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Land-sea surface air temperature (SAT) contrast, an index of tropospheric thermodynamic structure and dynamical circulation, has shown a significant increase in recent decades over East Asia during the boreal summer. In Part I of this two-part paper, observational data and the results of transient warming experiments conducted using coupled atmosphere-ocean general circulation models (GCMs) are analyzed to examine changes in land-sea thermal contrast and the associated atmospheric circulation over East Asia from the past to the future. The interannual variability of the land-sea SAT contrast over the Far East for 1950-2012 was found to be tightly coupled with a characteristic tripolar pattern of tropospheric circulation over East Asia, which manifests as anticyclonic anomalies over the Okhotsk Sea and around the Philippines, and a cyclonic anomaly over Japan during a positive phase, and vice versa. In response to CO increase, the cold northeasterly winds off the east coast of northern Japan and the East Asian rainband were strengthened with the circulation pattern well projected on the observed interannual variability. These results are commonly found in GCMs regardless of future forcing scenarios, indicating the robustness of the East Asian climate response to global warming. The physical mechanisms responsible for the increase of the land-sea contrast are examined in Part II. [ABSTRACT FROM AUTHOR]

Marine Isotope Stage (MIS) 13, an interglacial about 500,000 years ago, is unique due to an exceptionally strong East Asia summer monsoon (EASM) occurring in a relatively cool climate with low greenhouse gas concentrations (GHG). This paper attempts to find one of the possible mechanisms for this seeming paradox. Simulations with an Earth System model LOVECLIM show that the presence of ice sheets over North America and Eurasia during MIS-13 induces a positive phase of the winter North Atlantic Oscillation (NAO) like feature. The ocean having a longer memory than the atmosphere, the oceanic anomalies associated with NAO persists until summer. The signals of summer NAO are transmitted to East Asia to reinforce the monsoon there through the stationary waves excited at the Asian Jet entrance. The geopotential height shows clearly a mid-latitude wave train with positive anomalies over the eastern Mediterranean/Caspian Sea and the Okhotsk Sea and a negative anomaly over Lake Baikal. This reinforces the effect of the high-latitude wave train induced independently by the Eurasian ice sheet topography as shown in previous study. These features reinforce the Meiyu front and enhance the precipitation over East Asia. The results obtained from LOVECLIM are further confirmed by an atmospheric general circulation model, ARPEGE. [ABSTRACT FROM AUTHOR]

Transitional climate zone (TCZ) over East Asia is located between humid and arid regions, which is a highly sensitive and disaster-prone region especially under global climate change. Due to limited water resources, the atmospheric moisture availability has a dominant control on the precipitation variability. Hence, this study is motivated to reveal the key teleconnection patterns and associated moisture processes that govern the interannual variability of the summer precipitation over TCZ. In order to better diagnose moisture budget, the Lagrangian particle dispersion model FLEXPART is employed for quantifying contribution from moisture sources. Above all, the observational analysis highlights two critical modes, one is Eurasian teleconnection (EU) and the other is Circumglobal Teleconnection (CGT). As regards EU pattern, positive EU phase corresponds to ample precipitation in TCZ. In the presence of positive phase, it underlines a "+-+-" pattern of geopotential height anomalies stretching from western Europe to Mongolia plateau. In the context, the cyclonic flow and low pressure over Mongolia plateau act to enhance moisture flux from the west and the south and to prompt upward motions. Further moisture diagnoses illuminate largest increase of moisture uptake in monsoon dominated region, followed by the westerlies dominated region. However, the eventual contribution of summer monsoon is a little bit less than that of westerlies, due to the grand loss en route. In addition, the local evaporation exerts little impact. CGT propagates along the mid-latitude westerly jet, which is positively coupled with the precipitation in TCZ. Under the positive phase, there is an ascending motion over TCZ, which bears great resemblance to the EU case. However, unlike the result of moisture attribution in EU case, the southerly monsoon has the largest contribution followed by local effect, while the westerlies have little impact due to the cancellation of wetting and drying regimes along the pathway. [ABSTRACT FROM AUTHOR]

