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Large-scale Circulation (large-scale + circulation)
Selected AbstractsOn the climate and weather of mountain and sub-arctic lakes in Europe and their susceptibility to future climate changeFRESHWATER BIOLOGY, Issue 12 2009R. THOMPSON Summary 1.,The complex terrain and heterogeneous nature of the mountain environment coupled with remoteness from major centres of human activity makes mountains challenging locations for meteorological investigations. Mountainous areas tend to have more varied and more extreme weather than lowlands. 2.,The EMERGE program has the primary aim of assessing the status of remote mountain and sub-arctic lakes throughout Europe for the first time. In this study, we describe the main features of the climate, ice-cover durations and recent temperature trends of these areas. The main weather characteristics of European mountain and sub-arctic lakes are their cold temperatures and year-round precipitation. Mean annual temperatures are generally close to 0 °C, and maximum summer temperatures reasonably close to 10 °C. 3.,Maritime versus continental settings determine the main differences in annual-temperature range among lake districts (10.5 °C in Scotland to 26.7 °C in Northern Finland), and a similar factor for ice-cover duration. Radiation ranges from low (120 W m,2) in the high latitude sub-arctic and high (237 W m,2) in the southern ranges of the Pyrenees and Rila. Similarly, precipitation is high in the main Alpine chain (250 cm year,1 in the Central Southern Alps) and low in the continental sub-arctic (65 cm year,1 in Northern Finland). 4.,The main temporal patterns in air temperature follow those of the adjacent lowlands. All the lake districts warmed during the last century. Spring temperature trends were highest in Finland; summer trends were weak everywhere; autumn trends were strongest in the west, in the Pyrenees and western Alps; while winter trends varied markedly, being high in the Pyrenees and Alps, low in Scotland and Norway and negative in Finland. 5.,Two new, limnological case studies on Lake Redon, in the Pyrenees, highlight the sensitivity of remote lakes to projected changes in the global climate. These two case studies involve close linkages between extreme chemical-precipitation events and synoptic wind-patterns, and between thermocline behaviour and features of the large-scale circulation. 6.,Individual lakes can be ultra-responsive to climate change. Even modest changes in future air temperatures will lead to major changes in lake temperatures and ice-cover duration and hence probably affect their ecological status. [source] Towards ice-core-based synoptic reconstructions of west antarctic climate with artificial neural networksINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 5 2005David B. Reusch Abstract Ice cores have, in recent decades, produced a wealth of palaeoclimatic insights over widely ranging temporal and spatial scales. Nonetheless, interpretation of ice-core-based climate proxies is still problematic due to a variety of issues unrelated to the quality of the ice-core data. Instead, many of these problems are related to our poor understanding of key transfer functions that link the atmosphere to the ice. This study uses two tools from the field of artificial neural networks (ANNs) to investigate the relationship between the atmosphere and surface records of climate in West Antarctica. The first, self-organizing maps (SOMs), provides an unsupervised classification of variables from the mid-troposphere (700 hPa temperature, geopotential height and specific humidity) into groups of similar synoptic patterns. An SOM-based climatology at annual resolution (to match ice-core data) has been developed for the period 1979,93 based on the European Centre for Medium-Range Weather Forecasts (ECMWF) 15-year reanalysis (ERA-15) dataset. This analysis produced a robust mapping of years to annual-average synoptic conditions as generalized atmospheric patterns or states. Feed-forward ANNs, our second ANN-based tool, were then used to upscale from surface data to the SOM-based classifications, thereby relating the surface sampling of the atmosphere to the large-scale circulation of the mid-troposphere. Two recorders of surface climate were used in this step: automatic weather stations (AWSs) and ice cores. Six AWS sites provided 15 years of near-surface temperature and pressure data. Four ice-core sites provided 40 years of annual accumulation and major ion chemistry. Although the ANN training methodology was properly designed and followed standard principles, limited training data and noise in the ice-core data reduced the effectiveness of the upscaling predictions. Despite these shortcomings, which might be expected to preclude successful analyses, we find that the combined techniques do allow ice-core reconstruction of annual-average synoptic conditions with some skill. We thus consider the ANN-based approach to upscaling to be a useful tool, but one that