Pacific North American Pattern Research Articles

Winter precipitation isotope slopes of the contiguous USA and their relationship to the Pacific/North American (PNA) pattern

This study compares the synoptic-dynamic relationship between two phases of the Pacific/North American (PNA) pattern and winter precipitation isotopes at 73 sites across the contiguous USA. We use the spatial pattern of isotope slope—the rate of changes in precipitation isotope ratios with distance—to identify features in the seasonal precipitation isotope fields related to climatic patterns, PNA positive and PNA negative. Our results show relationships between zones of high isotope slopes and the spatial position of the polar jet stream and juxtaposition of air masses associated with the PNA pattern. During a positive PNA winter, zones of high isotope slope in the eastern USA shift southward. This change is coincident with a southward displacement of the polar jet stream in this region, which leads to a greater frequency of polar air masses and 18O-depleted isotope values of precipitation in the region. In the western USA, zones of high slope shift eastward during the positive PNA winter, associated with more frequent penetration of tropical air masses that bring 18O-enriched precipitation to the region. Differences in δ18O/temperature relationships between the PNA-positive and -negative winters and contrasting δ18O/temperature behaviors in the eastern and western USA provide support for the role of variation in moisture source and transport as a control on the isotopic patterns. These findings highlight the importance of synoptic climate driven by PNA pattern in determining the spatial patterns of precipitation isotopes and provide constraints on paleo-water isotope interpretation and modern isotope hydrological processes.

Data Clustering Reveals Climate Impacts on Local Wind Phenomena

AbstractThe authors demonstrate the utility of k-means clustering for identifying relationships between winds at turbine heights and climate oscillations, thereby developing a method suited for predicting the impacts of climate change on wind resources. Fourteen years of data from an 80-m tower at the National Wind Technology Center (NWTC) in Colorado have been reduced to four dominant flow phenomena using k-means clustering. At this location, this method identifies two clusters of westerly inflow (strong and weak), another cluster of flow from the north, and one of flow from the south. Similar clusters are found for the data at all heights on the tower, and each follow distinct seasonal cycles. Time series of each cluster, as well as the mean wind speed at the NWTC, are retained for comparison with climate oscillations along with the local 500-hPa pressure gradient. The mean wind speed in the surface layer is strongly correlated with the local north–south pressure gradient. The frequency of strong westerly flow is also negatively correlated with the Niño-3.4 index, whereas weaker westerly winds are negatively correlated with the Pacific–North American pattern (PNA) and Arctic Oscillation (AO). Northerly winds at the NWTC did not strongly correlate with any of the investigated climate indices (AO, PNA, and Niño-3.4). These northerly winds occur more frequently in the summer months, suggesting that these winds are more influenced by local conditions than by mesoscale forcing. This method of identifying clusters in wind data allows objective identification of wind phenomena that may benefit the deployment of wind turbines, for example, in choosing combinations of wind speed and direction to investigate for turbine siting.

Propagating Atmospheric Patterns Associated with Midwest Winter Precipitation

Abstract For winters over eastern North America, complex Hilbert empirical orthogonal function (HEOF) analysis was used to objectively identify propagating patterns in four atmospheric fields that have potential relevance to precipitation: jet stream–level wind speed, 850-hPa moisture transport (qv), temperature advection (TA), and vorticity advection (VA). A novel phase shift method was used to show the location where each propagating pattern was most correlated with Midwest precipitation, and each of the four phase-shifted HEOF patterns was compared to its respective high-precipitation composite view. The leading HEOFs of the three transport fields (qv, TA, and VA), which collectively represented the dynamics associated with a midlatitude cyclone, accounted for almost half of Midwest precipitation variability and were associated with lake effect snow when propagating downstream from the Midwest. Correlation and spectral analyses revealed how the propagating transport patterns were related to the Pacific–North American pattern and other teleconnections. The leading HEOF of jet stream–level wind speed, which represented the tendency for the jet stream to migrate equatorward over the study region during winter, accounted for only about 4% of Midwest daily precipitation variability. In contrast, the second HEOF of jet stream–level wind speed, which represented an eastward propagating trough dynamically consistent with a midlatitude cyclone, accounted for 16% of Midwest daily precipitation variability.

