Warming of an elevated layer over Africa

This paper analyzes trends of temperatures over Africa and seeks to quantify the most significant processes. Observations of air temperature reveal significant warming trends in the 925–600 hPa layer over tropical west Africa and the east Atlantic. This is related to the influence of desert dust and biomass burning emissions on the atmospheric energy budget. We calculate a net radiative absorption of ～??20 W m^???2. The southern (northern) plume is rich in short-lived greenhouse gases (dust aerosols), and the atmospheric response, according to a simplified radiative transfer model, is a >3°C heating of the 2–4 km layer. The observed pattern of warming coincides with a mixture of dust, black carbon and short-lived greenhouse gases in space, time and height. Physical forcing provides a secondary source of regional warming, with sinking motions over the Sahel. The elevated warm layer stabilizes the lower atmosphere over and west of Africa, so drying trends may be anticipated.     

Recent climatic trends in the tropical Atlantic

A homogeneous monthly data set of sea surface temperature (SST) and pseudo wind stress based on in situ observations is used to investigate the climatic trends over the tropical Atlantic during the last five decades (1964–2012). After a decrease of SST by about 1 °C during 1964–1975, most apparent in the northern tropical region, the entire tropical basin warmed up. That warming was the most substantial (>1 °C) in the eastern tropical ocean and in the longitudinal band of the intertropical convergence zone. Surprisingly, the trade wind system also strengthened over the peirod 1964–2012. Complementary information extracted from other observational data sources confirms the simultaneity of SST warming and the strengthening of the surface winds. Examining data sets of surface heat flux during the last few decades for the same region, we find that the SST warming was not a consequence of atmospheric heat flux forcing. Conversely, we suggest that long-term SST warming drives changes in atmosphere parameters at the sea surface, most notably an increase in latent heat flux, and that an acceleration of the hydrological cycle induces a strengthening of the trade winds and an acceleration of the Hadley circulation. These trends are also accompanied by rising sea levels and upper ocean heat content over similar multi-decadal time scales in the tropical Atlantic. Though more work is needed to fully understand these long term trends, especially what happens from the mid-1970’s, it is likely that changes in ocean circulation involving some combination of the Atlantic meridional overtuning circulation and the subtropical cells are required to explain the observations.     

Spatial and temporal analysis of climate change in Hispañola

Climate change in Hispañola is studied since 1900 using a variety of datasets. The longer station-observed temperature record has a significant trend of 0.012 °C/year, while the shorter reanalysis datasets exhibit faster warming, suggesting accelerating greenhouse radiative absorption and Hadley circulation. Rainfall trends are insignificant in the observed period, but a CMIP5 model simulation predicts a significant drying trend. The spatial pattern of climate trends was mapped with reanalysis fields and indicates a faster rate of warming over the eastern half of the island, where observations are dense and the drying trend is greatest. Northeasterly trade winds strengthen on the Atlantic side of the island. While trends intensify in the satellite era compared to the earlier 20th century, part of that effect is ascribed to an upturn in the Atlantic Multi-decadal Oscillation.     

Climate-induced changes in river water temperature in North Iberian Peninsula

With the surface air temperature (SAT) data at 37 stations on Central Yunnan Plateau (CYP) for 1961–2010 and the Defense Meteorological Satellite Program/Operational Linescan System (DMSP/OLS) nighttime light data, the temporal-spatial patterns of the SAT trends are detected using Sen’s Nonparametric Estimator of Slope approach and MK test, and the impact of urbanization on surface warming is analyzed by comparing the differences between the air temperature change trends of urban stations and their corresponding rural stations. Results indicated that annual mean air temperature showed a significant warming trend, which is equivalent to a rate of 0.17 °C/decade during the past 50 years. Seasonal mean air temperature presents a rising trend, and the trend was more significant in winter (0.31 °C/decade) than in other seasons. Annual/seasonal mean air temperature tends to increase in most areas, and higher warming trend appeared in urban areas, notably in Kunming city. The regional mean air temperature series was significantly impacted by urban warming, and the urbanization-induced warming contributed to approximately 32.3–62.9 % of the total regional warming during the past 50 years. Meantime, the urbanization-induced warming trend in winter and spring was more significant than that in summer and autumn. Since 1985, the urban heat island (UHI) intensity has gradually increased. And the urban temperatures always rise faster than rural temperatures on the CYP.     

