The regional sea level index is calculated in the general manner described in NOAA Technical Report NOS CO-OPS 065 with some modifications. A group of long-term water level stations with monthly mean sea level (MSL) data beginning at least 25 years before 1992 are selected. The pre-1992 series have a median length of 60 years.

For each station, a linear trend and average seasonal cycle, obtained only from the data up to 1992, are calculated and then subtracted from all the data to obtain a monthly residual series representing the multidecadal and interannual variation of the ocean. The post-1992 data, which can therefore depart from the pre-1992 trendline, will also be part of the residual series. These series are highly correlated among the stations in each group.

The residuals of all stations in the region are then averaged and the global sea level rate for the 20th century of 1.7 mm/yr relative to the base year of 1992 is applied. This implies that over the pre-1992 length of each index, the region followed the 20th century global rate (i.e., its regional multidecadal variations averaged out to the global rate). The annual index can then be obtained from the monthly index.