J. Clim. http://doi.org/bcd8 (2016)

Observations of rainfall have shown an increase in heavy precipitation events in the northern and eastern United States over the past few decades. It is unknown whether these increases are being driven by natural climate variability or human influence; climate models have varying success at simulating precipitation, particularly on a regional scale.

To investigate the cause, Martin Hoerling from NOAA Earth System Research Laboratory, USA, and colleagues analysed heavy precipitation observational time series from the beginning of the twentieth century to 2013. They then used climate model simulations to investigate human influence since 1979 by comparing the signal from external forcing to that of natural variability in the ocean and atmosphere.

Fluctuations in sea surface temperature, mostly related to natural internal variability, was found to be the main influence on the precipitation increases for both the summer and winter seasons. Anthropogenic influence was found to be a smaller contributor to the observed trends for the study period; however, its influence is expected to increase into the future.