Northern Hemisphere Summer Monsoon Research Articles

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.

An investigation of the relationship between tropical monsoon precipitation changes and stratospheric sulfate aerosol optical depth

Abstract Stratospheric aerosol geoengineering (SAG) is one of the several solar geoengineering options that have been proposed to counteract climate change. In the case of SAG, reflective aerosols injected into the stratosphere would reflect more sunlight and cool the planet. When assessing the potential efficacy and risks of SAG, the sensitivity of tropical monsoon precipitation changes should be also considered. Using a climate model, we perform several stylized simulations with different meridional distributions and amounts of volcanic sulfate aerosols in the stratosphere. Because tropical monsoon precipitation responds to global mean and interhemispheric difference in radiative forcing or temperature, we quantify the sensitivity of tropical monsoon precipitation to SAG in terms of two parameters: global mean aerosol optical depth (GMAOD) and interhemispheric AOD difference (IHAODD). For instance, we find that the simulated northern hemisphere monsoon precipitation has a sensitivity of −1.33 ± 0.95% per 0.1 increase in GMAOD and −7.62 ± 0.27% per 0.1 increase in IHAODD. Our estimated precipitation changes in terms of the two sensitivity parameters for the global mean precipitation and for the indices of tropical, northern hemisphere, southern hemisphere and Indian summer monsoon precipitation are in good agreement with the model simulated precipitation changes. Similar sensitivity estimates are also made for unit changes in global mean and interhemispheric differences in effective radiative forcing and surface temperature. Our study based on planetary energetics provides a simpler framework for understanding the tropical monsoon precipitation response to external forcing agents.

Investigating hydroclimatic impacts of the 168–158 BCE volcanic quartet and their relevance to the Nile River basin and Egyptian history

Abstract. The Ptolemaic era (305–30 BCE) is an important period of Ancient Egyptian history known for its material and scientific advances, but also intermittent political and social unrest in the form of (sometimes widespread) revolts against the Ptolemaic elites. While the role of environmental pressures has long been overlooked in this period of Egyptian history, ice-core-based volcanic histories have identified the period as experiencing multiple notable eruptions, and a repeated temporal association between explosive volcanism and revolt has recently been noted. Here we analyze the global and regional (Nile River basin) hydroclimatic response to a unique historical sequence of four large and closely timed volcanic eruptions (first a tropical one, followed by three extratropical northern hemispheric events) between 168 and 158 BCE, a particularly troubled period in Ptolemaic history for which we now provide a more detailed hydroclimatic context. The NASA (National Aeronautics and Space Administration) GISS (Goddard Institute for Space Studies) ModelE2.1 Earth system model simulates a strong radiative response with a radiative forcing (top of atmosphere) of −7.5 W m−2 (following the first eruption) and −2.5 W m−2 (after each of the three remaining eruptions) at a global scale. Associated with this, we observe a global surface cooling of the order of 1.5 ∘C following the first (tropical) eruption, with the following three extratropical eruptions extending the cooling period for more than 15 years. Consequently, this series of eruptions is observed to constrain the northward migration of the inter-tropical convergence zone (ITCZ) during the Northern Hemisphere summer monsoon season, and major monsoon zones (African, South Asian, and East Asian) were seen to experience a suppression of rainfall of >1 mm d−1 during the monsoon (JJAS) season averaged for 2 years after each eruption. A substantial suppression of the Indian and North African summer monsoon (over the Nile River headwater region) was seen to strongly affect the modeled river flow in the catchment and discharge at river mouth. River mass flow over the basin was observed to decrease by 29 % and 38 % relative to an unperturbed (non-volcanic) annual mean flow in the first and second year, respectively, after the first (i.e., tropical) eruption. A moderate decrease ranging between 5 % and 18 % was observed after the third and fourth (extratropical) eruptions. These results indicate, in sum, that the first eruption likely produced a strong hydroclimate response, with the following extratropical eruptions prolonging this. These results also support the recently hypothesized association between ice-core-based signals of explosive volcanism and hydroclimatic variability during the Ptolemaic era, including the suppression of the agriculturally critical Nile summer flooding.

