VARIATIONS OF SEA LEVEL AT VARNA TIDE GAUGE 1928 – 2025

Abstract

The sea level changes and global sea level rise are slow but permanent changes. They should be evaluated as a sign of ongoing climate changes. The Black Sea is a semi-enclosed sea, with the only connection to the world's oceans being the narrow strait of Bosporus. Sea level observations in the Bulgarian part of the Black Sea are carried out mainly at the Varna and Burgas tide gauge stations. The most representative of them, with the most complete set of measurements, is the Varna station. The series of observations spread to a period of 97 years (1928-2025). All sea level data from the tide gauge station are corrected for the Earth vertical movements at the site. As a result of the analysis is to determine mean sea level and its changes, as well as the influences of long-term and seasonal tides. Only monthly data are available for this time interval.  The monthly data series were analyzed using a least squares regression model. The model includes mean sea level, trend, decadal, annual, semiannual variations of sea level. The mean sea level was determined and a sea level of 1.3cm trend of about 1.4mm per year has been established. Hourly data since May 2013 are available for the tide gauge station. About 90,000 hourly values are available for the observation period 5.2013-12.2024. The period is sufficient to distinguish between multiple daily and semi-daily tides, as well as to establish the influence of the annual and semi-annual tides. The analysis is aimed to determine the influences of long-periods and short-periods tides. The least squares harmonic analysis was used to for data series of hourly sea level values. The influences of about seventy statistically significant short-term tidal have been identified. Obtained results from the harmonic analysis from hourly data for the annual and semi-annual tides were compared with those from the linear regression model -monthly data.