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We outline the familiar concept of a hierarchy of models for solving problems in climate dynamics. General circulation models (GCMs) occupy a special position at the apex of this hierarchy, and provide the main link between basic concepts—best captured by very simple, " toy " models—and the incomplete and inaccurate observations of climate variability in(More)
Thirty years ago, E. N. Lorenz provided some approximate limits to atmospheric predictability. The details---in space and time---of atmospheric flow fields are lost after about 10 days. Certain gross flow features recur, however, after times of the order of 10--50 days, giving hope for their prediction. Over the last two decades, numerous attempts have been(More)
A hidden Markov model (HMM) is used to describe daily rainfall occurrence at ten gauge stations in the state of Ceará in northeast Brazil during the February–April wet season 1975–2002. The model assumes that rainfall occurrence is governed by a few discrete states, with Markovian daily transitions between them. Four " hidden " rainfall states are(More)
A new probabilistic clustering technique, based on a regression mixture model, is used to describe tropical cyclone trajectories in the western North Pacific. Each component of the mixture model consists of a quadratic regression curve of cyclone position against time. The best-track 1950–2002 dataset is described by seven distinct clusters. These clusters(More)
Interannual variations of the summertime (January−March) atmospheric circulation over subtropical South America are examined during the period 1958–97 using the NCEP/NCAR Reanalysis data. It is found from an empirical orthogonal function (EOF) analysis that an anomalous upper-tropospheric stationary eddy in the lee of the Andes tends to accompany a dipole(More)
This study examines space-time characteristics of seasonal rainfall predictability in a tropical region, by analyzing observed data and model simulations over Senegal. Predictability is analyzed in terms of the spatial coherence of observed interannual variability at the station scale, and within-ensemble coherence of general circulation model (GCM)(More)
We have extended Raper's original work on the organizing ability of the tip of the Dictyostelium discoideum slug. Our new results are that tips from all multicellular (pseudoplasmodial) stages act as organizers; that the structure organized by a tip depends on the developmental stage of the cells responding to the tip's signal; that tips from all stages(More)
The influence of Atlantic sea surface temperature (SST) anomalies on the atmospheric circulation over the North Atlantic sector during winter is investigated by performing experiments with an atmospheric general circulation model (GCM). These consist of a 30-year run with observed SST anomalies for the period 1961–90 confined geographically to the Atlantic(More)
We consider the problem of modeling discrete-valued vector time series data using extensions of Chow-Liu tree models to capture both dependencies across time and dependencies across variables. We introduce conditional Chow-Liu tree models, an extension to standard Chow-Liu trees, for modeling conditional rather than joint densities. We describe learning(More)