A Rain Accumulation Timeseries at Ocean Weather Ship Papa in the Northeast Pacific Ocean

Station Papa (P) at 50 degrees N, 145 degrees W was occupied by an Ocean Weather Ship (OWS) from December 1949 until August 1981. We carried out a retrospective analysis of the precipitation at Station P during this occupation, with the ultimate objective of determining how mean precipitation rates and intensities in the past record compare with those observed at a moored buoy at Station P from 2007 to present. The search of the archives was an involved, multi-layered process. Two sources of OWS P meteorological data were found: the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) and the University of Washington Applied Physics Laboratory (APL) Data Set. Both data sets lack direct measurements of precipitation. Instead, “present weather” codes were recorded every three hours in order to estimate accumulations based on previous work relating these codes to actual precipitation rates. The two previously mentioned datasets are similar but includedifferences. It appears that when ICOADS removes “duplicate” ship measurements, it replaces the OWS measurements with transiting ship measurements. Our best estimates of past accumulations on seasonal time scales are consistent with published results. Importantly, it appears that these historical seasonal totals tend to be lower than their counterparts during 2007-2018 based on buoy measurements. An increase in precipitation is consistent with the freshening of the surface waters that has been observed at Station P over the historical record.