Atmospheric Circulation Systems Research Articles

Atmospheric feedbacks in North Africa from an irrigated, afforested Sahara

Afforestation of the Sahara has been proposed as a climate engineering method to sequester a substantial amount of carbon dioxide, potentially effective to mitigate climate change. Earlier studies predicted changes in the atmospheric circulation system. These atmospheric feedbacks raise questions about the self-sustainability of such an intervention, but have not been investigated in detail. Here, we investigate changes in precipitation and circulation in response to Saharan large-scale afforestation and irrigation with NCAR’s CESM-WACCM Earth system model. Our model results show a Saharan temperature reduction by 6 K and weak precipitation enhancement by 267 mm/year over the Sahara. Only 26% of the evapotranspirated water re-precipitates over the Saharan Desert, considerably large amounts are advected southward to the Sahel zone and enhance the West African monsoon (WAM). Different processes cause circulation and precipitation changes over North Africa. The increase in atmospheric moisture leads to radiative cooling above the Sahara and increased high-level cloud coverage as well as atmospheric warming above the Sahel zone. Both lead to a circulation anomaly with descending air over the Sahara and ascending air over the Sahel zone. Together with changes in the meridional temperature gradient, this results in a southward shift of the inner-tropical front. The strengthening of the Tropical easterly jet and the northward displacement of the African easterly jet is associated with a northward displacement and strengthening of the WAM precipitation. Our results suggest complex atmospheric circulation feedbacks, which reduce the precipitation potential over an afforested Sahara and enhance WAM precipitation.

Bi-decadal climate reconstruction derived from a 1200-year long pollen record from the NE Qinghai-Tibet Plateau and its implications for discussion of the late Holocene environmental dynamics

This study presents bi-decadal scale pollen record of climate change from the Daotang Pond (36°57′35.4 N, 100°74′48.4 E; 3205 m a.s.l.). The pollen record situated 2 km southeast of Lake Qinghai in the north-eastern part of the Qinghai-Tibet Plateau (QTP) reveals a pattern of consequent changes in the alpine meadow and steppe vegetation throughout the last 1200 years. Due to its location at the interface of the East Asian Summer Monsoon (EASM) and the North Atlantic Westerlies, the Daotang record reacts sensitively even to minor changes in precipitation. Consequently, impact of both these major atmospheric circulation systems on the regional environments can be analysed and discussed along with the other climatic proxies from the region. The EASM impact was generally strong throughout the Medieval Warm Period (MWP), which lasted from the beginning of the record at 850 CE up to roughly 1400 CE. This time interval was characterised by a favourable warm and rather moist climate with a precipitation peak around 950 CE. However, two dry spells indicating irregular EASM weakening occurred around 1050–1150 CE and 1250–1400 CE. Westerlies dominated the atmospheric circulation during the Little Ice Age (LIA). This cold phase started around 1450 CE and lasted roughly until 1950 CE. It was characterised by a drier regional climate compared to the humid MWP, though lower evaporation due to decreased summer temperatures helped in keeping locally wet environment at the study site. Three dry and cold intervals are detected in the Daotang record at 1450–1550 CE, 1650–1700 CE and 1800–1850 CE, based on an exceptionally low Artemisia/Chenopodiaceae (A/C) pollen ratio and a generally low pollen concentration. After 1950 CE a shift towards a warmer and wetter climate occurred interrupted by a pronounced dry spell during the recent years.

Southern Hemisphere anticyclonic circulation drives oceanic and climatic conditions in late Holocene southernmost Africa

