Strong Winter Monsoon Research Articles

Different provenance of separate loess sites in Yangtze River Basin and its paleoenvironmental implications

The dust deposits in the Yangtze River Basin (YRB) have been widely distributed along the Yangtze River valley. Previous studies mainly concentrated in its provenance of each loess site, however, the differences in provenance of loess among different sites in this area have rarely been studied. Here, the geochemical characteristics of the <20 µm fraction in the loess in separate Jiujiang, Xuancheng and Nanjing sites were analyzed. The result shows that the loess in Jiujiang and Nanjing sites, near Yangtze River riverway, share similar geochemical characteristics that may commonly derived from the detrital materials of Yangtze River. However, both sites have significantly different elemental compositions compared with Xuancheng site, which is ∼60 km away from the Yangtze River riverway. These sites share different geochemical characteristics with Eastern Qinling Mountains and Xifeng section on the Chinese Loess Plateau, North China. Combining the distribution of the aeolian sand hills and the loess in this area published by previous studies, we suggest that the exposed river valley floodplain caused by weak summer monsoon provided abundant materials to form local dust cells controlled by strong winter monsoon during the glacial periods. Therefore, those local loess deposits should be identified as the “valley-sourced loess”. These reveal the inherent connection between the monsoon system and the loess deposits in Yangtze River Basin.

Late Tortonian – Piacenzian multi-proxy record of Asian southwest monsoon intensification: evidence from Coastal Makran, southeast Iran

This study presents a long-term, multi-proxy reconstruction of the Asian southwest monsoon during the Tortonian to Piacenzian, based on a 4.78 Ma record from Coastal Makran, northwestern Gulf of Oman, southeast Iran. The integration of humidity proxies (clay minerals, Th/K, volume magnetic susceptibility, and grain size analysis), marine redox sensitivity (Th/U), total organic matter, carbonate content, 87Sr/86Sr ratio, and spectral gamma-ray data conducted here provide valuable information that fill the existing gap in marine palaeoclimate records. The results show that a strong winter monsoon condition associated with relatively low precipitation and subsequently low physical and chemical weathering dominated the region during late Tortonian – late Messinian (7.65–5.83 Ma). However, a few episodes of intense physical and chemical weathering related to high precipitation are observed during this period (6.23–6.01 Ma), which is consistent with increased organic matter input from continental reservoirs to the oceans. In addition, the data indicate that from the latest Messinian (5.82–5.33 Ma) to Zanclean–Piacenzian (5.33–2.87 Ma), a strong summer monsoon accompanied by a relatively wetter condition and higher physical and chemical weathering resulted in a high detrital input into the basin. This higher weathering period is associated with the highest rate of Himalayan uplift, causing enhanced precipitation. Wavelet analysis of spectral gamma-ray data revealed notable periodicities at 750 Ka and 1.7 Ma, with significant periodicities centered around 5.75–6.03 Ma over the latest Messinian – Zanclean. Comparison with palaeoclimate records from other sites indicates a teleconnection with respect to precipitation, weathering, and productivity, especially during the Messinian–Zanclean transition.

The vulnerability of the aquifer units in the flood-affected areas of the east coast Peninsula Malaysia