Impact of resolving the diurnal cycle in an ocean–atmosphere GCM. Part 2: A diurnally coupled CGCM

Coupled ocean atmosphere general circulation models (GCM) are typically coupled once every 24 h, excluding the diurnal cycle from the upper ocean. Previous studies attempting to examine the role of the diurnal cycle of the upper ocean and particularly of diurnal SST variability have used models unable to resolve the processes of interest. In part 1 of this study a high vertical resolution ocean GCM configuration with modified physics was developed that could resolve the diurnal cycle in the upper ocean. In this study it is coupled every 3 h to atmospheric GCM to examine the sensitivity of the mean climate simulation and aspects of its variability to the inclusion of diurnal ocean-atmosphere coupling. The inclusion of the diurnal cycle leads to a tropics wide increase in mean sea surface temperature (SST), with the strongest signal being across the equatorial Pacific where the warming increases from 0.2°C in the central and western Pacific to over 0.3°C in the eastern equatorial Pacific. Much of this warming is shown to be a direct consequence of the rectification of daily mean SST by the diurnal variability of SST. The warming of the equatorial Pacific leads to a redistribution of precipitation from the Inter tropical convergence zone (ITCZ) toward the equator. In the western Pacific there is an increase in precipitation between Papa new guinea and 170°E of up to 1.2 mm/day, improving the simulation compared to climatology. Pacific sub tropical cells are increased in strength by about 10%, in line with results of part 1 of this study, due to the modification of the exchange of momentum between the equatorially divergent Ekman currents and the geostropic convergence at depth, effectively increasing the dynamical response of the tropical Pacific to zonal wind stresses. During the spring relaxation of the Pacific trade winds, a large diurnal cycle of SST increases the seasonal warming of the equatorial Pacific. When the trade winds then re-intensify, the increase in the dynamical response of the ocean leads to a stronger equatorial upwelling. These two processes both lead to stronger seasonal basin scale feedbacks in the coupled system, increasing the strength of the seasonal cycle of the tropical Pacific sector by around 10%. This means that the diurnal cycle in the upper ocean plays a part in the coupled feedbacks between ocean and atmosphere that maintain the basic state and the timing of the seasonal cycle of SST and trade winds in the tropical Pacific. The Madden–Julian Oscillation (MJO) is examined by use of a large scale MJO index, lag correlations and composites of events. The inclusion of the diurnal cycle leads to a reduction in overall MJO activity. Precipitation composites show that the MJO is stronger and more coherent when the diurnal cycle of coupling is resolved, with the propagation and different phases being far more distinct both locally and to larger lead times across the tropical Indo-Pacific. Part one of this study showed that that diurnal variability of SST is modulated by the MJO and therefore increases the intraseasonal SST response to the different phases of the MJO. Precipitation-based composites of SST variability confirm this increase in the coupled simulations. It is argued that including this has increased the thermodynamical coupling of the ocean and atmosphere on the timescale of the MJO (20–100 days), accounting for the improvement in the MJO strength and coherency seen in composites of precipitation and SST. These results show that the diurnal cycle of ocean–atmosphere interaction has profound impact on a range of up-scale variability in the tropical climate and as such, it is an important feature of the modelled climate system which is currently either neglected or poorly resolved in state of the art coupled models.     

Statistical analysis of the temperature records for the Northern Territory of Australia

The Northern Territory of Australia has a unique situation of an extension larger than France and a population of 200,000, with only three meteorology stations open for more than 40 years, Darwin (DW), Alice Springs (AS) and Tennant Creek, and only two of them, DW and AS, providing data over 100 years, and from 500 to more than 1,000 km separating these stations and the stations in the neighbouring states of Australia. Homogenizations of data in between different measuring sites for the same location as well as the way to derive the missed data to complete at least 100 years from the neighbouring locations are analysed in details and the effects on the temperature trends are straightforwardly investigated. Using properly homogenised data over 130 years and a linear fitting, the warming maximum and minimum temperatures are +0.009 and +0.057 °C/10 years for Alice Springs and ?0.025 and 0.064 °C/10 years for Darwin. With the data available, the only option to produce warming trends is to overweight the cold years in the middle of the 1970s and the subsequent return to warmer temperatures. Starting from 1980, to compute trends, there is still a clear warming in Alice Springs, but also clear cooling in Tennant Creek, and a mixed behaviour with warming maximum temperatures and cooling minimum temperatures in Darwin.     

