Moisture Budget Research Articles

Amplified tropical Pacific rainfall variability related to background SST warming

Previous studies have found that under global warming, El Niño/Southern Oscillation (ENSO)-related rainfall variability will become enhanced over the tropical central-eastern Pacific and weakened over the western Pacific. The climatological sea surface temperature (SST) warming pattern exhibits a warming center in the equatorial eastern Pacific in projections. How this pattern contributes to projected changes in ENSO-driven rainfall variability has not been fully addressed. Here, we use “time-slice” experiments to investigate the response of interannual variability in tropical Pacific rainfall in boreal winter to a warming background SST and associated physical mechanisms. A high-resolution Atmosphere General Circulation Model is driven by the detrended observational SST (1979–2003) plus a warming pattern from the coupled model under the A1B emission scenario (2075–2099). The results show that precipitation interannual variability over the tropical central-eastern Pacific will be enhanced more than the surrounding regions under warming, which is mostly contributed by a faster increase in rainfall amount during the El Niño year relative to non-El Niño years. Based on a moisture budget analysis, both the dynamic and thermodynamic components in the vertical advection of climatological specific humidity contribute to the enhancement of El Niño-induced precipitation anomalies in the tropical central-eastern Pacific where the dynamic effect is dominant. Moist static energy budget analysis further illustrates that vertical velocity is enhanced due to the increased transport of moist static energy from the lower troposphere into the middle-upper troposphere and the intensified warming effect of cloud longwave radiation caused by the increase of high cloud amount and altitude.

Projected Changes to Extreme Precipitation Along North American West Coast From the CESM Large Ensemble

AbstractPrecipitation events along the North American (NA) west coast are strongly modulated by atmospheric rivers, yet the mechanisms of their influences on the probability distributions of precipitation events are not well studied. Simulations from the Community Earth System Model (CESM) large ensemble under global warming are investigated using a moisture budget conditioned onto precipitation events for the recurrence intervals ranging from 0.1 to 50 years. In the midlatitudes, the increases in precipitation intensity and accumulation for all events over the NA west coast are predominantly controlled by moisture increases. In contrast, changes in the subtropical precipitation distributions in southwestern NA are associated with moisture increases and duration decreases for all events, with additional dynamical amplification for the heaviest precipitation events. These interpretations from the conditional moisture budget are more consistent with future projection of atmospheric rivers than the conventional mean and transient decomposition of the moisture budget.

Effects of Moisture Initialization on MJO and its Teleconnection Prediction in BCC Subseasonal Coupled Model

AbstractIt is well recognized that moisture is of great importance in the representation of the Madden‐Julian Oscillation (MJO). In this study, a moisture initialization scheme is developed using the Beijing Climate Center (BCC) subseasonal operational coupled model, and its effects on MJO predictions are investigated. Three sets of hindcast experiments are conducted: a reference experiment without moisture initialization (REF) and two sensitivity experiments with moderate and strong moisture nudging schemes. We show that the prediction skill of the real‐time multivariate MJO index is significantly improved in the two sensitivity experiments, where the skillful prediction can reach up to 20 days using the moderate nudging scheme. Larger improvements can be achieved when MJO was initiated from Phases 2–3 and 5–7 or during La Niña episodes, implying that the MJO prediction skill is more sensitive to moisture conditions in active convection regions and backgrounds. The moisture budget analysis indicated that the MJO prediction improvement using the moisture initialization schemes comes from the realistic representation of vertical moisture structure and vertical moisture advection to the east of the MJO convection in the planetary boundary layer. The improved MJO prediction is also demonstrated to enable a more reliable subseasonal prediction of extratropical circulation and precipitation through a more realistic description of MJO‐related teleconnections. In addition, the ensemble mean of the three schemes can extend the MJO prediction skill to more than 22 days, showing the potential benefits of ensemble prediction of multiinitialization schemes.