During the periods of strong winter monsoon, the occurrence of extensive floods in the low-lying areas mostly in the east coast part of the Peninsula Malaysia continually results in huge destruction to public and private utilities, damage to farmlands, and in addition, leads to loss of lives with outbreak of epidemic which prompted evacuations of hundreds of thousands from their homes and communities. This paper presents research results on the impacts of subsurface water on the east coast Peninsula Malaysian floods, therefore complementing the existing data on the flooding events in this area. In our finding, we present the vulnerability of the aquifer units in the flood-affected communities to floods during intense precipitations and then proffer suggestions to prevent near future occurrence. The partial coverage of the alteration zone that is completely nonexistent in this area should have provided the needed essential storage and transmissivity if present in the east coast Peninsula Malaysia environments. Most of the aquifer units in this part are unconfined and are opened to surface water. We applied electrical resistivity tomography methods of geophysical prospecting and borehole lithologic logs to study the subsurface structures prone to flooding in addition to the information on the rainfall data. However, the GOD vulnerability Index rating method was modified and improved upon with the additions of fluctuations of borehole water heights FBWH and the river bank heights RBH together with the topography parameters to produce the GOD-FBRT Index that better refine the method by considering the literal movement of fluids within the subsurface. The configuration of the Kelantan River basin land system as delineated by this study makes the entire east coast areas vulnerable to flooding with the degree of vulnerability as computed in the study area that ranged between 0.1 (very low) and 1.0 (very high).

A two million year record of low-latitude aridity linked to continental weathering from the Maldives

Indian-Asian monsoon has oscillated between warm/wet interglacial periods and cool/dry glacial periods with periodicities closely linked to variations in Earth’s orbital parameters. However, processes that control wet versus dry, i.e. aridity cyclical periods on the orbital time-scale in the low latitudes of the Indian-Asian continent remain poorly understood because records over millions of years are scarce. The sedimentary record from International Ocean Discovery Program (IODP) Expedition 359 provides a well-preserved, high-resolution, continuous archive of lithogenic input from the Maldives reflecting on low-latitude aridity cycles. Variability within the lithogenic component of sedimentary deposits of the Maldives results from changes in monsoon-controlled sedimentary sources. Here, we present X-ray fluorescence (XRF) core-scanning results from IODP Site U1467 for the past two million years, allowing full investigation of orbital periodicities. We specifically use the Fe/K as a terrestrial climate proxy reflecting on wet versus dry conditions in the source areas of the Indian-Asian landmass, or from further afield. The Fe/K record shows orbitally forced cycles reflecting on changes in the relative importance of aeolian (stronger winter monsoon) during glacial periods versus fluvial supply (stronger summer monsoon) during interglacial periods. For our chronology, we tuned the Fe/K cycles to precessional insolation changes, linking Fe/K maxima/minima to insolation minima/maxima with zero phase lag. Wavelet and spectral analyses of the Fe/K record show increased dominance of the 100 kyr cycles after the Mid Pleistocene Transition (MPT) at 1.25 Ma in tandem with the global ice volume benthic δ18O data (LR04 record). In contrast to the LR04 record, the Fe/K profile resolves 100-kyr-like cycles around the 130 kyr frequency band in the interval from 1.25 to 2 million years. These 100-kyr-like cycles likely form by bundling of two or three obliquity cycles, indicating that low-latitude Indian-Asian climate variability reflects on increased tilt sensitivity to regional eccentricity insolation changes (pacing tilt cycles) prior to the MPT. The implication of appearance of the 100 kyr cycles in the LR04 and the Fe/K records since the MPT suggests strengthening of a climate link between the low and high latitudes during this period of climate transition.

