Black Sea subsurface temperature anomaly

'''DEFINITION''' The subsurface temperature anomaly (T') is the basin-averaged temperature (T) of a given year at a specific depth minus the reference temperature (T), which is a basin- and time-averaged temperature at the corresponding depth and reference period 1993-2014. Therefore, T'=T-T. The subsurface temperature anomaly is derived from the results of the Black Sea reanalysis (product reference BLKSEA_MULTIYEAR_PHY_007_004). This OMI has been discussed in Mulet et al. (2018).

'''CONTEXT''' The global warming is driving an unprecedented warming in the ocean (Johnson and Lyman, 2020; SROCC, 2019), which makes ocean temperature an important climate indicator. The anomaly of temperature is a key metric used to monitor the warming/cooling trends in the ocean (Casey and Cornillon, 2001). This metric has already revealed a warming of the Black Sea in recent years (Mulet et al., 2018). Analyzing the sea surface temperature, Shapiro et al. (2010) noticed a warming trend in the Black Sea starting from the 1990s. Miladinova et al. (2017) found that the surface layers do not display a trend in temperature while results of temperature at 200 m indicated a positive trend of 0.005°C year−1 over the years 1960–2015. The time evolution of the temperature anomalies also helps to understand the impact of climate change on water masses in the Black Sea such as Stanev et al. (2019), which related the disappearance of the Cold Intermediate Layer to the warmer winters in the last years.

'''CMEMS KEY FINDINGS''' Overall the upper 200 m of the Black Sea have experienced a warming trend superposed on the year-to-year changes leading to anomalous high temperature values exceeding 1.50 °C in 2019. Until 1998, there is a predominance of lower-than-average temperature values down to 200 m, with values below -1.25 °C at the beginning of the time series. In 1999-2009, temperature anomalies show strong year-to-year changes in the upper 200 m depth, fluctuating with negative and positive temperature anomaly values around the reference baseline. Particularly, a replenishment of the cold intermediate layer in 2006, 2012 and 2017 is reported, which is represented by the presence of very low or negative anomalies in the water column between 15 and 100 m depths. These years are considered anomalously cold, so that freezing events were registered during the winter in the northern part of the sea.