Spatial Shift of Greenland Moisture Sources Related to Enhanced Arctic Warming

AbstractUnderstanding the atmospheric moisture budget of Greenland is critical for predicting future Greenland climate. However, past attempts to quantify the provenance of Greenland precipitation have been limited to certain seasons. Here we present an analysis of Greenland moisture sources using water tracers in the Goddard Institute for Space Studies climate model, which can provide source estimates for all time periods. The North Atlantic is found to be the dominant moisture source, except during summer when continental sources increase substantially. The variability is also found to be partially correlated with the Greenland Blocking Index. The key finding, however, is a long‐term trend in the moisture source location for Northwest Greenland, with an increase in more locally sourced moisture over time during nonsummer months. This is at least partially related to sea ice loss in the Baffin Bay region, along with a larger increase in sea surface temperatures for the region relative to other locales.

Brief communication: Rare ambient saturation during drifting snow occurrences at a coastal location of East Antarctica

Abstract. Sublimation of snow particles during transport has been recognized as an important ablation process on the Antarctic ice sheet. The resulting increase in moisture content and cooling of the ambient air are thermodynamic negative feedbacks that both contribute to increase the relative humidity of the air, inhibiting further sublimation when saturation is reached. This self-limiting effect and the associated development of saturated near-surface air layers in drifting snow conditions have mainly been described through modelling studies and a few field observations. A set of meteorological data, including drifting snow mass fluxes and vertical profiles of relative humidity, collected at site D17 in coastal Adélie Land (East Antarctica) during 2013 is used to study the relationship between saturation of the near-surface atmosphere and the occurrence of drifting snow in a katabatic wind region that is among the most prone to snow transport by wind. Atmospheric moistening by the sublimation of the windborne snow particles generally results in a strong increase in relative humidity with the magnitude of drifting snow and a decrease in its vertical gradient, suggesting that windborne-snow sublimation can be an important contributor to the local near-surface moisture budget. Despite a high incidence of drifting snow at the measurement location (60.1 % of the time), saturation, when attained, is however most often limited to a thin air layer below 1 m above ground. The development of a near-surface saturated air layer up to the highest measurement level of 5.5 m is observed in only 8.2 % of the drifting snow occurrences or 6.3 % of the time and mainly occurs in strong wind speed and drift conditions. This relatively rare occurrence of ambient saturation is explained by the likely existence of moisture-removal mechanisms inherent to the katabatic and turbulent nature of the boundary-layer flow that weaken the negative feedback of windborne-snow sublimation. Such mechanisms, potentially quite active in katabatic-generated windborne-snow layers all over Antarctica, may be very important in understanding the surface mass and atmospheric moisture budgets of the ice sheet by enhancing windborne-snow sublimation.

The role of spring dry zonal advection in summer drought onset over the US Great Plains

Abstract. This study addresses the role of the atmospheric moisture budget in determining the onset and development of summer droughts over the North American Great Plains (GP) using two state-of-the-art reanalysis datasets. We identified zonal moisture advection as the main cause of severe tropospheric drying during the extreme droughts in the southern GP in 2011 and northern GP in 2012. For both events, the eastward advection of anomalously dry and warm air in the free troposphere in spring set the stage for summer drought. This led to a sharp drop in relative humidity above the boundary layer, enhancing dry entrainment and suppressing deep convection. Further breakdown of the zonal advection into dynamic (caused by circulation anomalies) and thermodynamic (caused by moisture anomalies) contributions reveals dominance of thermodynamic advection in the tropospheric drying observed during the onset of both 2011 and 2012 droughts. The dependence of thermodynamic advection on the moisture gradient links springtime precipitation in the Rockies and southwestern US, the source region of the anomalous dry advection, to the GP summer precipitation (with correlations > 0.4 using gauge-based data). Identifying this previously overlooked precursor of the GP summer droughts improves our predictive understanding of drought onset mechanisms over the region.

Future Changes in East Asian Summer Monsoon Circulation and Precipitation Under 1.5 to 5 °C of Warming