Climate Divisions for Alaska Based on Objective Methods

AbstractAlaska encompasses several climate types because of its vast size, high-latitude location, proximity to oceans, and complex topography. There is a great need to understand how climate varies regionally for climatic research and forecasting applications. Although climate-type zones have been established for Alaska on the basis of seasonal climatological mean behavior, there has been little attempt to construct climate divisions that identify regions with consistently homogeneous climatic variability. In this study, cluster analysis was applied to monthly-average temperature data from 1977 to 2010 at a robust set of weather stations to develop climate divisions for the state. Mean-adjusted Advanced Very High Resolution Radiometer surface temperature estimates were employed to fill in missing temperature data when possible. Thirteen climate divisions were identified on the basis of the cluster analysis and were subsequently refined using local expert knowledge. Divisional boundary lines were drawn that encompass the grouped stations by following major surrounding topographic boundaries. Correlation analysis between station and gridded downscaled temperature and precipitation data supported the division placement and boundaries. The new divisions north of the Alaska Range were the North Slope, West Coast, Central Interior, Northeast Interior, and Northwest Interior. Divisions south of the Alaska Range were Cook Inlet, Bristol Bay, Aleutians, Northeast Gulf, Northwest Gulf, North Panhandle, Central Panhandle, and South Panhandle. Correlations with various Pacific Ocean and Arctic climatic teleconnection indices showed numerous significant relationships between seasonal division average temperature and the Arctic Oscillation, Pacific–North American pattern, North Pacific index, and Pacific decadal oscillation.

Hydroclimatology of the US Intermountain West

The semi-arid US Intermountain West is characterized by complex hydroclimatic variability, influenced both by topography and by atmosphere and ocean processes operating over a large range of time and space scales. Understanding climate–hydrology interactions has become increasingly important as demands on water resources grow from both within and beyond the region, particularly in light of projected climate changes. This paper reviews key atmospheric and oceanic controls that impact the Intermountain West’s water supply, how those controls vary over multiple timescales, the tree-ring record of hydroclimatic variability in the region, projected climate change impacts, and research needs for the future. Water availability in the Intermountain West is largely influenced by interrelated atmospheric features that include the strength and position of the Pacific subtropical high, the intensity and geographic location of the Aleutian low, the latitude of the westerly storm track, and teleconnections such as the Pacific North American pattern and the El Niño−Southern Oscillation system. The tree-ring record of pre-instrumental conditions in the Intermountain West indicates that there have been droughts in the past that were more severe than those experienced in the historical record. Climate model projections of future moisture-related changes, including decreased snowpack and changing seasonality of precipitation, may exacerbate strain on the region’s water supply. Advances in climate modeling and in our understanding of climate variability over multiple time and space scales would improve capacity for water resource management in the Intermountain West.

Synoptic climatology and the analysis of atmospheric teleconnections

Over recent decades, analyses of the structure and impact of atmospheric teleconnections have substantially increased our understanding of the climate system and the role of climate variability. Moving beyond simple correlations between teleconnection indices and temperature and precipitation anomalies, synoptic climatology has been able to provide insight on the spatiotemporal manifestation of teleconnection anomalies, as well as further understanding in terms of teleconnection-related anomalies in circulation pattern frequencies that can lead to extreme events over individual areas. In this progress report, we focus on a number of recent papers that, broadly defined, assess two realms of teleconnections: the North Atlantic circulation, largely focusing on the North Atlantic Oscillation; and the north and tropical Pacific circulation, as manifest in El Niño–Southern Oscillation, the Pacific North American pattern, and the Pacific Decadal Oscillation. In each of these two regions, we highlight the major goals and results of recent synoptic research.

Eclipse Ice Core Accumulation and Stable Isotope Variability as an Indicator of North Pacific Climate

Abstract The high accumulation rate and negligible amount of melt at Eclipse Icefield (3017 m) in the Saint Elias Range of Yukon, Canada, allows for the preservation of a high-resolution isotopic and glaciochemical records valuable for reconstruction of climatic variables. Each of the three Eclipse ice cores have a well-constrained depth–age scale with dozens of reference horizons over the twentieth century that permits an exceptional level of confidence in the results of the current calibration exercise. Stacked time series of accumulation and stable isotopes were divided into cold and warm seasons and seasons of extreme high and extreme low accumulation and stable isotope values (eight groups). For each group, season-averaged composites of 500-hPa geopotential height grids, and the individual seasons that constitute them, were analyzed to elucidate common anomalous flow patterns. This analysis shows that the most fractionated isotopes and lowest accumulation cold seasons reflect a more zonal height pattern in the North Pacific associated with negative Pacific–North American (PNA) and Pacific decadal oscillation (PDO) indices. Conversely, the least fractionated isotopes and highest accumulation cold seasons are associated with a positive PNA pattern. Although only a maximum of approximately 20% of the total number of accumulation and stable isotope seasons exhibit a relatively consistent relationship with 500-hPa geopotential height patterns, these results support the hypothesis that the most extreme accumulation and extreme isotope cold-season values in the Saint Elias Mountains are related to consistent atmospheric circulation and oceanic sea surface temperature patterns.

