Increase In Monsoon Rainfall Research Articles

Emerging signals of climate change from the equator to the poles: new insights into a warming world

The reality of human-induced climate change is unequivocal and exerts an ever-increasing global impact. Access to the latest scientific information on current climate change and projection of future trends is important for planning adaptation measures and for informing international efforts to reduce emissions of greenhouse gases (GHGs). Identification of hazards and risks may be used to assess vulnerability, determine limits to adaptation, and enhance resilience to climate change. This article highlights how recent research programs are continuing to elucidate current processes and advance projections across major climate systems and identifies remaining knowledge gaps. Key findings include projected future increases in monsoon rainfall, resulting from a changing balance between the rainfall-reducing effect of aerosols and rainfall-increasing GHGs; a strengthening of the storm track in the North Atlantic; an increase in the fraction of precipitation that falls as rain at both poles; an increase in the frequency and severity of El Niño Southern Oscillation (ENSO) events, along with changes in ENSO teleconnections to North America and Europe; and an increase in the frequency of hazardous hot-humid extremes. These changes have the potential to increase risks to both human and natural systems. Nevertheless, these risks may be reduced via urgent, science-led adaptation and resilience measures and by reductions in GHGs.

Hydro-climatic variability and consequent vegetation response during CE 1219–1942 from the Western Ghats, India

We present a pollen proxy record of hydro-climatic variability in the Somwarpet area of Karnataka State (Western Ghats), India, receiving monsoon rainfall from both the southwest summer monsoon (SWM; June to September: JJAS) and the northeast winter monsoon (NEM; October to December: OND). This record covers the last 723 years (CE 1219–1942) and is based on high-resolution lacustrine sediment pollen proxy data. The pollen evidence indicates that between 731 and 576 cal yr BP (CE [AD] 1219–1374), the landscape was dominated by mixed moist/semi-evergreen–dry tropical deciduous forests under a warm and moist climate, possibly indicating increased monsoon rainfall. This phase is globally correlated with the Medieval Climate Anomaly (MCA; CE 750–1200). Between 576 and 279 cal yr BP (CE 1374–1671), dense mixed moist/semi-evergreen–dry tropical deciduous forests came into existence and transformed the existing forests of the previous phase under a warm and more moist climate with further increase in monsoon rainfall. Finally, between 279 and 8 cal yr BP (CE 1671–1942), mixed moist/semi-evergreen–dry tropical deciduous forests again came into being, replacing the existing dense mixed forests of the previous phase under a warm and comparatively less moist climate with increased monsoon (may be NEM) rainfall, corresponds to the global Little Ice Age (LIA; CE 1550–1850). The hydro-climatic variability during CE 1219–1942 offers valuable insights into the impact of monsoonal variability and human activities on ecosystems in the Western Ghats (India), which are subject to dual monsoon systems (SWM and NEM).

Insights from the bias-corrected simulations of CMIP6 in India's future climate

In this study, we have used thirteen statistically downscaled bias-corrected General Circulation Models (GCMs) from Coupled Model Intercomparison Project Phase 6 (CMIP6) under the Shared Socioeconomic Pathways (SSP) scenarios. Based on these models, the projected changes in mean rainfall and daily extremes (number of rainy days and simple daily intensity) during the southwest (SW) and northeast (NE) monsoon seasons in the near (2021–2050) and far future (2071–2100) under the SSP2-4.5 & SSP5-8.5 scenarios relative to the baseline period (1985–2014) are studied. In addition to the rainfall, the maximum, minimum, and mean temperature changes and their daily extremes are also examined. Our results show that the Multi-Model Mean (MMM) slightly underestimated (overestimated) the SW (NE) monsoon rainfall compared to the observed rainfall in the baseline period. A considerable increase in future rainfall during the SW monsoon season is noticed in central India, the Himalayan region, and over the northwestern parts of India. In the NE monsoon season, south peninsular India experiences more rainfall under SSP5-8.5 than SSP2-4.5. In general, both SSPs scenarios shows increase in monsoon rainfall from the mid-century onwards. Interestingly, by the end of the 21st century the number of rainy days are projected to reduce, whereas the intensity of rainfall projected to be drastically increase over many areas of India. Analysis of temperature revealed that there is increase in the projected warming with maximum temperature is around 4.5°C during summertime, with minimum temperatures of about 5°C in the wintertime in northern parts of India by the end of the 21st century. Under the SSP5-8.5 scenario, increase in the highest maximum temperatures is seen in the Himalayan region and elevated minimum temperature pattern over the Indo-Gangetic Basin (IGB) is noted. This increased warming may affect the agriculture, water, health, and power sectors severely.