Climate change affecting temperature and aridity zones: a case study in Eastern Inner Mongolia, China from 1960–2008

Recent climate change is substantially affecting the spatial pattern of geographical zones, and the temporal and spatial inconsistency of climatic warming and drying patterns contributes to the complexity of the shifting of temperature and aridity zones. Eastern Inner Mongolia, China, located in the interface region of different biomes and ecogeographic zones, has experienced dramatic drying and warming over the past several decades. In this study, the annual accumulated temperature above 10 °C (AAT10) and the aridity index, two key indicators in geographical regionalization, are used to assess warming and drying processes and track the movements of temperature and aridity zones from 1960 to 2008. The results show a significant warming at the regional level from 1960 to 2008 with an AAT10 increase rate of 7.89 °C·d/year (p?<?0.001) in Eastern Inner Mongolia, while the drying trend was not significant during this period. Spatial heterogeneity of warming and drying distributions was also evident. Analysis of warming and drying via piecewise regression revealed two separate, specific trends between the first 31 years (1960–1990) and the subsequent 18 years (1991–2008). Generally, mild warming and very slight wetting occurred prior to 1990, while after 1991 both warming and drying were significant and enhanced. Continuous warming drove a northward shift of temperature zones from the 1960s to 2000s, while aridity zones displayed enhanced temporal and spatial variability. Climate change effects on temperature and aridity zones imply that the patterns of cropping systems, macro-ecosystems, and human land use modes are potentially undergoing migration and modification due to climate change.     

Future climate of the Caribbean from a super-high-resolution atmospheric general circulation model

Present-day (1979–2003) and future (2075–2099) simulations of mean and extreme rainfall and temperature are examined using data from the Meteorological Research Institute super-high-resolution atmospheric general circulation model. Analyses are performed over the 20-km model grid for (1) a main Caribbean basin, (2) sub-regional zones, and (3) specific Caribbean islands. Though the model’s topography underestimates heights over the eastern Caribbean, it captures well the present-day spatial and temporal variations of seasonal and annual climates. Temperature underestimations range from 0.1 °C to 2 °C with respect to the Japanese Reanalysis and the Climatic Research Unit datasets. The model also captures fairly well sub-regional scale variations in the rainfall climatology. End-of-century projections under the Intergovernmental Panel on Climate Change SRES A1B scenario indicate declines in rainfall amounts by 10–20 % for most of the Caribbean during the early (May–July) and late (August–October) rainy seasons relative to the 1979–2003 baselines. The early dry season (November–January) is also projected to get wetter in the far north and south Caribbean by approximately 10 %. The model also projects a warming of 2–3 °C over the Caribbean region. Analysis of future climate extremes indicate a 5–10 % decrease in the simple daily precipitation intensity but no significant change in the number of consecutive dry days for Cuba, Jamaica, southern Bahamas, and Haiti. There is also indication that the number of hot days and nights will significantly increase over the main Caribbean basin.     

Accelerated climate change and its potential impact on Yak herding livelihoods in the eastern Tibetan plateau

The Tibetan Plateau has experienced rapid warming like most other alpine regions. Regional assessments show rates of warming comparable with the arctic region and decreasing Asian summer monsoons. We used meteorological station daily precipitation and daily maximum and minimum temperature data from 80 stations in the eastern Tibetan Plateau of southwest China to calculate local variation in the rates and seasonality of change over the last half century (1960–2008). Daily low temperatures during the growing season have increased greatly over the last 24 years (1984–2008). In sites of markedly increased warming (e.g., Deqin, Yunnan and Mangya, Qinghai), daily and growing season daily high temperatures have increased at a rate above 5 °C/100 years. In Deqin, precipitation prior to the 1980s fell as snow whereas in recent decades it has shifted to rain during March and April. These shifts to early spring rains are likely to affect plant communities. Animals like yaks adapted to cold climates are also expected to show impacts with these rising temperatures. This region deserves further investigation to determine how these shifts in climate are affecting local biodiversity and livelihoods.     

