Abstract

A new orthogonal decomposition based on the Schmidt decomposition approach has been applied to the barotropic equation linearized around the January 300-mb climatological flow. The Schmidt decomposition can be computed numerically performing a singular value decomposition of the numerical representation of the equation. The decomposition provides a set of positive real numbers whose minimum is linked to the singularity of the linear equation. A nonzero minimum singular value guarantees nonsingularity. Within the limits of the numerical precision and resolution used (R15 and R30) the nondivergent, global, barotropic equation linearized around the winter climatology is not singular, but it is very badly conditioned.

The Schmidt decomposition gives two sets of orthonormal basis functions, and a possible interpretation is offered by expressing the covariance matrix of forced responses in terms of Schmidt modes. An interpretation of the basis is obtained by showing that one set corresponds to the EOF of the responses forced by random sources and the second basis to the forcings that excite that particular EOF.

Abstract

An ensemble of atmospheric general circulation model (GCM) simulations with prescribed sea surface temperature (SST) generates a rich dataset. The main aim here is to advocate and demonstrate an approach to skill and reproducibility based on spatial anomaly patterns. Benefits and applications of this type of analysis include the efficient extraction of the model’s forced variability, guidance on systematic errors in the model’s response to SST forcing, clues to physical mechanisms, and a basis for model output statistics for seasonal forecasting. Some of the possible statistical techniques are illustrated, though the aim is not to provide an exhaustive comparison of the different spatial analysis techniques available. The examples are taken from an ensemble of three GCM integrations forced with observed SST through 1979–88. Boreal summer examples are given for the tropical Pacific and Europe, providing a contrast of a high and a low skill situation, respectively.

For model verification, a coupled pattern singular value decomposition analysis is performed between model and observed fields over the analysis domain. Over Europe, a model rainfall pattern is identified that specifies the contrasting rainfall anomalies associated with blocked and westerly summers observed through the period 1979–88, though statistical significance for the result cannot be proven using this small sample size. In the central and western tropical Pacific (CWTP), the leading model (rainfall) and observed (outgoing longwave radiation) modes have near-perfect temporal agreement, but the model’s spatial pattern lacks weight near Indonesia, which may be useful information for model developers.

Significant reproducibility of climate anomalies among ensemble members indicates potential seasonal forecast skill, because the similar atmospheric anomalies must derive from a common response to the anomalous SST forcing. To study reproducibility, the cross-covariance among all ensemble members is used to define a model base pattern (referred to as the forced pattern) that maximizes temporal covariance among ensemble members. The close relationship with the principal components of the ensemble mean anomaly is demonstrated. Monte Carlo tests show that the covariances among ensemble members associated with the CWTP and European forced patterns are highly statistically significant. It is suggested that this approach is an efficient way to identify statistically significant reproducibility.

Abstract

In the last years, ensemble methods have been widely popular in atmospheric, climate, and ocean dynamics investigations and forecasts as convenient methods to obtain statistical information on these systems. In many cases, ensembles have been used as an approximation to the probability distribution that has acquired more and more a central role, as the importance of a single trajectory, or member, was recognized as less informative. This paper shows that using results from the dynamical systems and more recent results from the machine learning and AI communities, we can arrive at a direct estimation of the probability distribution evolution and also at the formulation of predictor systems based on a nonlinear formulation. The paper introduces the theory and demonstrates its application to two examples. The first is a one-dimensional system based on the Niño-3 index; the second is a multidimensional case based on time series of monthly mean SST in the Pacific. We show that we can construct the probability distribution and set up a system to forecast its evolution and derive various quantities from it. The objective of the paper is not strict realism, but the introduction of these methods and the demonstration that they can be used also in the complex, multidimensional environment typical of atmosphere and ocean applications.

Abstract

An aquaplanet model is used to study the nature of the highly persistent low-frequency waves that have been observed in models forced by zonally symmetric boundary conditions.

Using the Hayashi spectral analysis of the extratropical waves, the authors find that a quasi-stationary wave 5 belongs to a wave packet obeying a well-defined dispersion relation with eastward group velocity. The components of the dispersion relation with k ≥ 5 baroclinically convert eddy available potential energy into eddy kinetic energy, whereas those with k < 5 are baroclinically neutral. In agreement with Green’s model of baroclinic instability, wave 5 is weakly unstable, and the inverse energy cascade, which had been previously proposed as a main forcing for this type of wave, only acts as a positive feedback on its predominantly baroclinic energetics. The quasi-stationary wave is reinforced by a phase lock to an analogous pattern in the tropical convection, which provides further amplification to the wave. It is also found that the Pedlosky bounds on the phase speed of unstable waves provide guidance in explaining the latitudinal structure of the energy conversion, which is shown to be more enhanced where the zonal westerly surface wind is weaker. The wave’s energy is then trapped in the waveguide created by the upper tropospheric jet stream. In agreement with Green’s theory, as the equator-to-pole SST difference is reduced, the stationary marginally stable component shifts toward higher wavenumbers, while wave 5 becomes neutral and westward propagating.