Antiphase response of the Indonesian–Australian monsoon to millennial-scale events of the last glacial period

Antiphase behaviour of monsoon systems in alternate hemispheres is well established at yearly and orbital scales in response to alternating sensible heating of continental landmasses. At intermediate timescales without a sensible heating mechanism both in-phase and antiphase behaviours of northern and southern hemisphere monsoon systems are recorded at different places and timescales. At present, there is no continuous, high resolution, precisely dated record of millennial-scale variability of the Indonesian–Australian monsoon during the last glacial period with which to test theories of paleomonsoon behaviour. Here, we present an extension of the Liang Luar, Flores, speleothem δ18O record of past changes in southern hemisphere summer monsoon intensity back to 55.7 kyr BP. Negative δ18O excursions (stronger monsoon) occur during Heinrich events whereas positive excursions (weaker monsoon) occur during Dansgaard-Oeschger interstadials—a first order antiphase relationship with northern hemisphere summer monsoon records. An association of negative δ18O excursions with speleothem growth phases in Liang Luar suggests that these stronger monsoons are related to higher rainfall amounts. However, the response to millennial-scale variability is inconsistent, including a particularly weak response to Heinrich event 3. We suggest that additional drivers such as underlying orbital-scale variability and drip hydrology influence the δ18O response.

Deforestation Drives Desiccation in Global Monsoon Region

AbstractGlobal monsoon system is a major component of the Earth's climate system and carries about two‐thirds of the world's population. How large‐scale deforestation induced changes in water cycle and energy balance may modify global monsoon system and its underlying mechanisms are not well understood. Here, we use simulations of 11 models participating in the Land Use Model Intercomparison Project in the framework of the Coupled Model Intercomparison Project, phase 6 to study the response of global monsoon precipitation and atmospheric circulation to large‐scale deforestation. Global deforestation of about 38% induces annual precipitation reduction of about −15.01 mm yr−1 (Multi‐Model Ensemble) in global monsoon regions and −26.15 mm yr−1 in land monsoon regions. The precipitation reduction is particularly pronounced in the Northern Hemisphere Summer Monsoon (NHSM) regions with decreased precipitation by 16.83 mm in the summer monsoon season from May to September along with weakened circulation intensity. Deforestation causes asymmetrical temperature response in the northern high latitudes and Southern Hemisphere Tropics, leading to decreased hemispheric thermal contrast, which reduces the meridional pressure gradient and weakens the convergence of cross‐equatorial flows into the NHSM trough regions, thereby decreasing the NHSM circulation and precipitation intensity. The deforestation‐induced hemispheric temperature asymmetry also increases the northward cross‐equatorial atmosphere heat transport (c‐eq AHT) that drives southward shift of Intertropical Convergence Zone, which suppresses the NHSM precipitation. Our results show that the drying monsoon region in response to deforestation is fairly consistent among the models implying the adverse effects of large‐scale deforestation beyond the region of land use change.

Multidecadal variation of northern hemisphere summer monsoon forced by the SST inter-hemispheric dipole

The sea surface temperature inter-hemispheric dipole (SSTID) is an important variability mode of global SST anomalies, characterized by an anti-phase variation of SST between the two hemispheres. In this study, the decadal variation of the northern hemisphere summer monsoon (NHSM) is found to be strongly regulated by the SSTID, with positive (negative) phases of the SSTID corresponding to the strengthening (weakening) of NHSM. Both observation and SST-forced atmospheric model simulations suggest that the SSTID related thermal forcing modulates the NHSM by causing planetary-scale atmospheric circulation adjustments. Positive SSTID events lead to coherent increase (decrease) of surface air temperature over the entire northern (southern) hemisphere, increasing the inter-hemispheric thermal contrast (ITC). As sea level pressure changes are just opposite to air temperature, the increase of ITC enhances the inter-hemispheric pressure gradient (southern hemisphere minus northern hemisphere), leading to the strengthening of summer monsoonal circulation and the increase of monsoon rainfall in the northern hemisphere.

