'''DEFINITION'''

The subsurface temperature anomaly has been derived from regional reanalysis results of the CMEMS BAL MFC group for the Baltic Sea (product references BALTICSEA_REANALYSIS_PHY_003_011). Horizontal averaging has been conducted over the Baltic Sea domain (13 °E - 31 °E and 53 °N - 66 °N). The profiles of the annual mean temperature anomaly have been calculated relative to the reference period of 1993-2014.

'''CONTEXT'''

The Baltic Sea is semi-enclosed sea in north-eastern Europe. The upper mixed layer temperature in the Baltic Sea is characterized by strong seasonal cycle driven by the annual cycle of solar radiation (Leppäranta and Myrberg, 2008). Maximum water temperature values are reached in July and August and the minima during February, when the Baltic Sea becomes partially frozen (Samuelson et al., 2018). The seasonal thermocline, developing at the depth range of 10-30 m in spring, holds strongest in summer and is eroded in autumn. In autumn and winter the Baltic Sea is thermally mixed down to a permanent halocline at 60-80 meters depth (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 layers, and temperature variations are predominantly driven by mixing processes and horizontal advection. Interannual temperature variability is governed by regional air-sea interactions, and water transport from the North Sea into the deep layers (Leppäranta and Myrberg, 2008; Raudsepp et al., 2018). Positive temperature anomaly (Mulet et al., 2018) in the surface layer is coupled with a smaller sea ice extent (Omstedt and Hansson, 2006) and more favourable conditions for cyanobacteria blooms (Laanemets et al., 2006). An increase of bottom layer temperature results in an increase of denitrification and nitrification rate in sediments (Seitzinger, 1988). In general, higher water temperatures can reinforce a cascade of biogeochemical processes (Meier et al., 2012) which is called the vicious circle (Vahtera et al. 2007; Savchuk 2010).

'''CMEMS KEY FINDINGS'''

Interannual variations of the subsurface temperature anomaly range from -1.5 °C to 1.5 °C. The period 1993-2006 is characterised by lower-than-average temperature, while the period 2007-2019 by higher-than-average temperature. The warm period started in 2015 with higher-than-average temperature over the whole water column. In 2019, the temperature anomaly exceeded 1 °C in the entire water column.