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ABSTRACT An unprecedentedly large ensemble of climate simulations with a 60-km atmospheric general circulation model and dynamical downscaling with a 20-km regional climate model has been performed to obtain probabilistic [...]

This paper provides an update on research in the relatively new and fast-moving field of decadal climate prediction, and addresses the use of decadal climate predictions not only for potential [...]

A new field of study, 'decadal prediction,' is emerging in climate science. Decadal prediction lies between seasonal/interannual forecasting and longer-term climate change projections, and focuses on time-evolving regional climate conditions over the next 10-30 yr. Numerous assessments of climate information user needs have identified this time scale as being important to infrastructure planners, water resource managers, and many others. It is central to the information portfolio required to adapt effectively to and through climatic changes. At least three factors influence time-evolving regional climate at the decadal time scale: 1) climate change commitment (further warming as the coupled climate system comes into adjustment with increases of greenhouse gases that have already occurred), 2) external forcing, particularly from future increases of greenhouse gases and recovery of the ozone hole, and 3) internally generated variability. Some decadal prediction skill has been demonstrated to arise from the first two of these factors, and there is evidence that initialized coupled climate models can capture mechanisms of internally generated decadal climate variations, thus increasing predictive skill globally and particularly regionally. Several methods have been proposed for initializing global coupled climate models for decadal predictions, all of which involve global time-evolving three-dimensional ocean data, including temperature and salinity. An experimental framework to address decadal predictability/prediction is described in this paper and has been incorporated into the coot dinated Coupled Model Intercomparison Model, phase 5 (CMIP5) experiments, some of which will be assessed for the IPCC Fifth Assessment Report (AR5). These experiments will likely guide work in this emerging field over the next 5 yr. DOI: 10.1175/2009BAMS2778.1

A detailed analysis of the bifurcation structure of a two-level, quasigeostrophic model is made by a combined use of a continuation algorithm and extensive time integrations. The model is formulated on a sphere with realistic Northern Hemisphere topography and is driven by Newtonian relaxation to axisymmetric radiative-equilibrium temperature that represents the equator-to-pole contrast of the diabatic heating. The strength of this heating contrast is varied as a primary control parameter. Bifurcations of not only stationary but also periodic, doubly periodic (quasi periodic), and chaotic solutions are followed for the model with triangular 15 (T15) horizontal resolution, corresponding to the number of (real) spherical harmonic coefficients, 240. It is found that the model possesses multiple attractors for a fairly broad, but realistic, range in the parameter space. Characteristic oscillatory modes are associated with each one of these attractors. Two frequency bands, one in synoptic and the other in intraseasonal timescales, are dominant in such oscillations. These attractors keep their identity even when the system is perturbed by stochastic forcing of fairly large amplitude. Furthermore, when some parameters are changed to make the system more turbulent, it is observed that the system starts transiting among the rains of attractors that used to he stable, a behavior reminiscent of the transitions among different flow regimes observed in the real atmosphere. Such behavior, called chaotic itinerancy, has recently attracted much attention as a characteristic of chaos in physical systems with many degrees of freedom. Chaotic itinerancy in the present model appears to be more complicated than the examples reported so far in that it consists of attractors with no obvious spatial symmetry and that preferred routes of transitions are observed. Evidences of multiple attractors and chaotic itinerancy are found in higher resolutions up to T21. Discussions are made about implications of such model behavior to the understanding of observed low-frequency variability.

This paper presents an observational analysis of recurrent flow patterns in the Northern Hemisphere (NH) winter, based on a 37-year series of daily 700-mb height anomalies. Large-scale anomaly patterns that appear repeatedly and persist beyond synoptic time scales are identified by searching for local maxima of probability density in a phase subspace, which is spanned by the leading empirical orthogonal functions (EOFs). By using an angular probability density function (PDF), we focus on the shape, not magnitude, of the anomaly patterns. The PDF estimate is nonparametric; that is, our algorithm makes no a priori assumption on symmetry with respect to the climatological mean as in one-point correlation and rotated EOF analyses. The local density maxima are searched by iterative bump hunting. Based on observed partial decoupling between the Pacific (PAC) and the Atlantic-Eurasian (ATL) sectors, the classification algorithm is applied separately to each of the two. Seven PAC and six ATL patterns are obtained. Anomaly maps that belong to the neighborhood of each PDF peak are associated with distinct flow regimes. These include regional blocked and zonal flows, and wave train-like anomaly patterns, some of them well known from previous studies, others revealed by our analysis for the first time. Successive appearances of flow regimes are generally separated by unclassifiable, transient periods. A Markov chain describes transitions between different flow regimes; highly likely, as well as unlikely routes of transition exist. Chains of preferred transitions may be related to the existence of oscillatory modes in the NH extratropics. A synoptic characterization of onsets and breaks for the flow regimes obtained is given by compositing. In situ evolutions of anomaly patterns, slow westward shifts of high-latitude anomaly centers, and successive downstream increase of anomaly magnitudes are the typical signatures of such events.