Terrestrial carbonate oxygen isotopes constraints on the interplay between westerlies and monsoonal rains modulating the Cenozoic climate on the northeastern Tibetan Plateau

The relative contribution of the westerlies and the Asian monsoon has played a critical role in regulating the evolution of the Cenozoic Asian interior environment in response to regional tectonic uplift and global climate change. Here, we reconstruct a long-term record of atmospheric moisture oxygen isotopes in the Asian interior using the oxygen isotopic composition of authigenic carbonates in terrestrial sediments from the Xining Basin on the northeastern Tibetan Plateau spanning the last 52 Myrs. On the VPDB scale, the average carbonate δ18O is characterised by a long-term decrease from −1.5‰ at ca. 43 Ma to −7.5‰ at 33 Ma and smaller variations from −7‰ to −8.5‰ after 33 Ma, with the exception of one period that contains samples with higher δ18O values at −6‰ at 17–14 Ma. At the same time, there is a corresponding long-term rise in average carbon isotope δ13C values from −9.5‰ at ca. 43 Ma to −3.5‰ at 33 Ma, followed by a less variable but similar variation in δ18O values after 33 Ma. The oxygen and carbonate isotopic compositions are negatively correlated before 33 Ma and positively after, suggesting a transition at ∼33 Ma in the context of atmospheric circulation coupled with regional aridity. The δ18O record can be explained by a simple binary mixing model of the atmospheric moisture between the recycling moisture with a positive δ18O value in the westerlies-dominated region and the monsoon rainfall or related moisture with a negative δ18O value from the Indo-Pacific Ocean. In that case, the first-order carbonate δ18O decrease from ca. 43 Ma to 33 Ma was primarily controlled by the retreat of the Para-Tethys Sea, which induced a remarkable decline in westerly rainfall relative to the monsoon. After ∼33 Ma, the contribution from the westerlies could be constant and the lower limits of the carbonate δ18O values are modelled by episodic increases in monsoon rainfall corresponding to enhanced Asian monsoon periods at 26–24 Ma, 17–14 Ma, and 9–8 Ma induced by the tectonic uplift of the Tibetan Plateau and the global climate. Our study, also, suggests that the Oligocene was a transitional stage of moisture sources from the dominance of Eocene westerlies to the Miocene-Quaternary monsoon in continental Asian environments.

Climate-Induced Perspective Variations in Irrigation Schedules and Design Water Requirements for the Rice–Wheat System

Conceptualizing the implications of climate change for crop evapotranspiration (ETc) and subsequent net irrigation water requirement (NIWR) is critical to sustaining Pakistan’s agriculture and food security. In this article, future ETc, NIWR, and design water requirements (DWR) were projected for the rice–wheat system of Punjab, Pakistan. Consistently increasing temperatures signify an impending hotter transition in the future thermal regime, accompanied by a substantial increase in monsoon rainfall. Future climate warming accelerated ETc and NIWR, which were compensated by 2–5 and 1–2 additional irrigations during the rice and wheat seasons, respectively. Future rice and wheat required 13–18 and 2–5 irrigations per season, respectively. Effective rainfall increments did not compensate for the warming-driven higher ETc and NIWR because of uneven and erratic rainfall distribution. Rainfall occurrence and the duration of peak irrigation demand were mismatched, resulting in surplus rainwater availability during the future rice season. The results suggest that DWR for 5- and 10-year return period droughts during the baseline period (965 and 1000 mm, respectively) should be revised to accommodate the additional 100–200 mm of irrigation water per season; otherwise, the study area will face an acute water shortage in the future.

