# Long-Term and Decadal Sea-Level Trends of the Baltic Sea Using Along-Track Satellite Altimetry
## 5 Results
### Inter-Comparisons of SA-Based ASL Trends
#### 5.4.3 Gridded vs. Instantaneous SA-Derived ASL Trends
These three gridded datasets consist of (i) a global SA gridded sea-level anomaly, (ii) hydrodynamic model gridded products obtained from Nemo (Baltic Sea Physical Reanalysis), and (iii) Baltic + SEAL gridded SA data over the Baltic Sea region obtained from the Baltic + SEAL dataset. More detailed information on these data sources can be found in Section 4.5. This examination amongst the different sea-level products is performed from 1995 to 2019 (the upper limit is due to the availability of the Baltic + SEAL project data for along-track and gridded data being available only up to June 2019, cf. Table 2). The result of this comparison is shown in Figure 12. The results of the inter-comparison show that most of the gridded products agreed well with the instantaneous SA-derived trend calculations in the present study. In Figure 12, the long-term (28 years) trend variations (the results of Figure 7 and Table 5) from TGs (\(DT_{TG}\)) and SA (\(DT_{SA\#TG}\)) are also illustrated (triangle marker) for comparison. The largest TG trend differences appear to occur with the Baltic+SEAL gridded data, with a difference of 4.7 mm/yr at the Ventspils (#3) station, as well as with the global gridded SLA for Dirhami (#1) of 3.3 mm/yr. The TG data at Ventspils (#3) and Rodvig (#8) also showed somewhat larger differences compared to the other products. Possible reasons are discussed in Section 5.3. Figure 12: Sea-level trend estimates based on instantaneous SA data (red) and three gridded datasets including global SLA data by Copernicus (purple), gridded Nemo model SLA by Copernicus (green), and Baltic + SEAL gridded SA data (yellow) together with the TG data (blue) in the period of 1995–2019. The whiskers represent uncertainties using 95% CI. The long-term (1995–2022) trend based on \(DT_{SA\#TG}\) (red triangle) and \(DT_{TG}\) (blue triangle) are added as a reference. Figure 12 shows that, in general, the instantaneous SA-derived ASL trend results match those of the gridded datasets. Note that the gridded SA data of the Copernicus dataset could not come close enough to the coastline to estimate the sea-level trend for seven TG stations. Hence, the RMSE is compared only for the six available stations. Although the instantaneous SA and gridded data from Baltic + SEAL have the same RMSE, the gridded data also could not come close to the two TG stations' (Kolobrzeg and Swinoujscie) estimate. This emphasizes the advantage of using the instantaneous SA-derived DT rather than gridded and SLA data for realistic sea-level trend calculations. Table 8 also includes the long-term trend (28 years) estimations of both SA and TG (the results from Figure 7 and Table 5). From this table, the instantaneous SA-derived trend has a better match with TG (bold fonts) for five stations (Ristna, Swinoujscie, Sassnitz, Hanko, and Pori) as compared to other datasets. These results show that the instantaneous along-track data are comparable to those of the gridded SA products, in which SA appears to work better for regional trend estimation over this area at several TG stations. This could be due to averaging the instantaneous DT within 50 km rather than taking one observation (of TG) for trend computation. In Table 8, the RMSE values are calculated between the pair (\(DT_{TG}\) and \(DT_{SARTG}\)) of long-term (28 years) and 1995-2019 (24 years) trends of 1.78 mm/yr (blue circles and blue triangles) and 0.64 mm/yr (red circle and red triangles), respectively. In Figure 12, the RMSE between \(DT_{TG}\) (blue circles) and other datasets over the same period (1995-2019) is also mentioned (stations with any NaN value are excluded). The RMSE between the \(DT_{TG}\) trend (blue circles) and the \(DT_{SARTG}\) trend (red circles) is 1.37 mm/yr. The RMSE between \(DT_{TG}\) and the Baltic+SEAL gridded data (yellow circles) is 1.37 mm/yr, that of the Nemo gridded data (green circles) is 1.12, and for the global gridded SLA data (purple circles), this is 1.65 mm/yr.