Assessing teleconnections patterns in climate models using a combination of point correlation maps and self-organizing maps

Abstract

A new method to identify and evaluate teleconnection patterns in gridded climate data is presented. A large set of point correlation maps (one for each grid point) is used to train a self-organizing map (SOM). This combines the teleconnection identification properties of point correlation maps with the ability of SOMs to group similar patterns together on a topological grid and provides a frequency of occurrence for each pattern. Once the SOM is trained it is used as a reference for comparison to other sets of correlation maps. A SOM trained using point correlation maps calculated from NCEP/NCAR sea level pressure reanalysis for the period 01.1948-12.2005 is presented and the patterns found compared to point correlation maps from several climate models. By matching each NCEP/NCAR correlation map and each model correlation map with their most similar pattern on the SOM, discrepancies between the datasets are revealed, such as differences in the frequency of occurrence or shifts in the spatial structure of teleconnections. The base points corresponding to the correlation maps for each teleconnection show the regions important for their existence. Differences in the base point locations between NCEP/NCAR and the models provide insight into the physical biases underlying the model deviations from reality. Prominent patterns are identified by the SOM, such as the NAO, ENSO and the PNA, however the flexibility of the SOM allows these patterns to be viewed as a continuum of patterns, each identifiable as a variation within a defined teleconnection. As the SOM is a non-linear method, asymmetries between patterns generated from opposite centres of action are revealed. Clustering the SOM patterns identifies the regions of the SOM corresponding to each teleconnection type by classifying similar patterns together, which retains the continuum of patterns, but allows general characterization of the teleconnections present in the data.

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