Neoglacial climate anomalies and the Harappan metamorphosis

Abstract. Climate exerted constraints on the growth and decline of past human societies but our knowledge of temporal and spatial climatic patterns is often too restricted to address causal connections. At a global scale, the inter-hemispheric thermal balance provides an emergent framework for understanding regional Holocene climate variability. As the thermal balance adjusted to gradual changes in the seasonality of insolation, the Intertropical Convergence Zone migrated southward accompanied by a weakening of the Indian summer monsoon. Superimposed on this trend, anomalies such as the Little Ice Age point to asymmetric changes in the extratropics of either hemisphere. Here we present a reconstruction of the Indian winter monsoon in the Arabian Sea for the last 6000 years based on paleobiological records in sediments from the continental margin of Pakistan at two levels of ecological complexity: sedimentary ancient DNA reflecting water column environmental states and planktonic foraminifers sensitive to winter conditions. We show that strong winter monsoons between ca. 4500 and 3000 years ago occurred during a period characterized by a series of weak interhemispheric temperature contrast intervals, which we identify as the early neoglacial anomalies (ENA). The strong winter monsoons during ENA were accompanied by changes in wind and precipitation patterns that are particularly evident across the eastern Northern Hemisphere and tropics. This coordinated climate reorganization may have helped trigger the metamorphosis of the urban Harappan civilization into a rural society through a push–pull migration from summer flood-deficient river valleys to the Himalayan piedmont plains with augmented winter rains. The decline in the winter monsoon between 3300 and 3000 years ago at the end of ENA could have played a role in the demise of the rural late Harappans during that time as the first Iron Age culture established itself on the Ghaggar-Hakra interfluve. Finally, we speculate that time-transgressive land cover changes due to aridification of the tropics may have led to a generalized instability of the global climate during ENA at the transition from the warmer Holocene thermal maximum to the cooler Neoglacial.

Seismic characteristics and sedimentary record of the late Pleistocene delta offshore southwestern Hainan Island, northwestern South China Sea

A late Pleistocene delta (Hainan palaeo river delta, HNPD hereafter) exceeding [Formula: see text] offshore Hainan Island, northwestern South China Sea (SCS), is investigated using high-resolution seismic and core data to understand the relationship between subaqueous delta development and climate drivers during the Last Glacial Period. The seismic data indicate general progradational configurations toward the southwest or southeast, indicating that the sediments from the eastern slope offshore southwestern Hainan Island were transported southwestward and southeastward. The average thickness of the delta sediment is approximately 35 m, and it has an arcuate shape surrounding the eastern slope. Therefore, the sediment provenances of the HNPD were mainly from the Red River drainage and Hainan Island. Comparison between the core dating results and the global sea-level curve indicates that the delta formed mainly during marine isotope stage 4 (65–56 ka). The topography of the basin, the sea-level change from low stand to high stand, and the southward oceanic currents driven by the glacial-period strong winter monsoon all contributed to the formation of the delta. Because the development of the delta required large riverine sediment input, we speculate that the main reason that the delta’s development ceased was the migration of the river channel along the eastern slope. Based on a comparison between the palaeobathymetric scenarios derived from published sea-level curves and the delta stratigraphy identified from the seismic profiles and cores, we have determined a possible range of relative sea level between 65 and 56 ka for the southwestern coast of Hainan Island, which might also be applicable for a broader region, i.e., the northern SCS.

Phase relationship between summer and winter monsoons over the South China Sea: Indian Ocean and ENSO forcing

This paper identifies the seasonal relationships between monsoon activities over South China (SC) in winter and summer, which can help improve seasonal predictability. A predefined unified monsoon index that can well represent monsoon characteristics in both winter and summer over SC is employed. A time series of this unified monsoon index in summer and winter, and a lead-lag correlation analysis of the unified monsoon index, found that from summer to winter, monsoon activities tend to have an out-of-phase relationship (weak summer with strong winter monsoons or strong summer with weak winter), while from winter to summer, they tend to be in-phase (weak winter with weak summer or strong winter with strong summer). The composite difference between strong and weak winter monsoons shows that in the preceding summer of strong winter monsoons, monsoon activities tend to be weak, due to the influence of developing La Nina-like events; in the ensuing summer, monsoon activities tend to be strong, modulated by an anomalous cyclone over the western North Pacific (WNP), which is triggered by that La Nina-like pattern over the East Pacific. From summer to winter, the Indian Ocean dipole (IOD) mode is evident over the Indian Ocean (IO); from winter to summer, the Indian Ocean Basin (IOB) mode is dominant, which confirms the important role of the IO in these seasonal relationships of monsoon activities. A negative IOD pattern acts together with La Nina-like forcing, enhances northerly anomalies over East Asia, strengthens winter monsoon winds, and thus enhances the out-of-phase relationship. IOB cooling can capture the anomalous signal of El Nino–Southern Oscillation (ENSO) and favor the persistence of an anomalous cyclone over the WNP until the following summer (“capacitor effect”), which is important for the in-phase relationship. These roles are also further verified by numerical experiments that prescribe ENSO-like, IOD-like, and IOB-like heat source anomalies over the Pacific and IO in an anomalous atmospheric general circulation model.

