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E I A t ase presentation 79-year-old woman presented with a regular tachycardia t 160 bpm with narrow QRS complexes alternating with ight bundle branch block (RBBB) in 2:1 fashion through ost of the ECG tracing. After reversion to sinus rhythm, 2-lead ECGs showed intermittent 1:1 RBBB occurring at andom. Figure 1 was recorded during a period of transient :1 RBBB that lasted a few minutes. The RBBB (QRS uration 0.12 second) was not rate dependent (bradycaria or tachycardia). When RBBB subsided, the ECG howed left anterior hemiblock, a tiny q wave in lead V2 robably related to left anterior hemiblock. Myocardial inarction was ruled out. Echocardiography and coronary aniography were normal. Electrophysiologic study revealed typical dual AV nodal athway physiology. Rapid atrial pacing easily induced AV odal reentrant tachycardia that was easily terminated by entricular burst pacing. More than 20 induced episodes of achycardia, mostly at the same rate, all displayed 2:1 BBB (QRS 0.12 second; Figure 2). The cycle length of ll episodes of tachycardia was constant for any given pisode. The HV interval during the tachycardia was contant at 55 ms (Figure 3). The tachycardia was no longer nducible after slow pathway ablation. What was the mechnism of the 2:1 RBBB during tachycardia?

The crystal structure of the type II restriction endonuclease BglI bound to DNA containing its specific recognition sequence has been determined at 2.2 A resolution. This is the first structure of a restriction endonuclease that recognizes and cleaves an interrupted DNA sequence, producing 3' overhanging ends. BglI is a homodimer that binds its specific DNA sequence with the minor groove facing the protein. Parts of the enzyme reach into both the major and minor grooves to contact the edges of the bases within the recognition half-sites. The arrangement of active site residues is strikingly similar to other restriction endonucleases, but the co-ordination of two calcium ions at the active site gives new insight into the catalytic mechanism. Surprisingly, the core of a BglI subunit displays a striking similarity to subunits of EcoRV and PvuII, but the dimer structure is dramatically different. The BglI-DNA complex demonstrates, for the first time, that a conserved subunit fold can dimerize in more than one way, resulting in different DNA cleavage patterns.

The impact of the climatological seasonally varying 300-mb flow on the North Pacific/North American response to remote anomalous forcing is considered in the context of a linear barotropic model. WKB theory suggests that the total wavenumber of stationary Rossby waves over the Pacific increases from about 7 in January to 8.5 by June, with the reverse occurring during fall. This change is accompanied by monthly changes in the location and shape of the Rossby waveguide itself. Using a diagnostic tool called the influence function, it is shown that the most sensitive area of forcing for producing a large response over the United States shifts from the east Pacific in late winter to the west Pacific by late spring. As spring progresses, there is also a marked increase in the sensitivity to smaller-scale forcing in both of these regions, particularly the west Pacific. The amplitude of the forced response can potentially be larger in June than any other month of the year. These results suggest that the evolution of extreme springtime weather events over North America may depend critically upon the precise timing and geographical structure of forcing anomalies over both the east and the west Pacific. In this model, low-frequency variability within and downstream of the Rossby waveguide is sensitive to the annual cycle of the ambient flow. This suggests that the impact of the annual cycle must be taken into account in any complete theory of low-frequency variability. The impact is large enough to raise the possibility of significant interactions across timescales. In other words, it is possible for a steady forcing to produce an unsteady response and, equally, for an unsteady forcing to produce a seasonal-mean response. In such situations, particularly during the northern spring and fall seasons, investigating low-frequency anomalies as departures from three-month seasonal climatologies may lead to confusion and may not be useful.

This study is concerned with assessing the extent to which extratropical low-frequency variability may be viewed as a response to geographically coherent stochastic forcing. This issue is examined with a barotropic model linearized about the long-term mean wintertime 300-mb flow with zonal and meridional structure. The perturbation eigenfunctions of the model are stable (i.e., decaying) for a realistic 5-day drag, so transient eddy activity can be maintained against the drag only with forcing. In a statistical steady state, a fluctuation–dissipation relation (FDR) links the covariance structure of the eddy vorticity to the covariance structure of the forcing. This relation is used in a forward sense to determine the covariance of eddy vorticity for a specified covariance of forcing. It is also used in a backward sense to infer the covariance of forcing required to maintain the observed covariance of eddy vorticity. The focus is on explaining the observed variability of 10-day running mean anomali...