Indian Ocean warming during 1958–2004 simulated by a climate system model and its mechanism

The mechanism responsible for Indian Ocean Sea surface temperature (SST) basin-wide warming trend during 1958–2004 is studied based on both observational data analysis and numerical experiments with a climate system model FGOALS-gl. To quantitatively estimate the relative contributions of external forcing (anthropogenic and natural forcing) and internal variability, three sets of numerical experiments are conducted, viz. an all forcing run forced by both anthropogenic forcing (greenhouse gases and sulfate aerosols) and natural forcing (solar constant and volcanic aerosols), a natural forcing run driven by only natural forcing, and a pre-industrial control run. The model results are compared to the observations. The results show that the observed warming trend during 1958–2004 (0.5 K (47-year)^?1) is largely attributed to the external forcing (more than 90 % of the total trend), while the residual is attributed to the internal variability. Model results indicate that the anthropogenic forcing accounts for approximately 98.8 % contribution of the external forcing trend. Heat budget analysis shows that the surface latent heat flux due to atmosphere and surface longwave radiation, which are mainly associated with anthropogenic forcing, are in favor of the basin-wide warming trend. The basin-wide warming is not spatially uniform, but with an equatorial IOD-like pattern in climate model. The atmospheric processes, oceanic processes and climatological latent heat flux together form an equatorial IOD-like warming pattern, and the oceanic process is the most important in forming the zonal dipole pattern. Both the anthropogenic forcing and natural forcing result in easterly wind anomalies over the equator, which reduce the wind speed, thereby lead to less evaporation and warmer SST in the equatorial western basin. Based on Bjerknes feedback, the easterly wind anomalies uplift the thermocline, which is unfavorable to SST warming in the eastern basin, and contribute to SST warming via deeper thermocline in the western basin. The easterly anomalies also drive westward anomalous equatorial currents, against the eastward climatology currents, which is in favor of the SST warming in the western basin via anomalous warm advection. Therefore, both the atmospheric and oceanic processes are in favor of the IOD-like warming pattern formation over the equator.     

Diurnal and seasonal variations of wind farm impacts on land surface temperature over western Texas

This paper analyzes seasonal and diurnal variations of MODerate resolution Imaging Spectroradiometer (MODIS) land surface temperature (LST) data at ~1.1 km for the period of 2003–2011 over a region in West-Central Texas, where four of the world’s largest wind farms are located. Seasonal anomalies are created from MODIS Terra (~10:30 a.m. and 10:30 p.m. local solar time) and Aqua (~1:30 a.m. and 1:30 p.m. local solar time) LSTs, and their spatiotemporal variability is analyzed by comparing the LST changes between wind farm pixels (WFPs) and nearby non wind farm pixels (NNWFPs) using different methods under different quality controls. Our analyses show consistently that there is a warming effect of 0.31–0.70 °C at nighttime for the nine-year period during which data was collected over WFPs relative to NNWFPs, in all seasons for both Terra and Aqua measurements, while the changes at daytime are much noisier. The nighttime warming effect is much larger in summer than winter and at ~10:30 p.m. than ~1:30 a.m. and hence the largest warming effect is observed at ~10:30 p.m. in summer. The spatial pattern and magnitude of this warming effect couple very well with the geographic distribution of wind turbines and such coupling is stronger at nighttime than daytime and in summer than winter. Together, these results suggest that the warming effect observed in MODIS over wind farms are very likely attributable to the development of wind farms. This inference is consistent with the increasing number of operational wind turbines with time during the study period, the diurnal and seasonal variations in the frequency of wind speed and direction distribution, and the changes in near-surface atmospheric boundary layer (ABL) conditions due to wind farm operations. The nocturnal ABL is typically stable and much thinner than the daytime ABL and hence the turbine enhanced vertical mixing produces a stronger nighttime effect. The stronger wind speed and the higher frequency of the wind speed within the optimal power generation range in summer than winter and at nighttime than daytime likely drives wind turbines to generate more electricity and turbulence and consequently results in the strongest warming effect at nighttime in summer. Similarly, the stronger wind speed and the higher frequency of optimal wind speed at ~10:30 p.m. than that at ~1:30 a.m. might help explain, to some extent, why the nighttime LST warming effect is slightly larger at ~10:30 p.m. than ~1:30 a.m. The nighttime warming effect seen in spring and fall are smaller than that in summer and can be explained similarly.     

