Learning a weather dictionary of atmospheric patterns using Latent Dirichlet Allocation

Mid-latitude circulation dynamics is often described in terms of weather regimes, represented by atmospheric field configurations extracted using pattern recognition techniques. Each pattern is given by a given combination of distinct elements, corresponding to synoptic objects (cyclones and anticyclones). Such intrication makes it arduous to detect or quantify shifts in atmospheric circulation - possibly due to anthropogenic forcings - impacting recurrence and intensity of climate extremes. Here we apply Latent Dirichlet Allocation (LDA), typically used for topic modeling in linguistic studies, to build a weather dictionary: in analogy with linguistics, we define daily maps of a gridded target observable as documents, and the grid-points composing the map as words. LDA provides a representation of documents in terms of a combination of spatial patterns named motifs, which are latent patterns inferred from the set of snapshots. For atmospheric data, we find that motifs correspond to pure synoptic objects (cyclones and anticyclones), that can be seen as building blocks of weather regimes. We show that LDA weights provide a natural way to characterize the impact of climate change on the recurrence of regimes associated with extreme events.