bluuebunny/crossref_metadata_2025_split huggingface dataset:
Observed Influence of North Pacific SST Anomalies on the Atmospheric Circulation
Authors: Frankignoul Claude , Sennéchael Nathalie 
Published: February 2007

Abstract
A lagged maximum covariance analysis (MCA) of monthly anomaly data from the NCEP–NCAR reanalysis shows significant relations between the large-scale atmospheric circulation in two seasons and prior North Pacific sea surface temperature (SST) anomalies, independent from the teleconnections associated with the ENSO phenomenon. Regression analysis based on the SST anomaly centers of action confirms these findings. In late summer, a hemispheric atmospheric signal that is primarily equivalent barotropic, except over the western subtropical Pacific, is significantly correlated with an SST anomaly mode up to at least 5 months earlier. Although the relation is most significant in the upper troposphere, significant temperature anomalies are found in the lower troposphere over North America, the North Atlantic, Europe, and Asia. The SST anomaly is largest in the Kuroshio Extension region and along the subtropical frontal zone, resembling the main mode of North Pacific SST anomaly variability in late winter and spring, and it is itself driven by the atmosphere. The predictability of the atmospheric signal, as estimated from cross-validated correlation, is highest when SST leads by 4 months because the SST anomaly pattern is more dominant in the spring than in the summer. In late fall and early winter, a signal resembling the Pacific–North American (PNA) pattern is found to be correlated with a quadripolar SST anomaly during summer, up to 4 months earlier, with comparable statistical significance throughout the troposphere. The SST anomaly changes shape and propagates eastward, and by early winter it resembles the SST anomaly that is generated by the PNA pattern. It is argued that this results via heat flux forcing and meridional Ekman advection from an active coupling between the SST and the PNA pattern that takes place throughout the fall. Correspondingly, the predictability of the PNA-like signal is highest when SST leads by 2 months. In late summer, the maximum atmospheric perturbation at 250 mb reaches 35 m K−1 in the MCA and 20 m K−1 in the regressions. In early winter, the maximum atmospheric perturbation at 250 mb ranges between 70 m K−1 in the MCA and about 35 m K−1 in the regressions. This suggests that North Pacific SST anomalies have a substantial impact on the Northern Hemisphere climate. The back interaction of the atmospheric response onto the ocean is also discussed.
Steric Sea Level Change in the Northern Seas
Authors: Steele Michael , Ermold Wendy 
Published: February 2007

Abstract
Ocean temperature and salinity data over the period 1950–2000 in the Northern Seas, defined here as the North Atlantic Ocean (north of 50°N), North Pacific Ocean (north of 40°N), and Arctic Oceans, are combined to diagnose the steric (i.e., density) contribution to sea level variation. The individual contributions to steric height from temperature (thermosteric height) and salinity (halosteric height) are also analyzed. It is found that during 1950–2000, steric height rose over the study’s domain, mostly as a result of halosteric increases (i.e., freshening). Over a shorter time period (late 1960s to early 1990s) during which climate indices changed dramatically, steric height gradients near the Nordic Seas minimum were reduced by 18%–32%. It is speculated that this may be associated with a local slowing of both the Meridional Overturning Circulation and the southward flow through Fram Strait. However, steric height increases in the North Pacific Ocean during this time imply a possible acceleration of flow through the poorly measured Canadian Arctic. Evidence that the Great Salinity Anomaly of the late 1960s and 1970s had two distinct Arctic Ocean sources is also found: a late 1960s export of sea ice, and a delayed but more sustained 1970s export of liquid (ocean) freshwater. A simple calculation indicates that these Arctic Ocean freshwater sources were not sufficient to create the 1970s freshening observed in the North Atlantic Ocean.
Ice Bridges on the St. Lawrence River as an Index of Winter Severity from 1620 to 1910
Authors: Houle Daniel , Moore Jean-David , Provencher Jean 
Published: February 2007

