Linear split trends of relative mean sea level at CO-OPS tide gauges

This memo outlines an expansion to the CO-OPS linear trend approach to more accurately capture changing RSL conditions. The new approach uses a split-trend method to fit two distinct linear trend segments joined at a breakpoint of 1993. The new approach was proposed for the following reasons: 1) it better reflects RSL change over the recent past than a single linear trend, 2) it enables comparing recent RSL change to historical change, 3) it provides a common baseline to compare against other U.S. stations within and across geographic regions, and 4) it provides a more direct comparison to ocean observations from satellite altimetry, which begin in 1993. Coinciding with the adoption of the split trend approach, CO-OPS also modified its 50-year trend plots to use 30-year intervals instead and to be applied at gauges with at least 40 years of historical record.