would benefit from additional training data. Copyright © 2005 Royal Meteorological Society. [source] Characteristics, evolution and mechanisms of the summer monsoon onset over Southeast AsiaINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 12 2004Zuqiang Zhang Abstract Based on the 1979,95 mean pentad reanalysis data from the US National Centers for Environmental Prediction, the climatological characteristics and physical mechanism of the Asian summer monsoon (ASM) onset are investigated. Special focus is given to whether the ASM onset starts earlier over the Indochina Peninsula than over the South China Sea (SCS) and why the ASM is established the earliest over Southeast Asia. An examination of the composite thermodynamic and dynamic quantities confirms that the ASM onset commences earliest over the Indochina Peninsula, as highlighted by active convection and rainfall resulting from the convergence of southwesterly flow from the Bay of Bengal (BOB) vortex and easterly winds associated with the subtropical anticyclone over the SCS. Two other important characteristics not previously noted are also identified: the earliest reversal of meridional temperature gradient throughout the entire troposphere and the corresponding establishment of an easterly vertical wind shear, which are due to upper level warming caused by eddy (convective) transport of latent heat. These changes in the large-scale circulation suggest that, in addition to rainfall, a reversal in the planetary-scale circulation should be included in determining the timing of the ASM onset. With such a consideration, the climatological ASM onset occurs first over southeastern BOB and southwestern Indochina Peninsula in early May, and then advances northeastward to reach the SCS by the fourth pentad of May (16,20 May). The monsoon then covers the entire Southeast Asia region by the end of May. Subsequently, a similar onset process begins over the eastern Arabian Sea, India and western BOB, and the complete establishment of the ASM over India is accomplished in mid June. In the process of the onset of each ASM component, the reversal of the upper level planetary-scale circulation depends strongly on that of the meridional temperature gradient. Over the Indochina Peninsula, the seasonal transition of upper level temperature results from convection-induced diabatic heating, whereas over western Asia it is attributed to subsidence warming induced by the active ascending motion over the former region. The steady increase in surface sensible heating over the Indian subcontinent and the latent heating over the tropical Indian Ocean in April to early May appear to be the major impetus for the development of the cyclonic vortex over the BOB. A similar enhancement over the Arabian Peninsula and the surrounding regions is also identified to be crucial to the development of the so-called onset vortex over the Arabian Sea, and then ultimately to the ASM onset over India. Copyright © 2004 Royal Meteorological Society [source] Decadal changes in the link between El Niño and springtime North Atlantic oscillation and European,North African rainfallINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 11 2003Peter Knippertz Abstract The link between El Niño,southern oscillation (ENSO) variability in boreal winter (represented by the NIÑO3 index, i.e. East Pacific sea-surface temperature anomalies) and the large-scale circulation and weather conditions over Europe,northwest Africa in spring is explored, considering station reports of precipitation, sea-level pressure (SLP) anomalies and two North Atlantic oscillation (NAO) indices. It is found that these relations have undergone consistent and simultaneous changes in the 20th century. Three characteristic periods can be identified. During 1900,25 and 1962,87, positive NIÑO3 index values are associated with enhanced precipitation over central Europe and reduced rainfall in southern Europe and northern Africa. The ENSO influence on precipitation over Scotland and Norway is small. The rainfall anomalies can be explained from the advective and dynamical implications of a north,south dipole in SLP correlations (warm ENSO events followed by low pressure in northern Europe and high pressure over the Mediterranean Sea,North Africa). This dipole hardly projects on the commonly used NAO centres (Iceland and Azores/Gibraltar) and thus ENSO,NAO correlations are insignificant. During 1931,56 the NIÑO3 index reveals little influence on precipitation over the Iberian Peninsula and Morocco, but there are large negative correlations with precipitation over Scotland and Norway. This is related to an alteration of the NIÑO3,SLP correlation pattern, which implies high pressure over northern Europe and low pressure over central Europe after warm events, and thus a virtually inverted dipole with respect to the other two periods. The large westward extension of the dipole leads to a significant NAO,NIÑO3 correlation of r = ,0.5. These alterations were accompanied by substantial large-scale