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

'''DEFINITION'''

Ocean Freshwater Content (OFC) ocean monitoring indicator was introduced in Copernicus Marine Service Ocean State Report, Issue 7 (Raudsepp et al, 2023) and is derived from regional reanalysis product BALTICSEA_MULTIYEAR_PHY_003_011. The OFC is calculated according to Boyer et al. (2007) OFC = ρ(Sref, Tref, p) / ρ(0, Tref, p ) · ( Sref - S) / Sref where S(x, y, z, t) and Sref (x, y, z) are actual salinity and reference salinity, respectively, and x,y,z,t are zonal, meridional, vertical and temporal coordinates, respectively. The density, ρ, is calculated according to the TEOS10 (IOC et al., 2010). The key issue of OFC calculations lies in how the reference salinity is defined. The climatological range of salinity in the Baltic Sea varies from the freshwater conditions in the northern and eastern parts to the oceanic water conditions in the Kattegat. We follow the Boyer et al. (2007) formulation and calculate the climatological OFC from the three-dimensional temperature (Tref) and salinity (Sref) fields averaged over the period of 1993–2014. The method for calculating the ocean freshwater content anomaly is based on the daily mean sea water salinity fields (S) derived from the Baltic Sea reanalysis product BALTICSEA_MULTIYEAR_PHY_003_011. The total freshwater content anomaly is determined using the following formula: OFC(t) = ∭V OFC(x, y, z, t) dx dy dz The vertical integral is computed using the static cell vertical thicknesses (dz) 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 integration is performed over the Baltic Sea spatial domain, defined as the region between 9° - 31° E and 53° - 66° N using product grid definition in cmems_mod_bal_phy_my_static.

We evaluate the uncertainty from the mean standard deviation of monthly mean OFC. The shaded area in the figure corresponds to the annual standard deviation of monthly mean OFC.

Linear trend (km3y-1) has been estimated from the annual anomalies with the uncertainty of 1.96-times standard error.

'''CONTEXT'''

Climate warming has resulted in the intensification of the global hydrological cycle but not necessarily on the regional scale (Pratap and Markonis, 2022). The increase of net precipitation over land and sea areas, decrease of ice cover, and increase of river runoff are the main components of the global hydrological cycle that increase freshwater content in the ocean (Boyer et al., 2007) and decrease ocean salinity. The Baltic Sea is one of the marginal seas where water salinity and OFC are strongly influenced by the water exchange with the North Sea. The Major Baltic Inflows (MBIs) are the most voluminous event-type sources of saline water to the Baltic Sea (Mohrholz, 2018). The frequency and intensity of the MBIs and other large volume inflows have no long-term trends but do have a multidecadal variability of about 30 years (Mohrholz, 2018; Lehmann and Post, 2015; Lehmann et al., 2017; Radtke et al., 2020). Smaller barotropic and baroclinically driven inflows transport saline water into the halocline or below it, depending on the density of the inflow water (Reissmann et al., 2009).

'''KEY FINDINGS'''

The Baltic Sea's ocean freshwater content is exhibiting a declining trend 38.06±8.69 km3/year, along with decadal fluctuations as also noted by Lehmann et al. (2022). Elevated freshwater levels were recorded prior to the Major Baltic Inflows of 1993, 2002, and 2013, which subsequently led to a swift decrease in freshwater content. The lowest ocean freshwater content was recorded in 2023. Over the past four years, the freshwater content anomaly has remained comparatively stable.

'''DOI (product):''' https://doi.org/10.48670/mds-00347