70-Year Trends in Ship-Reported Oceanic Precipitation Frequency

Abstract

Ship present-weather reports from 1950 through 2019 are used to assess trends in the reporting of precipitation occurrence over the global oceans. Annual reported precipitation frequency shows statistically significant positive trends of up to $\sim$15\% per decade throughout most ocean areas equatorward of 45 degrees. However, latitudes poleward of 45 degrees are dominated by negative trends, some areas of which meet the 95\% confidence threshold. Nine smaller regions were subjectively selected for further investigation, revealing that the observed trends, both positive and negative, are often but not always nearly linear, with the amplitude of interannual fluctuations usually being much larger than that expected from random sampling error alone. The annual time series reveal that four comparatively dry areas are associated with the largest overall positive trends, ranging from 8.3\% to 12.8\% (relative) per decade. Trends were also computed separately for each season, revealing remarkable overall consistency in trends across seasons.