Simulations of warm tropical conditions with application to middle Pliocene atmospheres

During the early and mid-Pliocene, the period from 5 to 3 million years ago, approximately, the Earth is believed to have been significantly warmer than it is today, but the reasons for the higher temperatures are unclear. This paper explores the impact of recent findings that suggest that, at that time, cold surface waters were absent from the tropical and subtropical oceanic upwelling zones. El Niño was in effect a perennial rather than intermittent phenomenon, and sea surface temperatures in low latitudes were essentially independent of longitude. When these conditions are specified as the lower boundary condition for an atmospheric GCM, we find that the trade winds along the equator, and hence the Walker Circulation, collapse. The low-level stratus clouds in low latitudes diminish greatly, thus reducing the albedo of the Earth. The atmospheric concentration of water vapor increases, and enhanced latent heat release due to stronger evaporation warms up the tropical atmosphere, particularly between 40°S and 20°N. Moreover, teleconnection patterns from the Pacific induce a warming over North America that is enhanced by surface albedo feedback, a process that may have helped to maintain this region ice-free before 3 Ma. The results presented here indicate that the suggested absence of cold surface waters from the tropical and subtropical oceanic upwelling zones could have contributed significantly to the Pliocene warmth.     

A study on the role of land-atmosphere coupling on the south Asian monsoon climate variability using a regional climate model

The spatial and temporal trends of 11 (7) temperature (precipitation) extreme indices are examined for the Loess Plateau Region (LPR) and its southeast and northwest sub-regions based on daily observations at 214 meteorological stations. Results show widespread significant warming trends for all the temperature extremes except for the diurnal temperature range (DTR) and the lowest daily maximum temperature in each year (TXn) during 1961–2010. When regionally averaged, a significant warming trend is detected for all the indices except for DTR and TXn in the past 50 years. Compared with the entire LPR, a significant warming trend is detected for all the indices except for DTR and TXn over the southeast sub-region of LPR; while it is observed for all the indices over the northwest. The trends for these indices are generally stronger in the northwest than in the southeast in the past 50 years. In contrast, for precipitation indices, only a small percentage of areas show significant drying or wetting trends and, when regionally averaged, none of them displays significant trends during the past 50 years. On the sub-regional scale, however, a larger percentage of areas show significant drying trends for precipitation indices generally over the southeast relative to the entire LPR, and noticeably, the sub-regional average heavy precipitation (R10mm) and wet day precipitation (PRCPTOT) display significant decreasing trends during the past 50 years; whereas only a slightly larger percentage of areas show significant wetting trends for these indices over the northwest compared with the entire LPR, and when sub-regionally averaged, none of the indices have significant trends during the past 50 years.     

冬季亚洲中纬度极端低温事件与巴伦支—喀拉海异常增暖的关系及联系机制

利用1979～2019年NCEP-DOE再分析资料,分析了亚洲中纬度冬季极端低温事件发生频次与巴伦支—喀拉海异常增暖的联系及可能机制.研究表明:巴伦支—喀拉海异常增暖通常对应西伯利亚高压异常增强及亚洲中纬度极端低温频发.进一步分析发现,伴随巴伦支—喀拉海异常增暖,新地岛、乌拉尔山附近出现位势高度正异常,在位势高度正异常...     

Anthropogenic changes of the thermal and zonal flow structure over Western Europe and Eastern North Atlantic in CMIP3 and CMIP5 models

The anthropogenic changes during boreal winter in the thermal and zonal flow structure over Eastern Atlantic and Western Europe (EAWE) have been investigated using an ensemble of CMIP3 and CMIP5 models. The ensemble mean change in the zonal wind at 500 hPa over this region is characterized by an eastward extension of the belt of zonal winds. Using the thermal wind relation these wind changes are found to be consistent with the changes in the tropospheric temperature profile. An enhanced warming is simulated in the subtropical upper troposphere and a relative surface cooling in the mid-latitudes. The subtropical upper tropospheric warming is related to the downward branch of the mean meridional circulation, whereas the mid-latitude lower tropospheric relative cooling is linked to the ocean processes that govern changes in its surface temperatures. Inter-model differences in the simulated change of the zonal wind over the EAWE by the CMIP3 and CMIP5 models relate well with differences in the upper tropospheric subtropical warming and the mid-latitude lower tropospheric relative cooling. The simulated change of the zonal wind over the EAWE region by the CMIP3 and CMIP5 models correlates well with changes in the meridional SST gradient. We conclude that uncertainties in the projected changes of the zonal flow over Europe are at least partly due to uncertainties in the response of the North Atlantic Ocean to increased levels of greenhouse gases.     

