Modes Of Atmospheric Circulation Research Articles

Implications of changes in the atmospheric circulation on the detection of regional surface air temperature trends

Previous studies have shown that observed significant warming trends in surface air temperature (SAT) consistent with the response to anthropogenic forcing are detected at scales on the order of 500 km in many regions of the globe. However, regional SAT trends project strongly on the dominant natural atmospheric circulation modes, such as the Arctic Oscillation (AO) and the hemispheric Pacific‐North America (PNA)‐like patterns. The warming associated with the changes of atmospheric circulation is not well simulated in current coupled climate models. In this study, we explore the influence of the exclusion of warming related to changes of the atmospheric circulation on the detection of a regional response to combined anthropogenic and natural forcings. We compare observed SAT trends over the second half of the 20th century with those simulated in response to natural and anthropogenic climate forcings in a suite of six current coupled general circulation models. Control runs from these models are used to provide estimates of the internal variability of trends. We find that the detection of the regional response to combined anthropogenic and natural forcing is robust to the exclusion of warming related to changes of the atmospheric circulation considered here.

Influence of teleconnection patterns on precipitation variability and on river flow regimes in the Miño River basin (NW Iberian Peninsula)

The influence of the most representative winter patterns of atmospheric variation in the Northern Hemisphere on precipitation variability and on river flow regimes was evaluated for the Mino River basin (NW Iberian Peninsula). The Arctic Oscillation (AO) was also considered in order to analyze the effect of a larger-scale atmospheric mode on precipitation and river flow. Precipitation data were obtained from 22 rain-gauge stations located within the Mino River basin from 1976 to 1997, and the Mino River flow data were recorded at Frieira gauge station from 1970 to 2005. Both hydrological-year (October to September) time series were averaged for the wet season (December to February). The precipitation variability and river flow in the Mino catchment were shown to have high positive correlation. Relationships with atmospheric circulation modes show that the main pre- cipitation variability, and consequently the river-flow regimes, during winter cannot be explained by a single atmospheric pattern. The temporal evolution (1970-2005) of the correlation between the teleconnection indices and river flow averaged for 22 yr periods showed how all atmospheric patterns evolve with a different trend structure—in such a way that the most prevalent pattern depends on the chosen period of time. Moreover, the time evolution of the correlation between the AO and river flow remains more stationary than for the rest of the indices, due to its larger scale.

Influence of seasonal pressure patterns on temporal variability of vegetation activity in Central Siberia

AbstractThis paper analyses the spatial distribution of the inter‐annual variability of vegetation activity in central Siberia and its relationship with atmospheric circulation variability. We used NOAA‐AVHRR NDVI series from Pathfinder Land Data Set at 1° of spatial resolution, and we calculated the annual vegetation activity in each pixel (aNDVI) from 1982 to 2001. Principal component analysis (PCA) was used to determine the general spatial patterns of inter‐annual variability of vegetation activity. We identified three main modes, which explain more than 50% of the total variance, each corresponding to a large region. By means of surface pressure grids, we analysed the main patterns of the seasonal atmospheric circulation in the study area: its variability was summarised by means of a few circulation modes and the patterns differ significantly between winter, spring and summer. However, a pattern with a North–South dipole structure represents the general spatial pattern of atmospheric circulation. We investigated the effect of seasonal atmospheric circulation patterns on the inter‐annual variation of vegetation activity. In general, the strongest relationships between the atmospheric circulation variability, climate and the aNDVI variability were found in areas where the climatic characteristics are more limiting for the vegetation development, such as the northern regions. This may be explained by the fact that in these areas the variability of atmospheric circulation modes determines summer temperatures, which have a direct impact on vegetation activity. Copyright © 2006 Royal Meteorological Society.