Comparing the impact of strong and weak East Asian winter monsoon on PM2.5 concentration in Beijing

We investigate the relationship between the wintertime PM2.5 concentration in Beijing from 2005 to 2016 and the East Asian winter monsoon (EAWM) strength based on PM2.5 observations and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data. The large-scale atmospheric circulations are compared and the differences in spatial distribution and source areas of Beijing PM2.5 concentration are studied in depth combined with GRAPES-CUACE atmospheric chemistry model and its aerosol adjoint model in strong and weak EAWM years. The results show that the monthly Beijing PM2.5 concentration in winter is significantly anti-correlated with the individual EAWM indices of winter month. According to three EAWM indices, December 2014 and 2016 are selected as the representatives of strong and weak EAWM years for further comparison. In December 2014, the strong EAWM circulation was characterized by the strengthened subtropical westerly jet stream at 300 hPa, the deepened 500 hPa East Asian trough, the strengthened northwesterly wind at 850 hPa in North China, and the developed Siberian High. Conversely, these anomalous patterns in December 2016 were much weaker, resulting in lower boundary layer height, more stable atmosphere stratification and more frequent inversion. The simulated PM2.5 concentration in the northern and southern part of Beijing were affected by the EAWM to different extents, with the values of 55–75 μg/m3 in the north and 95–125 μg/m3 in the south. There was no obvious difference in the locations of sensitive source areas between December 2014 and 2016. However, compared to December 2014, the accumulated sensitivity coefficients of Beijing, Tianjin, Hebei and Shanxi sources at 72 h ahead of the objective time period were 2.2, 5.9, 1.6, and 1.4 times higher, and the key discharge period was 2.6 times longer in December 2016. The contributions of Beijing and Tianjin sources had greater increase rates, stressing the importance of sensitive source areas in the direction of southeast Beijing in weak EAWM year.

Sedimentary Characteristics of the Second Marine Layer During the Late Marine Isotope Stage 3 in Southern Yellow Sea and Their Response to the East Asian Monsoon

We use the particle size of sediments in core YS01A to study the sedimentary environment of the mud deposit in the central South Yellow Sea of China during late Marine Isotope Stages 3 (MIS3; 40.5 kyr–31.3 kyr). In addition, the East Asian Monsoon and its relationship with the North Atlantic Ocean climate change are discussed based on the sensitive grain-size calculation and the spectrum analysis. The results show that during late MIS3, the muddy area in the central South Yellow Sea experienced the evolution of coastal facies, shallow marine facies, coastal facies, and continental facies, with weak hydrodynamic conditions. Compared with other climate indicators, we found that there were many century to millennium-scale climate signals documented in the muddy area sediments in the central South Yellow Sea. According to our particle size results, three strong winter monsoon events occurred at 37.6 kyr, 35.6 kyr and 32.2 kyr. The East Asian Winter Monsoon records in core YS01A are consistent with the Greenland ice core and the Hulu cave stalagmite δ18O. The millennial and centennial scale cycles, which are 55 yr, 72 yr, 115 yr, 262 yr respectively, correspond to solar activity cycles, while the 1049 yr and 2941 yr cycles correspond to the Dansgaard-Oeschger cycles. These cycles indicate that the paleoclimate evolution of the area was controlled by the solar activities, with the high-latitude driving thermohaline circulation as the main energy conveyor belt, followed by the sea-air-land amplification of the winter monsoon variation in the central Yellow Sea in the late MIS3.