Modeling the relative roles of the foehn wind and urban expansion in the 2002 Beijing heat wave and possible mitigation by high reflective roofs

Rapid urbanization has intensified summer heat waves in recent decades in Beijing, China. In this study, effectiveness of applying high-reflectance roofs on mitigating the warming effects caused by urban expansion and foehn wind was simulated for a record-breaking heat wave occurred in Beijing during July 13–15, 2002. Simulation experiments were performed using the Weather Research and Forecast (WRF version 3.0) model coupled with an urban canopy model. The modeled diurnal air temperatures were compared well with station observations in the city and the wind convergence caused by urban heat island (UHI) effect could be simulated clearly. By increasing urban roof albedo, the simulated UHI effect was reduced due to decreased net radiation, and the simulated wind convergence in the urban area was weakened. Using WRF3.0 model, the warming effects caused by urban expansion and foehn wind were quantified separately, and were compared with the cooling effect due to the increased roof albedo. Results illustrated that the foehn warming effect under the northwesterly wind contributed greatly to this heat wave event in Beijing, while contribution from urban expansion accompanied by anthropogenic heating was secondary, and was mostly evident at night. Increasing roof albedo could reduce air temperature both in the day and at night, and could more than offset the urban expansion effect. The combined warming caused by the urban expansion and the foehn wind could be potentially offset with high-reflectance roofs by 58.8 % or cooled by 1.4 °C in the early afternoon on July 14, 2002, the hottest day during the heat wave.     

Glacial/interglacial changes in the East Australian current

The East Australian Current (EAC) is the western boundary current of the south Pacific gyre transporting warm tropical waters to higher southern latitudes. Recent modelling shows that the partial separation of the EAC (~32°S) and the coupled formation of the Tasman Front (~34°S) are caused by a steep gradient in the zonally integrated wind stress curl. Analysis of oxygen isotope ratios (δ¹⁸O) in the planktonic foraminifer, Globigerinoides ruber, from sediment cores from the Coral Sea and Tasman Sea indicates that the EAC separation shifted northward to between 23 and 26°S during the last glacial. We suggest these results indicate a significant change in the Pacific wind stress curl during the glacial. Given recent evidence for El Niño-like conditions in the Pacific during the last glacial, with a reduction in the east–west sea surface temperature (SST) gradient, we suggest that weaker trade winds combined with more northerly, stronger westerlies were associated with a change to the wind stress curl, which repositioned the EAC separation and Tasman Front. In contrast, by ~11 ka BP, the EAC separation was forced south of 26°S. This southward shift was synchronous with a rapid warming of tropical SSTs, and the onset of a La Niña-like SST configuration across the tropical Pacific. It appears that the south Pacific trade winds strengthened accordingly, causing the EAC to readjust its flow. This readjustment of the EAC marks the onset of modern surface-ocean circulation in the southwest Pacific, but the present EAC transport was only achieved in the late Holocene, after 5 ka BP.     

Changing inland lakes responding to climate warming in Northeastern Tibetan Plateau

The main portion of Tibetan Plateau has experienced statistically significant warming over the past 50 years, especially in cold seasons. This paper aims to identify and characterize the dynamics of inland lakes that located in the hinterland of Tibetan Plateau responding to climate change. We compared satellite imageries in late 1970s and early 1990s with recent to inventory and track changes in lakes after three decades of rising temperatures in the region. It showed warm and dry trend in climate with significant accelerated increasing annual mean temperature over the last 30 years, however, decreasing periodically annual precipitation and no obvious trend in potential evapotranspiration during the same period. Our analysis indicated widespread declines in inland lake??s abundance and area in the whole origin of the Yellow River and southeastern origin of the Yangtze River. In contrast, the western and northern origin of the Yangtze River revealed completely reverse change. The regional lake surface area decreased by 11,499 ha or 1.72% from the late 1970s to the early 1990s, and increased by 6,866 ha or 1.04% from the early 1990s to 2004. Shrinking inland lakes may become a common feature in the discontinuous permafrost regions as a consequence of warming climate and thawing permafrost. Furthermore, obvious expanding were found in continuous permafrost regions due to climate warming and glacier retreating. The results may provide information for the scientific recognition of the responding events to the climate change recorded by the inland lakes.     