A Comparison of Three Prolonged Periods of Heavy Rainfall over the Hawaiian Islands

AbstractThe anomalous circulation patterns during an unusually prolonged stormy-weather period in Hawaii from 19 February to 2 April 2006 are analyzed and are compared with those of two previously known prolonged heavy-rainfall periods (March 1951 and February 1979). The circulation patterns for these three periods are characterized by 1) a negative Pacific–North American (PNA) pattern in the midlatitudes with a blocking high southwest of the Aleutian Islands, 2) retraction and splitting of the zonal jet into a polar jet north of 50°N and a persistent subtropical jet to the south over the central Pacific Ocean, 3) an anomalous low west of the Hawaiian Islands embedded in the subtropical jet, and 4) a weaker-than-normal Hadley circulation in the mid-Pacific. The moisture advected from low latitudes by the southerly wind component east of the persistent anomalous low, combined with upward motion, provides the large-scale setting for the unusually prolonged unsettled weather across the Hawaiian Islands. For all three cases, the prolonged stormy weather started after the onset of large-scale blocking and a negative PNA pattern over the North Pacific and the occurrence of a persistent anomalous low embedded in the subtropical jet west of the Hawaiian Islands. Furthermore, the persistent low was located at the optimal position to bring moisture from the central equatorial Pacific to Hawaii. The stormy weather ceased after the midlatitude blocking pattern weakened and the anomalous low in the subtropics decayed and/or shifted westward. There are no apparent common precursors in the 2-week period prior to the prolonged stormy weather among these three cases, however.

The impacts of the PNA and NAO on annual maximum snowpack over southern Canada during 1979–2009

AbstractThe Pacific/North American pattern (PNA) and North Atlantic Oscillation (NAO) are two of the most important atmospheric low‐frequency variability modes modulating interannual anomalies of snowpacks in North America. However, the quantitative degree to which the PNA and NAO affect the annual maximum snow water equivalent (SWEmax), a quantity directly relevant to the ground snow load calculation in National Building Code of Canada, remains unknown. This study explores the relationship of the winter PNA and NAO to the Canadian SWEmax anomalies and potential changes associated with atmospheric circulation patterns. Significant correlations are identified in eastern and western Canada, where climatological mean values are largest due to the high frequency of snow storm occurrence. The regions in Canada where SWEmax anomalies are influenced by the PNA and NAO experienced a significant change from pre‐1998 to post‐1998. This change is consistent with the observed changes in annual maximum snow depth and the winter atmospheric circulations during 1979–2009. Copyright © 2012 Royal Meteorological Society

The Impact of Climate Modes on Summer Temperature and Precipitation of Darwin, Australia, 1870-2011

Monthly mean summer (DJF) temperature and precipitation from Global Historical Climate Network (GHCN-V3) for the period of 1870-2011, are analyzed to assess the role of teleconnections on climate of Darwin, Australia. Indices of El Nino-Southern Oscillation (ENSO), Antarctic Oscillation (AAO), Pacific Decadal Oscillation (PDO), North Atlantic Oscillation (NAO), Arctic Oscillation (AO), and Pacific North American Oscillation (PNA) are extracted from monthly means and compared with climatic data of Darwin. Most of these climate modes are shown to have a strong influence on the monthly mean summer temperature and precipitation. ENSO is shown to have a positive relationship with the amount of precipitation received and a negative relationship with the temperature. Where an El Nino event produces warmer drier conditions and a La Nina event produces colder wetter conditions. The AAO is shown to cause cold and dry conditions during the positive phase and warm and wet conditions during the negative phase. The PDO is shown to cause El Nino like condition during the positive phase causing warmer, drier weather, and La Nina like conditions during the negative phase causing cooler, wetter weather. Through the analysis it is also shown that the NAO, AO, and PNA have little effect on the temperature and precipitation patterns of Darwin.

