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.
In recent years, calibrated satellite measurements have improved our understanding of global precipitation distribution and seasonal evolution, including that of ocean precipitation (Skofronick-Jackson et al., 2017). However, assessing long-term trends in oceanic precipitation remains challenging due in part to the comparatively short and heterogeneous satellite record (Nicolas & Bromwich, 2011). This is particularly true prior to the advent of operational passive microwave imagers in 1987 as well as up to the present at higher latitudes, where microwave sensors may miss shallower, lighter, and especially frozen precipitation (Panegrossi et al., 2022).Gu and Adler (2022) have undertaken an analysis of trends in precipitation amount covering the 42-year period from 1979 to 2020. Based on the Global Precipitation Climatology Project precipitation product (Adler et al., 2018), their findings reveal a generally weak but statistically significant long-term trend in global mean precipitation. On regional scales, both positive and negative trends have been observed, but statistical significance could not be established.Non-satellite-based attempts to estimate climatological oceanic precipitation have of necessity relied on subjective and qualitative reports of precipitation occurrence and type submitted by sparsely and unevenly distributed commercial and military vessels. Researchers produced climatologies of monthly precipitation amounts by assigning nominal intensities to each common present-weather code and aggregating ship reports over time
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