Some properties of the aquaplanet quasi-stationary waves are found to be in interesting agreement with a low frequency wave observed by Salby during December–February in the Southern Hemisphere so that this perspective on low frequency variability, apart from its value in terms of basic geophysical fluid dynamics, might be of specific interest for studying the earth’s atmosphere.

Abstract

The statistics of tropical intraseasonal variability are studied using European Centre for Medium-Range Weather Forecasts analyses and several ECHAM General Circulation Model experiments made with different model versions (ECHAM2 and ECHAM3, which have different convection schemes) and different horizontal resolutions (T21, T42, and T106). The study applies the principal oscillation pattern technique to the 200-mb equatorial velocity potential. Associated patterns of tropical outgoing longwave radiation, equatorial zonal wind, and equatorial divergence are also presented.

The intercomparison of ECHAM2 and ECHAM3 simulations at low (T21) resolution shows that the improved model physics has a beneficial impact on the simulated Madden–Julian oscillation (MJO). The MJO produced by the ECHAM2 model has an unrealistic spatial distribution of convection, whereas the MJO simulated by the ECHAM3 model appears to be related to convective activity over the Indian Ocean and the West Pacific, which is consistent with the observed MJO.

An increase of the horizontal resolution of the ECHAM3 model seems to actually degrade the results. At T42 and T106, the ECHAM3 MJO exhibits too much convective activity over central and equatorial America, with only a marginal effect of the MJO on the West Pacific–Indonesian region.

Abstract

Singular value decomposition analysis (SVDA) is used to analyze an ensemble of three 34-yr general circulation model (GCM) simulations forced with observed sea surface temperature. It is demonstrated how statistical postprocessing based on the leading SVDA modes of simulated and observed fields, primarily precipitation, can be applied to improve the skill of the simulation. For a given limited prediction region, the GCM has the potential to nonlinearly transform the SST information from around the globe and produce a dynamic solution over the region that can be statistically corrected to account for such features as systematic shifts in the location of anomaly dipoles. This paper does not address the separate question of whether the current generation of GCMs contain information above that which could be extracted using linear statistical relationships with SST.

For precipitation, examples are drawn from skillful tropical regions, as well as the moderate-to-low skill Pacific–North American and North Atlantic–European regions. Skill averaged across the analysis domain, as measured by the mean anomaly correlation, is notably improved by the statistical postprocessing in almost all situations where there is at least some real skill in the raw model fields. Postprocessing based on leading canonical correlation analysis (CCA) modes has been compared to postprocessing based on leading SVDA modes. The two methods show small differences, but neither one of the methods can be claimed to do better than the other. A third method, which is based on the leading empirical orthogonal functions of the simulations, has been tested on examples of tropical rainfall where it is shown to also be successful, but with skill generally a little below that based on SVDA or CCA modes.

The statistical postprocessing appears to have the greatest potential to improve skill for a variable like precipitation, which can have particularly strong anomaly gradients. Application of the postprocessing to large-scale atmospheric fields of 500-hPa geopotential height and sea level pressure produced smaller skill improvements relative to the skill of the raw model output.

Abstract

In this paper, Northern Hemisphere winter blocking is analyzed through the introduction of a set of new bidimensional diagnostics based on geopotential height that provide information about the occurrence, the duration, the intensity, and the wave breaking associated with the blocking. This analysis is performed with different reanalysis datasets in order to evaluate the sensitivity of the index and the diagnostics adopted. In this way, the authors are able to define a new category of blocking placed at low latitudes that is similar to midlatitude blocking in terms of the introduced diagnostics but is unable to divert or block the flow. Furthermore, over the Euro-Atlantic sector it is shown that it is possible to phenomenologically distinguish between high-latitude blocking occurring over Greenland, north of the jet stream and dominated by cyclonic wave breaking, and the traditional midlatitude blocking localized over Europe and driven by anticyclonic wave breaking. These latter events are uniformly present in a band ranging from the Azores up to Scandinavia. Interestingly, a similar distinction cannot be pointed out over the Pacific basin where the blocking activity is dominated by high-latitude blocking occurring over eastern Siberia. Finally, considering the large impact that blocking may have on the Northern Hemisphere, an analysis of the variability and the trend is carried out. This shows a significant increase of Atlantic low-latitude blocking frequency and an eastward displacement of the strongest blocking events over both the Atlantic and Pacific Oceans.