Understanding the cloud feedback (CFB) process remains a critical challenge for climate projection. While previous studies have mainly focused on the globe, the tropics, or the Southern Ocean, this study analyzes the long-term CFB over East Asia (EA) using data from the Coupled Model Intercomparison Project Phase 6 (CMIP6) and radiative kernels newly developed. We find that the overall CFBs over EA act to amplify warming in an annual scale, with mean values of 0.66 ± 0.47 and 0.63 ± 0.39 W m−2 K−1 for the SSP2-4.5 and SSP5-8.5 scenarios, respectively, although remarkable seasonal discrepancies. There are two distinct subregions: the positive center over the East Asian monsoon region, primarily due to the significant reduction in low cloud cover; the negative center over the Tibet Plateau, largely owing to the reduced high cloud cover and the ice-to-liquid conversion of cloud phase. Furthermore, the CFB over EA changes from negative to positive in the early 1980s, and then enhance at a rate faster than the global average until the early years of the twenty-first century, which may be linked with the weakening and subsequent recovery of the East Asian summer monsoon. Generally, inter-kernel uncertainty is markedly smaller than inter-model uncertainty; however, in regional scale, inter-kernel uncertainty is larger and inter-model uncertainty is smaller than those in the global scale. This feature implies that ignoring inter-kernel uncertainty may bring additional errors to regional CFB quantification. Notably, these results are comparable between the SSP2-4.5 and SSP5-8.5 scenarios, suggesting that CFB processes may be insensitive to emission scenarios. [ABSTRACT FROM AUTHOR]

Transitional Climate Zone (TCZ) over East Asia, characterized by semi-arid climate, is ecologically fragile environment with limited water resources, making atmospheric moisture supply being the key influential factor. This study investigates the moisture sources of summer (JJA) rainfall in the TCZ over East Asia and associated impact during 1979–2010 with the Lagrangian particle dispersion model. Seven moisture source regions and associated contribution are quantified: Eurasia continent to northwest of the TCZ (EC, 32.14%), central-eastern China (CEC, 16.62%), western Pacific Ocean (WPO, 8.58%), South China Sea and Indonesia (SCSI, 2.99%), Bay of Bengal (BOB, 1.4%), Arabian Sea (AS, 0.71%) and local evaporation (TCZ, 32.58%). The direct moisture contribution from ocean (13.67%) is much less than those from the continent (81.34%), due to the great loss en-route. In particular, the local evaporation not only contributes the most moisture among 7 selected source regions, but also exerts the greatest influence in summer precipitation variability in TCZ. Furthermore, the moisture related to summer rainfall over TCZ conveyed by westerlies and monsoon are discriminated according to the dominant system of moisture source regions. It is found that moisture contributions from summer monsoon system (30.3%) and the mid-latitude westerlies (32.14%) dominant areas are quite close. However, further analysis shows that summer monsoon system takes more responsibility for inter-annual fluctuation of summer precipitation in TCZ from the perspective of moisture supply, followed by local evaporation and mid-latitude westerlies. [ABSTRACT FROM AUTHOR]

The future vegetation–climate system over East Asia, as well as its dependence on Representative Concentration Pathways (RCPs), is investigated using a regional climate–vegetation model driven with boundary conditions from Flexible Global Ocean–Atmosphere–Land System Model: Grid-point Version 2. Over most of the region, due to the rising CO2 concentration and climate changes, the model projects greater vegetation density (leaf area index) and gradual shifts of vegetation type from bare ground to grass or from grass to trees; the projected spatial extent of the vegetation shift increases from RCP2.6 to RCP8.5. Abrupt shifts are projected under RCP8.5 over northeast China (with grass replacing boreal needleleaf evergreen trees due to heat stress) and India (with tropical deciduous trees replacing grass due to increased water availability). The impact of vegetation feedback on future precipitation is relatively weak, while its impact on temperature is more evident, especially during DJF over northeast China and India with differing mechanisms. In northeast China, the projected forest loss induces a cooling through increased albedo, and daytime high temperature (Tmax) is influenced more than nighttime low temperature (Tmin); in India, increased vegetation cover induces an evaporative cooling that outweighs the warming effect of an albedo decrease in DJF, leading to a weaker impact on Tmax than on Tmin. Based on a single model, the qualitative aspects of these results may hold while quantitative assessment will benefit from a follow-up regional model ensemble study driven by multiple general circulation models. [ABSTRACT FROM AUTHOR]

