Investigating the Temporal and Spatial Variability of Precipitation

To explore the temporal and spatial variability of precipitation, a statistical tool called Principal Component Analysis (PCA) is applied to precipitation data from Tropical Rainfall Measuring Mission (TRMM), Global Precipitation Climatology Project (GPCP), and Community Atmosphere Model (CAM5). Results for the tropical domain reveal the first leading mode is related to the El Niño Southern Oscillation (ENSO). Further, it is found that the second principal component mode demonstrates correlation with a separate phenomenon, named El Niño Modoki. Results show a positive phase of El Niño Modoki produces positive precipitation anomalies over central Pacific and negative over western and eastern Pacific, analogous to those of typical ENSO episodes. Both observations and the CAM5 model are able to capture the ENSO and El Niño Modoki signals in the tropical precipitation, although the signals in the model are weaker than the observation. In the polar regions, spatial analysis and time series correlations with Northern Annular Mode/Southern Annular Mode Indices suggest the strength of the polar vortex can influence the temporal and spatial variability of precipitation in the high and mid-latitudes. The CAM5 precipitation simulations demonstrate patterns similar to that of the observed GPCP, although they slightly under predict magnitudes. Next, high and low precipitation areas are defined with climatological monthly mean precipitation larger than 200 millimeters per month (mm/mon) and less than 50 mm/mon. Observed temporal variation reveals that precipitation has an increasing tendency in the wet areas and a decreasing tendency in the dry areas. The NASA Goddard Institute for Space Studies (GISS) model is utilized, and simulations imply that the increasing greenhouse gases can affect the temporal variation of precipitation over the wet and dry areas, consistent with the observed “rich-get-richer” mechanism. Results further reveal that the atmospheric dynamics related to the convective stability, and hence the vertical motions, contribute to the increased precipitation over the tropical area as a result of global warming. However, the vertical motion in the dry areas does not demonstrate significant change, making the physics of the negative trend of precipitation in these regions more complicated.