Long Hydroclimate Records from Tree-Rings in Western Canada: Potential, Problems and Prospects

Tree-ring chronologies from moisture-sensitive sites can provide annually resolved proxy records for precipitation, streamflow and other hydrological variables for periods several hundred years longer than available instrumental records. This paper reviews some previous North American work in this field (concentrating on western Canada), examining the limitations and potential of these studies with specific examples from ongoing research in the Canadian Cordillera. Although they cannot be used to predict future flows or precipitation, proxy data can document the long-term natural variability and periodicities of these phenomena—information vital for better approximations of future probabilities. In addition, by establishing relationships between hydrometeorological variables and persistent modes of atmospheric and oceanic circulation, such research may provide some predictive skill in short-term forecasting.

Climate and hydrological changes in tropical Africa during the past million years

Tropical African climate has oscillated between markedly wetter and drier conditions on all timescales in response to global climate disturbances. A step-like increase in aridity over the past 3 Ma has been primarily paced by orbital cycles coupled with the onset and amplification of high-latitude glacial cycles. On the 10 4–10 3-ka timescales, observed changes imply interactions between insolation, sea-surface conditions and vegetation. High-frequency variations could be linked to oscillations in major atmospheric circulation modes, in solar output, or to major volcanic events. To cite this article: F. Gasse, C. R. Palevol 5 (2006) .

Arctic and North Atlantic Oscillation phase changes are recorded in the isotopes (δ18O and δ13C) of Cassiope tetragona plants

AbstractThe Arctic and North Atlantic Oscillations (AO/NAO) are large‐scale annual modes of atmospheric circulation that have shifted in the last 30 years. Recent changes in arctic climate, including increasing surface air temperature, declining sea ice extent, and shifts in the amounts seasonality of precipitation are linked to the strong positive phase of the AO/NAO. Here, we show that phase changes in the AO/NAO are recorded in the isotopic (δ18O and Δ‐carbon isotope discrimination) characteristics of the long‐lived circum‐arctic plant, Cassiope tetragona, as summer rain has become a more important water source than snowmelt water which in turn has lead to decreases in Δ and reductions in plant stem growth. These isotopic records in C. tetragona may facilitate reconstructions of climate, plant–soil water relations, plant gas exchange attributes and a mechanistic understanding of growth responses to shifts in atmospheric circulation. If plant specimens were available for populations across the arctic as part of the International Polar Year, these archives could provide a circum‐arctic record of historical climate change and associated shifts in physiological plant performance and growth.

Dendrochronological Potential of the Arctic Dwarf-Shrub Cassiope tetragona

In this report, we describe the use of dendrochronological techniques on the circumpolar, evergreen dwarf-shrub, Cassiope tetragona. Using techniques such as crossdating and standardization, and the software programs COFECHA and ARSTAN, we developedC. tetragona growth and reproduction chronologies for sites in the Canadian High Arctic. High-resolution chronologies may be used to reconstruct past climate and phase changes in large-scale modes of atmospheric circulation (e.g. Arctic Oscillation, North Atlantic Oscillation), to investigate the growth and reproductive responses of the plant to ambient and manipulated environmental variables, and to reconstruct the plant’s past ecohydrology ( d18O, dD, d13C), gas exchange (d13C) and mineral nutrition (d15N). As C. tetragona is a circumpolar species, chronologies may be developed throughout the Arctic at sites where no trees exist, and thus provide new information on the past climate and environmental history of sites and regions previously unstudied.