Abstract. Due to the high sensitivity of southern Africa to climate change, a reliable understanding of its hydrological system is crucial. Recent studies of the regional climatic system have revealed a highly complex interplay of forcing factors on precipitation regimes. This includes the influence of the tropical easterlies, the strength of the southern hemispheric westerlies as well as sea surface temperatures along the coast of the subcontinent. However, very few marine records have been available in order to study the coupling of marine and atmospheric circulation systems. Here we present results from a marine sediment core, recovered in shallow waters off the Gouritz River mouth on the south coast of South Africa. Core GeoB18308-1 allows a closer view of the last ∼ 4 kyr. Climate sensitive organic proxies, like the distribution and isotopic composition of plant-wax lipids as well as indicators for sea surface temperatures and soil input, give information on oceanographic and hydrologic changes during the recorded time period. Moreover, the micropaleontology, mineralogical and elemental composition of the sediments reflect the variability of the terrigenous input to the core site. The combination of down-core sediment signatures and a catchment-wide provenance study indicate that the Little Ice Age ( ∼ 300–650 cal yr BP) was characterized by climatic conditions favorable to torrential flood events. The Medieval Climate Anomaly ( ∼ 950–650 cal yr BP) is expressed by lower sea surface temperatures in the Mossel Bay area and humid conditions in the Gouritz River catchment. These new results suggest that the coincidence of humid conditions and cooler sea surface temperatures along the south coast of South Africa resulted from a strengthened and more southerly anticyclonic circulation. Most probably, the transport of moisture from the Indian Ocean by strong subtropical easterlies was coupled with Agulhas Bank upwelling pulses, which were initiated by an increase in Agulhas Current strength.

Distribution of ostracods in west-central Argentina related to host-water chemistry and climate: implications for paleolimnology

Ecological and biogeographical studies of Neotropical non-marine ostracods are rare, although such information is needed to develop reliable paleoecological and paleoclimatic reconstructions for the region. An extensive, yet little explored South American area of paleoclimatic interest, is the arid-semiarid ecotone (Arid Diagonal) that separates arid Patagonia from subtropical/tropical northern South America, and lies at the intersection of the Pacific and Atlantic atmospheric circulation systems. This study focused on the Laguna Llancanelo basin, Argentina, a Ramsar site located within the Arid Diagonal, and was designed to build a modern dataset using ostracods (diversity, spatial distribution, seasonality, habitat preferences) and water chemistry. Cluster and multivariate analysis of the data indicated that salinity is the most significant variable segregating two ostracod groups. Limnocythere aff. staplini is the only species that develops abundant populations in the saline ephemeral Laguna Llancanelo during almost all seasons, and is accompanied by scarce Cypridopsis vidua in summer. The latter species is abundant in freshwater lotic sites, where Ilyocypris ramirezi, Herpetocypris helenae, and Cyprididae indet. are also found in large numbers. Darwinula stevensoni, Penthesilenula incae, Heterocypris incongruens, Chlamydotheca arcuata, Chlamydotheca sp., Herpetocypris helenae, and Potamocypris smaragdina prefer freshwater lentic conditions (springs), with C. arcuata and Chlamydotheca sp. found only in the Carapacho warm spring, which has a year-round constant temperature of ~20 °C. Seasonal sampling was necessary because some taxa display a highly seasonal distribution. Species that were recorded have either subtropical or Patagonian affinities, although a few taxa are endemic or common to both regions. These data can serve as modern analogues for reconstructing the late Quaternary history of the area, and to investigate the extent and position of the arid/semiarid ecotone (Arid Diagonal) during past glacial/interglacial cycles.

Biome changes in Asia since the mid-Holocene – an analysis of different transient Earth system model simulations