Abstract
Temperature reconstruction of the last thousand years suggests that an unprecedented warming (+0.6°C) occurred over the globe in the last century. However, regional variations in climate are not resolved by Northern Hemisphere reconstructions. In northeastern North America, past climate and, particularly, past winter variations are poorly known. Here, the authors report on the variation of a winter temperature index during the 1620–1910 period, based on the ice bridge formation (IBF) rate on the Saint Lawrence River at Québec City (Canada), combined with instrumental data (1876–2000). During this 300-yr period, the IBF rate shows that winters in the seventeenth and eighteenth centuries were warmer than those in the nineteenth century. In particular, the IBF rate suggests that winter severity culminated in the 1850–1900 period, while very few ice bridges were reported between 1620 and 1740, presumably because of warmer temperatures and the relative scarcity of historical documents for the 1680–1740 period. These data suggest that winter temperature, particularly between ∼1800 and 1910, was 2.4° to 4.0°C colder than the last 30-yr average. Major volcanic eruptions had a significant positive impact on IBF rates, which is consistent with their role as important climate-forcing events.
Simulations of Arctic Temperature and Pressure by Global Coupled Models
Authors: Chapman William L. , Walsh John E. 
Published: February 2007

Abstract
Simulations of Arctic surface air temperature and sea level pressure by 14 global climate models used in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change are synthesized in an analysis of biases and trends. Simulated composite GCM surface air temperatures for 1981–2000 are generally 1°–2°C colder than corresponding observations with the exception of a cold bias maximum of 6°–8°C in the Barents Sea. The Barents Sea bias, most prominent in winter and spring, occurs in 12 of the 14 GCMs and corresponds to a region of oversimulated sea ice. All models project a twenty-first-century warming that is largest in the autumn and winter, although the rates of the projected warming vary considerably among the models. The across-model and across-scenario uncertainties in the projected temperatures are comparable through the first half of the twenty-first century, but increases in variability associated with the choice of scenario begin to outpace increases in across-model variability by about the year 2070. By the end of the twenty-first century, the cross-scenario variability is about 50% greater than the across-model variability. The biases of sea level pressure are smaller than in the previous generation of global climate models, although the models still show a positive bias of sea level pressure in the Eurasian sector of the Arctic Ocean, surrounded by an area of negative pressure biases. This bias is consistent with an inability of the North Atlantic storm track to penetrate the Eurasian portion of the Arctic Ocean. The changes of sea level pressure projected for the twenty-first century are negative over essentially the entire Arctic. The most significant decreases of pressure are projected for the Bering Strait region, primarily in autumn and winter.
The Isotopic Composition of Present-Day Antarctic Snow in a Lagrangian Atmospheric Simulation*
Authors: Helsen M. M. , Van de Wal R. S. W. , Van den Broeke M. R. 
Published: February 2007

Abstract
The isotopic composition of present-day Antarctic snow is simulated for the period September 1980–August 2002 using a Rayleigh-type isotope distillation model in combination with backward trajectory calculations with 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data as meteorological input. Observed spatial isotopic gradients are correctly reproduced, especially in West Antarctica and in the coastal areas. However, isotopic depletion of snow on the East Antarctic plateau is underestimated, a problem that is also observed in general circulation models equipped with isotope tracers. The spatial isotope–temperature relation varies strongly, which indicates that this widely used relation is not applicable to all sites and temporal scales. Spatial differences in the seasonal amplitude are identified, with maximum values in the Antarctic interior and hardly any seasonal isotope signature in Marie Byrd Land, West Antarctica. The modeled signature of deuterium excess remains largely preserved during the last phase of transport, though the simulated relation of deuterium excess with δ18O suggests that parameterizations of kinetic isotopic fractionation can be improved.
Trends in Southern Hemisphere Circulation in IPCC AR4 Models over 1950–99: Ozone Depletion versus Greenhouse Forcing
Authors: Cai Wenju , Cowan Tim 
Published: February 2007