A 59-year-old cachectic male was referred to the surgical outpatient department with intermittent haematochezia and a longstanding change in bowel habit with associated weight loss and anaemia. Following investigation, he was diagnosed with a large rectal tumour with multiple metastases. 7 days later, the patient presented again with fevers, bilious vomiting, abdominal pain and distension. On examination, he had a generally tender abdomen,= although no peritonism, but an enlarged, extremely tender hemiscrotum with no cough reflex. Imaging revealed a perforated rectum and subsequent abscess formation, which tracked via an unusual anatomical route to present as scrotal swelling.

The crystal structure of restriction endonuclease Bam HI complexed to DNA has been determined at 2.2 angstrom resolution. The DNA binds in the cleft and retains a B-DNA type of conformation. The enzyme, however, undergoes a series of conformational changes, including rotation of subunits and folding of disordered regions. The most striking conformational change is the unraveling of carboxyl-terminal alpha helices to form partially disordered "arms." The arm from one subunit fits into the minor groove while the arm from the symmetry related subunit follows the DNA sugar-phosphate backbone. Recognition of DNA base pairs occurs primarily in the major groove, with a few interactions occurring in the minor groove. Tightly bound water molecules play an equally important role as side chain and main chain atoms in the recognition of base pairs. The complex also provides new insights into the mechanism by which the enzyme catalyzes the hydrolysis of DNA phosphodiester groups.

A 2.91-billion base pair (bp) consensus sequence of the euchromatic portion of the human genome was generated by the whole-genome shotgun sequencing method. The 14.8-billion bp DNA sequence was generated over 9 months from 27,271,853 high-quality sequence reads (5.11-fold coverage of the genome) from both ends of plasmid clones made from the DNA of five individuals. Two assembly strategies—a whole-genome assembly and a regional chromosome assembly—were used, each combining sequence data from Celera and the publicly funded genome effort. The public data were shredded into 550-bp segments to create a 2.9-fold coverage of those genome regions that had been sequenced, without including biases inherent in the cloning and assembly procedure used by the publicly funded group. This brought the effective coverage in the assemblies to eightfold, reducing the number and size of gaps in the final assembly over what would be obtained with 5.11-fold coverage. The two assembly strategies yielded very similar results that largely agree with independent mapping data. The assemblies effectively cover the euchromatic regions of the human chromosomes. More than 90% of the genome is in scaffold assemblies of 100,000 bp or more, and 25% of the genome is in scaffolds of 10 million bp or larger. Analysis of the genome sequence revealed 26,588 protein-encoding transcripts for which there was strong corroborating evidence and an additional ∼12,000 computationally derived genes with mouse matches or other weak supporting evidence. Although gene-dense clusters are obvious, almost half the genes are dispersed in low G+C sequence separated by large tracts of apparently noncoding sequence. Only 1.1% of the genome is spanned by exons, whereas 24% is in introns, with 75% of the genome being intergenic DNA. Duplications of segmental blocks, ranging in size up to chromosomal lengths, are abundant throughout the genome and reveal a complex evolutionary history. Comparative genomic analysis indicates vertebrate expansions of genes associated with neuronal function, with tissue-specific developmental regulation, and with the hemostasis and immune systems. DNA sequence comparisons between the consensus sequence and publicly funded genome data provided locations of 2.1 million single-nucleotide polymorphisms (SNPs). A random pair of human haploid genomes differed at a rate of 1 bp per 1250 on average, but there was marked heterogeneity in the level of polymorphism across the genome. Less than 1% of all SNPs resulted in variation in proteins, but the task of determining which SNPs have functional consequences remains an open challenge.

Summary: Accurately mapping RNA-Seq reads to the reference genome is a critical step for performing downstream analysis such as transcript assembly, isoform detection and quantification. Many tools have been developed; however, given the huge size of the next generation sequencing datasets and the complexity of the transcriptome, RNA-Seq read mapping remains a challenge with the ever-increasing amount of data. We develop Omicsoft sequence aligner (OSA), a fast and accurate alignment tool for RNA-Seq data. Benchmarked with existing methods, OSA improves mapping speed 4–10-fold with better sensitivity and less false positives. Availability: OSA can be downloaded from http://omicsoft.com/osa. It is free to academic users. OSA has been tested extensively on Linux, Mac OS X and Windows platforms. Contact: john.hu@omicsoft.com; jhu7@ncsu.edu; jack.liu@omicsoft.com Supplementary information: Supplementary data are available at Bioinformatics online.