Vegetation response to the Indian Summer Monsoon (ISM) variability during the Late-Holocene from the central Indian core monsoon zone

Pollen analysis and radiocarbon dating of a 1.4 m deep lacustrine sediment profile from Chhattisgarh State, central India, in the core monsoon zone (CMZ), has revealed the vegetation history, associated climate change and the Indian Summer Monsoon (ISM) rainfall variability during the Late-Holocene. The pollen evidence suggests that between ca. 3000 and 2600 cal year BP, tree-savannah vegetation occurred in the region having a comparatively lesser monsoon rainfall. The forest expanded and culminated into an open-mixed tropical deciduous forest between ca. 2600 and 2200 cal year BP under a warm and moderately humid climate with an increase in monsoon rainfall. Subsequently, between ca. 2200 and 2000 cal year BP, the existing open-mixed tropical deciduous forest transformed into a mixed tropical deciduous forest under a warm and humid climate with further increase in monsoon rainfall. Finally, between ca. 2000 and 1800 cal year BP, a dense mixed tropical deciduous forest occupied the landscape under a regime of a warm and relatively more humid climate with further strengthening of the ISM. The gradual warming, and the climatic amelioration with the intensification of the ISM, during the Late-Holocene (ca. 2600–1800 cal year BP; ~650 BC to AD150) corresponds to the Roman Warm Period (RWP), recorded globally between 2500 and1600 cal year BP (~550 BC to AD ~350). Human activities were present around the study area, which varied according to the ISM variations. The present study provides insights into the gradual intensification of the monsoon since the last ca. 2600 cal year BP (between ca. 2600 and1800 cal year BP), and an increase in the ISM strength in the CMZ of India, against the generally weakening trend during the Late-Holocene.

Robust increase of Indian monsoon rainfall and its variability under future warming in CMIP6 models

Abstract. The Indian summer monsoon is an integral part of the global climate system. As its seasonal rainfall plays a crucial role in India's agriculture and shapes many other aspects of life, it affects the livelihood of a fifth of the world's population. It is therefore highly relevant to assess its change under potential future climate change. Global climate models within the Coupled Model Intercomparison Project Phase 5 (CMIP5) indicated a consistent increase in monsoon rainfall and its variability under global warming. Since the range of the results of CMIP5 was still large and the confidence in the models was limited due to partly poor representation of observed rainfall, the updates within the latest generation of climate models in CMIP6 are of interest. Here, we analyze 32 models of the latest CMIP6 exercise with regard to their annual mean monsoon rainfall and its variability. All of these models show a substantial increase in June-to-September (JJAS) mean rainfall under unabated climate change (SSP5-8.5) and most do also for the other three Shared Socioeconomic Pathways analyzed (SSP1-2.6, SSP2-4.5, SSP3-7.0). Moreover, the simulation ensemble indicates a linear dependence of rainfall on global mean temperature with a high agreement between the models independent of the SSP if global warming is the dominant forcing of the monsoon dynamics as it is in the 21st century; the multi-model mean for JJAS projects an increase of 0.33 mm d−1 and 5.3 % per kelvin of global warming. This is significantly higher than in the CMIP5 projections. Most models project that the increase will contribute to the precipitation especially in the Himalaya region and to the northeast of the Bay of Bengal, as well as the west coast of India. Interannual variability is found to be increasing in the higher-warming scenarios by almost all models. The CMIP6 simulations largely confirm the findings from CMIP5 models, but show an increased robustness across models with reduced uncertainties and updated magnitudes towards a stronger increase in monsoon rainfall.

Asian monsoon evolution linked to Pacific temperature gradients since the Late Miocene

Considerable uncertainty remains over the nature and causes(s) of East Asian monsoon evolution since the Late Miocene, a significantly warmer period characterized by substantially weaker meridional and zonal Pacific sea surface temperature (SST) gradients than today and therefore regarded as a potential analog for current and future global warming. However, the extent to which these temperature gradients impacted rainfall patterns across East Asia, and particularly the northern extent of the monsoon domain, remains controversial. Here we present the first hydrological record extending back eight million years (Ma) for North China Plain derived from organic biomarkers preserved in a terrestrial sediment sequence. Our record shows a significant increase in monsoon rainfall during the Early Pliocene (∼4.2-4.5 Ma), coincident with strengthening of Pacific meridional and zonal SST gradients, and the eastern equatorial Pacific cold tongue. This marked intensification of the monsoon rainfall in northern China is also observed in paleoclimate archives from southern Asia, but anti-phased with those from central-eastern China (including southern China), indicating a ‘tripole-like’ rainfall pattern over East Asia. Through a set of climate model experiments, we show that this redistribution of monsoon rainfall across East Asia during the Early Pliocene (5.33-3.6 Ma) was likely due to an equatorward contraction of the western Pacific warm pool, reduced summer convection in the western subtropical Pacific, and the strengthening of the Hadley and Walker circulations. Our study thus highlights the strong influence of Pacific Ocean temperature gradients on East Asian hydroclimate.