Variability in Fusarium Head Blight Epidemics in Relation to Global Climate Fluctuations as Represented by the El Niño-Southern Oscillation and Other Atmospheric Patterns

Cross-spectral analysis was used to characterize the relationship between climate variability, represented by atmospheric patterns, and annual fluctuations of Fusarium head blight (FHB) disease intensity in wheat. Time series investigated were the Oceanic Niño Index (ONI), which is a measure of the El Niño-Southern Oscillation (ENSO), the Pacific-North American (PNA) pattern and the North Atlantic Oscillation (NAO), which are known to have strong influences on the Northern Hemisphere climate, and FHB disease intensity observations in Ohio from 1965 to 2010 and in Indiana from 1973 to 2008. For each climate variable, mean climate index values for the boreal winter (December to February) and spring (March to May) were utilized. The spectral density of each time series and the (squared) coherency of each pair of FHB-climate-index series were estimated. Significance for coherency was determined by a nonparametric permutation procedure. Results showed that winter and spring ONI were significantly coherent with FHB in Ohio, with a period of about 5.1 years (as well as for some adjacent periods). The estimated phase-shift distribution indicated that there was a generally negative relation between the two series, with high values of FHB (an indication of a major epidemic) estimated to occur about 1 year following low values of ONI (indication of a La Niña); equivalently, low values of FHB were estimated to occur about 1 year after high values of ONI (El Niño). There was also limited evidence that winter ONI had significant coherency with FHB in Indiana. At periods between 2 and 7 years, the PNA and NAO indices were coherent with FHB in both Ohio and Indiana, although results for phase shift and period depended on the specific location, climate index, and time span used in calculating the climate index. Differences in results for Ohio and Indiana were expected because the FHB disease series for the two states were not similar. Results suggest that global climate indices and models could be used to identify potential years with high (or low) risk for FHB development, although the most accurate risk predictions will need to be customized for a region and will also require use of local weather data during key time periods for sporulation and infection by the fungal pathogen.

A vorticity-based analysis of the spatial and temporal characteristics of the Beaufort Anticyclone

[1] The Beaufort Anticyclone is the dominant pressure feature over the Arctic Ocean in all seasons and has a large influence on the surface wind regime and sea ice motion. Sea level pressure (SLP) from the National Center for Atmospheric Research/National Centers for Environmental Prediction Reanalysis is used to create a vorticity metric to investigate the spatial and temporal characteristics of the Beaufort Anticyclone from 1948 to 2008. Correlations between vorticity averaged over the Beaufort Anticyclone region and Northern Hemisphere SLP are strongest south of Alaska and north of Siberia. These two areas are also present in SLP composite maps based on extremes of the vorticity metric. Temporal characteristics are investigated using running means and spectral analysis in which the only predominant peak is the annual cycle. If 30 year time periods are compared, little to no trend in the vorticity metric is found except in October–December, where the more recent time period has slightly larger vorticity magnitudes. The Beaufort Anticyclone vorticity metric is correlated with teleconnection index values; the Pacific Ocean patterns show a larger influence than the Atlantic patterns, including the Arctic Oscillation, which has been shown in recent studies to have little correspondence to changes in sea ice in the early 21st century. A significant correlation is found with the Pacific North American pattern in all seasons except summer. The El Nino–Southern Oscillation shows a significant correlation with the vorticity metric in winter, and the Pacific Decadal Oscillation shows a significant correlation in winter and spring.

Pacific/North American teleconnection controls on precipitation isotope ratios across the contiguous United States

Large-scale climate teleconnections such as the Pacific/North American (PNA) pattern strongly influence atmospheric processes and continental climate. Here we show that precipitation δ 18O values in the contiguous United States are correlated with an index of the PNA pattern. The δ 18O/PNA relationship varies across the study region and exhibits two prominent modes, with positive correlation in the western USA and negative correlation in the east. This spatial pattern appears not to reflect variation in local climate variables, but rather primarily reflects differences in atmospheric circulation and moisture sources associated with PNA. Our results suggest that strong antiphase variation in paired paleo-water δ 18O proxy records from regions characterized by the two modes of δ 18O/PNA correlation, especially in the Midwest and southwestern USA, may provide a robust basis for reconstruction of past variation in the PNA pattern.