Pollen record of early- to mid-Holocene vegetation and climate dynamics on the eastern coast of the Yellow Sea, South Korea

To understand the early- to mid-Holocene vegetation and climate dynamics on the eastern coast of the Yellow Sea, we obtained a sedimentary core with high-resolution accelerator mass spectrometry (AMS) carbon 14 (14C) data from the Gunsan coast in South Korea. The palynological analysis demonstrated that the riverine wetland meadow from 12.1 to 9.8 cal. kyr BP changed to temperate deciduous broad-leaved forest in 9.8–2.8 cal. kyr BP. In addition, the cold climate from 12.1 to 9.8 cal. kyr BP became warmer from 8.5 to 7.3 cal. kyr BP. This was followed by another relatively cold period from 7.3 to 2.8 cal. kyr BP. The temperature change was mainly in response to solar factors. However, there are two relatively humid periods from 12.1 to 9.8 and 8.5 to 7.3 cal. kyr BP, which arose for different reasons. The earlier humid period resulted from strong westerlies and a rapidly rising sea level. The later humid period was produced mainly by the strong East Asian summer monsoon (EASM) and may also be linked to La Niña–like activity. The cold ‘Younger Dryas’ event from 12.0 to 11.4 cal. kyr BP recorded in this study may have been produced by a North Atlantic meltwater pulse. This would have reduced temperatures that were already low because of weak insolation, and the strong winter monsoons would have increased the precipitation.

Effects of the Wind and Ice Conditions on the Upwelling along the Western Kamchatka Coast (Sea of Okhotsk) Based on the Satellite Data

The variability of the upwelling along the western coast of the Kamchatka Peninsula (northeastern part of the Sea of Okhotsk) has been studied based on an analysis of the multisensor satellite data. The intensity of upwelling is estimated on the basis of wind-forced offshore Ekman transport (upwelling index). The wind data for studying the seasonal variation of upwelling were collected in 1999–2009 using a Quik-SCAT/SeaWinds scatterometer. The upwelling events along the western Kamchatka coast were observed in December at the beginning of the winter monsoon period. During the period of strong winter monsoon northern winds from January to the middle of March, the drifting ice prevents the upwelling of the deep water at the western Kamchatka shelf edge under the mean conditions. The oceanographic data show that upwelling in the western coastal zone of Kamchatka was also observed during the transitional periods from winter to the summer monsoon (April). In summer, upwelling events are rarely observed in this region. The main cause of the summer upwelling is the propagation of the atmospheric cyclones over the Kamchatka Peninsula.

The climate change during early Holocene recorded by trace elements of Shenmu section, Mu Us Desert

Through nine trace elements of the Shenmu section, Mu Us Desert is analyzed for the climate changes during Early Holocene. Except Sr, a migration process of P, Mn, Cr, Rb, Nb, Cu, Zn, and V is found in the dune sands, indicating the accumulation processes of the Asian winter monsoon-driven wind erosion and transport and deposition under the cold climatic phase. And accumulation of these elements in different layers of paleo sol dune sands is related to regional geomorphology and element properties. 8.5 winter-summer monsoon cycles in the early Holocene are observed. The duration time and climate of the stronger winter monsoon phases can be compared with the cold climate in the North Atlantic and other parts of China. The result of Red fit 3.8 power spectra shows 310a, 210a, 90a, and 70a centennial cycles for the element, which are consistent with the solar oscillation cycles. These suggest that the external solar activities of the Earth may have great influences on driving the winter and summer monsoon cycles during early Holocene.