Abstract. The large variety of atmospheric circulation systems affecting the eastern Asian climate is reflected by the complex Asian vegetation distribution. Particularly in the transition zones of these circulation systems, vegetation is supposed to be very sensitive to climate change. Since proxy records are scarce, hitherto a mechanistic understanding of the past spatio-temporal climate–vegetation relationship is lacking. To assess the Holocene vegetation change and to obtain an ensemble of potential mid-Holocene biome distributions for eastern Asia, we forced the diagnostic biome model BIOME4 with climate anomalies of different transient Holocene climate simulations performed in coupled atmosphere–ocean(–vegetation) models. The simulated biome changes are compared with pollen-based biome records for different key regions.In all simulations, substantial biome shifts during the last 6000 years are confined to the high northern latitudes and the monsoon–westerly wind transition zone, but the temporal evolution and amplitude of change strongly depend on the climate forcing. Large parts of the southern tundra are replaced by taiga during the mid-Holocene due to a warmer growing season and the boreal treeline in northern Asia is shifted northward by approx. 4° in the ensemble mean, ranging from 1.5 to 6° in the individual simulations, respectively. This simulated treeline shift is in agreement with pollen-based reconstructions from northern Siberia. The desert fraction in the transition zone is reduced by 21 % during the mid-Holocene compared to pre-industrial due to enhanced precipitation. The desert–steppe margin is shifted westward by 5° (1–9° in the individual simulations). The forest biomes are expanded north-westward by 2°, ranging from 0 to 4° in the single simulations. These results corroborate pollen-based reconstructions indicating an extended forest area in north-central China during the mid-Holocene. According to the model, the forest-to-non-forest and steppe-to-desert changes in the climate transition zones are spatially not uniform and not linear since the mid-Holocene.

Circulation system configuration characteristics of four rainfall patterns in summer over the East China

In this study, the simultaneous atmospheric circulation system configuration characteristics of the four rainfall patterns (FRP) over the East China during the period 1951–2015 are analyzed in order to investigate their formation mechanisms. The results confirm that the FRP possess obvious differences in the upper-level, middle-level, and lower-level troposphere. In northern China rainfall pattern (NCP) years, the East Asian subtropical westerly jet stream (EAJS) shows a northward trend, with a higher intensity than normal; the blocking high (BH) in the mid-high latitudes is inactive; and the western Pacific subtropical high (WPSH) tends to be stronger, with a location to the north of its normal position. The East Asian summer monsoon (EASM) is stronger, which promotes vapor transport to northern China, and this leads to increased rainfall. In intermediate rainfall pattern (IRP) years, the EAJS position is close to that in normal years; the BH is inactive; the WPSH tends to be weaker, with a location to the east of its normal position; and the EASM is stronger, which is conducive to increased rainfall over the Huaihe River Basin. In Yangtze River rainfall pattern (YRP) years, the circulations are found to be almost opposite in their features to those in NCP years. In South China rainfall pattern (SCP) years, the circulations are found to be almost opposite in their features to those in IRP years. This leads to increased rainfall over South China. Therefore, the different circulation system configuration characteristics lead to the different rainfall patterns.

Temperature Surface of Guaíba Lake, RS, from time series of MODIS images

ABSTRACT The Guaíba lake is located in an area of complex weather variation and is influenced by many atmospheric circulation systems, bringing about violent occluded fronts, and, sometimes, intense precipitation. In Rio Grande do Sul, during El Niño, air temperatures and the precipitation index are higher, contrary to La Niña. Moreover, the Guaíba Lake receives water from the Guaíba's Hydrographical Region, which corresponds to 1/3 of Rio Grande do Sul State, and is thus an important water body to the metropolitan region of Porto Alegre. Methods that seek to understand the behavior of Guaíba lake surface water temperature (LSWT) may lead to relevant information to identify periods of more or less water warming, as well as the relations between LSWT, water quality deterioration and risks to human health. This paper aims to comprehend the behavior of Guaíba LSWT during periods of climatic anomalies (El Niño/La Niña). Therefore, 418 sea surface temperature (SST) images from the MODIS sensor were processed with SeaDas 7.2 software. The quarterly averages of LSWT were obtained and compared to the climatological anomalies in Equatorial Pacific Ocean. LSWT behavior is more complex in El Niño/La Niña periods. The results show that during climatic abnormality periods there are no direct relationship between the warming/cooling of Guaíba LSWT and the warming/cooling of Equatorial Pacific Ocean’s SST. The precipitation indices were more significant to the behavior of LSWT during El Niño periods, but for all periods (of climatic normality and abnormality), air temperature is what most influences LSWT. This relation occurs with climatic factors of water retention time, water entry and precipitation, and air temperature. There is a major correspondence during La Niña periods with the cooling of Guaíba LSWT only for some years. On the other hand, during El Niño periods there are no correspondences of this phenomenon with the warming of Guaíba LSWT. There are only more intense oscillations in surface temperatures than during regular and La Niña periods, but with a tendency to LSWT warming.