AbstractUnderstanding the link between future changes in East Asian summer monsoon (EASM) and global warming levels is of great importance for regional climate change adaptation and mitigation in East Asia. Here, we analyze the projected changes in EASM circulation and precipitation under different warming levels from 1.5 to 5 °C above the preindustrial global mean temperature, using large‐ensemble simulations conducted with Canadian Earth System Model version 2. We find that the model projects enhanced monsoon circulation and precipitation with global warming. The 850‐hPa meridional winds, precipitation, and 500‐hPa vertical ascending motion will be enhanced nonlinearly, while the total column precipitable water will increase quasi‐linearly. The increase in precipitable water in the wet EASM region is only slightly greater than global average but the increase in precipitation is much greater than global one, with enhanced 500‐hPa vertical ascending motion contrary to global mean. The increased low‐level land‐sea thermal contrast leads to the enhanced EASM meridional circulation and thus bring a large amount of moisture into Eastern China, providing favorable conditions for additional increase in precipitation. A simplified moisture budget analysis shows that the dynamic component related to strengthening monsoon circulation plays dominant role in the increase in EASM precipitation when the global temperature increases by more than approximately 2 °C, while the thermodynamic component caused by increased water vapor is important when the warming is smaller.

Nonlinear optimal moisture perturbations as excitation of primary MJO events in a hybrid coupled climate model

Although the moisture feedback has been well known to be essential in the Madden–Julian Oscillation (MJO) dynamics, whether its pre-moistening effect plays a key role in exciting the onset of primary MJO events, as has been confirmed in the successive initiation, remains elusive. In this study, using a hybrid coupled climate model that has a good fidelity in simulating the intraseasonal variability, we develop a new framework of methodology to investigate the nonlinear excitation of primary MJO event, of which the key achievement is the successful implementation of the conditional nonlinear optimal perturbation (CNOP). In an application of this new framework, the CNOP-type moisture perturbations are calculated for the pre-chosen non-MJO reference states and generally favor a moistening in the equatorial region while drying in the poleward. Comparisons of the model simulation with observation give credibility to the existence of moisture signals several weeks before some primary MJO events. A suite of numerical experiments confirms that the CNOPs of moisture can contribute to the excitation and propagation of strong primary MJO events while random perturbations cannot. The moisture budget analysis further reveals the central importance of the horizontal moisture advection, especially the nonlinearly upscaled moisture transports associated with the high-frequency disturbances on the quasi-3–4-day and 6–8-day synoptic time scales, in supporting the nonlinear excitation of the primary MJO events. The subgrid-scale processes of evaporation, condensation and eddy transport of moisture are found to be critical for the pre-moistening effect in the boundary layer as well. This study directly supports the vital importance of the moisture perturbations, which are characterized by a particular pattern concentrated at low levels, to the nonlinear growth and propagation of the primary MJO events.

Simulation and analysis of the moist vortex associated with the extreme rain event of Ouagadougou in 2009

AbstractThis study analyses the mechanisms involved in the formation of a moist meso‐vortex associated with an extreme rain event that occurred in Burkina Faso on 1 September 2009, on the basis of high‐resolution convection‐permitting simulations. After an evaluation of the 6‐day simulation skill to capture the main characteristics of this event, budgets of heat, moisture and relative vorticity are calculated. Results allow us to propose the following scenario for the occurrence of this extreme rain event, completing the large‐scale analysis of the same event performed in the companion article. Firstly, the arrival of a large‐scale wet spell is a key factor, creating a quite unusual wet environment over the Sahel, resulting in weak rain evaporation. As over oceanic regions, it favours intense heating at low levels and thus strong mean ascent and convergence. Secondly, the combination with the arrival of the trough of an African easterly wave provides a second key factor. The associated relative vorticity maximum on the southern flank of the African easterly jet (AEJ) core is boosted by the strong low‐level convergence through the stretching term. The vortex deepening is favoured by the tilting and by eddies at upper levels. Thirdly, in turn the quasi‐balanced vortex circulation brings warm and moist air from the north, favouring the convection triggering to the west of the vortex in the down‐shear direction, in agreement with Raymond and Jiang's theory for long‐lived mesoscale convective systems. This scenario exhibits a positive feedback that could explain its extreme character.

Contributions of Extreme and Non‐Extreme Precipitation to California Precipitation Seasonality Changes Under Warming

AbstractChanges in seasonality of extreme precipitation have important implications for flood and fire hazards and water resources. Here we explore the contributions of extreme and non‐extreme precipitation in the sharpened wet season in California under warming and the underlying mechanisms. Modeling evidence shows that increased extreme precipitation due to enhanced extreme intensity and more extreme days dominates the wetter winter, while decreased non‐extreme precipitation due to fewer wet days induces the dryer spring and fall. Moisture budget analysis indicates that increased moisture dominates the extreme precipitation increase in winter, while weakened circulation offsets the moisture increase, resulting in no changes in spring and fall. The sharpened seasonal cycle of extreme precipitation via both dynamic and thermodynamic effects is consistent with the seasonality changes in atmospheric river days and is mainly attributed to the seasonality changes in the number of extreme days rather than the intensity during extreme days.