Origin of Indian summer monsoon rainfall biases in CMIP5 multimodel ensemble

Significant biases of coupled general circulation models (CGCMs) lead to considerable uncertainty in climate predictions and projections. Based on the historical simulations in the phase 5 of the Coupled Model Intercomparison Project (CMIP5), we identify that state-of-the-art CGCMs commonly simulate insufficient Indian summer monsoon (ISM) rainfall along with too weak monsoon circulations. Such ISM rainfall/circulation biases, however, are absent in the Atmospheric Model Intercomparison Project simulations forced by observed sea surface temperature (SST), suggesting that the common ISM biases are not intrinsic errors of the atmospheric models but arise from the interaction between the monsoon and the oceans. A multimodel statistical analysis further shows that the ISM rainfall/circulation biases in CMIP5 models can be traced back to the excessively cold SST over the northern Indian Ocean (NIO). The systemic cold SST biases in the NIO suppress local convective activity and reduce air temperature, resulting in a weak north–south thermal contrast in the mid-upper troposphere. This would induce an excessively weak ISM circulation and resultant insufficient monsoon rainfall. Furthermore, the dynamic effect of cold NIO SST biases on the ISM rainfall/circulation simulations is also confirmed through several sensitivity experiments by using the widely-applied Weather Research and Forecasting model. To the extent that the cold SST biases over the NIO may originate from an excessively strong Indian winter monsoon, improving the winter monsoon simulation is an important prerequisite for better summer climate simulations and predictions/projections over the broad ISM region.

A numerical study of the South China Sea Warm Current during winter monsoon relaxation

Using a Finite-Volume Community Ocean Model, we investigated the dynamic mechanism of the South China Sea Warm Current (SCSWC) in the northern South China Sea (NSCS) during winter monsoon relaxation. The model reproduces the mean surface circulation of the NSCS during winter, while model-simulated subtidal currents generally capture its current pattern. The model shows that the current over the continental shelf is generally southwestward, under a strong winter monsoon condition, but a northeastward counter-wind current usually develops between 50-and 100-m isobaths, when the monsoon relaxes. Model experiments, focusing on the wind relaxation process, show that sea level is elevated in the northwestern South China Sea (SCS), related to the persistent northeasterly monsoon. Following wind relaxation, a high sea level band builds up along the mid-shelf, and a northeastward current develops, having an obvious vertical barotropic structure. Momentum balance analysis indicates that an along-shelf pressure gradient provides the initial driving force for the SCSWC during the first few days following wind relaxation. The SCSWC subsequently reaches a steady quasi-geostrophic balance in the cross-shelf direction, mainly linked to sea level adjustment over the shelf. Lagrangian particle tracking experiments show that both the southwestward coastal current and slope current contribute to the northeastward movement of the SCSWC during winter monsoon relaxation.

Development of a regional ocean–atmosphere‐wave coupled model and its preliminary evaluation over the CORDEX East Asia domain

ABSTRACTIn this study, a regional ocean–atmosphere‐wave coupled model was developed using the Ocean Atmosphere Sea Ice Soil (OASIS) version 3 coupler and applied to the Coordinated Regional Downscaling Experiment (CORDEX) Phase 1 East Asia domain. The impacts of the non‐breaking surface wave‐induced mixing Bv on the model performance were investigated. The inclusion of the surface wave model enhanced the vertical mixing in the upper 100 m of the ocean and deepened the simulated ocean mixed layer. The too‐shallow ocean mixed layer in the simulation without surface waves was reduced in some places, e.g. over the Bay of Bengal. The coupling of the surface wave model also reduced the cold biases of the ocean temperature in the lower 30 m of the ocean over the southern Northwest Pacific. Due to the shallower ocean mixed layer depth in summer, the effects of Bv were stronger in summer than in winter. For the monsoon simulation, the wave‐coupled simulation favoured an enhanced East Asian summer monsoon due to the colder simulated sea surface temperature (SST) over the Northwest Pacific, whereas in winter, the warmer simulated SST over the area north of 40°N led to a slightly stronger winter monsoon over Far East Asia than did the no wave simulation.