TYPE II restriction endonucleases are characterized by the remarkable specificity with which they cleave specific DNA sequences. Surprisingly, their protein sequences are in most cases unrelated, and no recurring structural motif has yet been identified1,2. We have determined the structure of restriction endonuclease BamHI at 1.95 A resolution. BamHI shows striking resemblance to the structure of endonuclease EcoRI (refs 3, 4), despite the lack of sequence similarity between them. We also observe some curious differences between the two structures, and propose an evolutionary scheme that may explain them. The active site of BamHI is structurally similar to the active sites of EcoRI and EcoRV (ref. 5), but the mechanism by which BamHI activates a water molecule for nucleophilic attack may be different.

[1] Using a multivariate, “patterns-based”, red noise approach to 42 years of observed tropical SST, thermocline depth, and zonal wind stress seasonal anomalies, it is shown that natural random variations can account for the observed variability of Central Pacific (CP) and Eastern Pacific (EP) ENSO events. The recent multidecadal increase in the number of CP events relative to EP events, which has been hypothesized to be connected to anthropogenic change in the state of the ocean, is also found to be consistent with multivariate red noise and hence with stationary statistics. ENSO “flavors” are the consequence of differing combinations of two initially orthogonal spatial patterns that are precursors to CP or EP events of both signs. These precursors can be excited by random weather forcing and subsequently result in SST anomaly amplification primarily through surface or thermocline feedbacks, respectively.

Motivation: Next generation sequencing technology generates high-throughput data, which allows us to detect fusion genes at both transcript and genomic levels. To detect fusion genes, the current bioinformatics tools heavily rely on paired-end approaches and overlook the importance of reads that span fusion junctions. Thus there is a need to develop an efficient aligner to detect fusion events by accurate mapping of these junction-spanning single reads, particularly when the read gets longer with the improvement in sequencing technology. Results: We present a novel method, FusionMap, which aligns fusion reads directly to the genome without prior knowledge of potential fusion regions. FusionMap can detect fusion events in both single- and paired-end datasets from either RNA-Seq or gDNA-Seq studies and characterize fusion junctions at base-pair resolution. We showed that FusionMap achieved high sensitivity and specificity in fusion detection on two simulated RNA-Seq datasets, which contained 75 nt paired-end reads. FusionMap achieved substantially higher sensitivity and specificity than the paired-end approach when the inner distance between read pairs was small. Using FusionMap to characterize fusion genes in K562 chronic myeloid leukemia cell line, we further demonstrated its accuracy in fusion detection in both single-end RNA-Seq and gDNA-Seq datasets. These combined results show that FusionMap provides an accurate and systematic solution to detecting fusion events through junction-spanning reads. Availability: FusionMap includes reference indexing, read filtering, fusion alignment and reporting in one package. The software is free for noncommercial use at (http://www.omicsoft.com/fusionmap). Contact: ge@amgen.com Supplementary information: Supplementary data are available at Bioinformatics online.

[1] This study quantifies the impact of ENSO on the decadal predictability of tropical Indo‐Pacific Ocean trends in a very large ensemble of NCAR CCSM3 anthropogenically‐forced (A1B scenario) simulations, by decomposing upper ocean temperatures into “ENSO” and “non‐ENSO” variability. On decadal time scales, the ENSO pattern primarily contributes totheensemblespreadandhasatrendwhoseamplitudeisnot predictable.However,the non‐ENSOcomponentofthetrend has much smaller spread and is predictable after 10 years, much sooner than the total trend, which is predictable after 25 years. The non‐ENSO component of the trend explains 96% of the total trend and has a structure that is distinct from ENSO, including cooling in the South Pacific due to increased southeast trades, warming of the warm pool, and strengthening of the equatorial Pacific near‐surface temperature gradient superimposedupon a uniform warming. Citation: Solomon, A., and M. Newman (2011), Decadal predictability of tropical Indo‐Pacific Ocean temperature trends due to anthropogenic forcing in a coupled climate model, Geophys. Res. Lett., 38, L02703, doi:10.1029/2010GL045978.

Computer models and some theories suggest that global warming causes tropical Pacific trade winds to weaken, even as western tropical Pacific rainfall increases. A new observational study indicates otherwise.

The XPF/Mus81 structure-specific endonucleases cleave double-stranded DNA (dsDNA) within asymmetric branched DNA substrates and play an essential role in nucleotide excision repair, recombination and genome integrity. We report the structure of an archaeal XPF homodimer alone and bound to dsDNA. Superposition of these structures reveals a large domain movement upon binding DNA, indicating how the (HhH)(2) domain and the nuclease domain are coupled to allow the recognition of double-stranded/single-stranded DNA junctions. We identify two nonequivalent DNA-binding sites and propose a model in which XPF distorts the 3' flap substrate in order to engage both binding sites and promote strand cleavage. The model rationalises published biochemical data and implies a novel role for the ERCC1 subunit of eukaryotic XPF complexes.