Statistical Modeling of Extreme Rainfall in Southwest Western Australia

Abstract Rainfall over southwest Western Australia (SWWA; 32°S southward and 118°E westward) has been decreasing over the past decades, putting further constraints on water resources in an already dry area. In this study, daily rainfall over five geographically dispersed and homogenized weather stations within SWWA are analyzed. A peak over threshold method from the extreme value theory is used to model daily rainfall above a given threshold. The Mann–Whitney–Pittitt (change point) test was applied to detect changes in annual, winter (May–October), and summer (November–April) maximum daily rainfall. Change points for winter extreme daily rainfall were found around 1965, based on different individual stations, with the extreme daily rainfall reduced since then. To demonstrate the degree of change in the winter extreme daily rainfall, at 1965 the data were stratified, and generalized Pareto distributions were fitted to the tails of the distributions for daily rainfall in the prechange period of 1930–65 (including 1965) and the postchange period of 1966–2001. The fitted tail distributions also allow the estimation of probabilities and return periods of the daily rainfall extreme. Results show that return periods for the winter extreme daily rainfall have increased after 1965, implying that winter daily rainfall extremes in SWWA are lower after 1965 than they were before. There has been vigorous debate as to what forces the drying trend, that is, whether it is part of multidecadal variability or whether it is driven by secular forcings, such as increasing atmospheric CO2 concentration. In this paper, statistical modeling is also used to identify possible associated changes in atmospheric circulation. It is found that there is a change point near 1965 in a dominant atmospheric circulation mode of the Antarctic Oscillation (AAO). The result offers qualified support for the argument that the AAO may contribute to the drying trend.

Temperature and precipitation variability in the European Alps since 1500

High-resolution temperature and precipitation variations and their seasonal extremes since 1500 are presented for the European Alps (43.25–48.25°N and 4.25–16.25°E). The spatial resolution of the gridded reconstruction is given by 0.5° × 0.5° and monthly (seasonal) grids are reconstructed back to 1659 (1500–1658). The reconstructions are based on a combination of long instrumental station data and documentary proxy evidence applying principal component regression analysis. Annual, winter and summer Alpine temperatures indicate a transition from cold conditions prior to 1900 to present day warmth. Very harsh winters occurred at the turn of the seventeenth century. Warm summers were recorded around 1550, during the second half of the eighteenth century and towards the end of the twentieth century. The years 1994, 2000, 2002, and particularly 2003 were the warmest since 1500. Unlike temperature, precipitation variation over the European Alps showed no significant low-frequency trend and increased uncertainty back to 1500. The years 1540, 1921 and 2003 were very likely the driest in the context of the last 500 years. Running correlations between the North Atlantic Oscillation Index (NAOI) and the Alpine temperature and precipitation reconstructions demonstrate the importance of this mode in explaining Alpine winter climate over the last centuries. Winter NAOI correlates positively with Alpine temperatures and negatively with precipitation. These correlations, however, are temporally unstable. We conclude that the Alps are situated in a band of varying influence of the NAO, and that other atmospheric circulation modes controled Alpine temperature and precipitation variability through the recent past. Copyright © 2005 Royal Meteorological Society

Early peak in Antarctic oscillation index

The principal extratropical atmospheric circulation mode in the Southern Hemisphere, the Antarctic oscillation (or Southern Hemisphere annular mode), represents fluctuations in the strength of the circumpolar vortex and has shown a trend towards a positive index in austral summer in recent decades, which has been linked to stratospheric ozone depletion and to increased atmospheric greenhouse-gas concentrations. Here we reconstruct the austral summer (December-January) Antarctic oscillation index from sea-level pressure measurements over the twentieth century and find that large positive values, and positive trends of a similar magnitude to those of past decades, also occurred around 1960, and that strong negative trends occurred afterwards. This positive Antarctic oscillation index and large positive trend during a period before ozone-depleting chemicals were released into the atmosphere and before marked anthropogenic warming, together with the later negative trend, indicate that natural forcing factors or internal mechanisms in the climate system must also strongly influence the state of the Antarctic oscillation.

