# Long-Term and Decadal Sea-Level Trends of the Baltic Sea Using Along-Track Satellite Altimetry
## 5 Results
### Decadal Regional ASL Trend
#### 5.3.2 Decadal Trend from SA and TG  
Certainly, while the decadal sea-level trends appear to be higher than the long-term values, a closer examination also reveals a wide variation in the calculated decadal trend values from the two sources of TG and SA data. For instance, the decadal trend rates derived from SA (\(DT_{SA\#TG}\)) for both periods (2000-2009 and 2010-2019) ranged from 2.7 to 9.6 mm/yr. However, the TG-based trends, utilizing both \(DT_{TG\text{@}SA\ cycle}\) and \(DT_{TG}\), showed a broader range of \(-4.6\) to 16 mm/yr. The TG data exhibited notably extreme higher and lower values, notably evident at specific stations. For instance, the Dirhami (#1) station displayed an ASL rate of 12.9 mm/yr for 2000-2009 and \(-1.7\) mm/yr for 2010-2019. During 2000-2009, Swinoujscie (#6) and Rodvig (#8) showed an ASL rate of \(-1.5\) mm/yr and 16 mm/yr, respectively. Similarly, Kungsholmsfort (#10) indicated an ASL rate of 10.6 mm/yr during 2000-2009, whereas Kolobrzeg (#5) showed an ASL of \(-0.4\) mm/yr during 2010-2019. Notably, a majority of these extreme TG-based trend values were observed during the decade 2000-2009, suggesting potential issues with the reliability of the TG data. Investigating the TG data for the period 2000-2009 (detailed in Table 4) revealed notable data gaps at specific TG stations. Particularly, some of them were limited to sea-level readings only twice a day (cf. Table A1). For instance, Dirhami (#1) experienced a missing data rate of 61%, Ventspils (#3) 6%, Kolobrzeg (#5) 9%, and Rodvig (#8) 20%. These gaps potentially influenced the calculated trend estimates at these TG stations. Insufficient data availability at these stations potentially impacted the comprehensive assessment of sea-level trends for the respective periods under study. However, stations Swinoujscie (#6, ASL rate of \(-1.5\) mm/yr during 2000-2009) and Kungsholmsfort (#10, ASL rate of 10.6 mm/yr during 2000-2009) displayed extreme sea-level trends during 2000-2009, yet major TG data gaps were not identified at these stations. This suggests that other underlying issues might be affecting the data quality at these specific stations. TG data have traditionally served as a common method to calculate sea-level trend estimates. However, their reliability can become questionable due to inherent issues, like data gaps, often stemming from resolution limitations and missing data. As can be seen from the results mentioned above, there seems to be a correlation between TG data gaps and the resulting calculated trends. The outcomes of Table 5 indicate that despite SA data potentially representing only a single value at a specific time of the month (depending on the SA cycle), they are still able to capture a realistic trend comparable to the TG data. For instance, during the period of 2000-2009, the average ASL rate ranged around 2.7 mm/yrfor TG data and 3.9 mm/yr for SA data. Similarly, for the period of 2010-2019, these rates were 8.1 and 7.6 mm/yr, respectively. As highlighted in Section 5.2, there appears to be an approximately 1 to 1.5 mm/yr difference between the source and method utilized for calculating sea-level results. Another major observation, confirmed by both TG and SA, is that seven out of the thirteen examined stations showed a difference in their decadal trends (2000-2009 vs. 2010-2019) exceeding 1 mm/yr. Specifically, higher values were indicated during the 2000-2009 period at Dirhami (#1), Ustka, (#4), Kolobrze (#5), Sassnitz (#7), Rodvig (#8), Kungsholmsfort (#10), and Pori (#13). These stations are dispersed across the Baltic Proper, and there is no obvious geographic correlation. The RMSE value was also found to be larger for 2000-2009, ranging from 4.2 to 4.7 mm/yr, while for 2010-2019, the RMSE was 3.0 to 3.7 mm/yr (Figure 9). This discrepancy might indicate some phenomena or problems during this decade that may have impacted these stations. Furthermore, there were stations where