Near‐Global Variability of Stratospheric Water Vapor Observed by SAGE III/ISS

AbstractThe Stratospheric Aerosol and Gas Experiment III instrument on the International Space Station (SAGE III/ISS) has been making high quality solar occultation measurements of stratospheric water vapor since June 2017. Here we evaluate the large‐scale geophysical variability of the SAGE III/ISS water vapor measurements for the first 3 years of observations (2017–2020) as part of data validation for retrieval version 5.1 (v5.1). Detailed comparisons of SAGE III/ISS v5.1 with the Aura Microwave Limb Sounder (MLS) version 5 retrievals show overall excellent agreement in terms of seasonal mean structure and large‐scale variability. SAGE III/ISS data capture the well‐known seasonal variations in water vapor including the vertically propagating “tape recorder” in the tropics and lower stratospheric maxima linked to the NH summer monsoons. The high vertical resolution (∼2 km) measurements from SAGE III/ISS demonstrate contributions of the monsoons to the wet phase of “tape recorder” during the Northern Hemisphere (NH) summer. Interannual variations over the short data record are also consistent between SAGE III/ISS and MLS in the stratosphere between 16 and 30 km. We furthermore evaluate large‐scale variations in relative humidity (RH) derived from the high vertical resolution SAGE III/ISS water vapor measurements, highlighting the detailed seasonal behavior and links to thermal structure near the tropical tropopause. Spatial distributions of RH at the cold point tropopause (CPT) show a close link between high RH and the minimum CPT temperature in both the NH winter and summer, consistent with temperature control of water vapor near the tropopause.

Sources of the Intermodel Spread in Projected Global Monsoon Hydrological Sensitivity

AbstractThe projected monsoon hydrological sensitivity, namely, the precipitation change rate per kelvin of global warming, shows substantial intermodel spread among 40 Coupled Model Intercomparison Project phase 5 models. The hydrological sensitivity of the Northern Hemisphere summer monsoon is negatively correlated with that of the Southern Hemisphere summer monsoon. The intermodel spread of the Northern Hemisphere summer monsoon hydrological sensitivity is mainly attributed to the projected interhemispheric temperature gradients and the associated low‐level cross‐equatorial flows. The intermodel spread of the Afro‐Asia summer monsoon sensitivity is rooted in the projected continent‐ocean thermal gradients, while the spread of the North American monsoon sensitivity is related to the projected sea surface temperature pattern in the tropical eastern Pacific and Atlantic. These findings suggest that further constraining monsoon hydrological sensitivity requires a better projection of the warming rate between the Northern and Southern Hemispheres and between the land and ocean, and the sea surface warming pattern in the tropical eastern Pacific and Atlantic.

FIO-ESM v2.0 Outputs for the CMIP6 Global Monsoons Model Intercomparison Project Experiments

Three tiers of experiments in the Global Monsoons Model Intercomparison Project (GMMIP), one of the endorsed model intercomparison projects of phase 6 of the Coupled Model Intercomparison Project (CMIP6), are implemented by the First Institute of Oceanography Earth System Model version 2 (FIO-ESM v2.0), following the GMMIP protocols. Evaluation of global mean surface air temperature from 1870 to 2014 and climatological precipitation (1979–2014) in tier-1 shows that the atmosphere model of FIO-ESM v2.0 can reproduce the basic observed atmospheric features. In tier-2, the internal variability is captured by the coupled model, with the SST restoring to the model climatology plus the observed anomalies in the tropical Pacific and North Atlantic. Simulation of the Northern Hemisphere summer monsoon circulation is significantly improved by the SST restoration in the North Atlantic. In tier-3, five orographic perturbation experiments are conducted covering the period 1979–2014 by modifying the surface elevation or vertical heating in the prescribed region. In particular, the strength of the South Asian summer monsoon is reduced by removing the topography or thermal forcing above 500 m over the Asian continent. Monthly and daily simulated outputs of FIO-ESM v2.0 are provided through the Earth System Grid Federation (ESGF) node to contribute to a better understanding of the global monsoon system.

Global Monsoon Responses to Decadal Sea Surface Temperature Variations during the Twentieth Century: Evidence from AGCM Simulations