Abstract

In this work, the authors investigate possible changes in the distribution of heavy precipitation events under a warmer climate, using the results of a set of 20 climate models taking part in phase 5 of Coupled Model Intercomparison Project (CMIP5). Future changes are evaluated as the difference between the last four decades of the twenty-first century and the twentieth century, assuming the representative concentration pathway 8.5 (RCP8.5) scenario. As a measure of the width of the right tail of the precipitation distribution, the authors use the difference between the 99th and the 90th percentiles. Despite a slight tendency to underestimate the observed heavy precipitation, the considered CMIP5 models well represent the observed patterns in terms of the ensemble average, during both boreal summer and winter seasons for the 1997–2005 period. Future changes in average precipitation are consistent with previous findings based on models from phase 3 of CMIP (CMIP3). CMIP5 models show a projected increase for the end of the twenty-first century of the width of the right tail of the precipitation distribution, particularly pronounced over India, Southeast Asia, Indonesia, and central Africa during boreal summer, as well as over South America and southern Africa during boreal winter.

Abstract

The mechanisms leading to El Niño onset and termination in the ECHAM4/OPA coupled GCM are assessed and compared to observations and existing ENSO paradigms. At the equator as well as off equator, the patterns and timing of modeled El Niño composites are in good agreement with those observed. Heat content of the west Pacific is confirmed as a precursor to ENSO phase change, and the present work emphasizes the role of its northern off-equator part [west North Pacific (WNP) region, 5°–15°N, 120°–170°E]. The associated heat content changes appear to be dominated by a local Ekman pumping (or forced Rossby waves) rather than the accumulation of remotely generated free Rossby waves, as proposed by many theories. The heat content decrease in the WNP region, which triggers El Niño termination, is due to the negative feedback of the atmospheric Gill's response to the increased equatorial SST in the east Pacific, in agreement with most paradigms (delayed, recharged, west Pacific oscillators). The present study introduces the advection of the off-equator signal to the equatorial waveguide by the mean currents of the western Pacific as an additional process. A similar feedback (with opposite sign) also seems to drive the modeled El Niño onset, favoring a too strong and regular biennial ENSO in the model. This is due to the stronger-than-observed Walker circulation that isolates the WNP region from other remote influences (like monsoons). The model also exhibits “aborted” ENSO events where the warming peaks in late spring instead of late autumn and is quickly terminated by the Gill's negative feedback. The abort event occurs too frequently in the coupled model due to too strong and too zonal a convergence zone south of the equator (“double ITCZ”). It bears some resemblance to the spring 1993 warming, when the southern Tropics were also warm. The results of this paper provide additional insight into the El Niño seasonal phase lock mechanisms.

Abstract

The Indian Ocean dipole mode (IODM) is examined by comparing the characteristics of oceanic and atmospheric circulations, heat budgets, and possible mechanisms of IODM between El Niño and non–El Niño years. Forty-year ECMWF Re-Analysis (ERA-40) data, Reynolds SST data, and ocean assimilation data from the Modular Ocean Model are used to form composites of the IODM that occur during El Niño (1972, 1982, and 1997) and non–El Niño (1961, 1967, and 1994) years. In El Niño years, two off-equatorial, anticyclonic circulations develop, associated with the increased pressure over the eastern Indian Ocean. The anticyclonic circulation over the Northern Hemisphere enhances the easterly component of the winds in the northwestern Indian Ocean. This enhanced easterly component increases the mixed layer temperature by inducing an anomalous westward ocean current that advects the warm mean mixed layer from the central to the western Indian Ocean. Meanwhile, the anticyclonic circulation over the southeastern Indian Ocean strengthens southeasterlies, thereby causing oceanic meridional and vertical advection of the cold mean temperature. Consequently, the IODM in El Niño years is characterized by the warming in the northwestern and the cooling in the southeastern Indian Ocean. In non–El Niño years, a monsoonlike wind flow increases the westerly and southeasterly components of the wind over the northwestern and southeastern Indian Ocean, respectively. Oceanic currents induced by these winds result in anomalous cold advection in both of these regions. In addition, the monsoonlike wind flow over the southeastern Indian Ocean enhances the anomalous latent and sensible heat fluxes in non–El Niño years. Hence, the cooling of the eastern tropical Indian Ocean, rather than the warming of the western Indian Ocean, becomes the major feature of the IODM during non–El Niño years.