Baltic Sea Ocean Heat Content Anomaly (0-300m) from Reanalysis

'''DEFINITION'''

Ocean heat content ocean monitoring indicator was introduced in Copernicus Marine Service Ocean State Report, Issue 6 (Raudsepp et al, 2022) and is derived from regional reanalysis product BALTICSEA_MULTIYEAR_PHY_003_011. The method for calculating the ocean heat content anomaly is based on the daily mean sea water potential temperature fields (Tp) derived from the Baltic Sea reanalysis product BALTICSEA_MULTIYEAR_PHY_003_011. The total heat content is determined using the following formula:

HC = ρ * cp * ( Tp +273.15).

Here, ρ and cp represent spatially varying sea water density and specific heat, respectively, which are computed based on potential temperature, salinity and pressure using the UNESCO 1983 polynomial developed by Fofonoff and Millard (1983). The vertical integral is computed using the static cell vertical thicknesses sourced from the reanalysis product BALTICSEA_MULTIYEAR_PHY_003_011 dataset cmems_mod_bal_phy_my_static, spanning from the sea surface to the 300 m depth. Spatial averaging is performed over the Baltic Sea spatial domain, defined as the region between 13° - 31° E and 53° - 66° N. To obtain the OHC annual anomaly time series in (J/m2), the mean heat content over the reference period of 1993-2014 was subtracted from the annual mean total heat content.

We evaluate the uncertainty from the mean annual error of the potential temperature compared to the observations from the Baltic Sea (Giorgetti et al., 2020). The shade corresponds to the RMSD of the annual mean heat content biases (± 35.3 MJ/m²) evaluated from the observed temperatures and corresponding model values.

Linear trend (W/m2) has been estimated from the annual anomalies with the uncertainty of 1.96-times standard error.

'''CONTEXT'''

Ocean heat content is a key ocean climate change indicator. It accounts for the energy absorbed and stored by oceans. Ocean Heat Content in the upper 2,000 m of the World Ocean has increased with the rate of 0.35 ± 0.08 W/m2 in the period 1955–2019, while during the last decade of 2010–2019 the warming rate was 0.70 ± 0.07 W/m2 (Garcia-Soto et al., 2021). The high variability in the local climate of the Baltic Sea region is attributed to the interplay between a temperate marine zone and a subarctic continental zone. Therefore, the Ocean Heat Content of the Baltic Sea could exhibit strong interannual variability and the trend could be less pronounced than in the ocean.

'''KEY FINDINGS'''

The ocean heat content (OHC) of the Baltic Sea exhibits an increasing trend of 0.34±0.09 W/m², along with multi-year oscillations. This increase is less pronounced than the global OHC trend (Holland et al. 2019; von Schuckmann et al. 2019) and that of some other marginal seas (von Schuckmann et al. 2018; Lima et al. 2020). The relatively low trend values are attributed to the Baltic Sea's shallowness, which constrains heat accumulation in its waters. The most significant OHC anomaly was recorded in 2020, and following a decline from this peak, the anomaly has now stabilized at approximately 250 MJ/m².

'''DOI (product):''' https://doi.org/10.48670/mds-00322