BIBO stability of fractional-order controlled nonlinear systems

ABSTRACTThe BIBO stability of fractional-order controlled nonlinear systems is investigated in this paper. By introducing the properties of some Mittag-Leffler functions and using an inequality satisfied by the Caputo derivative of a Lyapunov function, sufficient conditions to guarantee the BIBO stability are firstly presented. Then, the boundedness of solutions of the fractional financial system and the fractional low-order atmospheric circulation system are proved by the established stability results. Besides, two illustrative examples verify the theoretical results.

Variation of the mid-latitude westerlies and Indian summer monsoon since the last deglaciation: Evidence from grain-size data from Linggo Co, central Tibet

The mid-latitude westerlies and Indian summer monsoon (ISM) have a significant impact on the water budget and general environment of the Tibetan Plateau (TP), and much work has been conducted in order to characterize variations in these two atmospheric circulation systems on various time scales. However, the nature of the interplay between the Asian summer monsoon and the westerlies during the last deglaciation remains controversial. In this study, a sediment core from Linggo Co, a closed lake in the central TP, was analyzed for grain size and ostracod content. The results suggest that environmental change in the Linggo Co basin during the past 17 ka can be divided into four stages: during Stage 1 (17–11.7 ka) Linggo Co was a shallow lake characterized by high amplitude hydrological fluctuations. Strong winds prevailed in winter, correlative with cold events in the North Atlantic region, and indicating the dominance of the westerlies. During Stage 2 (~11.7 ka), in the early Holocene, Linggo Co expanded rapidly and there was the disappearance of the shallow-water ostracod species Limnocythere inopinata together with a decrease in mean grain-size. These changes indicate that ISM was enhanced and that it was the dominant atmospheric circulation affecting TP rather than the westerlies. During Stage 3 (10–8 ka) Linggo Co experienced several intervals with a negative water budget, which may be attributed to a weakening of the weaken ISM or to the reduced influx of glacial meltwater as a result of the influence of cold climatic events. During Stage 4, after 8 ka, Linggo Co shrank progressively in parallel with a gradual weakening of the ISM and finally a transition to the present environment occurred at ~2.5 ka. A comparison of the record from Linggo Co and other records from the TP reveals that prior to 16.5 ka the mid-latitude westerlies was the dominant atmospheric circulation system controlling the entire TP. From 16.5 to 11.5 ka, the mid-latitude westerlies still controlled the central-northern TP; however, the ISM controlled the southern TP, which had a similar climatic pattern to the present day. During the early-middle Holocene, the ISM was the dominant system affecting the entire TP; however, its influence was considerably reduced during the late Holocene, which was probably accompanied by a strengthening of the westerlies.

Precipitation evolution of Central Asia during the last 5000 years

Central Asia is located at the confluence of large-scale atmospheric circulation systems. However, the number of Holocene climate records is still low in most parts of this region and insufficient to allow detailed discussion and comparisons to disentangle the complex climate history and interplays between the different climatic systems. Here, we present the first stalagmite record from arid Central Asia (south-western Kyrgyzstan) by using δ18O, δ13C, and micro x-ray fluorescence (µXRF)-sulfur data spanning the last 5000 years. The cave hosting stalagmite Uluu-2 is ideally suited to identify past shifts in seasonal variations in precipitation in this part of the world. Comparison of instrumental and paleo-isotopic studies demonstrates that the Uluu-2 speleothem isotope composition faithfully records climate changes and responds to shifts in the proportion of moisture derived from mid-latitude Westerlies during the winter/spring season. The reconstructions suggest that the area was characterized by a dry climate from 4700 to 3900 yr BP, interrupted by a wet episode around 4200 yr BP. Further drier conditions also occurred between 4000 and 3500 yr BP. Wetter conditions were re-established at ca. 2500 yr BP, after another dry episode between 3000 and 2500 yr BP. With the exception of two short dry events (1150 and 1300 yr BP), the period after 1700 yr BP shows moderate to wet conditions. Regional comparisons suggest that the strength and position of the Westerly winds control climatic shifts in arid Central Asia, leading to complex local responses.