Future Climate Change Scenario over Maharashtra, Western India: Implications of the Regional Climate Model (REMO-2009) for the Understanding of Agricultural Vulnerability

The present investigation attempts to understand the near-term (up to 2050) and distant (up to 2100) future climatic changes over Maharashtra. Trend analysis of projected monsoon rainfall and temperature was carried out with the use of parametric and non-parametric statistical techniques. All the meteorological sub-divisions in Maharashtra reveal a significant increase in monsoon rainfall during 2015–2100 (by 150–210 mm), except Konkan. In a near-term future, parts of the Vidarbha Sub-division and Western Ghats exhibit a significant increase in rainfall by 82–225 mm. Almost the entire state is very likely to experience a rise in annual mean temperature (AMT) by 0.5–2.5 °C up to 2050. The state is very likely to experience considerably warmer conditions post-2033. In particular, parts of Konkan and Madhya Maharashtra Sub-divisions will register significant warming (by 1–2.5 °C). The estimations also signify a marginal increase in AMT during the post-2070 period. The annual maximum temperature (AMXT) does not show a considerable rise; however, the annual minimum temperature (AMNT) is expected to increase (by < 1.2 °C) significantly over about 80% of the districts in Maharashtra. These climatic changes are very likely to affect the productivity of principal crops in Maharashtra including sorghum, pearl millet, sugarcane, wheat, rice and cotton. Under the future climate change scenario, therefore, it will be a great challenge for agronomist and policymakers to formulate a judicial plan for sustainable agriculture.

Marginal Benefit to South Asian Economies from SO2 Emissions Mitigation and Subsequent Increase in Monsoon Rainfall

Sulphate aerosols are dominated by SO2 emissions from coal-burning for the Indian electricity sector and they are thought to have a short term but significant, negative impact on South Asian Summer Monsoon rainfall. This reduction in precipitation in turn can lead to reduced economic outputs, primarily through smaller agricultural yields. By bringing together estimates of (a) the impact of sulphate aerosols on precipitation and (b) the observed relationship between monsoon rainfall and GDP, we present a methodology to estimate the possible financial cost of this effect on the Indian economy and on its agricultural sector. Our preliminary estimate is that the derived benefits could be large enough that around 50% of India’s SO2 emissions could be economically mitigated at no cost or net benefit, although it should be noted that the large uncertainties in the underlying relationships mean that the overall uncertainty is also large. Comparison of the 1952–1981 and 1982–2011 periods indicates that the Indian economy may now be more resilient to variability of the monsoon rainfall. As such, a case could be made for action to reduce SO2 emissions, particularly in the crucial monsoon period. This would have a significant, positive effect on a crucial and large sector in India’s economy and the effects would be visible almost instantly. The recent growth in renewable energy sources in India and the consequent, reduced increase in coal burning means that further financial costs have already been avoided. This impact should be further investigated so that it can be included in cost-benefit analyses of different fuel types in the region. The significant uncertainties associated with these calculations are discussed.

Evaluation and interpolation of rainfall trends over the semi-arid upper Godavari basin

Dams are the major source of water for irrigation in India. As a result, any variability in rainfall over the catchment area of dams leads to huge economic loss. It also triggers water-sharing conflicts. Hence, understating the variation in rainfall in association with dams becomes essential. For the present study, the upper Godavari basin, primarily representing the semi-arid region in India, is selected. To understand the rainfall variation, the Mann–Kendall trend test is employed after checking the homogeneity of the data. Using the inverse distance weighted technique, the monthly, seasonal and annual trends in rainfall are interpolated. These interpolated results are then superimposed over the location of the dams. The results depict that the rainfall decreases in June, while it increases in August for most of the stations. The highest increase in monsoon rainfall is experienced in Akole (11.7 mm/year), while Khuldabad has experienced the maximum decline (− 6.58 mm/year) in rainfall. When the spatial variability of annual rainfall is considered, it is mostly declining in the eastern part of the basin. This area of decreasing rainfall accounts for more than 35% of the total basin area. Around 34 dams with capacity of more than 0.5 MCM (million cubic meter) are situated here. As a result of declining rainfall, the water supply from these dams might get adversely affected. Hence, for the upper Godavari basin, based on the rainfall variability analysis, policies should focus on demand side management of water resources.