The xeroderma pigmentosa group F protein (XPF) is a founding member of a family of 3′-flap endonucleases that play an essential role in nucleotide-excision repair, DNA replication and recombination. The XPF gene has been cloned from Aeropyrum pernix, encoding a 254-residue protein (apXPF). Recombinant protein was produced in Escherichia coli and purified by three chromatographic steps. Three different crystal forms of apXPF were grown in trigonal, monoclinic and triclinic systems. The trigonal crystals diffracted to 2.8 A and were grown in the presence of double-stranded DNA. Monoclinic crystals were grown without DNA and diffracted to 3.2 A. Triclinic crystals were grown from a truncated apXPF protein lacking the tandem helix–hairpin–helix motifs and diffracted to 2.1 A.

It is well known that rapid fluctuations of the zonal mean wind speed can contribute significantly to the variability of planetary-scale atmospheric waves (Rossby waves). What seems to be less appreciated is that these same fluctuations can also contribute to the climatological mean wave pattern. In this study, the extent to which stochastic zonal wind fluctuations affect the mean and variance of the wave response to steady forcing is investigated in the context of a simple numerical model of the global atmosphere, i.e. a barotropic vorticity equation model. It is first shown that rapid variations of the zonal wind can justifiably be treated as stochastic. Then, the appropriate modifications to the barotropic vorticity equation are derived using the classical theory of stochastic differential equations. Finally, the effect of the stochastic fluctuations on the mean Rossby-wave response is illustrated in two qualitatively different cases. The first experiment considers stochastic fluctuations in the zonally-symmetric component of the wind, and it is shown that those fluctuations amount to a net damping of the Rossby wave. In the second experiment, random fluctuations in the frictional damping are considered, and are shown to have a net destabilizing influence.

LONG-TERM GOAL. Our long-term goal is to improve the accuracy of numerical prediction models of weather and climate. We shall concentrate on sources of prediction error involving interactions between physical phenomena having different timescales. OBJECTIVES. We examine how inadequate representation of rapidly-varying (i.e., stochastic) atmospheric effects in General Circulation Models (GCMs) can systematically affect prediction on a variety of scales, and how errors arising from this inadequacy may be ameliorated. Concurrently, we develop useful, efficient and accurate methods of accounting for these systematic effects of stochastic forcing, and also provide methods for estimating the spread of ensemble predictions taking into account multiplicative stochastic effects. APPROACH. We approach the problem both analytically and numerically. Simple barotropic models present cases that are nontrivial, yet simple enough to solve analytically, and we use these models to identify and solve some of the numerical problems that will arise in realistic, more complex models. We intend to apply our results to studying the impact of stochastic forcing in more complete models, culminating with long runs of stochastically forced, full-fledged GCMs and numerical weather prediction (NWP) models. Although routinely ignored in current models, the theory of stochastic white noise is well known and analytic solutions are available, at least in principle. We have evaluated the effect of white stochastic forcing on the mean response of a global barotropic model to steady forcing. In addition, we have considered the more realistic problem of stochastic colored noise forcing, deriving analytic estimations of the systematic change in the mean response, and have compared these analytic approximations with results from stochastically forced numerical models.

X-ray structure analyses of proteins and computational approaches to the comparison of three-dimensional structures provide a basis for understanding the nature of restraints on the diversity of sequences in families of homologous proteins. Detailed examples are provided by structures defined by X-ray analysis at Birkbeck for two families of homologous proteins, the beta/gamma crystallins (five proteins) and aspartic proteinases (five enzymes). In addition all families of proteins, for which two or more well-refined high-resolution structures are available in the Brookhaven Databank, have been compared. Residue to residue substitution tables have been calculated for amino acids classified according to residue type, secondary structure, accessibility of the sidechain, and existence of hydrogen bonds from sidechain to other sidechains or peptide carbonyl or amide functions. Distinct patterns of substitution characterize most classes especially where amino acid residues are both solvent inaccessible and hydrogen-bonded through their sidechains.