Teleconnections and interannual variability in Canadian groundwater levels

Summary Relatively little is known about the impact of climate change on groundwater. The analysis of large-scale climatic patterns such as the well-known El Nino Southern Oscillation Index could provide a better comprehension of the causes of recharge variations in aquifers and therefore improve the knowledge related to processes behind the variability of groundwater level time series. This topic has been studied to a limited extent in other parts of the world, but no study has yet used this approach across Canada. In this work, correlation and wavelet analyses and wavelet coherence are used to gain a broader understanding of the interannual dynamics of recharge through the use of groundwater level records in three Canadian regions, namely Prince Edward Island (PEI), southern Manitoba in the vicinity of Winnipeg (MB) and Vancouver Island (VI). Cause and effect linkages between four climatic indices (the North Atlantic Oscillation (NAO), the Arctic Oscillation (AO), the Pacific-North American pattern (PNA) and the El Nino Southern Oscillation represented by the Multivariate ENSO Index (MEI)), groundwater level time series, as well as precipitation and temperature time series are investigated. The length of the available historical records of groundwater levels at the three sites is 32 years (1974–2005). The three Canadian regions studied show drastically different patterns of variability evolution for the hydrogeological records. Groundwater level variability on PEI is mostly influenced by the NAO and the AO. The variability of groundwater levels in the MB region is partly explained by PNA. In the VI region, NAO, AO and MEI appear to impact the pattern of groundwater variability.

Northern Hemisphere Modes of Variability and the Timing of Spring in Western North America

Abstract Spatial and temporal patterns of variability in spring onset are identified across western North America using a spring index (SI) model based on weather station minimum and maximum temperatures (Tmin and Tmax, respectively). Principal component analysis shows that two significant and independent patterns explain roughly half of the total variance in the timing of spring onset from 1920 to 2005. However, these patterns of spring onset do not appear to be linear responses to the primary modes of variability in the Northern Hemisphere: the Pacific–North American pattern (PNA) and the northern annular mode (NAM). Instead, over the period when reanalysis data and the spring index model overlap (1950–2005), the patterns of spring onset are local responses to the state of both the PNA and NAM, which together modulate the onset date of spring by 10–20 days on interannual time scales. They do so by controlling the number and intensity of warm days. There is also a regionwide trend in spring advancement of about −1.5 days decade−1 from 1950 to 2005. Trends in the NAM and PNA can only explain about one-third (−0.5 day decade−1) of this trend.

Circulation system complex networks and teleconnections

In terms of the characteristic topology parameters of climate complex networks, the spatial connection structural complexity of the circulation system and the influence of four teleconnection patterns are quantitatively described. Results of node degrees for the Northern Hemisphere (NH) mid-high latitude (30° N-90° N) circulation system (NHS) networks with and without the Arctic Oscillations (AO), the North Atlantic Oscillations (NAO) and the Pacific—North American pattern (PNA) demonstrate that the teleconnections greatly shorten the mean shortest path length of the networks, thus being advantageous to the rapid transfer of local fluctuation information over the network and to the stability of the NHS. The impact of the AO on the NHS connection structure is most important and the impact of the NAO is the next important. The PNA is a relatively independent teleconnection, and its role in the NHS is mainly manifested in the connection between the NHS and the tropical circulation system (TRS). As to the Southern Hemisphere mid-high latitude (30° S-90° S) circulation system (SHS), the impact of the Antarctic Arctic Oscillations (AAO) on the structural stability of the system is most important. In addition, there might be a stable correlation dipole (AACD) in the SHS, which also has important influence on the structure of the SHS networks.

Eastward shift of the Pacific/North American pattern on an interdecadal time scale and an associated synoptic eddy feedback

AbstractIn this study, we investigate the interdecadal changes in the Pacific/North American (PNA) pattern during the winter season (November–February) that accompanies the global‐scale climate shift in the late 1970s. In the Empirical Orthogonal Function (EOF) analysis, the PNA pattern shows three distinctive features during the period of 1983–2002 in contrast with that of the previous decades (1959–1977): the extension of the central North Pacific cell into the Gulf of Alaska, the eastward shift of the action centre of the Canadian cell, and weakening of the North America cell. Pacific/North American structural changes are closely related to the southeastward shift of the storm track that causes the vigorous synoptic eddy feedback region to move farther east. Eddy activity feeds the cyclonic flow at the eastern part of conventional central North Pacific cell, following the pattern change of storm track. Copyright © 2011 Royal Meteorological Society

Projection of changes in the frequency of heavy rain events over Hawaii based on leading Pacific climate modes