Projecting regional rainfall changes in a warmer climate attracts ongoing attention. However, large uncertainty still exists in multi-model projection. In this study, we introduce a bias-corrected method to correct the multi-model projection of changes in East Asian summer monsoon (EASM) rainfall based on the historical and RCP8.5 runs of 25 models from phase 5 of Coupled Model Intercomparison Project. Firstly, the total rainfall changes are separated into the thermodynamic component due to increased specific humidity and the dynamic component due to circulation changes. The thermodynamic component is corrected using the observed present-day rainfall and the increase rate of specific humidity based on the wet-get-wetter mechanism. On the other hand, the dynamic component with the circulation changes is corrected based on a "spatial emergent constraint" method, which is further validated by the perfect model approach. Together, these corrections give an integrated projection for EASM rainfall changes under global warming. Such an approach can improve the signal-to-noise ratio of projection effectively, from the original 0.73 of the multimodel mean to around 1.9. The corrected projection of EASM rainfall changes shows a pronounced increase in southern China, the northwest Pacific and a belt from northern China to northeastern China, and a weak increase in other EASM regions. [ABSTRACT FROM AUTHOR]

Recently, there has been increasing evidence that Arctic warming and extreme cold events over the northern continents are intimately related. Here, we examine how well current climate models simulate the observed relationship between Arctic warming and extratropical cold winters based on historical simulations of the Coupled Model Intercomparison Project Phase 5 (CMIP5). It is shown that most climate models simulate the observed relation reasonably well. In the models, the regional centers of the observed Arctic warming such as the Barents–Kara Seas and the East Siberian–Chukchi Seas are closely associated with the cold winters over East Asia and North America, respectively. However, we also found significant diversity among models. Analyses from the inter-model diversity suggest that atmospheric teleconnection patterns, such as the eastward expansion of the surface high and downstream development of the upper-level cyclonic flow, are keys to the link between Arctic variation and extratropical climate. Temperature budget analysis reveals that these two lower- and upper-level circulations contribute to the leading anomalous cold advection into the downstream regions. [ABSTRACT FROM AUTHOR]

Surface air temperature is an important factor for human quality of life and is a key marker of global climate change. Understanding multidecadal changes in surface air temperature, and accurately predicting future trends, are therefore important for economic development. In this work, we explore multidecadal variability in East Asian surface air temperature (EASAT). We find that EASAT shows a strong multidecadal variability between 1900 and 2017. Observational analysis shows that annual EASAT multidecadal variability is highly associated with the North Atlantic oscillation (NAO) and the NAO leads detrended annual EASAT by 15–20 years. Further analysis illustrates that the NAO precedes annual EASAT multidecadal variability through its leading effect on the Atlantic Multidecadal oscillation (AMO). The AMO influences annual EASAT multidecadal variability through the Africa–Asia multidecadal teleconnection (AAMT) pattern. An NAO-based linear model is therefore established to predict annual EASAT. The model is able to better hindcast annual EASAT based on different periods of the time-series. Due to the joint influences of NAO multidecadal variability and the forcing associated with anthropogenic greenhouse gas emissions, annual EASAT for 2018–2034 is predicted to remain at its current level or even slightly lower, followed by a period of fast warming over the following decades. [ABSTRACT FROM AUTHOR]