circulation changes during the period 1931,56, as revealed by anomalously high pressure and dry conditions over central,western Europe, a change in precipitation-producing SLP patterns for Morocco and an anomalously low number of positive NAO and NIÑO3 index values. It is left for discussion as to whether the decadal variations described are due to a change in the physics of the teleconnection or to stochastic fluctuations. Copyright © 2003 Royal Meteorological Society [source] Variability of extreme temperature events in south,central Europe during the 20th century and its relationship with large-scale circulationINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 9 2003Peter Domonkos Abstract The variability of winter extreme low-temperature events and summer extreme high-temperature events was investigated using daily temperature series (1901,98) from 11 sites in central and southern Europe. An extreme temperature event (EXTE) is defined by various threshold values of daily temperature or daily temperature anomaly. Systematic changes in the frequencies of EXTEs are investigated by the Mann,Kendall test and a method based on the Wilcoxon test. The catalogue of macrocirculation types over central Europe (the Hess,Brezowsky classification) is applied to investigate the connections between EXTEs and large-scale circulation. Circulation classes (HBC) are defined, and mostly spatial averages of EXTEs are examined. There were large long-term fluctuations in the frequencies of both winter extreme cold events (EXCEs) and summer extreme warm events (EXWEs) during the 20th century. The systematic changes referring to the entire period indicate a slight warming tendency, but only a few of the changes, mostly in the northernmost sites, are statistically significant. Strong connections are present between the frequencies of EXTEs and the large-scale circulation on various time scales, particularly for EXCEs. The spatial differences of EXTE fluctuations and EXTE,HBC connections are small within the study area. Northerlies and easterlies, as well as meridional and anticyclonic situations, are favourable for EXCEs, whereas southerlies and persistent anticyclonic situations are favourable for EXWE occurrences. In the latest decades, a decline in the frequency of EXCEs and a sharp increase in the frequency of EXWEs happened, and the residence times of the circulation patterns over central Europe became longer both in winter and summer. Copyright © 2003 Royal Meteorological Society [source] The effect of large-scale circulation on precipitation and streamflow in the Gulf of California continental watershedINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 7 2003Luis Brito-Castillo Abstract The interannual variability of summer and winter rainfall and streamflow in the Gulf of California continental watershed is compared. Varimax-rotated empirical orthogonal function (EOF) analysis is applied to 15 streamflow series, in the period from 1960 to 1990, and two regions are defined: a central region and a southern region. Results show that in both regions, between 1944 and 1999, the long-term rainfall variability is well explained by the long-term streamflow variability in both seasons, the result being statistically significant at the 95% level. We conclude that regional streamflows in that period are climate driven. This conclusion is reinforced when we show that the large-scale circulation (700 hPa heights) explains: (i) wet and dry conditions in both regions; (ii) conditions of wet and dry years with the same signal of El Niño and La Niña events; and (iii) long-term periods in association with the Pacific decadal oscillation (PDO). When the PDO is in its warm phase, summers are likely to be dry with an El Niño event and wet with a La Niña event. In the cool phase of the PDO, summers are influenced by more localized events (i.e. the position of the subtropical continental ridge). In winter, warm and cool phases of the PDO are likely to be associated with wet and dry winters respectively. Copyright © 2003 Royal Meteorological Society [source] Changes in seasonal mean maximum air temperature in Romania and their connection with large-scale circulationINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 10 2002Rodica Tomozeiu Abstract This paper investigates the temporal and spatial variability of the seasonal mean of maximum air temperature in Romania and its links with the large,scale atmospheric circulation. The Romanian data sets are represented by time series at 14 stations. The large-scale parameters are represented by the observed sea-level pressure (SLP) and geopotential height at 500 hPa (Z500). The period analysed was 1922,98 for winter and 1960,98 for all seasons. Before analysis, the original temperature data were tested to detect for inhomogeneity using the standard normal homogeneity test. Empirical orthogonal functions (EOFs) were used to analyse the spatial and temporal variability of the local and large-scale parameters and to eliminate noise from the