AbstractMultidecadal variations in the global land monsoon were observed during the twentieth century, with an overall increasing trend from 1901 to 1955 that was followed by a decreasing trend up to 1990, but the mechanisms governing the above changes remain inconclusive. Based on the outputs of two atmospheric general circulation models (AGCMs) forced by historical sea surface temperature (SST) covering the twentieth century, supplemented with AGCM simulations forced by idealized SST anomalies representing different conditions of the North Atlantic and tropical Pacific, evidence shows that the observed changes can be partly reproduced, particularly over the Northern Hemisphere summer monsoon (NHSM) domain, demonstrating the modulation of decadal SST changes on the long-term variations in monsoon precipitation. Moisture budget analysis is performed to understand the interdecadal changes in monsoon precipitation, and the dynamic term associated with atmospheric circulation changes is found to be prominent, while the contribution of the thermodynamic term associated with humidity changes can lead to coincident wetting over the NHSM domain. The increase (decrease) in NHSM land precipitation during 1901–55 (1956–90) is associated with the strengthening (weakening) of NHSM circulation and Walker circulation. The multidecadal scale changes in atmospheric circulation are driven by SST anomalies over the North Atlantic and the Pacific. A warmer North Atlantic together with a colder eastern tropical Pacific and a warmer western subtropical Pacific can lead to a strengthened meridional gradient in mid-to-upper-tropospheric thickness and strengthened trade winds, which transport more water vapor into monsoon regions, leading to an increase in monsoon precipitation.

Attribution of Global Monsoon Response to the Last Glacial Maximum Forcings

Abstract Investigation of global monsoon (GM) responses to external forcings is instrumental for understanding its formation mechanism and projected future changes. Coupled climate model experiments are performed to assess how the individual and full Last Glacial Maximum (LGM) forcings change GM precipitation. Under the full LGM forcing, the annual and local summer-mean GM precipitation are reduced by 8.5% and 10.8%, respectively, compared to the results in the preindustrial control run; and the reduction of Northern Hemisphere (NH) summer monsoon (NHSM) precipitation is twice as large as its Southern Hemisphere (SH) counterpart (SHSM). The NH–SH asymmetric response is mainly caused by the monsoon circulation change–induced moisture convergence rather than the reduction of moisture content, but the root cause is the continental ice sheet forcing. The NHSM precipitation changes dramatically differ among various single-forcing experiments, while this is not the case for their SH counterparts. The moisture budget analysis indicates the NHSM is dynamically oriented, but SHSM is thermodynamically oriented. The markedly different NHSM circulation changes are caused by different forcing-induced sea surface temperature (SST) patterns, including the North Atlantic cooling pattern forced by the continental ice sheet, the mega–La Niña–like pattern resulting from the greenhouse gas forcing, and the Indian Ocean dipole–like SST pattern caused by the land–sea configuration forcing. Moreover, the distinctive change of “monsoonality” in the Australian–Indonesian monsoon is predominantly forced by the exposure of the land shelf, which enhances precipitation during early summer (November–December) but weakens it in the rest of the year.

Inter‐decadal change of the middle‐upper tropospheric land–sea thermal contrast in the late 1990s and the associated Northern Hemisphere hydroclimate

Although the middle‐upper tropospheric land–sea thermal contrasts play very important roles in modifying global monsoon, the inter‐decadal influences of the middle‐upper tropospheric land–sea thermal contrasts in the Northern Hemisphere on the global monsoon have not investigated specifically. In this study, it is found that the summer middle‐upper tropospheric zonal land–sea thermal contrast in the Eurasia–Pacific region changed from weak during the late 1980s and earlier and middle 1990s to strong after the late 1990s. This feature indicates a strengthened zonal land–sea thermal contrast during summer, which drives a stronger than normal Northern Hemisphere summer monsoon, with a hypothermal and wet climate over most of the subtropical East Asian and tropical African monsoon regions. In addition, the extensive anticyclonic circulation anomalies together with the less precipitation results in a warm and dry climate over the arid and semiarid regions to the west and north of the monsoon region, such as central Asia, Mongolia, and Northeast Asia. The inter‐decadal change of the middle‐upper tropospheric zonal thermal contrast is also closely associated with the warming trends of the extratropical North Pacific and North Atlantic during summer. However, this link possibly reflects a response of the warming oceans to the local atmospheric variability.

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.