Detecting spatio-temporal modes in multivariate data by entropy field decomposition

A new data analysis method that addresses a general problem of detecting spatio-temporal variations in multivariate data is presented. The method utilizes two recent and complimentary general approaches to data analysis, information field theory (IFT) and entropy spectrum pathways (ESPs). Both methods reformulate and incorporate Bayesian theory, thus use prior information to uncover underlying structure of the unknown signal. Unification of ESP and IFT creates an approach that is non-Gaussian and nonlinear by construction and is found to produce unique spatio-temporal modes of signal behavior that can be ranked according to their significance, from which space–time trajectories of parameter variations can be constructed and quantified. Two brief examples of real world applications of the theory to the analysis of data bearing completely different, unrelated nature, lacking any underlying similarity, are also presented. The first example provides an analysis of resting state functional magnetic resonance imaging data that allowed us to create an efficient and accurate computational method for assessing and categorizing brain activity. The second example demonstrates the potential of the method in the application to the analysis of a strong atmospheric storm circulation system during the complicated stage of tornado development and formation using data recorded by a mobile Doppler radar. Reference implementation of the method will be made available as a part of the QUEST toolkit that is currently under development at the Center for Scientific Computation in Imaging.

Seasonal Atmospheric Responses to Reduced Arctic Sea Ice in an Ensemble of Coupled Model Simulations

Abstract Arctic sea ice decline is expected to continue throughout the twenty-first century as a result of increased greenhouse gas concentrations. Here we investigate the impact of a strong Arctic sea ice decline on the atmospheric circulation and low pressure systems in the Northern Hemisphere through numerical experimentation with a coupled climate model. More specifically, a large ensemble of 1-yr-long integrations, initialized on 1 June with Arctic sea ice thickness artificially reduced by 80%, is compared to corresponding unperturbed control experiments. The sensitivity experiment shows an ice-free Arctic from July to October; during autumn the largest near-surface temperature increase of about 15 K is found in the central Arctic, which goes along with a reduced meridional temperature gradient, a decreased jet stream, and a southward shifted Northern Hemisphere storm track; and the near-surface temperature response in winter and spring reduces substantially due to relatively fast sea ice growth during the freezing season. Changes in the maximum Eady growth rate are generally below 5% and hardly significant, with reduced vertical wind shear and reduced vertical stability counteracting each other. The reduced vertical wind shear manifests itself in a decrease of synoptic activity by up to 10% and shallower cyclones while the reduced vertical stability along with stronger diabatic heating due to more available moisture may be responsible for the stronger deepening rates and thus faster cyclone development once a cyclone starts to form. Furthermore, precipitation minus evaporation decreases over the Arctic because the increase in evaporation outweighs that for precipitation, with implications for the ocean stratification and hence ocean circulation.

Pollen‐based reconstructions of Late Holocene climate on the southern slopes of the central Tianshan Mountains, Xinjiang,NWChina

ABSTRACTTo quantitatively describe the Late Holocene palaeoclimatic history of the southern slopes of the Tianshan Mountains in Xinjiang, China, we collected 74 surface pollen samples (from −154 to 4285 m), together with 81 fossil samples from Swan Lake. Based on modern pollen assemblage and redundancy analysis (RDA) of surface pollen data, five pollen zones were divided (alpine cushion vegetation, alpine meadow, subalpine meadow steppe, montane desert, and typical desert zones). Modern pollen–climate transfer functions were developed to quantitatively reconstruct palaeoclimate of the area using the weighted averaging partial least squares regression method. Reconstructed mean annual precipitation and mean annual temperature results suggest that a dry/warm or humid/cold climate pattern prevailed over the past 4200 aBP. Notably, three cold and humid stages were detected at 4200–4000 cal. aBP, 3700–2000 cal. aBP, and 1170–930 cal. aBP. Moreover, a humid ‘Medieval Warm Period’ (MWP) occurred not only on the northern slopes but also on the southern slopes of the Tianshan Mountains. However, based on palaeo‐environmental records and on archaeological documents, it would appear that a dryMWPoccurred in southern parts of south Xinjiang, with this being different from theMWPon the Tianshan Mountains. Interactions between monsoon and the westerly airflows might account for this difference. The data will improve understanding of past atmospheric circulation systems in the different parts of Xinjiang and will help to predict the future climate in the world.