Recurrent and persistent flow patterns are identified by examining multivariate probability density functions (PDFs) in the phase space of large-scale atmospheric motions. This idea is pursued systematically here in the hope of clarifying the extent to which intraseasonal variability can be described and understood in terms of multiple flow regimes. Bivariate PDFs of the Northern Hemisphere (NH) wintertime anomaly heights at 700 mb are examined in the present paper, using a 37-year dataset. The two-dimensional phase plane is defined by the two leading empirical orthogonal functions (EOFs) of the anomaly fields. PDFs on this plane exhibit synoptically intriguing and statistically significant inhomogeneities on the periphery of the distribution. It is shown that these inhomogeneities are due to the existence of persistent and recurrent anomaly patterns, well-known as dominant teleconnection patterns; that is, the Pacific/North American (PNA) pattern, its reverse, and zonal and blocked phases of the North Atlantic Oscillation (NAO). It is argued that the inhomogeneities are obscured when PDFs are examined in a smaller-dimensional subspace than dynamically desired.

A two-layer shallow-water model with R15 truncation and topographic forcing is used to study intraseasonal variability in the Northern Hemisphere's (NH's) extratropical atmosphere. The model's variability is dominated by oscillations with average periods near 65-70 and 40-50 days. These periods are also found in 13.5 years of daily upper-air data from January 1980 to July 1993. The spatial variability associated with these oscillations is examined by compositing the streamfunction-anomaly fields of the model and the observations. The model's 70-day oscillation is strongest in the Euro-Atlantic sector, where it bears a close resemblance to observed streamfunction composites of the North Atlantic oscillation. The observed 70-day mode exhibits similar features in the Euro-Atlantic sector, accompanied by a north-south 'seesaw' over the Pacific and Eurasia. Previous authors, in their analyses of geopotential height observations, also found these features to be present in an empirical orthogonal function that contains aspects of both the North Pacific and North Atlantic oscillations. The 40-day oscillation is characterized, in both the model simulations and observed data, by a zonal wavenumber-2 pattern anchored over the NH topography. This pattern undergoes a tilted-trough vacillation in both the model and observations. This midlatitude vacillation is strongest in the Pacific-North American sector, where it resembles a 40-day oscillation in the University of California, Los Angeles, general circulation model that is largely driven by mountain torques over the Rockies. Comparisons with observational data show a clear separation between a tropical 50-day oscillation, not present in the authors' model results, and a 40-day NH extratropical oscillation, which resembles the topographically induced oscillation that arises in their two-layer model.

The dynamical basis of extratropical low-frequency variability (LFV) is investigated using a quasigeostrophic model on a sphere with realistic Northern Hemisphere topography. The model is driven by Newtonian relaxation to an axisymmetric radiative equilibrium temperature. Two versions of the model are used: one with two vertical levels and horizontal T15 resolution and the other with five levels and T21 resolution. In previous investigations, by the authors, the former model has been found to possess multiple attractors in a stable range of the model parameters and to wander irregularly among attractor ruins for unstable parameter sets. A similar behavior is found for the higher-resolution model as well. Three aspects of LFV are considered in this paper. The first two are intermittent appearances of quasi-stationary weather regimes and low-frequency oscillations. The third is the dominance of a few principal patterns of variability that show red noise-like temporal behavior. In the real atmosphere, the first two can be found by careful examinations in the general predominance of the last aspect. The model is able to simulate these aspects with a certain level of realism. It is found that the first two are associated with the existence of multiple attractors and oscillations intrinsic to them. As for the two-level model, attractors that used to be confined to small regions in phase space correspond to quasi-stationary weather regimes and those located in regions where the phase space structure is flat support the oscillations with sizable amplitudes at a more turbulent stage. The five-level model shows a more complicated behavior, but the relevance of multiple attractors and associated oscillations has been confirmed as well. It is found in the realistic range of parameters that singular modes of linearized equations with time-mean basic states form the basis of principal spatial patterns for which low-frequency temporal variations dominate. The singular vector with the smallest singular value roughly coincides with the first mode of empirical orthogonal function (EOF) for the lower-resolution model. The smallness of the singular value guarantees the small rate of time change so that the system spends a long time along the linear axis in phase space. The singular vector is found to be relatively insensitive to the changes in the basic state for the linearization. The relevance of the singular vector is more controversial in the higher-resolution model. The similarity with the leading EOFs is lost considerably. However, it is found that leading singular vectors still play a role in determining a low-dimensional linear subspace with which most of the low-frequency variance is associated. The interaction between singular modes and forcing due to transients is suggested to be responsible for the deviation of the principal patterns from singular modes.

The article discusses the extent to which anthropogenic sea surface temperature (SST) and atmospheric circulation changes influenced heavy rainfall in southwestern Japan or Pacific-Japan (PJ) in July 2012. Characteristics of the stronger northwestern Pacific subtropical high (PASH) during the 2012 rainy season include a dominant teleconnection pattern over PJ. It is suggested that Japan's heavy rainfall in 2012 was not greatly influenced by anthropogenic climate change.