Role of the West African westerly jet in the seasonal and diurnal cycles of precipitation over West Africa

The West African westerly jet (WAWJ) is a low-level jet centered near 10° N over the West African coast from June to September. This study quantifies the low-level moisture transport associated with the WAWJ on seasonal timescales, examines the relationship between the jet and West African precipitation using a moisture budget analysis, and further distinguishes the WAWJ from the West African monsoon (WAM) flow. Three reanalyses and five observational rainfall datasets are examined to build confidence. The WAWJ has a diurnal cycle with a minimum at 12 UTC and a maximum at 18 UTC, dominated by the ageostrophic wind component. According to the momentum budget analysis, the diurnal acceleration/deceleration of the WAWJ is controlled by variations in geopotential height gradient forces associated with the continental thermal low and its westward, offshore extension over the eastern Atlantic. The WAWJ wind speed and rainfall across a large region of the western Sahel (0°–10°W, 8°–18° N) are significantly and positively correlated. In this region, the moisture flux associated with the WAWJ is stronger than that associated with the southerly WAM flow from July 5 to August 20 (45 days). The moisture budget analysis reveals that the seasonal evolution of the rainfall in this analysis region is associated with zonal moisture convergence related to changes of the WAWJ. Enhanced (reduced) rainfall occurs in months with a strong (weak) WAWJ, accompanied by low-level moisture flux anomalies associated with the WAWJ instead of the southerly WAM.

Effects of a stochastic multicloud parameterization on the simulated Asian‐Australian monsoon rainfall in an AGCM

AbstractThe Asian‐Australian monsoon (AAM) plays a vital role in modulating the global and local climates. Current numerical models, including climate system models (CSMs) and atmospheric general circulation models (AGCMs), have difficulties in capturing the fundamental and intrinsic features of the AAM. By applying the stochastic multicloud model (SMCM) to the state‐of‐the‐art ECHAM6.3 atm model, we investigated the impacts of SMCM on the simulation of AAM precipitation. Though the modified model with the SMCM improves simulations of the AAM precipitation in June–July–August–September season, the solstice mode, the AAM domain, and the AAM precipitation intensity, deficiencies also exist in other aspects, that is, simulations of rainfall in December–January–February–March and April–May seasons, and the equinox mode. Analyses also unravelled the modified model improves in simulating the spatial patterns of seasonal variation of precipitation in the Indian monsoon region, western North Pacific monsoon region, and Australian monsoon region in terms of pattern correlation coefficient and normalized root‐mean square error scores. The enhanced simulation of solar radiation in the modified model favours improving the performance of the simulated solstice mode. In addition, moisture budget analysis was applied to investigate physical processes that modulate the variations and distributions of AAM precipitation. Analyses revealed that variations and distributions of the vertical integral of moisture flux convergence is closely associated with changes in precipitation, while the stratification of vertical moist transport is associated with changes in precipitation over land in the AAM region. In addition, the modified model influences the AAM precipitation mainly through the variations of moisture flux convergence. The present work potentially provides a method to improve the simulated capability of AGCMs.

Improved leading modes of interannual variability of the Asian-Australian monsoon in an AGCM via incorporating a stochastic multicloud model

In the present study, we investigated improvements in simulating the two major modes of the Asian-Australian monsoon (AAM) interannual variability by incorporating the stochastic multicloud model (SMCM) into the state-of-the-art ECHAM6.3 atmospheric model. Model results showed that the modified ECHAM6.3, i.e., with the SMCM, improves the simulation of seasonal evolution of anomalous rainfall and low-level circulation. Analysis revealed that the improvement in the simulation of precipitation anomalies of the first mode is associated with the improvement in both enhanced and suppressed convection. In addition, the enhanced easterly vertical shear over the Maritime Continent in the modified ECHAM6.3 contributes to the improvement in an equatorial asymmetry of precipitation anomalies of the first mode. Moreover, the moisture budget analysis demonstrated that the modified ECHAM6.3 improves the seasonal anomalous rainfall of the first mode by ameliorating the vertical mass integral of the moist flux divergence. In addition, the second mode of the AAM interannual variability produced by the modified ECHAM6.3 potentially exerts stronger influence on the ENSO variability compared to the default ECHAM6.3.