Changes in south-east African tropical cyclone landfall frequency over the past 161 years

Variations in the sediment input to the Namaqualand mudbelt during the Holocene are assessed using an integrative terrestrial to marine, source to sink approach. Geochemical and Sr and Nd isotopic signatures are used to distinguish fluvial sediment source areas. Relative to the sediments of the Olifants River, craton outcrops in the northern Orange River catchment have a more radiogenic Sr and a more unradiogenic Nd isotopic signature. Furthermore, upper Orange River sediments are rich in heavier elements such as Ti and Fe derived from the chemical weathering of Drakensberg flood basalt. Suspension load signatures change along the Orange River's westward transit as northern catchments contribute physical weathering products from the Fish and Molopo River catchment area. Marine cores offshore of the Olifants (GeoB8323-2) and Orange (GeoB8331-4) River mouths show pulses of increased contribution of Olifants River and upper Orange River input, respectively. These pulses coincide with intervals of increased terrestrial organic matter flux and increased paleo-production at the respective core sites. We attribute this to an increase in fluvial activity and vegetation cover in the adjacent catchments during more humid climate conditions. The contrast in the timing of these wet phases in the catchment areas reflects the bipolar behavior of the South African summer and winter rainfall zones. While rainfall in the Orange River catchment is related to southward shifts in the ICTZ, rainfall in the Olifants catchment is linked to northward shifts in Southern Hemisphere Westerly storm tracks. The later may also have increased southern Benguela upwelling in the past by reducing the shedding of Agulhas eddies into the Atlantic. The high-resolution records of latitudinal shifts in these atmospheric circulation systems correspond to late Holocene centennial–millennial scale climate variability evident in Antarctic ice core records. The mudbelt cores indicate that phases of high summer rainfall zone and low winter rainfall zone humidity (at ca. 2.8 and 1 ka BP) may be synchronous with Antarctic warming events. On the other hand, dry conditions in the summer rainfall zone along with wet conditions in the winter rainfall zone (at ca 3.3, 2 and 0.5 ka BP) may be associated with Antarctic cooling events.

Holocene paleo-climatic record from the South African Namaqualand mudbelt: A source to sink approach

Variations in the sediment input to the Namaqualand mudbelt during the Holocene are assessed using an integrative terrestrial to marine, source to sink approach. Geochemical and Sr and Nd isotopic signatures are used to distinguish fluvial sediment source areas. Relative to the sediments of the Olifants River, craton outcrops in the northern Orange River catchment have a more radiogenic Sr and a more unradiogenic Nd isotopic signature. Furthermore, upper Orange River sediments are rich in heavier elements such as Ti and Fe derived from the chemical weathering of Drakensberg flood basalt. Suspension load signatures change along the Orange River's westward transit as northern catchments contribute physical weathering products from the Fish and Molopo River catchment area. Marine cores offshore of the Olifants (GeoB8323-2) and Orange (GeoB8331-4) River mouths show pulses of increased contribution of Olifants River and upper Orange River input, respectively. These pulses coincide with intervals of increased terrestrial organic matter flux and increased paleo-production at the respective core sites. We attribute this to an increase in fluvial activity and vegetation cover in the adjacent catchments during more humid climate conditions. The contrast in the timing of these wet phases in the catchment areas reflects the bipolar behavior of the South African summer and winter rainfall zones. While rainfall in the Orange River catchment is related to southward shifts in the ICTZ, rainfall in the Olifants catchment is linked to northward shifts in Southern Hemisphere Westerly storm tracks. The later may also have increased southern Benguela upwelling in the past by reducing the shedding of Agulhas eddies into the Atlantic. The high-resolution records of latitudinal shifts in these atmospheric circulation systems correspond to late Holocene centennial–millennial scale climate variability evident in Antarctic ice core records. The mudbelt cores indicate that phases of high summer rainfall zone and low winter rainfall zone humidity (at ca. 2.8 and 1 ka BP) may be synchronous with Antarctic warming events. On the other hand, dry conditions in the summer rainfall zone along with wet conditions in the winter rainfall zone (at ca 3.3, 2 and 0.5 ka BP) may be associated with Antarctic cooling events.