Abstract
Simulations by the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) models on the Southern Hemisphere (SH) circulation are assessed over the period 1950–99, focusing on the seasonality of the trend and the level of its congruency with the southern annular mode (SAM) in terms of surface zonal wind stress. It is found that, as a group, the models realistically produce the seasonality of the trend, which is strongest in the SH summer season, December–February (DJF). The modeled DJF trend is principally congruent with the modeled SAM trend, as in observations. The majority of models produce a statistically significant positive trend, with decreasing westerlies in the midlatitudes and increasing westerlies in the high latitudes. The trend pattern from an all-experiment mean achieves highest correlation with that from the National Centers for Environmental Prediction (NCEP) data. A total of 48 out of the 71 experiments were run with ozone-depletion forcing, which offers an opportunity to assess the importance of ozone depletion in driving the late-twentieth-century trends. The AR4 model ensemble that contains an ozone-depletion forcing produces an averaged trend that is comparable to the trend from the NCEP outputs corrected by station-based observations. The trend is largely generated after the mid-1970s. Without ozone depletion the trend is less than half of that in the corrected NCEP, although the errors in the observed trend are large. The impact on oceanic circulation is inferred from wind stress curl in the group with ozone-depletion forcing. The result shows an intensification of the southern midlatitude supergyre circulation, including a strengthening East Australian Current flowing through the Tasman Sea. Thus, ozone depletion also plays an important role in the subtropical gyre circulation change over the past decades.
A Multifaceted Climatology of Atmospheric Blocking and Its Recent Linear Trend
Authors: Croci-Maspoli M. , Schwierz C. , Davies H. C. 
Published: February 2007

Abstract
A dynamically based climatology is derived for Northern Hemisphere atmospheric blocking events. Blocks are viewed as large amplitude, long-lasting, and negative potential vorticity (PV) anomalies located beneath the dynamical tropopause. The derived climatology [based on the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40)] provides a concise, coherent, and illuminating description of the main physical characteristics of blocks and the accompanying linear trends. The latitude–longitude distribution of blocking frequency captures the standard bimodal geographical distribution with major peaks over the North Atlantic and eastern North Pacific in all four seasons. The accompanying pattern for the age distribution, the genesis–lysis regions, and the track of blocks reveals that 1) younger blocks (1–4 days) are more prevalent at lower latitudes whereas significantly older blocks (up to 12 days) are located at higher latitudes; 2) genesis is confined predominantly to the two major ocean basins and in a zonal band between 40° and 50°N latitude, whereas lysis is more dispersed but with clear preference to higher latitudes; and 3) the general northeastward–west-northwest movement of blocks in the genesis–lysis phase also exhibits subtle seasonal and intra- and interbasin differences. Examination of the intensity and spatial-scale changes during the blocking life cycle suggests that in the mean a block’s evolution is independent of the genesis region and its eventual duration. A novel analysis of blocking trends reveals significant negative trends in winter over Greenland and in spring over the North Pacific. It is shown that the changes over Greenland are linked to the number of blocking episodes, whereas a neighboring trend signal to the south is linked to higher-frequency anticyclonic systems. Furthermore, evidence is adduced that changes in blocking frequency contribute seminally to tropopause height trends.
Conditional Exceedance Probabilities
Authors: Mason Simon J. , Galpin Jacqueline S. , Goddard Lisa , Graham Nicholas E. , Rajartnam Balakanapathy 
Published: February 2007