[1] This study investigates the frequency of heavy rainfall events in Hawaii during the wet season (October-April) 1958–2005 and their conditional dependence on the Pacific-North American (PNA) pattern and El Nino-Southern Oscillation (ENSO). Heavy rain events are defined by the 95% quantile in the rainfall distribution of the wet seasons. Twelve stations with daily reports of rainfall amounts were used to count the number of heavy rain days during wet seasons. Multiple linear regression (MLR) indicated that the PNA index (PNAI) and the Southern Oscillation Index (SOI) can explain a significant amount of the interannual to interdecadal variability for 9 out of 12 stations. Cross validation showed that PNAI and SOI together explain about 18–44% of the variability in the number of heavy rain events. Furthermore, the MLR model reproduces the trend toward fewer heavy rain events in the years after the Pacific climate shift in the mid-1970s. The MLR model was applied to the projected PNAI and SOI indices that were obtained from six IPCC AR4 climate models. The current suite of AR4 simulations based on the A1B and A2 emissions scenarios projects small and equivocal changes in the mean state of the SOI and PNAI during the 21st century. The covariance between PNAI and SOI in these simulations appears to be stable. To the extent that variations in the frequency and magnitude of ENSO and the PNA mode are responsible for modulating extreme rainfall occurrence in Hawaii, our results indicate small changes in the projected number of heavy rainfall days with large uncertainties resulting from disparities among the climate models.

Relationships between large-scale circulation patterns and carbon dioxide exchange by a deciduous forest

[1] In this study, we focus on a deciduous forest in central Massachusetts and investigate the relationships between global climate indices and CO2 exchange using eddy-covariance flux measurements from 1992 to 2007. Results suggest that large-scale circulation patterns influence the annual CO2 exchange in the forest through their effects on the local surface climate. Annual gross ecosystem exchange (GEE) in the forest is closely associated with spring El Nino–Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO), previous fall Atlantic Multidecadal Oscillation (AMO), and previous winter East Pacific–North Pacific (EP-NP) pattern. Annual net ecosystem exchange (NEE) responds to previous fall AMO and PDO, while annual respiration (R) is impacted by previous fall ENSO and Pacific/North American Oscillation (PNA). Regressions based on these relationships are developed to simulate the annual GEE, NEE, and R. To avoid problems of multicollinearity, we compute a “Composite Index for GEE (CIGEE)” based on a linear combination of spring ENSO and PDO, fall AMO, and winter EP-NP and a “Composite Index for R (CIR)” based on a linear combination of fall ENSO and PNA. CIGEE, CIR, and fall AMO and PDO can explain 41, 27, and 40% of the variance of the annual GEE, R, and NEE, respectively. We further apply the methodology to two other northern midlatitude forests and find that interannual variabilities in NEE of the two forests are largely controlled by large-scale circulation patterns. This study suggests that global climate indices provide the potential for predicting CO2 exchange variability in the northern midlatitude forests.

The Twentieth Century Reanalysis Project

AbstractThe Twentieth Century Reanalysis (20CR) project is an international effort to produce a comprehensive global atmospheric circulation dataset spanning the twentieth century, assimilating only surface pressure reports and using observed monthly sea‐surface temperature and sea‐ice distributions as boundary conditions. It is chiefly motivated by a need to provide an observational dataset with quantified uncertainties for validations of climate model simulations of the twentieth century on all time‐scales, with emphasis on the statistics of daily weather. It uses an Ensemble Kalman Filter data assimilation method with background ‘first guess’ fields supplied by an ensemble of forecasts from a global numerical weather prediction model. This directly yields a global analysis every 6 hours as the most likely state of the atmosphere, and also an uncertainty estimate of that analysis.The 20CR dataset provides the first estimates of global tropospheric variability, and of the dataset's time‐varying quality, from 1871 to the present at 6‐hourly temporal and 2° spatial resolutions. Intercomparisons with independent radiosonde data indicate that the reanalyses are generally of high quality. The quality in the extratropical Northern Hemisphere throughout the century is similar to that of current three‐day operational NWP forecasts. Intercomparisons over the second half‐century of these surface‐based reanalyses with other reanalyses that also make use of upper‐air and satellite data are equally encouraging.It is anticipated that the 20CR dataset will be a valuable resource to the climate research community for both model validations and diagnostic studies. Some surprising results are already evident. For instance, the long‐term trends of indices representing the North Atlantic Oscillation, the tropical Pacific Walker Circulation, and the Pacific–North American pattern are weak or non‐existent over the full period of record. The long‐term trends of zonally averaged precipitation minus evaporation also differ in character from those in climate model simulations of the twentieth century. Copyright © 2011 Royal Meteorological Society and Crown Copyright.