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Abstract:
Accurate determination of absolute dynamic topography (ADT) has been mostly under-utilized because of limited access to high resolution geoid models. However, quantification of ADT is essential for advancing in realistically understanding ocean dynamic (e.g., ocean current patterns, sea level trends etc.). Most importantly it allows conformity of a stable vertical reference datum to be used amongst nations. As a result, this study develops and explores a method that derives ADT by employing both a geodetic and oceanographic approach. The geodesy approach utilizes multi-satellite along track satellite altimetry (SA) data in conjunction with the NKG2015 geoid model (developed by Baltic Sea countries). For the oceanographic approach the vertical datum of HDMs are often undisclosed, thus a method is applied using geoid referred tide gauges (TG), that corrects the HDM. The study site is that of the entire Baltic Sea for the period 2017‒2019.The statistical results showed average discrepancies between SA and HDM within range of ±20 cm; RMSE between 6‒ 9 cm and a standard deviation between 2‒16 cm. The method employed different sources; thus it identified problems with SA, HDM, TG, and geoid. For instance, possible geoid problems were discovered in the eastern Baltic Sea whereas for SA the northern Baltic sea-ice may be problematic. The comparison also revealed that SA along-track data has potential to show more realistic variation of ADT compared to that of HDM (which tended to generate a smooth surface). Also, in most cases, the HDM tended to underestimate the ADT.
