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Five initialization and ensemble generation methods are investigated with respect to their impact on the prediction skill of the German decadal prediction system “Mittelfristige Klimaprognose” (MiKlip). Among the tested methods, three tackle aspects of model-consistent initialization using the ensemble Kalman filter, the filtered anomaly initialization, and the initialization method by partially coupled spin-up (MODINI). The remaining two methods alter the ensemble generation: the ensemble dispersion filter corrects each ensemble member with the ensemble mean during model integration. And the bred vectors perturb the climate state using the fastest growing modes. The new methods are compared against the latest MiKlip system in the low-resolution configuration (Preop-LR), which uses lagging the climate state by a few days for ensemble generation and nudging toward ocean and atmosphere reanalyses for initialization. Results show that the tested methods provide an added value for the prediction skill as compared to Preop-LR in that they improve prediction skill over the eastern and central Pacific and different regions in the North Atlantic Ocean. In this respect, the ensemble Kalman filter and filtered anomaly initialization show the most distinct improvements over Preop-LR for surface temperatures and upper ocean heat content, followed by the bred vectors, the ensemble dispersion filter, and MODINI. However, no single method exists that is superior to the others with respect to all metrics considered. In particular, all methods affect the Atlantic Meridional Overturning Circulation in different ways, both with respect to the basin-wide long-term mean and variability and with respect to the temporal evolution at the 26° N latitude. ©2019. The Authors.

Decadal climate predictions and their verification are part of ongoing research. This article studies different methods applied to decadal hindcasts of three-dimensional atmospheric variables to evaluate the MiKlip (Mittelfristige Klimaprognosen) prediction system. Variables such as upper air temperature are tight to the core of the prediction system and hence help to reveal its power and deficiencies. The verification uses both, necessary and sufficient probabilistic measures. We analyze annual and multi-year averages of air temperature and geopotential height and the parametrized quantity net water flux at the ocean surface, the so-called freshwater flux, also known as E‑P (evaporation minus precipitation), as an important variable for atmosphere-ocean coupling. The model data stem from various versions of the MiKlip prediction system and constitute different sets of ensemble hindcasts covering 1979–2012. The results reveal that the freshwater flux is far more sensitive to model deficiencies than the basic dynamical variables and the predictability decays much earlier with prediction lead time. Initializing the atmospheric component is more important for the predictability than the difference in resolution between two model versions. The combined initialization of atmosphere and ocean has the effect of increasing the predictability in the inner tropics from 1 to 2 years compared to the ocean only initialization. For prediction year 7–10, the hindcasts are still closer to each other than to the uninitialized historical runs indicating that the prediction system is still influenced by the initial conditions. The skill for prediction year 7–10 is, however, only marginally larger than the skill of the uninitialized ensemble. The three-dimensional skill analysis reveals a clear indication of a mid-tropospheric temperature error developing in the tropical Pacific area.

A time series of wind stresses computed from European Centre for Medium Range Weather Forecast (ECMWF) wind data is compared to the climatology of Hellermann & Rosenstein (HR) for the Southern Hemisphere. ECMWF stresses are generally stronger, especially in the westerly belt. However they have an overall lower meridional component than the HR data. The dominance of the half annual cycle relative to the annual wave in the zonal stress at middle to high latitudes, which is documented for independent data sets, is seen in the ECMWF but not in the HR data. ECMWF winds are also compared with measurements from three expeditions to the Weddell Sea by RV Polarstern. Good correlations between Polarstern and ECMWF winds are found but for single dates the differences are above 10 ms −1. The differences are found to be uncorrelated in space and are thus due to observational errors and to the unresolved small scale variance in the ECMWF analysed winds.

Analyses indicate that the Atlantic Ocean SST was considerably colder at the beginning than in the middle of the century. Parallel to this a systematic change of the North Atlantic SLP pattern was observed. To find out whether the analyzed SST and SLP changes are consistent, which would indicate that the SST change was real and not an instrumental artifact, a response experiment with a low resolution (T21) atmospheric GCM was performed. Two perpetual January simulations were conducted which differ solely in Atlantic Ocean (40$ - 60N) SST: the ”cold” simulation utilizes the SSTs for the period 1904-13, the ”warm” simulation uses the SST’s for the period 1951—60. Also a ”control” run, with the model’s standard SST somewhat between the ”cold” and ”warm” SST, was made. For the response analysis a rigorous statistical approach was taken: First the null hypothesis of identical horizontal distributions was subjected to a multivariate significance test. Second, the level of recurrence was estimated. The multivariate statistical approaches are based on hierarchies of test models. We examined three different hierarchies: a scale dependent hierarchy based on spherical harmonics (S), and two physically motivated ones, one based on the barotropic normal modes of the mean 300 hPa flow (B) and one based on the eigenmodes of the advection diffusion operator at 1000 hPa (A). The intercomparison of the ”cold” and ”warm” experiments indicates a signal in the geostrophic stream function that is in the S-hierarchy significantly nonzero and highly recurrent. In the A—hierarchy, the low level temperature field is identi— fied as being significantly and recurrently affected by the altered SST distribution. The SLP signal is reasonably similar to the observed SLP change. Unexpectedly the upper level streamfunction signal does not appear significantly nonzero in the B—hierarchy. If, however the pairs of experiments ”warm vs. control” and ”cold vs. control” are examined in the B—hierarchy, a highly significant and recurrent signal emerges. We conclude that the ”cold vs. warm” response is not a ”small disturbance” which would allow the signal to be described by eigenmodes of the linearized system. An analysis of the three—dimensional structure of the signal leads to the hy— pothesis that two different mechanisms are acting to modify the model’s mean state. At low levels, the local heating and advection are dominant, but at upper levels the extratropical signal is a remote response to modifications of the tropical convection.

An analysis of the signature of the North Atlantic Oscillation (NAO) in the Atlantic-European sea level pressure (SLP) is presented for observed (German Weather Service) and ECMWF T21 model data. The former time series consists of 1881–1984 January to December fields and the latter of 42 monthly fields from 3 permanent January simulations. The NAO is shown to be one of the dominant eigenmodes of SLP for all calendar months. A very similar NAO anomaly pattern is filtered from the T21 model data, whereby the correspondence between model and reality is far better than for the mean states. Typical extreme states of the NAO, i.e. high- and low-index situations are reconstructed from the EOF (empirical orthogonal functions) and the mean fields. The annual cycle of these states and the differences between observed and simulated January extremes of the NAO are discussed. DOI: 10.1034/j.1600-0870.1990.t01-3-00001.x