Winter snow anomaly and atmospheric circulation in Mongolia

AbstractAn observational study was conducted of winter snow anomaly in Mongolia and the associated atmospheric circulation. Monthly data of snow depth and temperature at 23 Mongolian stations from 1940 to 1992 were used for a statistical analysis. The Mongolian snow amount is fairly large in the northern mountainous regions and decreases to the south. On average, there is a substantial seasonal increase in snow depth during November and December. The deepest snow occurs in January, with an average depth of 3.4 cm averaged for all 23 stations. The correlation coefficients for November and December between the snow depth and temperature are significant and negative over wide areas.A principal component analysis was applied to interannual anomalies of January snow depth. The first mode (PC1) reveals widespread loadings of the same sign, and the second mode (PC2) shows a northeast–southwest dipole pattern. A correlation analysis was employed to examine relationships between PC1, PC2 and the preceding atmospheric circulation modes derived from a principal component analysis of the Northern Hemisphere 500 hPa geopotential heights. The PC1 time series for January is highly correlated with the North Atlantic oscillation for November, and the PC2 time series is significantly correlated with the Eurasian pattern for December. These relationships will provide a potential basis for long‐range forecasts of mid‐winter anomalous snow that leads to a significant loss of livestock (so‐called white dzud) in Mongolia. Copyright © 2003 Royal Meteorological Society

Links between flood events in central Europe since AD 1500 and large‐scale atmospheric circulation modes

Based on documentary sources incidence variations of flood events can be reconstructed back to AD 1500 for several catchment areas in Central Europe. Links to atmospheric circulation modes, derived from reconstructed large‐scale sea level pressure (SLP) grids for the last 500 years, have been identified on climatic time scales (monthly to seasonal). These relations are expressed in terms of several indices describing the particular importance of atmospheric circulation modes as a dynamical background for the varying incidence of flood events. During winter, the zonal circulation mode covers the largest part of these events, in relation to mode‐frequency, however, other circulation modes become important during historical periods of increased flood frequency, e.g. modes characterised by Atlantic low and Russian high pressure centres. Different subtypes of this mode reached maxima in flood‐association during different climatic periods of the past.

The role of physical forcing in initiation of spring blooms in the northeast Atlantic

In this paper, the relationship between changes in the intensity of the spring bloom and changes in the physical forcing, mediated by the upper ocean mixing, is examined and illustrated. An idealized mixed-layer model coupled with an NPZD model with two different detritus compartments provides a conceptual framework, predicting the biological response to vertical mixing anomalies in the ocean–surface boundary layer. The model of coupled biological–hydrodynamic processes is used to search for general physical principles of phytoplankton bloom regulation in open ocean waters. The study addresses three questions: How does an individual winter differ from climatologically averaged ones? What are the dynamics of stratification in open ocean systems as influenced by variable atmospheric forcing? How does phytoplankton growth respond to these dynamics? The calculations suggest that changes in the balance of the atmospheric forcing on a daily scale could lead to less pronounced fluctuations of the mixed layer in winter as during most of the day the heat input by the sun exceeds or compensates the heat loss by nonsolar heat flux components (i.e. transient or moderate conditions). Furthermore, the model results show that the springtime shallowing of the mixed layer is not typically a smooth transition, but is interrupted by mixing events stimulated by passage of weather systems. The frequency and intensity of these synoptic events varies from year to year due to shift in storm tracks and modes of atmospheric circulation. The simulations suggest that such synoptic events (storms) associated with time scales of several days, and the period of the year when the heat flux changes sign, are important for the development of the phytoplankton population. Enhanced or reduced storm frequency associated with changes in vertical mixing intensity during the late winter/spring period leads to an increase/decrease in amplitude and duration of the bloom caused by enhanced/reduced nutrient supply or retard it due to light limitation. Finally, supported by the model results and observations, the role of interannual variability in physical forcing on timing, amplitude and structure of the phytoplankton bloom for the period 1989–1997 is examined.

Holocene fluvial response to climate change and human activities; Burgundy, France.