The present investigation is concerned with the derivation of zonal winds for the middle atmosphere (40-80 km) of Venus, using Pioneer Venus (PV) radio occultation data. The considered altitude range is of particular interest because the atmospheric circulation of Venus is uncertain above the cloud tops (65-70 km). The Pioneer Venus orbiter began circling the planet every 24 hours on December 4, 1978. Four seasons of occultation data have now been collected. It is found that the assumption of cyclostrophic balance, in conjunction with radio occultation derived temperature data, yields a zonal wind field for most of the middle atmosphere of Venus. An intense midlatitude jet at and above the cloud tops is the most notable feature of the winds.

Temperatures and pressures inferred from radio occultation data acquired by the Pioneer Venus orbiter between September 1982 and November 1983 are used to derive cyclostrophic zonal winds in the middle atmosphere of Venus (1350 to 2.1 mb, 10 deg to 70 deg latitude). The main feature of the wind field is a jet positioned just above the cloud tops at 70 km and approximately 48 deg latitude. The maximum speed of the jet is about 130 m/s. A comparison with results of similar analyses on Pioneer Venus radio occultation data obtained between December 1978 and October 1981 suggests an equatorward shift of the jet and a decrease in jet speed during this five-year time interval. It is proposed that the poleward transport of westward zonal momentum by the upper branch of the cloud level Hadley cell supplies the excess momentum of the jet and maintains it against dissipation. The location of the jet thereby provides a minimum estimate of the latitudinal extent of the Hadley cell. Cyclostrophic zonal wind velocities decrease with height above about 70-75 km. It is suggested that this deceleration of the superrotation in equatorial latitudes is due to the dissipation of vertically propagating thermal tides forced primarily at altitudes around 65 km.

The cloud-level atmosphere of Venus takes little more than 4 days to complete one rotation, whereas the solid planet below has a 243-day period. Computer simulations of the circulation of the Venus middle atmosphere between 40 and 85 kilometers, as driven by solar radiation absorbed in the clouds, reproduce (i) the observed cloud-level rotation rate, (ii) strong vertical shears above and below the cloud tops, and (iii) midlatitude jets and strong poleward flow on the day side. Simulated circulations converge to yield nearly the same zonal winds when initialized with both stronger or weaker rotation rates. These results support the hypothesis that the observed cloud-top rotation rate is maintained by statistical balance between fluxes of momentum by thermal tides and momentum advection by mean meridional circulation.

During El Nino-Southern Oscillation (ENSO) events, the atmospheric response to sea surface temperature (SST) anomalies in the equatorial Pacific influences ocean conditions over the remainder of the globe. This connection between ocean basins via the ''atmospheric bridge'' is reviewed through an examination of previous work augmented by analyses of 50 years of data from the National Centers for Environmental Prediction- National Center for Atmospheric Research (NCEP-NCAR) reanalysis project and coupled atmospheric general circulation (AGCM)-mixed layer ocean model experiments. Observational and modeling studies have now established a clear link between SST anomalies in the equatorial Pacific with those in the North Pacific, north tropical Atlantic, and Indian Oceans in boreal winter and spring. ENSO-related SST anomalies also appear to be robust in the western North Pacific during summer and in the Indian Ocean during fall. While surface heat fluxes are the key component of the atmospheric bridge driving SST anomalies, Ekman transport also creates SST anomalies in the central North Pacific although the full extent of its impact requires further study. The atmospheric bridge not only influences SSTs on interannual timescales but also affects mixed layer depth (MLD), salinity, the seasonal evolution of upper-ocean temperatures, and North Pacific SST variability at lower fre- quencies. The model results indicate that a significant fraction of the dominant pattern of low-frequency (.10 yr) SST variability in the North Pacific is associated with tropical forcing. AGCM experiments suggest that the oceanic feedback on the extratropical response to ENSO is complex, but of modest amplitude. Atmosphere- ocean coupling outside of the tropical Pacific slightly modifies the atmospheric circulation anomalies in the Pacific-North America (PNA) region but these modifications appear to depend on the seasonal cycle and air- sea interactions both within and beyond the North Pacific Ocean.

The upper-tropospheric circulation is investigated for the three months of April, May, and June 1988 during which the Great Plains region of the United States experienced one of its most severe droughts in history. It is found that during this period the April–June (AMJ) seasonal-mean anomaly was not representative of the variability of 10-day low-pass anomalies. Rather, over North America large fluctuations on monthly and shorter timescales occurred, with the dominant streamfunction anomalies not strongly anticyclonic until June. In fact, the AMJ anomaly was dominated by two episodes of rapidly developing, intense anomalous anticyclones in early and late June. Examination of the daily 10-day low-pass streamfunction anomalies at 300 mb suggests that propagating Rossby waves originating in the west Pacific played a dominant role in the initiation of these intense anomalous anticyclones. Numerical experiments with a linear, time-dependent, barotropic model also support this hypothesis. These result...