AbstractProbabilistic forecasts of variables measured on a categorical or ordinal scale, such as precipitation occurrence or temperatures exceeding a threshold, are typically verified by comparing the relative frequency with which the target event occurs given different levels of forecast confidence. The degree to which this conditional (on the forecast probability) relative frequency of an event corresponds with the actual forecast probabilities is known as reliability, or calibration. Forecast reliability for binary variables can be measured using the Murphy decomposition of the (half) Brier score, and can be presented graphically using reliability and attributes diagrams. For forecasts of variables on continuous scales, however, an alternative measure of reliability is required. The binned probability histogram and the reliability component of the continuous ranked probability score have been proposed as appropriate verification procedures in this context, but are subject to some limitations. A procedure is proposed that is applicable in the context of forecast ensembles and is an extension of the binned probability histogram. Individual ensemble members are treated as estimates of quantiles of the forecast distribution, and the conditional probability that the observed precipitation, for example, exceeds the amount forecast [the conditional exceedance probability (CEP)] is calculated. Generalized linear regression is used to estimate these conditional probabilities. A diagram showing the CEPs for ranked ensemble members is suggested as a useful method for indicating reliability when forecasts are on a continuous scale, and various statistical tests are suggested for quantifying the reliability.
The Characteristics of Key Analysis Errors. Part I: Dynamical Balance and Comparison with Observations
Authors: Caron Jean-François , Yau M. K. , Laroche Stéphane , Zwack Peter 
Published: February 2007

Abstract
The characteristics of the initial corrections obtained from the Canadian Meteorological Centre (CMC) energy-norm-based key analysis error algorithm that minimizes short-range (24 h) forecast errors were investigated for four specific CMC operational analyses. The results show that both the rotational and the divergent components of the initial corrections are strongly out of balance. Some dispersive modes are also present in the mass component of the initial corrections. The results from one experiment where the initial state errors were known suggest that the current algorithm always selects a set of unbalanced initial corrections with more mass correction than wind correction, regardless of the characteristics of the real initial condition errors. Comparison with observational data showed that the corrected analysis is systematically farther away from the observations than the control analysis even in large forecast error events where most of the forecast errors are believed to have originated from errors in the initial state.
Inferential, Nonparametric Statistics to Assess the Quality of Probabilistic Forecast Systems
Authors: Maia Alinede H. N. , Meinke Holger , Lennox Sarah , Stone Roger 
Published: February 2007

AbstractMany statistical forecast systems are available to interested users. To be useful for decision making, these systems must be based on evidence of underlying mechanisms. Once causal connections between the mechanism and its statistical manifestation have been firmly established, the forecasts must also provide some quantitative evidence of “quality.” However, the quality of statistical climate forecast systems (forecast quality) is an ill-defined and frequently misunderstood property. Often, providers and users of such forecast systems are unclear about what quality entails and how to measure it, leading to confusion and misinformation. A generic framework is presented that quantifies aspects of forecast quality using an inferential approach to calculate nominal significance levels (p values), which can be obtained either by directly applying nonparametric statistical tests such as Kruskal–Wallis (KW) or Kolmogorov–Smirnov (KS) or by using Monte Carlo methods (in the case of forecast skill scores). Once converted to p values, these forecast quality measures provide a means to objectively evaluate and compare temporal and spatial patterns of forecast quality across datasets and forecast systems. The analysis demonstrates the importance of providing p values rather than adopting some arbitrarily chosen significance levels such as 0.05 or 0.01, which is still common practice. This is illustrated by applying nonparametric tests (such as KW and KS) and skill scoring methods [linear error in the probability space (LEPS) and ranked probability skill score (RPSS)] to the five-phase Southern Oscillation index classification system using historical rainfall data from Australia, South Africa, and India. The selection of quality measures is solely based on their common use and does not constitute endorsement. It is found that nonparametric statistical tests can be adequate proxies for skill measures such as LEPS or RPSS. The framework can be implemented anywhere, regardless of dataset, forecast system, or quality measure. Eventually such inferential evidence should be complemented by descriptive statistical methods in order to fully assist in operational risk management.
Neural Network Retrieval of Ocean Surface Parameters from SSM/I Data
Authors: Meng Lei , He Yijun , Chen Jinnian , Wu Yumei 
Published: February 2007