Assessing the Impact of Volcanic Eruptions on Climate Extremes Using CMIP5 Models

Abstract This study analyzes extreme temperature and precipitation responses over the global land to five explosive tropical volcanic eruptions that occurred since the 1880s, using CMIP5 multimodel simulations. Changes in annual extreme indices during posteruption years are examined using a composite analysis. First, a robust global decrease in extreme temperature is found, which is stronger than the internal variability ranges (estimated from random bootstrap sampling). Intermodel correlation analysis shows a close relationship between annual extreme and mean temperature responses to volcanic forcing, indicating a similar mechanism at work. The cooling responses exhibit strong intermodel correlation with a decrease in surface humidity, consistent with the Clausius–Clapeyron relation. Second, extreme and mean precipitation reductions are observed during posteruption years, especially in Northern and Southern Hemisphere summer monsoon regions, with good intermodel agreement. The precipitation decreases are also larger than the internal variability ranges and are dominated by the monsoon regions. Moisture budget analysis further reveals that most of the precipitation decrease over the monsoon regions is explained by evaporation decrease, as well as dynamic and thermodynamic contributions. Interestingly, the dynamic effect is found to have a large influence on intermodel spread in precipitation responses, with high intermodel correlation with mean and extreme precipitation changes. These model-based results are largely supported by an observational analysis based on the Hadley Centre Global Climate Extremes Index 2 (HadEX2) dataset for the recent three volcanic eruptions. Our results demonstrate that temperature and precipitation extremes significantly respond to volcanic eruptions, largely resembling mean climate responses, which have important implications for geoengineering based on solar radiation management.

Northern Hemisphere Winter Warming and Summer Monsoon Reduction after Volcanic Eruptions over the Last Millennium.

Observations show that all recent large tropical volcanic eruptions (1850-present) were followed by surface winter warming in the first Northern Hemisphere (NH) winter after the eruption. Recent studies show that climate models produce a surface winter warming response in the first winter after the largest eruptions, but require a large ensemble of simulations to see significant changes. It is also generally required that the eruption be very large, and only two such eruptions occurred in the historical period: Krakatau in 1883 and Pinatubo in 1991. Here we examine surface winter warming patterns after the 10 largest volcanic eruptions between 850 and 1850 in the Paleoclimate Modeling Intercomparison Project 3 last millennium simulations and in the Community Earth System Model Last Millennium Ensemble. These eruptions were all larger than those since 1850. Though the results depend on both the individual models and the forcing data set used, we have found that models produce a surface winter warming signal in the first winter after large volcanic eruptions, with higher temperatures over NH continents and a stronger polar vortex in the lower stratosphere. We also examined NH summer precipitation responses in the first year after the eruptions, and find clear reductions of summer Asian and African monsoon rainfall.

Intraseasonal rainfall variability in North Sumatra and its relationship with Boreal Summer Intraseasonal Oscillation (BSISO)

The Boreal Summer Intraseasonal Oscillation (BSISO) is a mode of climate variability on intraseasonal timescales that dominantly occurs during Northern Hemisphere summer monsoon. Its propagation and activity can be monitored by using indices namely as BSISO1 and BSISO2. The rainfall anomalies and extremes in North Sumatra may be potentially affected by BSISO due to its location within the path of their propagations. This study analyzes the relationship of BSISO with daily rainfall anomalies and extremes during different phases and types of BSISO. The results of spectral analysis show some agreements in the periodicity of BSISO1 and BSISO2 with daily rainfall anomaly. This study also identifies extreme rainfall during different phases of BSISO, showing the increase in the number of extreme rainfall events. High number of extreme rainfall events beyond thresholds at 90th and 95th percentiles are mostly found during phase 1, 2 and 3 of BSISO1 and phase 1 and 2 of BSISO2. Composite analysis conducted separately for spatial standard deviations of daily rainfall anomalies are also performed in this study. Similar results are found, where BSISO influences on rainfall variability especially during phase 1, 2 and 3 of BSISO1, and phase 1 and 2 of BSISO2.

Different Impacts of Typical and Atypical ENSO on the Indian Summer Rainfall: ENSO-Developing Phase

ABSTRACTThe El Niño–Southern Oscillation (ENSO) is a main driving force of the northern hemisphere summer monsoon rainfall, including the Indian Summer Rainfall (ISR). The impacts of typical ENSO and atypical ENSO events on the ISR remain unclear during their developing summers. This study examines the different linkages between a typical ENSO and the ISR and between an a typical ENSO and the ISR. During the developing summer of a typical El Niño, negative rainfall anomalies are seen over the northeastern Indian subcontinent, while the anomalous rainfall pattern is almost the opposite for a typical La Niña; as for an atypical ENSO, the approximate “linear opposite” phenomenon vanishes. Furthermore, an anomalous global zonal wave train is found at mid-latitudes, with a local tripole circulation pattern over central–eastern Eurasia during the developing summer of a typical ENSO, which might explain the corresponding rainfall response over the Indian Peninsula. By contrast, such features are not obvious during the developing summer of an atypical ENSO. Among 106-year historical runs (1900–2005) of nine state-of-the-art models from the Coupled Model Intercomparison Project, Phase 5 (CMIP5), HadGEM2-ES exhibits promising skill in simulating the anomalous circulation pattern over mid-latitudes and central–eastern Eurasia. Probably, it is the model’s ability to capture the linkage between a typical ENSO and the ISR and the characteristics of a typical ENSO that makes the difference.