Three very different views of the mean structure and variability of deep convection over the tropical east Indian and west Pacific Oceans, provided by three different reanalysis datasets for 1980–93, are highlighted. The datasets were generated at the National Centers for Environmental Prediction, the National Aeronautics and Space Administration's Goddard Laboratory for Atmospheres, and the European Centre for Medium-Range Weather Forecasts (ECMWF). Precipitation, outgoing longwave radiation (OLR), and 200-mb wind divergence fields from the three datasets are compared with one another and with satellite observations. Climatological means as well as interannual and intraseasonal (30–70 day) variability are discussed. For brevity the focus is restricted to northern winter (DJF). The internal consistency of the datasets is high, in the sense that the geographical extremes of rainfall, OLR, and divergence in each dataset correspond closely to one another. On the other hand, the external consistency, that is,...

The classic Frankignoul–Hasselmann hypothesis for sea surface temperature (SST) variability of an oceanic mixed layer assumes that the surface heat flux can be simply parameterized as noise induced by atmospheric variability plus a linear temperature relaxation rate. It is suggested here, however, that rapid fluctuations in this rate, as might be expected, for example, from gustiness of the sea surface winds, are large enough that they cannot be ignored. Such fluctuations cannot be fully modeled by noise that is independent of the state of the SST anomaly itself. Rather, they require the inclusion of a state-dependent (i.e., multiplicative) noise term, which can be expected to affect both persistence and the relative occurrence of high-amplitude anomalies. As a test of this hypothesis, daily observations at several Ocean Weather Stations (OWSs) are examined. Significant skewness and kurtosis of the distributions of SST anomalies is found, which is shown to be consistent with a multiplicative noise model. The observed wintertime SST distribution at OWS P is reproduced using a single-column variable-depth mixed layer model; the resulting non-Gaussianity is found to be largely due to the state dependence of rapidly varying (effectively stochastic) sensible and latent heat flux anomalies. The authors’ model for the non-Gaussianity of anomalous SST variability (counterintuitively) implies that the multiplicative noise increases the persistence, predictability, and variance of midlatitude SST anomalies. The effect is strongest on annual and longer time scales and may, therefore, be important to the understanding and modeling of interannual and interdecadal SST and related climate variability.

Atmospheric circulation statistics are not strictly Gaussian. Small bumps and other deviations from Gaussian probability distributions are often interpreted as implying the existence of distinct and persistent nonlinear circulation regimes associated with higher-than-average levels of predictability. In this paper it is shown that such deviations from Gaussianity can, however, also result from linear stochastically perturbed dynamics with multiplicative noise statistics. Such systems can be associated with much lower levels of predictability. Multiplicative noise is often identified with state-dependent variations of stochastic feedbacks from unresolved system components, and may be treated as stochastic perturbations of system parameters. It is shown that including such perturbations in the damping of large-scale linear Rossby waves can lead to deviations from Gaussianity very similar to those observed in the joint probability distribution of the first two principal components (PCs) of weekly averaged 750-hPa streamfunction data for the past 52 winters. A closer examination of the Fokker–Planck probability budget in the plane spanned by these two PCs shows that the observed deviations from Gaussianity can be modeled with multiplicative noise, and are unlikely the results of slow nonlinear interactions of the two PCs. It is concluded that the observed non-Gaussian probability distributions do not necessarily imply the existence of persistent nonlinear circulation regimes, and are consistent with those expected for a linear system perturbed by multiplicative noise.

A linear inverse model (LIM) suitable for studies of atmospheric extratropical variability on longer than weekly timescales is constructed using observations of the past 30 years. Notably, it includes tropical diabatic heating as an evolving model variable rather than as a forcing, and also includes, in effect, the feedback of the extratropical weather systems on the more slowly varying circulation. Both of these features are shown to be important contributors to the model's realism. Forecast skill is an important test of any model's usefulness as a diagnostic tool. The LIM is better at forecasting week 2 anomalies than a dynamical model based on the linearized baroclinic equations of motion (with many more than the LIM's 37 degrees of freedom) that is forced with observed (as opposed to the LIM's predicted) tropical heating throughout the forecast. Indeed, at week 2 the LIM's skill is competitive with that of the global nonlinear medium-range forecast (MRF) model with nominally O(106) degrees of...