Hydro-meteorological and agricultural trends in water-scarce Karha Basin, western India: current and future scenario

The scientific understanding of hydro-meteorological and agricultural changes in the water-scarce region is crucial to formulate a plan for sustainable management of water resources and cropping pattern under the scenario of climate change. The present investigation, therefore, has attempted to detect the trends in the monsoon rainfall, runoff, dam storage, post-monsoon groundwater (PMGW) levels, and agricultural data in the Karha Basin, located in the rain shadow zone of the Western Ghat. Additionally, trends in projected monsoon rainfall and average annual temperature were also analyzed. The study indicates no significant increase in monsoon rainfall over the basin during past (1981–2013) and future (2015–2050). The noteworthy decline in monsoon runoff has mainly resulted due to the effective implementation of surface water harvesting in the basin, which has a significant positive change in PMGW table in the basin. The study also provides evidence of the shift in agricultural pattern, where the rainfed crops are getting replaced by the high water-requiring/cash crops. It may increase the future agricultural water demand and pressurize the limited water resources in the basin. On the other hand, significant increase in near-term future (up to 2050) temperature can augment the water demand along with the adverse impacts on the agricultural productivity. The Karha Basin is already suffering from water deficiency. The man-induced increase in agricultural water demand and the adverse climatic changes in the Karha Basin are very likely to aggravate the future drought conditions and further add to an already difficult water management challenge in the basin.

Assessment of future rainfall for the Brahmani-Baitarani river basin – practical implications of limited data availability

AbstractSevere floods are common in the Brahmani-Baitarani river basin in India. Insights into the implications of climate change on rainfall extremes and resulting floods are of major importance to improve flood risk analysis and water system design. A wide range of statistical and dynamical downscaling and bias-correction methods for the generation of local climate projections exists. Yet, the applicability of these methods highly depends on availability of meteorological data. In developing countries, data availability is often limited, either because data do not exist or because of restrictions on use. We here present a climate change analysis for the Brahmani-Baitarani river basin focusing on changes in rainfall using data from three GCMs from the Fifth Coupled Model Intercomparison Project (CMIP5) that were selected based on their performance. We apply and compare two widely used and easy to implement bias-correction methods. These were selected because reliable open historical meteorological datasets required for advanced methods were not available. The results indicate likely increases in monsoon rainfall especially in the mountainous regions and likely increases in the number of heavy rain days. We conclude with a discussion on the gap between state-of-the-art downscaling techniques and the actual options in regional climate change assessments.

Impacts of 20th century aerosol emissions on the South Asian monsoon in the CMIP5 models

Abstract. Comparison of single-forcing varieties of 20th century historical experiments in a subset of models from the Fifth Coupled Model Intercomparison Project (CMIP5) reveals that South Asian summer monsoon rainfall increases towards the present day in Greenhouse Gas (GHG)-only experiments with respect to pre-industrial levels, while it decreases in anthropogenic aerosol-only experiments. Comparison of these single-forcing experiments with the all-forcings historical experiment suggests aerosol emissions have dominated South Asian monsoon rainfall trends in recent decades, especially during the 1950s to 1970s. The variations in South Asian monsoon rainfall in these experiments follows approximately the time evolution of inter-hemispheric temperature gradient over the same period, suggesting a contribution from the large-scale background state relating to the asymmetric distribution of aerosol emissions about the equator. By examining the 24 available all-forcings historical experiments, we show that models including aerosol indirect effects dominate the negative rainfall trend. Indeed, models including only the direct radiative effect of aerosol show an increase in monsoon rainfall, consistent with the dominance of increasing greenhouse gas emissions and planetary warming on monsoon rainfall in those models. For South Asia, reduced rainfall in the models with indirect effects is related to decreased evaporation at the land surface rather than from anomalies in horizontal moisture flux, suggesting the impact of indirect effects on local aerosol emissions. This is confirmed by examination of aerosol loading and cloud droplet number trends over the South Asia region. Thus, while remote aerosols and their asymmetric distribution about the equator play a role in setting the inter-hemispheric temperature distribution on which the South Asian monsoon, as one of the global monsoons, operates, the addition of indirect aerosol effects acting on very local aerosol emissions also plays a role in declining monsoon rainfall. The disparity between the response of monsoon rainfall to increasing aerosol emissions in models containing direct aerosol effects only and those also containing indirect effects needs to be urgently investigated since the suggested future decline in Asian anthropogenic aerosol emissions inherent to the representative concentration pathways (RCPs) used for future climate projection may turn out to be optimistic. In addition, both groups of models show declining rainfall over China, also relating to local aerosol mechanisms. We hypothesize that aerosol emissions over China are large enough, in the CMIP5 models, to cause declining monsoon rainfall even in the absence of indirect aerosol effects. The same is not true for India.