Thermodynamic and dynamic effects of increased moisture sources over the Tropical Indian Ocean in recent decades

In the present work, the mechanisms for the changes in moisture sources (evaporation minus precipitation; EmP) during boreal summer (May–September) are explored over the tropical Indian Ocean during 1979–2016. We apply a moisture budget analysis to quantify the thermodynamic and dynamic effects. Our results show that the EmP in the tropical central-eastern and southwestern Indian Oceans experienced significant increasing trends during boreal summer. The increased EmP in the tropical central-eastern Indian Ocean is due to the enhanced dynamic divergence (account for approximately 51%), while a stronger dynamic advection contributes more moisture supply to the southwestern Indian Ocean (account for approximately 34%). We find that during recent decades, the enhanced east–west thermal gradient in the Pacific strengthens the Walker Circulation, which leads to a westward shift in convection over the Indian Ocean warm pool, resulting in weakened convection and ascent over the tropical central-eastern Indian Ocean. The weakened convection leads to an anomalous low-level atmospheric divergent circulation, which intensifies the dynamic divergence contributing to the enhanced EmP over the tropical central-eastern Indian Ocean. Additionally, the warming climate during recent decades also increases the land–sea thermal contrast in the vicinity of the Indian Ocean, which enhances the southeastern wind in the low-level troposphere and leads to an enhanced EmP over the southwestern Indian Ocean.

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.

Modulation of ENSO on Fast and Slow MJO Modes during Boreal Winter

AbstractThis study investigates modulation of El Niño–Southern Oscillation (ENSO) on the Madden–Julian oscillation (MJO) propagation during boreal winter. Results show that the spatiotemporal evolution of MJO manifests as a fast equatorially symmetric propagation from the Indian Ocean to the equatorial western Pacific (EWP) during El Niño, whereas the MJO during La Niña is very slow and tends to frequently “detour” via the southern Maritime Continent (MC). The westward group velocity of the MJO is also more significant during El Niño. Based on the dynamics-oriented diagnostics, it is found that, during El Niño, the much stronger leading suppressed convection over the EWP excites a significant front Walker cell, which further triggers a larger Kelvin wave easterly wind anomaly and premoistening and heating effects to the east. However, the equatorial Rossby wave to the west tends to decouple with the MJO convection. Both effects can result in fast MJO propagation. The opposite holds during La Niña. A column-integrated moisture budget analysis reveals that the sea surface temperature anomaly driving both the eastward and equatorward gradients of the low-frequency moisture anomaly during El Niño, as opposed to the westward and poleward gradients during La Niña, induces moist advection over the equatorial eastern MC–EWP region due to the intraseasonal wind anomaly and therefore enhances the zonal asymmetry of the moisture tendency, supporting fast propagation. The role of nonlinear advection by synoptic-scale Kelvin waves is also nonnegligible in distinguishing fast and slow MJO modes. This study emphasizes the crucial roles of dynamical wave feedback and moisture–convection feedback in modulating the MJO propagation by ENSO.

Changes in the sensitivity of tropical rainfall response to local sea surface temperature anomalies under global warming

