Means and Long-Term Trends of Global Coastal Zone Precipitation

Precipitation in the coastal zone is important to the socio-economic and ecological well-being of the world. Meteorologically, precipitation is generated by unique mechanisms at the land-sea interface, which is why coastal zone precipitation is not well resolved by global climate models. Yet, to date, much more effort has been placed in analyzing global precipitation over the oceans and land. In this study, global coastal zone precipitation is quantified by selecting Global Precipitation Climatology Centre V2018 0.5° grid cells in 50 km zones from the shoreline into the interior. The transition from maritime to continental precipitation regimes is revealed in the long-term (1931-2010) average, as there is a pronounced coast-to-interior decline in rainfall from approximately 911.5 mm yr^-1 within 50 km of the coast to 727.2 mm yr^-1 from 100 to 150 km away from the coast. Globally, coastal zone precipitation peaks in boreal summer, extending into fall for precipitation at the coastline. Dividing the long-term record into early and late 40-year periods reveals an increasing trend in precipitation in the coastal zone, with the interior increasing faster than at the coastline. Averaging over 30-year climate normals from 1931-60 to 1981-2010 further confirms this result. A seasonal analysis reveals that the upward trends, and discrepancy between the coast and inland are maximized in the austral summer season. Interestingly, from May to September there is a declining trend in rainfall at the coastline, whereas the interior only shows minimal declines in August and September. Potential forcing mechanisms that could favor a wetter interior coastal zone include changes in the sea breeze circulation, urban heat island effect, or precipitation content associated with synoptic systems or monsoonal circulations.