Stalagmite δ18O variations in southern India reveal divergent trends of Indian Summer Monsoon and East Asian Summer Monsoon during the last interglacial

Indian monsoon variations for the period around ∼100ka have not been documented from terrestrial archives, while limited data do exist from the Indian Ocean. Some paleoceanographic studies have reported abrupt and stepwise changes in the monsoon intensity during the last deglaciation, but similar observations have not been possible for earlier periods such as the interglacial to glacial transition, mainly because of the coarse resolution (∼500 to 1000 years) provided by marine archives of that age. Here, we report a new δ18O data set from a stalagmite (3-ACN) that grew in the Belum caves, Andhra Pradesh, a semi-arid region in southern India, with a resolution of ∼85 years. Age assignment to different growth layers consistent with associated analytical errors indicates that the record could span ∼9ky during part of Marine Isotope Stage (MIS) 5d-c, from ∼108 to ∼99ka. δ18O variations in this stalagmite, if interpreted using the ‘amount effect’, i.e., 18O depletion of ∼1.5‰ per 100 mm increase in monsoon rainfall, indicate a step-like increase during the transition from the cooler stadial MIS 5d to warmer interstadial MIS 5c, signifying an abrupt reduction in monsoon rain. The new data presented highlight divergent trends between the Indian and the East Asian Summer Monsoons for a time period for which not many high-resolution comparisons are available.

Consistent increase in Indian monsoon rainfall and its variability across CMIP-5 models

Abstract. The possibility of an impact of global warming on the Indian monsoon is of critical importance for the large population of this region. Future projections within the Coupled Model Intercomparison Project Phase 3 (CMIP-3) showed a wide range of trends with varying magnitude and sign across models. Here the Indian summer monsoon rainfall is evaluated in 20 CMIP-5 models for the period 1850 to 2100. In the new generation of climate models, a consistent increase in seasonal mean rainfall during the summer monsoon periods arises. All models simulate stronger seasonal mean rainfall in the future compared to the historic period under the strongest warming scenario RCP-8.5. Increase in seasonal mean rainfall is the largest for the RCP-8.5 scenario compared to other RCPs. Most of the models show a northward shift in monsoon circulation by the end of the 21st century compared to the historic period under the RCP-8.5 scenario. The interannual variability of the Indian monsoon rainfall also shows a consistent positive trend under unabated global warming. Since both the long-term increase in monsoon rainfall as well as the increase in interannual variability in the future is robust across a wide range of models, some confidence can be attributed to these projected trends.

Abrupt increase in east Indonesian rainfall from flooding of the Sunda Shelf ∼9500 years ago

We present a precisely dated, multi-proxy stalagmite record from Liang Luar Cave, Flores (southeast Indonesia) that reveals a rapid increase in Indonesian monsoon rainfall at ∼9.5 ka. A “ramp-fitting” method for detecting statistically significant inflections in a time-series was applied to the stalagmite δ18O, Mg/Ca, and Sr/Ca profiles to quantify the precise timing and magnitude of an abrupt increase in monsoon strength over a period of ∼350 years. Previously published lake-level records from the monsoon-affected Australian interior show a sudden intensification of the Australian monsoon at ∼14 ka. However, our records indicate that monsoon intensification in Flores occured ∼4–5 kyr later. The timing of the monsoon shift in Flores is synchronous with the rapid expansion of rainforest in northeast Australia and regional freshening of the southern Makassar Strait which, under present-day conditions, is sensitive to monsoon variability. The freshening of southern Makassar was coeval with an abrupt ∼1.5 °C cooling in the upper thermocline of the Timor Sea ∼9.5 ka, indicative of reduced surface heat transport by the Indonesian Throughflow (ITF) when the Java Sea opened during postglacial sea-level rise. This suggests that the abrupt increase in monsoon rainfall on Flores was not due to a change in the ITF – because a decrease in rainfall would be expected to accompany cooler local sea surface temperatures (SSTs) – but rather by the sudden increase in ocean surface area and/or temperature in the monsoon source region as the Sunda Shelf flooded during deglaciation. We propose that it was the abrupt intensification of the monsoon through the late deglaciation that maintained the subsequent structure of the ITF following the flooding of the Sunda Shelf at ∼9.5 ka.