Feedback process responsible for the suppression of ENSO activity during the mid-Holocene

Using the output of 12 models from the Paleoclimate Modeling Intercomparison Project Phase 3, we investigate the feedback process responsible for changes in El Niño-Southern Oscillation activity during the mid-Holocene based on a linear stability index (Bjerknes stability index; BJ index) analysis. The multi-model ensemble mean (MME) variance of the Niño-3.4 index (sea surface temperature anomalies averaged over 5°S–5°N, 170°–120°W) simulated for 6000 years ago (6 kya) was 13% lower than that for the pre-industrial era (0 kya), while changes in the MME BJ index were negligible. This is due to a balance between enhanced damping by anomalous thermal advection by mean currents (MA) and enhanced positive thermocline feedback (TH). Seven of the models show that MME variance of the Niño-3.4 and BJ indexes for the 6 kya run is 21 and 70% lower, respectively, than the 0 kya run. However, two models show the opposite change. Interestingly, MA in both model groups increases, especially due to the mean meridional current associated with enhanced trade winds, indicating a robust mechanism. The opposite tendency between the two groups is mainly due to large TH in the second group 6 kya, as a result of enhanced air-sea coupling and strongly reduced ocean stratification due to subsurface warming, which led to increased sensitivity of the zonal thermocline contrast to surface zonal wind stress.     

Meso-beta scale numerical simulations of terrain drag-induced along-stream circulations Part I: Midtropospheric frontogenesis

Summary The problem of along-stream ageostrophic frontogenesis is studied by employing a numerical model at meso-alpha and meso-beta scales in simulations of the downstream circulations over the Front Range of the Rocky Mountains. Three-dimensional real data simulations at these two scales of motion are used to diagnose the transition from semigeostrophic cross-stream frontogenesis accompanying a propagating baroclinic upper-level jet streak to midtropospheric along-stream ageostrophic frontogenesis. This along-stream ageostrophic frontogenesis results from the perturbation of the jet streak by the Rocky Mountain range. The case study represents an example of internal wave dynamics which are forced by the drag of the Rocky Mountains on a strong jet streak in the presence of a low-level inversion.The simulation results indicate that, unlike semi-geostrophic frontogenesis, a front (which is alligned perpendicular to the axis of the jet stream) may form when significant adiabatic heating occurs within a stratified shear flow over horizontal length scales shorter than the Rossby radius of deformation. The mechanism responsible for the frontogenesis is the growth of the divergent along-stream wind velocity component which becomes coupled to the front's along-stream pressure gradient force. This nonlinear interaction produces hydrostatic mesoscale frontogenesis as follows: 1) vertical wind shear in the along-stream plane strengthens resulting in the increasingly nonuniform vertical variation of horizontal temperature advection as the ageostrophic wind component grows in magnitude downstream of the meso-scale terrain-induced adiabatic heating, 2) increasing along-stream differential vertical motions (i.e., along-stream thermally indirect circulation with warm air sinking to the west and cold air rising to the east) tilt the vertical gradient of isentropes into the horizontal as the vertical temperature gradient increases due to the previous process in proximity to horizontal gradients in the along-stream component of the ageostrophic wind, 3) as tilting motions act to increase the along-stream horizontal temperature gradient, the along-stream confluence acts to nonuniformly increase the along-stream frontal temperature gradient which increases the along-stream pressure gradient force resulting in further accelerations, ageostrophy, and frontal steepening as part of a scale contraction process.The evolution of the aforementioned processes results in the three-dimensional hydrostatic frontogenesis accompanying the overturning of isentropic surfaces. These adjustments act to turn air parcels to the right of the southwesterly geostrophic wind vector at successively lower atmospheric levels as the scale contraction continues. This simulated along-stream front is verified from diagnostic analysis of the profiler-derived temperature and wind fields.With 17 Figures