AbstractAlluvial deposits of the Loire/Arroux trunk/tributary system record distinct, synchronous episodes of regional fluvial adjustment. Changes in facies and depositional style through time can be interpreted with a modern analogue model that relates vegetative cover/human influence with sediment supply, and modes of atmospheric circulation with the paths and styles of storms that drive variable discharge regimes across western Europe.Zonal atmospheric circulation results in a Mediterranean style climate over southern Burgundy, producing dry conditions punctuated by infrequent, large floods. Episodic overbank sedimentation and the burial of thin paleosols in sandy overbank facies is indicative of this style of fluvial activity, ca 1300 years BP. Humans may have increased the available volume of fine grained sediment at this times through increased agricultural activity along valley axes, however facies match that expected from a ‘flashy’ discharge regime.In contrast, meridional circulation patterns result in a maritime style climate over southern Burgundy, with the intrusion of storms, moist conditions and frequent, moderate magnitude discharges. Wide, deep channels, thick channel facies and thin overbank facies are indicative of this style of fluvial activity, recorded in deposits dating to ca 4050 to 3200 years BP. Strong meridional conditions and extreme climatic variability during the Little Ice Age resulted in very large discharges that straightened and widened channels, while scouring and obscuring older terraces (ca 500 years BP). Deposition over the last two centuries is related to increasingly zonal circulation and infrequent, large (over-bank) floods. Changes in fluvial dynamics over the last 300 years can be attributed primarily to climatic control, as there has been very little change in land-use over that period.

Dynamics of recent climate change in the Arctic.

The pattern of recent surface warming observed in the Arctic exhibits both polar amplification and a strong relation with trends in the Arctic Oscillation mode of atmospheric circulation. Paleoclimate analyses indicate that Arctic surface temperatures were higher during the 20th century than during the preceding few centuries and that polar amplification is a common feature of the past. Paleoclimate evidence for Holocene variations in the Arctic Oscillation is mixed. Current understanding of physical mechanisms controlling atmospheric dynamics suggests that anthropogenic influences could have forced the recent trend in the Arctic Oscillation, but simulations with global climate models do not agree. In most simulations, the trend in the Arctic Oscillation is much weaker than observed. In addition, the simulated warming tends to be largest in autumn over the Arctic Ocean, whereas observed warming appears to be largest in winter and spring over the continents.

Mixture Model For Overdispersion of Precipitation

Abstract Stochastic models fit to time series of daily precipitation amount generally ignore any year-to-year (i.e., low frequency) source of random variation, and such models are known to underestimate the interannual variance of monthly or seasonal total precipitation. To explicitly account for this “overdispersion” phenomenon, a mixture model is proposed. A hidden index, taking on one of two possible states, is assumed to exist (perhaps representing different modes of atmospheric circulation). To represent the intermittency of precipitation and the tendency of wet or dry spells to persist, a stochastic model known as a chain-dependent process is applied. The parameters of this stochastic model are permitted to vary conditionally on the hidden index. Data for one location in California (whose previous study motivated the present approach), as well as for another location in New Zealand, are analyzed. To estimate the parameters of a mixture of two conditional chain-dependent processes by maximum likeliho...

Warming over the Eurasia continent and Variations of the Atmospheric Circulation in Recent

The singular value decomposition (SVD) analysis is applied to examine relationship between lower-tropospheric temperature over the Eurasia continent and atmospheric circulation in the Northern Hemisphere for 30 years from 1967-1996. The cluster analysis using 5-days mean data is applied to detect the change of regime transition. The first mode of the temperature (SVD1) has amplitude in East Asia, the Scandinavian Peninsula and Polar region. The first mode of the atmospheric circulation is closely related to the teleconnection pattern known as PEA with decadal scale. The score time series of this mode shows climatic jump in winters of 1976/77, 1988/89. Similar interannual and interdecadal variabilities are appeared in the regime of 5-days mean as well. On the other hand, the second mode of temperature (SVD2) shows recent warming in the Eurasia continent. This atmospheric circulation mode is similar to the NA and NAO of the teleconnection pattern and seems to be connected strongly with the variability of the Iceland low than the Siberian high. Consequently, fluctuation of the recent lower-tropospheric temperature over the Eurasia continent is probably related with Polar low and atmospheric circulation in the North Atlantic Ocean region. It is found that NAO might play a role for the warming over the Eurasian continent by the isolating to two components; decadal-scale jump and increasing trend with different processes.