A mechanism for land–ocean contrasts in global monsoon trends in a warming climate

A central paradox of the global monsoon record involves reported decreases in rainfall over land during an era in which the global hydrologic cycle is both expected and observed to intensify. It is within this context that this work develops a physical basis for both interpreting the observed record and anticipating changes in the monsoons in a warming climate while bolstering the concept of the global monsoon in the context of shared feedbacks. The global-land monsoon record across multiple reanalyses is first assessed. Trends that in other studies have been taken as real are shown to likely be spurious as a result of changes in the assimilated data streams both prior to and during the satellite era. Nonetheless, based on satellite estimates, robust increases in monsoon rainfall over ocean do exist and a physical basis for this land–ocean contrast remains lacking. To address the contrast’s causes, simulated trends are therefore assessed. While projections of total rainfall are inconsistent across models, the robust land–ocean contrast identified in observations is confirmed. A feedback mechanism is proposed rooted in the facts that land areas warm disproportionately relative to ocean, and onshore flow is the chief source of monsoonal moisture. Reductions in lower tropospheric relative humidity over land domains are therefore inevitable and these have direct consequences for the monsoonal convective environment including an increase in the lifting condensation level and a shift in the distribution of convection generally towards less frequent and potentially more intense events. The mechanism is interpreted as an important modulating influence on the “rich-get-richer” mechanism. Caveats for regional monsoons exist and are discussed.

The effect of Arabian Sea optical properties on SST biases and the South Asian summer monsoon in a coupled GCM

This study examines the effect of seasonally varying chlorophyll on the climate of the Arabian Sea and South Asian monsoon. The effect of such seasonality on the radiative properties of the upper ocean is often a missing process in coupled general circulation models and its large amplitude in the region makes it a pertinent choice for study to determine any impact on systematic biases in the mean and seasonality of the Arabian Sea. In this study we examine the effects of incorporating a seasonal cycle in chlorophyll due to phytoplankton blooms in the UK Met Office coupled atmosphere-ocean GCM HadCM3. This is achieved by performing experiments in which the optical properties of water in the Arabian Sea—a key signal of the semi-annual cycle of phytoplankton blooms in the region—are calculated from a chlorophyll climatology derived from Sea-viewing Wide Field-of-View Sensor (SeaWiFS) data. The SeaWiFS chlorophyll is prescribed in annual mean and seasonally-varying experiments. In response to the chlorophyll bloom in late spring, biases in mixed layer depth are reduced by up to 50% and the surface is warmed, leading to increases in monsoon rainfall during the onset period. However when the monsoons are fully established in boreal winter and summer and there are strong surface winds and a deep mixed layer, biases in the mixed layer depth are reduced but the surface undergoes cooling. The seasonality of the response of SST to chlorophyll is found to depend on the relative depth of the mixed layer to that of the anomalous penetration depth of solar fluxes. Thus the inclusion of the effects of chlorophyll on radiative properties of the upper ocean acts to reduce biases in mixed layer depth and increase seasonality in SST.

Plant migration and plant communities at the time of the “green Sahara”

Around 8500 cal years BP, at the time of the maximum of the African Humid Period, lakes and wetlands expanded in the present-day Sahara while large paleodrainages were formed or re-actived, in response to an orbitally-induced increase in monsoon rainfall. It has been suggested that the direct consequence of this increase in rainfall was the northward displacement of the Sahara/Sahel boundary, thought to have reached 23°N in central and eastern Africa. Here, we show a more complex situation characterized by an increase in biodiversity as the desert accommodated more humid-adapted species from tropical forests and wooded grasslands: tropical plant species now found some 400 to 500 km to the south probably entered the desert as gallery-forest formations along rivers and lakes where they benefited from permanent fresh water. At the same time, Saharan trees and shrubs persisted, giving rise to a vegetation that has no analogue today. In this article, we present distribution maps of selected plant species to show both the amplitude of the vegetation change compared to the present and the composition of the past plant communities. We also estimate the migration rate of tropical plant taxa to their northernmost position in the Sahara. This study is based on the use of several data sets: a data set of the modern plant distribution in northern Africa and a data set of modern and fossil pollen sites (from the African Pollen Database, http://fpd.mediasfrance.org/ and http://medias.obs-mip.fr/apd/).