990). It lies between 20° 12' N in the north of Navpur or somewhat north of river Tapati and Kanyakumari (8° 06' N, 77° 35' E) in the south and spread over six states. The Western Ghats complex encompasses southern Gujarat, western Maharashtra, Goa, western Karnataka, Kerala and part of Tamil Nadu (Fig. 1). It is located between the Tropical African (Ethiopian) and the IndoMalayan biogeographic regions. Physiographically the area is somewhat flattened or flat -topped range of hills rise from the Arabian Sea and runs more or less parallel with it. The major hill ranges are the Nilgiri, Annamalai, Palanis and Cardamom hills. The Western Ghats, being an important part of the monsoon land, is typically characterized by the development of the luxuriant tropical rainforests. The vegetation is influenced more by the abundance of seasonal rainfall than atmospheric temperature. This important biogeographic zone offers most suitable niche to the avifauna and it contains about 59% of Indian forms. Out of which 3 genera and 4 species of hornbills inhabit in Western Ghats. Hornbill is a large bird with considerably huge curved bills surmounted by casque in most of the species. It harbours in the semi-evergreen forests. The avifauna of Western Ghats and South India was studied by Davison (1883), Dewar (1904), Ali (1942-43), Ali (1969), Ali and Ripley (1970), Nicholas (1937). In recent years the birds are studied by Mahabal and Vasanth (2001), Pande et 01 (2003). This report deals with the distribution and abundance of our species of hornbills namely Malabar Grey Hornbill, Indian Grey Hornbill, Malabar Pied Hornbill and Great Pied Hornbill (Family Bucerodidae) in some areas of Western Ghats.

Decreasing trend in global land monsoon precipitation over the past 50 years simulated by a coupled climate model

The authors examine the effects of external forcing agents such as greenhouse gases (GHGs) and aerosols, as well as solar variability and ozone, on global land monsoon precipitation by using a coupled climate model HadGEM1, which was developed by the Met Office Hadley Centre for Climate Research. The results indicate that HadGEM1 performs well in simulating the observed decreasing trend of global land monsoon precipitation over the past 50 years. This trend mainly occurred in the Northern Hemisphere and is significantly different from the trend of natural variability due to ocean-atmosphere-land interactions. The coherence between the simulation and the observations indicates that the specified external forcing agents, including GHGs and aerosols as well as solar variability and ozone, are important factors that have affected the decreasing trend of global land monsoon precipitation in the past 50 years.

The Global Monsoon Variability Simulated by CMIP3 Coupled Climate Models*

Abstract The global monsoon climate variability during the second half of the twentieth century simulated by 21 coupled global climate models (CGCMs) that participated in the World Climate Research Programme’s Coupled Model Intercomparison Project phase 3 (CMIP3) is evaluated. Emphasis was placed on climatology, multidecadal trend, and the response of the global monsoon precipitation to volcanic aerosols. The impact of the atmospheric model’s horizontal resolution on the group ensemble mean (GEM; obtained from the three resolution groups) simulations of global monsoon climate is also examined. The CMIP3 CGCMs’ multimodel ensemble simulates a reasonably realistic climatology of the global monsoon precipitation and circulation. The GEMs are also able to capture the gross features of the global monsoon precipitation and westerly domains. However, the spreading among the rainfall GEMs is large, particularly at the windward side of narrow mountains (e.g., the western coast of India, the Philippines, Mexico, and the steep slope of the Tibetan Plateau). Main common biases in modeling rainfall climatology include a northeastward shift of the intertropical convergence zone (ITCZ) in the tropical North Pacific and a southward migration of the North Atlantic ITCZ during boreal winter. The trend in the Northern Hemisphere land monsoon index (NHMI) detected in the CMIP3 models is generally consistent with the observations, albeit with much weaker magnitude. The significant decreasing NHMI trend during 1951–85 and 1951–99 occurs mainly in the models with volcanic aerosols (VOL models). This volcanic signal is detectable by comparison of the forced and free runs. It is estimated that from about one-quarter to one-third of the drying trend in the Northern Hemisphere land monsoon precipitation over the latter half of the twentieth century was likely due to the effects of the external volcanic forcings. On the other hand, the significant increasing trend in the global ocean monsoon index (GOMI) during 1980–99 appears chiefly in those models that are free of volcanic aerosols (No-VOL models). The exclusion of the volcanic aerosols is significant in simulating the positive GOMI trend against the internal variability of each model. These results suggest the climatic importance of the volcanic forcings in the global monsoon precipitation variability.

Ocean Forcing to Changes in Global Monsoon Precipitation over the Recent Half-Century

Abstract Previous examination of changes in global monsoon precipitation over land reveals an overall weakening over the recent half-century (1950–2000). The present study suggests that this significant change in global land monsoon precipitation is deducible from the atmosphere’s response to the observed SST variations. When forced by historical sea surface temperatures covering the same period, the ensemble simulation with the NCAR Community Atmosphere Model, version 2 (CAM2) model successfully reproduced the weakening tendency of global land monsoon precipitation. This decreasing tendency was mainly caused by the warming trend over the central-eastern Pacific and the western tropical Indian Ocean. At the interannual time scale, the global land monsoon precipitation is closely correlated with ENSO. The simulated interannual variation of the global land monsoon index matches well with the observation, indicating that most monsoon precipitation variations arise from the ocean forcing. There are uncertainties between the GPCP and the CMAP data in describing the evolution of global ocean monsoon precipitation. There is very little correspondence between the simulated and the observed global monsoon index over the ocean area. Uncertainties in the satellite data and model deficiencies in describing the ocean monsoon domain are partly to blame. Among the components of global monsoon systems, the Asian–Australian monsoon system has the lowest reproducibility with prescribed SST forcing due to the neglect of air–sea feedback.

Changes in global land monsoon area and total rainfall accumulation over the last half century

This study reports the changes of global land monsoon coverage and monsoon rainfall accumulation. We showed that the combination of monsoon area and rainfall intensity change has led to an overall weakening trend of global land monsoon rainfall accumulation during the last 54 years. This decreasing tendency is mainly caused by the North African monsoon and South Asian monsoon, due to the significant decreasing tendencies of both rainfall intensity and monsoon coverage. The long‐term changes of the other monsoon subsystems are not significant in the context of regional average. The monsoon area and rainfall accumulation indices are consistent with the rainfall intensity index in revealing both the inter‐decadal and interannual variability. The former appears as an inter‐decadal oscillation with a peak phase through 1970‐1980; the latter appears as the North African, North American and Australian monsoons having stronger spectrum power in the LF (3‐7yr) band, while the South African, South and East Asian monsoons having greater powers in the QB (2‐3yr) band.