Holocene lake level fluctuations and environmental changes at Taro Co, southwestern Tibet, based on ostracod-inferred water depth reconstruction

Lake level change is an important indicator of changes in the hydrological status of lakes triggered by the influence of atmospheric circulation systems. Previous studies show that the interaction of the Indian monsoon and the Westerlies, and their influence on environmental changes in the Tibetan Plateau region, remain unclear. Here, we reconstruct the Holocene lake level history of a lake, Taro Co, in southwestern Tibet, based on the relationship between ostracod assemblages and water depth from 34 surface sediment samples spanning the depth range 0 to 125 m. Measurement and analysis of multi-proxy data from a 310 cm sediment core from Taro Co lead to the following conclusions: (1) From 10,200 to 9400 cal. a BP, the lake received large inflows of freshwater (mainly glacial melt water) resulting in a rapid rise in lake level. Following a cold and dry interval and low lake level from 9400 to 8800 cal. a BP, the lake maintained a high level between 8800 and 7500 cal. a BP. This was followed by a less warm and wet period until 6400 cal. a BP. From 6400 to 1200 cal. a BP, the lake probably overflowed after reaching a high level, and after 1200 cal. a BP, the lake level decreased and fluctuated around the modern depth of 60 m at the coring site. (2) The region experienced warm and wet conditions from 10,200 to 6400 cal. a BP, with a cold event at around 9000 cal. a BP, which probably resulted from the influence of the Indian monsoon. However, the inferred wet conditions between 6400 and 1200 cal. a BP are not observed in other records from the monsoon region; they are consistent with numerous records from the regions under the influence of the Westerlies, which probably indicate an interval of Westerlies-domination in the Taro Co region.

The joint return period analysis of natural disasters based on monitoring and statistical modeling of multidimensional hazard factors

As a random event, a natural disaster has the complex occurrence mechanism. The comprehensive analysis of multiple hazard factors is important in disaster risk assessment. In order to improve the accuracy of risk analysis and forecasting, the formation mechanism of a disaster should be considered in the analysis and calculation of multi-factors. Based on the consideration of the importance and deficiencies of multivariate analysis of dust storm disasters, 91 severe dust storm disasters in Inner Mongolia from 1990 to 2013 were selected as study cases in the paper. Main hazard factors from 500-hPa atmospheric circulation system, near-surface meteorological system, and underlying surface conditions were selected to simulate and calculate the multidimensional joint return periods. After comparing the simulation results with actual dust storm events in 54years, we found that the two-dimensional Frank Copula function showed the better fitting results at the lower tail of hazard factors and that three-dimensional Frank Copula function displayed the better fitting results at the middle and upper tails of hazard factors. However, for dust storm disasters with the short return period, three-dimensional joint return period simulation shows no obvious advantage. If the return period is longer than 10years, it shows significant advantages in extreme value fitting. Therefore, we suggest the multivariate analysis method may be adopted in forecasting and risk analysis of serious disasters with the longer return period, such as earthquake and tsunami. Furthermore, the exploration of this method laid the foundation for the prediction and warning of other nature disasters.

Climate change on the Tibetan Plateau in response to shifting atmospheric circulation since the LGM.