Abstract
A new algorithm based on the multiparameter neural network is proposed to retrieve wind speed (WS), sea surface temperature (SST), sea surface air temperature, and relative humidity (RH) simultaneously over the global oceans from Special Sensor Microwave Imager (SSM/I) observations. The retrieved geophysical parameters are used to estimate the surface latent heat flux and sensible heat flux using a bulk method over the global oceans. The neural network is trained and validated with the matchups of SSM/I overpasses and National Data Buoy Center buoys under both clear and cloudy weather conditions. In addition, the data acquired by the 85.5-GHz channels of SSM/I are used as the input variables of the neural network to improve its performance. The root-mean-square (rms) errors between the estimated WS, SST, sea surface air temperature, and RH from SSM/I observations and the buoy measurements are 1.48 m s−1, 1.54°C, 1.47°C, and 7.85, respectively. The rms errors between the estimated latent and sensible heat fluxes from SSM/I observations and the Xisha Island (in the South China Sea) measurements are 3.21 and 30.54 W m−2, whereas those between the SSM/I estimates and the buoy data are 4.9 and 37.85 W m−2, respectively. Both of these errors (those for WS, SST, and sea surface air temperature, in particular) are smaller than those by previous retrieval algorithms of SSM/I observations over the global oceans. Unlike previous methods, the present algorithm is capable of producing near-real-time estimates of surface latent and sensible heat fluxes for the global oceans from SSM/I data.
The Structure of Tornadoes near Attica, Kansas, on 12 May 2004: High-Resolution, Mobile, Doppler Radar Observations
Authors: Bluestein Howard B. , Weiss Christopher C. , French Michael M. , Holthaus Eric M. , Tanamachi Robin L. , Frasier Stephen , Pazmany Andrew L. 
Published: February 2007

Abstract
The University of Massachusetts W- and X-band, mobile, Doppler radars scanned several tornadoes at close range in south-central Kansas on 12 May 2004. The detailed vertical structure of the Doppler wind and radar reflectivity fields of one of the tornadoes is described with the aid of boresighted video. The inside wall of a weak-echo hole inside the tornado was terminated at the bottom as a bowl-shaped boundary within several tens of meters of the ground. Doppler signatures of horizontal vortices were noted along one edge in the lowest 500 m of the tornado. The vertical structure of Doppler velocity displayed significant variations on the 100-m scale. Near the center of the tornado, a quasi-horizontal, radial bulge of the weak-echo hole at ∼500–600 m AGL dropped to about 400 m above the ground and was evident as a weak-echo band to the south of the tornado. It is suggested that this feature represents echo-weak material transported radially outward by a vertical circulation. Significant vertical variations of Doppler velocity were found in the surface friction layer both inside and outside the tornado core. The shape of a weak-echo notch that was associated with a hook echo wrapped around the tornado is described. Highest Doppler velocities were located outside the condensation funnel. The structure of the other tornadoes is also described, but with much less detail. Some of the analyses are compared with numerical simulations of tornado-like vortices done elsewhere.
The Fidelity of NCEP CFS Seasonal Hindcasts over Nordeste
Authors: Misra Vasubandhu , Zhang Yuning 
Published: February 2007

Abstract
The fidelity of the interannual variability of precipitation over Nordeste is examined using the suite of the NCEP Climate Forecast System (CFS) seasonal hindcasts. These are coupled ocean–land–atmosphere multiseasonal integrations. It is shown that the Nordeste rainfall variability in the season of February–April has relatively low skill in the CFS seasonal hindcasts. Although Nordeste is a comparatively small region in the northeast of Brazil, the analysis indicates that the model has a large-scale error in the tropical Atlantic Ocean. The CFS exhibits a widespread El Niño–Southern Oscillation (ENSO) forcing over the tropical Atlantic Ocean. As a consequence of this remote ENSO forcing, the CFS builds an erroneous meridional SST gradient in the tropical Atlantic that is known from observations to be a critical forcing for the rainfall variability over Nordeste.