original data set. The time series associated with the first EOF pattern of the SLP and mean maximum temperature in Romania were analysed from trend and shifts point of view using the Pettitt and Mann,Kendall tests respectively. The covariance map computed using the Z500 and the seasonal mean of maximum temperature in Romania were used as additional methods to identify the large-scale circulation patterns influencing the local variability. Significant increasing trends were found for winter and summer mean maximum temperature in Romania, with upward shifts around 1947 and 1985 respectively. During autumn, a decreasing trend with a downward shift around 1969 was detected. These changes seem to be real, since they are connected to similar changes in the large-scale circulation. So, the intensification of the southwesterly circulation over Europe since 1933 overlapped with the enhancement of westerly circulation after the 1940s could be the reason for the change in winter mean maximum temperature. The slight weakening of the southwesterly circulation during autumn could be one of the reasons for the decrease in the regime of the mean maximum temperature for autumn seasons. Additionally, the covariance map technique reveals the influence of the North Atlantic oscillation in winter, East Atlantic Jet in summer and Scandinavian (or Euroasia-1) circulation pattern in autumn upon mean maximum air temperature. Copyright © 2002 Royal Meteorological Society. [source] Bioluminescent monitoring of turbulent bioconvectionLUMINESCENCE: THE JOURNAL OF BIOLOGICAL AND CHEMICAL LUMINESCENCE, Issue 2 2006imkus Abstract Under adjusted experimental conditions, open-to-air cultures of lux gene-engineered Ralstonia eutropha (wholecell biosensors of copper) exhibit bioconvection, which accounts for fluctuating bioluminescence. The power spectrum of bioluminescence intensity fluctuations recorded from a cylindrical sample 9 mm in diameter and ,10 mm in height is characterized by a dominant low-frequency oscillation (with a characteristic period of ,8,12 min), which is occasionally accompanied by a few weaker oscillations. The corresponding spectral peaks emerge on a high-noise background. The spectra of bioluminescence intensity fluctuations qualitatively resemble the spectra of temperature or fluid velocity fluctuations in an appropriate turbulent thermal convection system. It has been suggested that in a bioconvective system, like in thermal convection systems, the emergence of oscillation reflects the large-scale convective circulation that spans the height of the cylindrical cell. The velocity of large-scale bioconvective circulation was estimated to be 37,48 µm/s. The occasional emergence of weaker-than-dominant oscillations was explained through the coexistence and interaction of the large-scale circulation with, presumably, a gene-expression-related cyclic process (with a characteristic period of ,25,50 min). Copyright © 2006 John Wiley & Sons, Ltd. [source] Recent developments in gravity-wave effects in climate models and the global distribution of gravity-wave momentum flux from observations and modelsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 650 2010M. J. Alexander Abstract Recent observational and theoretical studies of the global properties of small-scale atmospheric gravity waves have highlighted the global effects of these waves on the circulation from the surface to the middle atmosphere. The effects of gravity waves on the large-scale circulation have long been treated via parametrizations in both climate and weather-forecasting applications. In these parametrizations, key parameters describe the global distributions of gravity-wave momentum flux, wavelengths and frequencies. Until recently, global observations could not define the required parameters because the waves are small in scale and intermittent in occurrence. Recent satellite and other global datasets with improved resolution, along with innovative analysis methods, are now providing constraints for the parametrizations that can improve the treatment of these waves in climate-prediction models. Research using very-high-resolution global models has also recently demonstrated the capability to resolve gravity waves and their circulation effects, and when tested against observations these models show some very realistic properties. Here we review recent studies on gravity-wave effects in stratosphere-resolving climate models, recent observations and analysis methods that reveal global patterns in gravity-wave momentum fluxes and results of very-high-resolution model studies, and we outline some future research requirements to improve the treatment of these waves in climate simulations. Copyright © 2010 Royal Meteorological Society and Crown in the right of Canada [source] Moisture,convection feedback in the tropicsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 604 2004W. W. Grabowski Abstract This paper discusses the large-scale moisture,convection feedback in the tropics, where spatial fluctuations