The impacts of global warming/climate change on extreme rainfall events during the Baiu period in Japan and their dependency on weather patterns (WPs) were examined using the self-organizing map (SOM) method. To investigate the differences in climate change impacts on daily rainfall among the WPs, a SOM was applied to surface atmospheric circulation data from the Database for Policy Decision Making for Future Climate Change (d4PDF) to determine the dominant heavy rainfall WPs. The obtained SOM shows that different WPs are associated with regional variations in extreme rainfall events. Projected future changes in the occurrence of heavy rainfall displayed a non-uniform spatial distribution. To understand the spatial heterogeneous rainfall changes, the sensitivity of heavy rainfall WPs to climate forcing was evaluated. Results of the SOM analysis suggest that this regional variation in the future changes in extreme rainfall events could be attributed to sensitivity differences between WPs to the climate changes. These sensitivity differences can be attributed to the non-uniform spatial changes in the large-scale climatological background state in East Asia via modulations in the moist air intrusion into Japan. [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) in the Indo-Pacific warm pool (IPWP) plays a key role in influencing East Asian climate, and even affects global-scale climate change. This study defines IPWP Niño and IPWP Niña events to represent the warm and cold phases of IPWP SST anomalies, respectively, and investigates the effects of these events on stratospheric circulation and temperature. Results from simulations forced by observed SST anomalies during IPWP Niño and Niña events show that the tropical lower stratosphere tends to cool during IPWP Niño events and warm during IPWP Niña events. The responses of the northern and southern polar vortices to IPWP Niño events are fairly symmetric, as both vortices are significantly warmed and weakened. However, the responses of the two polar vortices to IPWP Niña events are of opposite sign: the northern polar vortex is warmed and weakened, but the southern polar vortex is cooled and strengthened. These features are further confirmed by composite analysis using reanalysis data. A possible dynamical mechanism connecting IPWP SST to the stratosphere is suggested, in which IPWP Niño and Niña events excite teleconnections, one similar to the Pacific-North America pattern in the Northern Hemisphere and a Rossby wave train in the Southern Hemisphere, which project onto the climatological wave in the mid-high latitudes, intensifying the upward propagation of planetary waves into the stratosphere and, in turn, affecting the polar vortex. [ABSTRACT FROM AUTHOR]

We conducted a statistical study to characterize the nonlinear response of the East Asian summer monsoon (EASM) to its potential forcing factors over the last 260 ka on orbital timescales. We find that both variation in solar insolation and global ice volume were responsible for the nonlinear forcing of orbital-scale monsoonal variations, accounting for ~80% of the total variance. Specifically, EASM records with dominated precession variance exhibit a more sensitive response to changes in solar insolation during intervals of enhanced monsoon strength, but are less sensitive during intervals of reduced monsoon strength. In the case of global ice volume with 100-ka variance, this difference is not one of sensitivity but rather a difference in baseline conditions, such as the relative areas of land and sea which affected the land-sea thermal gradient. We therefore suggest that EASM records with dominated precession variance recorded the signal of a shift in the location of the Inter-tropical Convergence Zone, and the associated changes in the incidence of torrential rainfall; while for proxies with dominated 100-ka variance, it recorded changes in the land-sea thermal gradient via its effects on non-torrential precipitation. [ABSTRACT FROM AUTHOR]

As the leading mode of the global atmospheric mass inter-annual variability, the Southern Hemisphere annular mode (SAM) may exert potential influences to the Northern Hemisphere (NH) climate, but the related physical mechanism is not yet clear. In this study, it is found that the November-December (ND) SAM exhibits a significant inverse relationship with the winter precipitation over East Asia, particularly southern China. Observational and numerical evidences show that anomalous ND SAM is usually associated with a South Atlantic-Pacific dipole sea surface temperature anomaly (SSTA) which persists into ensuring winter. The dipole SSTA can modulate the variability of the Inter-tropical Convergence Zone (ITCZ) in Pacific. Subsequently, a distinguished atmospheric tele-connection pattern is induced and prevails over the NH mid-latitude region as a response to the anomalous ITCZ. Large areas of high pressure anomalies are triggered at upper troposphere over East Asia and centered over southern China, which favors less precipitation over East Asia, particularly southern China, and vice versa. Through such a physical mechanism, the notable influence of the ND SAM can sustain through the following season and impact on the NH winter climate. [ABSTRACT FROM AUTHOR]

Two dominant types of East Asian mobile trough (EAMT) are identified by a novel mobile trough detection algorithm. The two major EAMTs likely pass through to the north and the south of Lake Baikal. In this study, both of synoptic and planetary time scales influences on East Asia are studied. For synoptic scale, southern path of mobile trough shows a higher rate of intensification that of northern path. Southern path has stronger impact on Southeast Asia temperature fluctuation because of more southern pathway and stronger magnitude of wave train. But duration of fluctuation is shorter in southern due to the downstream development. For planetary scale, the northern path shows a large warm anomaly over Southeast Asia and a cold anomaly over Northeast Asia, which is associated with the northward shift of the jet stream. The southern path shows a cold anomaly over East Asia due to a northwesterly anomaly. The forcing of high frequency eddy on low frequency eddy is estimated in terms of momentum and energy. The reinforcement of high frequency eddy flux on low frequency variation can be observed in northern path, but not in southern path. The energy difference for two paths is also caused by the variations of barotropic and baroclinic conversion. The energy tendency of interaction between two frequency eddies is only responsible for sustention of energy anomaly over southwestern Japan. [ABSTRACT FROM AUTHOR]

