03 Feb 2016
Today, we have introduced a change to some of the longest time series at the PSMSL. In some of the older time series, the sea level values were reported as means of high and low waters, typically called mean tidal level (MTL). This is in contrast to the average of higher frequency reading taken over the entire tidal cycle, which is called mean sea level (MSL). MTL and MSL differ primarily due to the M4 harmonic of the M2 semidiurnal tide and the combination of the diurnal tides K1 and O1 (Woodworth, in press). In some of the more extreme cases, the annual average difference between these two mean levels can be over 10 cm. If a time series had both MTL and MSL, this difference between levels could introduce an artifact into estimates of the long-term trends. For this reason, one generally should not mix these two types of values in a single record.
However, it is worth noting that about 40 older time series at the PSMSL do contain both MTL and MSL. While these differences are noted in the documentation, it is our impression that that the difference in levels frequently is not taken into account when using the records to study long-term trends. Indeed, our own trend results did not explicitly account for these differences. Thus, some better solution is needed.
In an attempt to make these combined records more transparent, and to cause the minimum disruption to the current set of records, we have introduced a flag indicating MTL values in a MSL record. In addition, we have applied an estimate of the annual average difference (MTL-MSL) to the RLR time series, so that these values should be more directly comparable with the rest of the MSL records. Note that the metric files have the MTL values flagged but do not have the correction applied.
While we may be unsure of the exact correction, we believe that this is better than the current practice of ignoring the difference. The flagging will allow people to easily identify these MTL values within a MSL time series. Thus, if a researcher wishes to remove the correction, use their own correction, or ignore these sea-level values completely, it will become much easier to do.
For the differences between MTL and MSL, we are using results provided by Philip Woodworth in conjunction with his manuscript on the subject. The difference does have a seasonal component, and we are only using an annual average for the correction. Thus, the seasonal portion of the MTL and MSL values still cannot be compared, even after we have applied the correction. Typically, we are using recent data to estimate a correction applied to data in the 1800s or early 1900s. Thus, we are explicitly assuming that the difference has not changed in time. In some cases, there is not any high-frequency data needed to conduct an analysis. In others, we have reason to believe that the correction is highly uncertain (at greater than the 1 cm level). Both cases will be addressed through flagging described below.
For details of the change, please read the appropriate section in the psmsl.hel file. Additional information on the flags can be found on the notes page. We encourage all users of the data to review these changes, as the changes to the flags in particular may cause issues for some software.
We would like to Peter Hogarth, for making us realise that these combined time series needed our attention, Philip Woodworth, for advice and contributing the corrections, and to Thomas Wahl, for providing us with two corrections for stations in the German Bight.