Response of inland lake dynamics over the Tibetan Plateau to climate change

The water balance of inland lakes on the Tibetan Plateau (TP) involves complex hydrological processes; their dynamics over recent decades is a good indicator of changes in water cycle under rapid global warming. Based on satellite images and extensive field investigations, we demonstrate that a coherent lake growth on the TP interior (TPI) has occurred since the late 1990s in response to a significant global climate change. Closed lakes on the TPI varied heterogeneously during 1976-1999, but expanded coherently and signifi- cantly in both lake area and water depth during 1999-2010. Although the decreased potential evaporation and glacier mass loss may contribute to the lake growth since the late 1990s, the significant water surplus is mainly attributed to increased regional precipitation, which, in turn, may be related to changes in large-scale atmospheric circulation, including the intensified Northern Hemisphere summer monsoon (NHSM) circulation and the poleward shift of the Eastern Asian westerlies jet stream.

Interhemispheric Influence of the Northern Summer Monsoons on Southern Subtropical Anticyclones

Abstract The southern subtropical anticyclones are notably stronger in austral winter than summer, particularly over the Atlantic and Indian Ocean basins. This is in contrast with the Northern Hemisphere (NH), in which subtropical anticyclones are more intense in summer according to the “monsoon heating” paradigm. To better understand the winter intensification of southern subtropical anticyclones, the present study explores the interhemispheric response to monsoon heating in the NH during austral winter. A specially designed suite of numerical model experiments is performed in which summer monsoons in the NH are artificially weakened. These experiments are performed with both an atmospheric general circulation model and a simple two-layer model. The highlight of the findings presented here is that during the boreal summer enhanced tropical convection activity in the NH plays important roles in either maintaining or strengthening the southern subtropical anticyclones. Enhanced NH convection largely associated with the major summer monsoons produces subsidence over the equatorial oceans and the tropical Southern Hemisphere via interhemispheric meridional overturning circulations and increases the sea level pressure locally. In addition, suppressed convection over some regions of climatological subsidence produces stationary barotropic Rossby waves that propagate far beyond the tropics. These stationary barotropic Rossby waves and those forced directly by the summer heating in the NH are spatially phased to strengthen the southern subtropical anticyclones over all three oceans. The interhemispheric response to the NH summer monsoons is most dramatic in the South Pacific, where the subtropical anticyclone nearly disappears in the austral winter without the influence of the NH.

Northern Hemisphere summer monsoon intensified by mega-El Niño/southern oscillation and Atlantic multidecadal oscillation

Prediction of monsoon changes in the coming decades is important for infrastructure planning and sustainable economic development. The decadal prediction involves both natural decadal variability and anthropogenic forcing. Hitherto, the causes of the decadal variability of Northern Hemisphere summer monsoon (NHSM) are largely unknown because the monsoons over Asia, West Africa, and North America have been studied primarily on a regional basis, which is unable to identify coherent decadal changes and the overriding controls on planetary scales. Here, we show that, during the recent global warming of about 0.4 °C since the late 1970s, a coherent decadal change of precipitation and circulation emerges in the entirety of the NHSM system. Surprisingly, the NHSM as well as the Hadley and Walker circulations have all shown substantial intensification, with a striking increase of NHSM rainfall by 9.5% per degree of global warming. This is unexpected from recent theoretical prediction and model projections of the 21st century. The intensification is primarily attributed to a mega-El Niño/Southern Oscillation (a leading mode of interannual-to-interdecadal variation of global sea surface temperature) and the Atlantic Multidecadal Oscillation, and further influenced by hemispherical asymmetric global warming. These factors driving the present changes of the NHSM system are instrumental for understanding and predicting future decadal changes and determining the proportions of climate change that are attributable to anthropogenic effects and long-term internal variability in the complex climate system.