Surface and 700 hPa Atmospheric Circulation Patterns for the Great Lakes Basin and Eastern North America and Relationship to Atmospheric Teleconnections

Many studies have identified continental-scale atmospheric circulation regimes, and some have been employed for various regions, but none have involved a regional categorization of circulation around the Great Lakes basin. Such an analysis is important not only because of the economic and recreational importance of the lakes, but in an effort to relate the regional circulation types to the broaderscale modes of atmospheric circulation, such as that forced by El Niño (ENSO). In this study, rotated principal components analysis (RPCA) is performed on the monthly mean sea-level pressure field around the Great Lakes basin, and in a separate analysis, on the mean 700 hPa field in eastern North America. An average-linkage clustering algorithm is applied to the RPCA scores to classify the monthly surface circulation in the Great Lakes region and the 700 hPa circulation over eastern North America. The classification is used to determine whether the various categories of regional circulation patterns are coincident with distinct hemispheric-scale flow regimes. To do this, indices of the modes of variability in some of the most well-known atmospheric teleconnections during months that fall within each circulation mode are subjected to ANOVA tests by cluster. Results suggest that the regional atmosphere over the Great Lakes basin undergoes long-term shifts in preferred modes of circulation. Furthermore, flow variability associated with the 700 hPa North Atlantic Oscillation (NAO) and Pacific/North American (PNA) teleconnections are more strongly tied to variability in both the Great Lakes regional surface circulation and the 700 mb eastern North American flow regimes than is the ENSO-forced Southern Oscillation.

The poleward propagation of atmospheric angular momentum with a 30–60-day time scale

Abstract The existence of the intraseasonal variation of relative atmospheric angular momentum and the poleward propagation of this physical quantity by the intraseasonal mode of atmospheric circulation have been amply demonstrated in previous studies. However, the temporal relationship between the intraseasonal variation and poleward propagation is not well understood and the possible mechanism causing the poleward propagation is unknown. Analyzing the 15-year (1979–1993) upper-air data of the National Meteorological Center (currently the National Centers for Environmental Prediction), we explored these two intriguing issues related to atmospheric angular momentum via case-study analyses. The major findings are as follows: 1. 1. The maximum (minimum) 30–60-day globally integrated atmospheric angular momentum occurs when the positive (negative) angular momentum anomalies of a 30–60-day cycle appear in the tropics-subtropics and those of the previous 30–60-day cycle propagate to the northern and southern midlatitudes. 2. 2. The eastward propagation of the 30–60-day global divergent circulation induces a 30–60-day flip-flop oscillation in the meridional circulation which in turn results in a similar alternation in the angular momentum tendency caused by the Coriolis torque. It was inferred from the zonal-mean angular momentum budget analysis that the 30–60-day variation of atmospheric angular momentum and its poleward propagation are results of the 30–60-day variation of the aforementioned momentum tendency.

The influence of synoptic scale meteorology on transport of urban air to remote locations in the SouthWestern United States Of America

Linking air quality at remote sites with synoptic scale atmospheric circulation can provide information regarding the sources of pollutant and may be used to assess the frequency of (and trends in) conditions conducive to long range transport. A methodology for combining tracer studies and regional scale synoptic climatologies is devised herein. Using principal components analysis (PCA) of the 700 and 800 mb heights and precipitable water at 11 radiosonde sites and sea-level reduced surface pressure at 13 surface sites, the dominant modes of atmospheric circulation over the southwestern United States of America are identified and indices of the synoptic scale circulation calculated. These indices are then used as predictor parameters in regression equations of the average daily methylchloroform concentration (a tracer of urban emissions) at two remote sites located to the west of the Grand Canyon National Park, to test the] dependence of ambient methylchloroform concentrations upon prevailing synoptic scale meteorology. It is shown that the influence of synoptic scale conditions is seasonal; the relationship between the average daily methylchloroform concentration at the remote sites and synoptic scale conditions is stronger during the winter season (December-February) than during the summer (June-September).