Feedback Attributions to the Dominant Modes of East Asian Winter Monsoon Variations

Abstract This study investigates the variations and feedback attributions of changes in surface temperature between strong and weak East Asian winter monsoons. The variations of winter-mean surface air temperature are dominated by two distinct principal modes that account for 70.9% of the total variance. The first mode features high correlation with the high-latitude atmospheric circulation, including a correlation coefficient of −0.53 with the Arctic Oscillation in January, and the second mode is significantly linked to El Niño–Southern Oscillation, with a correlation coefficient of −0.37. The surface temperature anomalies of each mode are decomposed into partial temperature anomalies resulting from radiative and nonradiative feedback processes by applying a coupled climate feedback–response analysis method to quantify contributions from thermodynamic and dynamic processes. Results indicate that the surface cooling associated with both modes is mainly attributed to the nonradiative feedback processes of atmospheric dynamics and surface sensible heating and to the radiative feedback processes of water vapor and clouds. The first mode exhibits a deep barotropic anomalous high that weakens the high-latitude westerly jet stream but strengthens the midlatitude westerly jet stream. This circulation feature traps cold and dry air over northern East Asia. For the second mode, the ocean and land heat storage processes induce a large thermal gradient over eastern China and the northwestern Pacific, resulting in a large pressure gradient. Northerly anomalies further reinforce the pressure gradient, which favors cold air intruding southward into the tropics.

Winter monsoon variability and its impact on aerosol concentrations in East Asia

We investigate the relationship between winter aerosol concentrations over East Asia and variability in the East Asian winter monsoon (EAWM) using GEOS-Chem 3-D global chemical transport model simulations and ground-based aerosol concentration data. We find that both observed and modeled surface aerosol concentrations have strong relationships with the intensity of the EAWM over northern (30–50°N, 100–140°E) and southern (20–30°N, 100–140°E) East Asia. In strong winter monsoon years, compared to weak winter monsoon years, lower and higher surface PM2.5 concentrations by up to 25% are shown over northern and southern East Asia, respectively. Analysis of the simulated results indicates that the southward transport of aerosols is a key process controlling changes in aerosol concentrations over East Asia associated with the EAWM. Variability in the EAWM is found to play a major role in interannual variations in aerosol concentrations; consequently, changes in the EAWM will be important for understanding future changes in wintertime air quality over East Asia.

Winter climate changes over East Asian region under RCP scenarios using East Asian winter monsoon indices

The changes in the winter climatology and variability of the East Asian winter monsoon (EAWM) for the late 21st century (2070–2099) under the Representative Concentration Pathway (RCP) 4.5 and 8.5 scenarios are projected in terms of EAWM indices (EAWMIs). Firstly, the capability of the climate models participating in the Coupled Model Intercomparison Project phase 5 (CMIP5) in simulating the boreal winter climatology and the interannual variability of the EAWM for the late 20th century (1971–2000) is examined. Nine of twenty-three climate models are selected based on the pattern correlations with observation and a multi-model ensemble is applied to the nine model data. Three of twelve EAWMIs that show the most significant temporal correlations between the observation and CMIP5 surface air temperatures are utilized. The ensemble CMIP5 is capable of reproducing the overall features of the EAWM in spite of some biases in the region. The negative correlations between the EAWMIs and boreal winter temperature are well reproduced and 3–5 years of the major interannual variation observed in this region are also well simulated according to power spectral analyses of the simulated indices. The fields regressed onto the indices that resemble the composite strong winter monsoon pattern are simulated more or less weakly in CMIP5 compared to the observation. However, the regressed fields of sea level pressure, surface air temperature, 500-hPa geopotential height, and 300-hPa zonal wind are well established with pattern correlations above 0.83 between CMIP5 and observation data. The differences between RCPs and Historical indicate strong warming, which increases with latitude, ranging from 1 to 5 °C under RCP4.5 and from 3 to 7 °C under RCP8.5 in the East Asian region. The anomalous southerly winds generally become stronger, implying weaker EAWMs in both scenarios. These features are also identified with fields regressed onto the indices in RCPs. The future projections reveal that the interannual variability of the indices will be maintained with an intensity similar to that of the present. The correlation between monsoon indices and Arctic Oscillation increases over time. On the other hand, the correlation between monsoon indices and North Atlantic Oscillation decreases.

