[ARCHIVE] Baltic Sea Subsurface Temperature trend from Reanalysis

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'''DEFINITION'''

The subsurface temperature trends have been derived from regional reanalysis results for the Baltic Sea (product references BALTICSEA_MULTIYEAR_PHY_003_011). Horizontal averaging has been done over the Baltic Sea domain (13 °E - 31 °E and 53 °N - 66 °N; excluding the Skagerrak strait). The temperature trend has been obtained through a linear fit for each time series of horizontally averaged annual temperature and at each depth level.

'''CONTEXT'''

The Baltic Sea is a semi-enclosed sea in North-Eastern Europe. The temperature of the upper mixed layer of the Baltic Sea is characterised by a strong seasonal cycle driven by the annual course of solar radiation (Leppäranta and Myrberg, 2008). The maximum water temperatures in the upper layer are reached in July and August and the minimum during February, when the Baltic Sea becomes partially frozen (CMEMS OMI Baltic Sea Sea Ice Extent, CMEMS OMI Baltic Sea Sea Ice Volume). Seasonal thermocline, developing in the depth range of 10-30 m in spring, reaches its maximum strength in summer and is eroded in autumn. During autumn and winter the Baltic Sea is thermally mixed down to the permanent halocline in the depth range of 60-80 metres (Matthäus, 1984). The 20–50 m thick cold intermediate layer forms below the upper mixed layer in March and is observed until October within the 15-65 m depth range (Chubarenko and Stepanova, 2018; Liblik and Lips, 2011). The deep layers of the Baltic Sea are disconnected from the ventilated upper ocean layers, and temperature variations are predominantly driven by mixing processes and horizontal advection. A warming trend of the sea surface waters is positively correlated with the increasing trend of diffuse attenuation of light (Kd490) and satellite-detected chlorophyll concentration (Kahru et al., 2016). Temperature increase in the water column could accelerate oxygen consumption during organic matter oxidation (Savchuk, 2018).

'''KEY FINDINGS'''

Analysis of subsurface temperatures from 1993 to 2023 indicates that the Baltic Sea is experiencing warming across all depth intervals. The temperature trend in the upper mixed layer (0-25 m) is approximately 0.055 °C/year, decreasing to 0.045 °C/year within the seasonal thermocline layer. A peak temperature trend of 0.065 °C/year is observed at a depth of 70 m, aligning with the base of the cold intermediate layer. Beyond this depth, the trend stabilizes, closely matching the 0.065 °C/year value. At a 95% confidence level, it can be stated that the Baltic Sea's warming is consistent with depth, averaging around 0.06 °C/year. Notably, recent trends show a significant increase; for instance, Savchuk's 2018 measurements indicate an average temperature trend of 0.04 °C/year in the Baltic Proper's deep layers (>60m) from 1979 to 2016.

'''DOI (product):''' https://doi.org/10.48670/moi-00208