The Tibetan Plateau (TP) is primarily influenced by the northern hemispheric middle latitude Westerlies and the Indian summer monsoon (ISM). The extent, long-distance effects and potential long-term changes of these two atmospheric circulations are not yet fully understood. Here, we analyse modern airborne pollen in a transition zone of seasonally alternating dominance of the Westerlies and the ISM to develop a pollen discrimination index (PDI) that allows us to distinguish between the intensities of the two circulation systems. This index is applied to interpret a continuous lacustrine sedimentary record from Lake Nam Co covering the past 24 cal kyr BP to investigate long-term variations in the atmospheric circulation systems. Climatic variations on the central TP widely correspond to those of the North Atlantic (NA) realm, but are controlled through different mechanisms resulting from the changing climatic conditions since the Last Glacial Maximum (LGM). During the LGM, until 16.5 cal kyr BP, the TP was dominated by the Westerlies. After 16.5 cal kyr BP, the climatic conditions were mainly controlled by the ISM. From 11.6 to 9 cal kyr BP, the TP was exposed to enhanced solar radiation at the low latitudes, resulting in greater water availability.

Magnetic variations in surface soils in the NE Tibetan Plateau indicating the climatic boundary between the Westerly and East Asian summer monsoon regimes in NW China

Two atmospheric circulation systems, i.e., the mid-latitude Westerlies and the East Asian summer monsoon (EASM), dominate climate change in the NE Tibetan Plateau. However, controversy remains with regard to the climatic boundary between these westerly and monsoon regimes. In this paper, detailed rock magnetic and geochemical Rb/Sr analyses for a larger number of surface soils from a wide area in the NE Tibetan Plateau are presented. They show that surface soils in this region evince relatively significant regional variations in magnetic properties and geochemical characteristics. The magnetic parameters χfd, χARM and geochemical Rb/Sr ratios exhibit higher values in EASM-controlled (humid) areas, and lower values in Westerlies-influenced (arid) areas. These characteristics indicate that such variations in χfd, χARM and Rb/Sr ratios can be regarded as effective indicators of the degree of pedogenesis and weathering in the NE Tibetan Plateau. Our results also demonstrate that the climatic boundary between the westerly and monsoon regimes approximately overlaps with the boundary of highland meadow and temperate steppe desert/desert in NW China.

High-resolution leaf wax carbon and hydrogen isotopic record of the late Holocene paleoclimate in arid Central Asia

Abstract. Central Asia is located at the confluence of large-scale atmospheric circulation systems. It is thus likely to be highly susceptible to changes in the dynamics of those systems; however, little is still known about the regional paleoclimate history. Here we present carbon and hydrogen isotopic compositions of n-alkanoic acids from a late Holocene sediment core from Lake Karakuli (eastern Pamir, Xinjiang Province, China). Instrumental evidence and isotope-enabled climate model experiments with the Laboratoire de Météorologie Dynamique Zoom model version 4 (LMDZ4) demonstrate that δ D values of precipitation in the region are influenced by both temperature and precipitation amount. We find that these parameters are inversely correlated on an annual scale, i.e., the climate has varied between relatively cool and wet and more warm and dry over the last 50 years. Since the isotopic signals of these changes are in the same direction and therefore additive, isotopes in precipitation are sensitive recorders of climatic changes in the region. Additionally, we infer that plants use year-round precipitation (including snowmelt), and thus leaf wax δ D values must also respond to shifts in the proportion of moisture derived from westerly storms during late winter and early spring. Downcore results give evidence for a gradual shift to cooler and wetter climates between 3.5 and 2.5 cal kyr BP, interrupted by a warm and dry episode between 3.0 and 2.7 kyr BP. Further cool and wet episodes occur between 1.9 and 1.5 and between 0.6 and 0.1 kyr BP, the latter coeval with the Little Ice Age. Warm and dry episodes from 2.5 to 1.9 and 1.5 to 0.6 kyr BP coincide with the Roman Warm Period and Medieval Climate Anomaly, respectively. Finally, we find a drying tend in recent decades. Regional comparisons lead us to infer that the strength and position of the westerlies, and wider northern hemispheric climate dynamics, control climatic shifts in arid Central Asia, leading to complex local responses. Our new archive from Lake Karakuli provides a detailed record of the local signatures of these climate transitions in the eastern Pamir.