of deep convection cause perturbations of free-tropospheric moisture which, in turn, affect the spatial distribution of deep convection. A simple heuristic argument using the timescale of free-tropospheric humidity change explains why moisture,convection feedback is particularly relevant for tropical intraseasonal oscillations. The large-scale dynamical context for moisture,convection feedback is investigated in idealized rotating constant-sea-surface-temperature (,tropics everywhere') aquaplanet using cloud-resolving convection parametrization (CRCP; super-parametrization) and a traditional convective parametrization (the Emanuel scheme). The large-scale organization of convection within the equatorial waveguide takes the form of MJO-like (Madden,Julian Oscillation) coherent structures. First, CRCP simulations are performed in which development of large-scale free-tropospheric moisture perturbations is artificially suppressed using relaxation with a timescale of one day. As in previous simulations where much shorter relaxation timescale was used, MJO-like coherences do not develop and, if already present, they disintegrate rapidly. Second, CRCP simulations that start from planetary-scale moisture perturbation in the free troposphere are conducted. The ensuing large-scale velocity perturbations have e-folding times of five and seven days, respectively, for interactive and prescribed radiation simulations. This supports the conjecture that interactive radiation enhances moisture,convection feedback; an enhanced large-scale circulation results from differences in radiative cooling between areas having enhanced and suppressed convectively-generated moisture and cloudiness. Additional support for the role of moisture,convection feedback in intraseasonal oscillations is seen in simulations that apply the Emanuel scheme. The standard configuration of the Emanuel scheme is insensitive to free-tropospheric humidity and results in weak MJO-like coherences. A simple modification of the Emanuel scheme that enhances its sensitivity to free-tropospheric humidity dramatically improves the simulated MJO-like coherences. Copyright © 2004 Royal Meteorological Society [source] High-resolution regional climate simulations of the long-term decrease in September rainfall over IndochinaATMOSPHERIC SCIENCE LETTERS, Issue 1 2009Hiroshi G. Takahashi Abstract We address the long-term decrease in September rainfall over the Indochina Peninsula. Distinct long-term decreases in rainfall along the monsoon trough across the Indochina Peninsula have been observed. We performed long-term simulations and discuss the effects of long-term changes in both the local surface conditions and large-scale circulation. Using a 30-year simulation for September for the period from 1966 to 1995 with land-use conditions fixed at present-day values and neglecting the recorded deforestation, we successfully simulated the observed long-term decrease in rainfall. We therefore conclude that the weakening tropical-cyclone activity over the Indochina Peninsula region is probably responsible for the decrease in rainfall. Copyright © 2008 Royal Meteorological Society [source] Factors governing the interannual variation and the long-term trend of the 850 hPa temperature over IsraelTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 647 2010H. Saaroni Abstract This study examines the ability of the interannual variability in the occurrence of synoptic types, intensity of large-scale circulations and global temperature to explain that of the 850 hPa temperature in Israel for the summer and the winter. The synoptic factor was represented by 19 types defined by Alpert et al(2004b). For the summer, the deep and the weak Persian Trough explained 35% of the interannual temperature variance. For the winter, the lows to the east and to the north explained 44% of the interannual temperature variance. Two additional factors were incorporated: large-scale circulations, the North Atlantic Oscillation for the summer and the Arctic Oscillation for the winter; and global radiative forcing, represented by the global temperature. Both of them were found to be significant, and the variance explained by all of them is 56% for the summer and 64% for the winter. In the summer the variation is dominated by warm and cool types whereas in the winter the cold systems dominate. The individual contribution of each factor to the long-term temperature trend was estimated. While the global radiative forcing contribution was positive and large in both seasons, the synoptic contribution was positive, four times larger in the summer. The large-scale contribution was negative, three times larger in the winter. The considerable warming in the summer results from a rapid increase in the occurrence of the weak Persian Trough, which is a warm type. The study approach may be useful for predicting future temperature regimes, based on predicted synoptic features in climatic models. Copyright © 2010 Royal Meteorological Society [source] | ||||||||||