Why has Precipitation Increased in the Last 120 Years in Norway?

Abstract

We use a data set with daily precipitation observations from 55 homogeneity-tested stations in Norway from 1900 to 2019 available from MET-Norway. These observations show that precipitation in Norway has increased by 19% since 1900. Notably, over half of the overall increase occurred within the decade of 1980–1990 and is happening across all precipitation rates. To examine possible mechanisms behind the precipitation increase, we use a diagnostic model to separate the effects of changes in vertical velocity, temperature and relative humidity. We use daily vertical velocity, near-surface temperature and relative humidity from two reanalysis products, ERA-20C and 20th Century Reanalysis. The model-based precipitation correlates significantly with the observed precipitation on an annual timescale (r > 0.9), as well as captures the trend in all reanalysis products. The diagnostic model indicates that the variability in vertical velocity chiefly determines the interannual variability and long-term trend. The trend in vertical velocities contributes to more than 80% of the total modeled trend in precipitation between 1900 and 2019. However, over the last two decades (1995–2015), changes in temperature and relative humidity are the main contributors to the modeled trend in precipitation.