Decadal predictions have a high profile in the climate science community and beyond, yet very little is known about their skill. Nor is there any agreed protocol for estimating their skill. This paper proposes a sound and coordinated framework for verification of decadal hindcast experiments. The framework is illustrated for decadal hindcasts tailored to meet the requirements and specifications of CMIP5 (Coupled Model Intercomparison Project phase 5). The chosen metrics address key questions about the information content in initialized decadal hindcasts. These questions are: (1) Do the initial conditions in the hindcasts lead to more accurate predictions of the climate, compared to un-initialized climate change projections? and (2) Is the prediction model's ensemble spread an appropriate representation of forecast uncertainty on average? The first question is addressed through deterministic metrics that compare the initialized and uninitialized hindcasts. The second question is addressed through a probabilistic

Because of the action of various geophysical excitation mechanisms, the Earth does not rotate about its figure axis, so it wobbles as it rotates. Here, the effectiveness of atmospheric and oceanic processes in exciting the Earth's wobbles during 1980–2000 is evaluated using estimates of atmospheric angular momentum from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis project and estimates of oceanic angular momentum from the Estimating the Circulation and Climate of the Ocean (ECCO) consortium's simulation of the general circulation of the oceans. On intraseasonal timescales, atmospheric surface pressure changes are found to be the single most effective process exciting the Earth's wobbles, explaining about twice as much of the observed variance as do either atmospheric wind or ocean bottom pressure changes and nearly 4 times as much of the observed variance as do oceanic currents. However, on interannual timescales, ocean bottom pressure changes are found to be the single most effective process exciting the Earth's wobbles, explaining more than 5 times as much of the observed variance as do atmospheric wind and pressure changes combined, and more than twice as much of the observed variance as do oceanic currents. Within the Chandler band it is found that during 1980–2000 atmospheric and oceanic processes have enough power to excite the Chandler wobble and are significantly coherent with it. The single most important mechanism exciting the Chandler wobble is found to be ocean bottom pressure variations. Atmospheric and oceanic processes do not appear to have enough power to excite the Earth's wobbles to their observed levels on pentadal and longer timescales, although series longer than the 21‐yearlong series used here need to be studied in order to obtain greater statistical significance of this result.

Green's functions provide a simple yet effective method to test and to calibrate general circulation model (GCM) parameterizations, to study and to quantify model and data errors, to correct model biases and trends, and to blend estimates from different solutions and data products. The method is applied to an ocean GCM, resulting in substantial improvements of the solution relative to observations when compared to prior estimates: overall model bias and drift are reduced and there is a 10%-30% increase in explained variance. Within the context of this optimization, the following new estimates for commonly used ocean GCM parameters are obtained. Background vertical diffusivity is (15.1 Ϯ 0.1) ϫ 10 Ϫ6 m 2 s Ϫ2 . Background vertical viscosity is (18 Ϯ 3) ϫ 10 Ϫ6 m 2 s Ϫ2 . The critical bulk Richardson number, which sets boundary layer depth, is Ri c ϭ 0.354 Ϯ 0.004. The threshold gradient Richardson number for shear instability vertical mixing is Ri 0 ϭ 0.699 Ϯ 0.008. The estimated isopycnal diffusivity coefficient ranges from 550 to 1350 m 2 s Ϫ2 , with the largest values occurring at depth in regions of increased mesoscale eddy activity. Surprisingly, the estimated isopycnal diffusivity exhibits a 5%-35% decrease near the surface. Improved estimates of initial and boundary conditions are also obtained. The above estimates are the backbone of a quasi-operational, global-ocean circulation analysis system.

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