In this the second of a two-part study, we examine the physical mechanisms responsible for the increasing contrast of the land-sea surface air temperature (SAT) in summertime over the Far East, as observed in recent decades and revealed in future climate projections obtained from a series of transient warming and sensitivity experiments conducted under the umbrella of the Coupled Model Intercomparison Project phase 5. On a global perspective, a strengthening of land-sea SAT contrast in the transient warming simulations of coupled atmosphere-ocean general circulation models is attributed to an increase in sea surface temperature (SST). However, in boreal summer, the strengthened contrast over the Far East is reproduced only by increasing atmospheric CO concentration. In response to SST increase alone, the tropospheric warming over the interior of the mid- to high-latitude continents including Eurasia are weaker than those over the surrounding oceans, leading to a weakening of the land-sea SAT contrast over the Far East. Thus, the increasing contrast and associated change in atmospheric circulation over East Asia is explained by CO-induced continental warming. The degree of strengthening of the land-sea SAT contrast varies in different transient warming scenarios, but is reproduced through a combination of the CO-induced positive and SST-induced negative contributions to the land-sea contrast. These results imply that changes of climate patterns over the land-ocean boundary regions are sensitive to future scenarios of CO concentration pathways including extreme cases. [ABSTRACT FROM AUTHOR]

This study assesses future climate change over East Asia using the Global/Regional Integrated Model system-Regional Model Program (RMP). The RMP is forced by two types of future climate scenarios produced by the Hadley Center Global Environmental Model version 2 (HG2); the representative concentration pathways (RCP) 4.5 and 8.5 scenarios for the intergovernmental panel on climate change fifth assessment report (AR5). Analyses for the current (1980-2005) climate are performed to evaluate the RMP's ability to reproduce precipitation and temperature. Two different future (2006-2050) simulations are compared with the current climatology to investigate the climatic change over East Asia centered in Korea. The RMP satisfactorily reproduces the observed seasonal mean and variation of precipitation and temperature. The spatial distribution of the simulated large-scale features and precipitation by the RMP is generally less reflective of current climatic conditions than that is given by the HG2, but their inter-annual variations in East Asia are better captured by the RMP. Furthermore, the RMP shows higher reproducibility of climate extremes including excessive heat wave and precipitation events over South Korea. In the future, strong warming is distinctly coupled with intensified monsoonal precipitation over East Asia. In particular, extreme weather conditions are increased and intensified over South Korea as follows: (1) The frequency of heat wave events with temperature greater than 30 °C is projected to increase by 131 and 111 % in the RCP 8.5 and 4.5 downscaling, relative to the current climate. (2) The RCP 8.5 downscaling shows the frequency and variability of heavy rainfall to increase by 24 and 31.5 %, respectively, while the statistics given by the RCP 4.5 downscaling are similar to those of the current climate. [ABSTRACT FROM AUTHOR]

An interactive system coupling the Beijing Climate Center atmospheric general circulation model (BCC_AGCM2.0.1) and the Canadian Aerosol Module (CAM) with updated aerosol emission sources was developed to investigate the global distributions of optical properties and direct radiative forcing (DRF) of typical aerosols and their impacts on East Asian climate. The simulated total aerosol optical depth (AOD), single scattering albedo, and asymmetry parameter were generally consistent with the ground-based measurements. Under all-sky conditions, the simulated global annual mean DRF at the top of the atmosphere was −2.03 W m for all aerosols including sulfate, organic carbon (OC), black carbon (BC), dust, and sea salt; the global annual mean DRF was −0.23 W m for sulfate, BC, and OC aerosols. The sulfate, BC, and OC aerosols led to decreases of 0.58° and 0.14 mm day in the JJA means of surface temperature and precipitation rate in East Asia. The differences of land-sea surface temperature and surface pressure were reduced in East Asian monsoon region due to these aerosols, thus leading to the weakening of East Asian summer monsoon. Atmospheric dynamic and thermodynamic were affected due to the three types of aerosol, and the southward motion between 15°N and 30°N in lower troposphere was increased, which slowed down the northward transport of moist air carried by the East Asian summer monsoon, and moreover decreased the summer monsoon precipitation in south and east China. [ABSTRACT FROM AUTHOR]