AbstractChanges in tropical rainfall variability under global warming are crucial to rainfall changes induced by tropical sea surface temperature (SST) variability modes, which exerts severe influences on human society and the natural environment. As the main factor influencing tropical rainfall variability, changes in the sensitivity of tropical rainfall response to local SST anomalies under global warming remain unclear. This study reveals its two dominant spatial features based on the multi‐model ensemble mean result from phase 5 of the Coupled Model Intercomparison Project. The sensitivity of tropical rainfall response to SST anomalies is enhanced in the equatorial central–eastern Pacific, and exhibits a positive Indian Ocean dipole‐like pattern in the Indian Ocean. A simplified moisture budget decomposition shows that such change is composed of the thermodynamic effect induced by the increase in mean‐state moisture and the dynamic effect induced by the changes in the sensitivity of atmospheric circulation response to SST anomalies. When the thermodynamic effect, displaying a ‘wet‐get‐wetter’ paradigm, is basically offset with a part of the dynamic effect by the general weakening of atmospheric circulation response to SST anomalies under global warming, changes in the sensitivity of rainfall response to SST anomalies is dominated by a residual dynamic effect that reflects the roles of two factors: the spatially non‐uniform SST warming and the climatological SST. As a result, the ‘warmer‐get‐wetter’ paradigm is more important to changes in the sensitivity of rainfall rasponse to SST anomalies than to climatological rainfall changes. Moreover, the non‐uniform SST warming pattern and the climatological SST are also important sources of intermodel uncertainty in the changes of sensitivity of rainfall rasponse to SST anomalies.

Northern Hemisphere land monsoon precipitation changes in the twentieth century revealed by multiple reanalysis datasets

Observations are required to understand monsoon changes over time. Reanalyses are useful supplements to observations; however, their ability to reveal long-term monsoon changes remains unknown. Here, we evaluate against observations the performance of two reanalysis datasets covering the whole twentieth century, the ECMWF reanalysis of the twentieth century (ERA20C) and the Twentieth Century Reanalysis Project (20CR), in terms of reproducing the climatological averages, the centennial co-variation, and long-term trends of Northern Hemisphere land monsoon rainfall (NHLMR). Their performance is compared with three other widely used reanalyses, the NCEP–NCAR reanalysis (NCEP1), the ECMWF 40-year reanalysis (ERA40), and the Japanese 55-year Reanalysis Project (JRA55), for the period after 1955. Results show that the five reanalysis datasets reasonably reproduce the climatological NHLMR and NHLM domains. The leading co-variation mode of NHLMR, with an overall increasing tendency before 1955 and a decreasing tendency afterwards, is captured by the corresponding principal components of the two long-term datasets, with some spatial biases. For the long-term precipitation trends, both the ERA20C and 20CR reproduce the increasing trend during 1901–1955, except for parts of the North African (NAF) monsoon region. However, neither dataset captures the decreasing trend of NHLMR after 1955 because of limitations in the NAF and North American (NAM) monsoon region. The NCEP1, ERA40, and JRA55 datasets also have shortcomings in the NAM region. An overall decreasing NHLMR is only shown in the NCEP1 dataset. A moisture budget analysis reveals that the long-term trends of NHLMR are dominated by the dynamic component of moisture convergence, suggesting a prominent role for atmospheric circulation changes. The influence of residual terms related to observations assimilated in the reanalyses is also discussed.

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.

Evolution of the Asian–African Monsoonal Precipitation over the last 21 kyr and the Associated Dynamic Mechanisms

Abstract The Asian–African monsoonal precipitation (AAMP) has a significant impact on the water availability, biodiversity, and livelihoods of billions of people. A comprehensive understanding of the AAMP behavior over Earth’s history will help to make better future projections. Using a set of transient climate simulations over the last 21 000 years (21 ka), the variation of the AAMP and its responses to various external forcings, including orbital insolation, greenhouse gases (GHGs), and ice sheets, are explored. The precipitation evolutions in the individual monsoon domains have the characteristic of hemispheric synchrony over the last 21 ka. Specifically, the AAMP increased from the Last Glacial Maximum to the early Holocene with several abrupt events and then decreased subsequently. The raised orbital insolation and GHGs lead to an overall AAMP increase, but the enhanced insolation tends to induce a systematic northward shift of the Asian–African monsoon domain. Decreased meltwater discharge could promote the African and Indian monsoonal precipitation through strengthening the Atlantic Ocean meridional overturning circulation. However, the lowering of ice sheets (i.e., orographic effect) results in an anomalous dipole precipitation pattern between North China and India. An analysis of the moisture budget suggests that, although different external forcings may lead to the same sign of precipitation change (e.g., both increased insolation and GHGs can cause the enhanced AAMP), the thermodynamic and dynamic contributions to precipitation could vary greatly by region and forcing. This study provides a reference for the long-term behavior of the AAMP with rising GHGs, higher insolation, and potential melting of the Greenland Ice Sheet.