Climate-dependent fluvial architecture and processes on a suborbital timescale in areas of rapid tectonic uplift: An example from the NE Tibetan Plateau

The substantial tectonic uplift (1000–2500m in a few million years) of the Northeastern Tibetan Plateau (NETP), together with the major climatic changes during the Quaternary, provides an opportunity to study the impact of tectonic and climatic changes on the morphological development and sedimentary architecture of fluvial deposits. The effects of these processes are revealed by a terrace staircase, together with the stratigraphy of each individual terrace, in the confluence zone of the Huang Shui and Yellow Rivers in the NETP, during the late Quaternary. On the basis of morphological mapping and OSL-dating, at least seven strath terraces were identified that formed during the last glacial cycle, which are preserved at locations where tectonic uplift was sufficient to separate them altitudinally from one another. The terraces are composed of stacked fluvial gravels, sands and alluvial loams. The principal result is that we demonstrate that the terraces were formed in response to climatic cycles on a suborbital timescale. For each terrace, the lower coarse-grained sediments (gravel and sand) were deposited during cold periods (such as the LGM, MIS3b, MIS4 and MIS5d) associated with a strong Asian winter monsoon. The aggradation during cold periods was associated with floodplain widening. The river incised slightly during the transitions from cold to warm phases, resulting in the transformation of the previous river plain into a terrace. The coarse grained cold phase deposits are covered by inter-bedded, horizontally-laminated silt and sand (representing flood sediments that often contain reworked soil material), during the (cold to warm) transitional phases. The floodplain accumulation on the terrace continued during the subsequent warm period. The warm periods (such as MIS3a, MIS3c, and MIS5a) of the climatic cycles are associated with a strong Asian summer monsoon. Pronounced incision took place at the subsequent warm–cold transitions. After this warm–cold transition, aeolian loess accumulated on the abandoned terrace without any further fluvial reworking. Our results demonstrate that critical thresholds for fluvial response can be crossed at climatic changes on a suborbital timescale given conditions of accelerated tectonic uplift in the NETP. In addition, based on the OSL ages of different units in the sedimentary sequences of the terraces, we conclude that the durations of terrace aggradations and floodplain widening lasted much longer than the periods of fluvial downcutting.

Two Opposite Extreme Events in Seasonal Mean Winter Rainfall over East China during the Past Three Decades

In this study, the extremes of winter seasonal mean precipitation have been investigated by using daily precipitation data from 91 stations in East China, the Na- tional Centers for Environmental Prediction/the National Center for Atmospheric Research (NCEP/NCAR) month- ly reanalysis, and sea surface temperature data from the Hadley Centre for 1979-2007. The largest anomalous rainfall amount was observed in regions south of the Yangtze River. In the most recent three decades, extreme events in the seasonal mean winter precipitation occurred in 1985 and 1997. Because it was influenced mainly by a La Nina event, the precipitation in 1985 showed a deficit following a stronger winter monsoon. The rainfall amount in 1997 was influenced by El Nino and was significantly larger than normal with a weaker winter monsoon. Both the circulation anomalies and wave energy dispersions during the winters of 1985 and 1997 differed significantly. In 1985, the North Atlantic Oscillation anomalously ex- cited the Eurasian-Pacific teleconnection and circum- global teleconnection phenomena. Consequently, Rossby wave energy propagated along the north and south branches of the westerlies, strengthening the East Asian trough along with a stronger winter monsoon, which fa- cilitated the wintertime dry extreme in East China. In 1997, however, Rossby wave energy propagated from low latitudes northeastward into the southern part of China, resulting in a weaker winter monsoon and the wettest winter. The results of this study will be helpful for future monitoring and prediction of extreme winter rainfall events in East China.