Global warming projection experiments were conducted using a 20-km mesh global atmospheric model, focusing on the change in the rain band of East Asian summer monsoon. To assess the uncertainty of climate change projections, we performed ensemble simulations with the 60-km resolution model combining four different SSTs and three atmospheric initial conditions. In the present-day climate simulations, the 20-km model reproduces the rain band of East Asian summer monsoon better than lower resolution models in terms of geographical distribution and seasonal march. In the future climate simulation by the 20-km model, precipitation increases over the Yangtze River valley in May through July, Korean peninsula in May, and Japan in July. The termination of rainy season over Japan tends to be delayed until August. Ensemble simulations by the 60-km model show that precipitation in the future climate for July increases over the Yangtze River valley, the East China Sea and Japan. These changes in precipitation are partly consistent with those projected by the 20-km model. Simulations by the 20-km and 60-km models consistently show that in the future climate the termination of rainy season over Japan tends to be delayed until August. The changes in the vertically integrated water vapor flux show the intensification of clockwise moisture transport over the western Pacific subtropical high. Most precipitation changes over the East Asia can be interpreted as the moisture convergence resulting from change in the horizontal transport of water vapor. [ABSTRACT FROM AUTHOR]

This study aims at (1) exploring dominant atmospheric dynamical processes which are responsible for climate model-simulated land-use impacts on Asian monsoon; and (2) assessing uncertainty in such model simulations due to their skills in simulating detailed monsoon circulations in the region. Firstly, results from a series of the Australian Bureau of Meteorology Research Centre (BMRC) global model simulations of land-use vegetation changes (LUC) in China are analysed. The model showed consistent signals of changes in atmospheric low-level vertical profile and regional circulations responding to LUC. In northern winter, the model-simulated rainfall reduction and surface cooling are associated with an enhanced southward penetration of dry and cold air mass, which impedes warm and humid air reaching the region for generating cold-front rainfall. In its summer, an enhanced cyclonic circulation responding to LUC further blocks the northeast penetration of southwestly summer monsoon flow into the region and results in rainfall decreases and a surface warming. Secondly, we have explored uncertainties in the proposed mechanism operating in the global model. By comparing its results with a set of high-resolution regional model simulations using the same vegetation datasets, it reveals similar changes in winter rainfall but opposite features in summer rainfall responses. In the global model, there is a cyclonic low-level circulation pattern over the South China Sea and adjacent region, an unsatisfactory feature commonly seen in other global climate models. With the reduction in surface roughness following LUC, such a deficiency becomes more prominent which further results in a weakened south/southwestly summer monsoon flow and rainfall reduction. In contrast, in the regional model, its southwestly summer monsoon flow is further enhanced due to the same process as reduced surface roughness. The enhanced monsoon flow further pushes the East Asian monsoon rainfall belt more northward and increases summer rainfall in the Yangtze River region. This study highlights the need for better monsoon simulations in climate models to produce reliable climate change projections in the region. [ABSTRACT FROM AUTHOR]

A modelling experiment with changes in vegetation types at the Last Glacial Maximum (21 ka BP) is performed by using an AGCM coupled with a simplified simple biosphere model (AGCM+SSiB). The results show that the decrease in temperature is large in Tibet and Eastern Asia at 21 ka. The amplitudes of the decrease vary with seasons and locations. Annual mean temperature decreases between 13 °C and 2 °C. Summer and annual precipitation in eastern China at 21 ka is only about 50% of that at present; while little more or unchanged precipitation appears in northwest, west and southwest of China. The effective precipitation (P–E) shows much drier conditions in eastern and southeastern China than today. A comparison with geological evidences of lake-status and pollen-based vegetation shows that this simulation is in broad scale agreement with the geological records. The changes by using palaeo-vegetation data to prescribe land surface show improved signals in the 21 ka climate simulations in East Asia. This result encourages us to study the potential causes, feedbacks and mechanisms of palaeoclimate changes using the AGCM+SSiB modelling approach continuously. [ABSTRACT FROM AUTHOR]