Mediterranean depth-time anomaly map of Sub Surface Temperature

'''DEFINITION''' Mediterranean Sub-Surface Temperature is the temperature of the model water column from surface up to 1000m depth averaged in the whole basin. The annual subsurface temperature anomaly is derived from the Mediterranean Sea regional reanalysis modelling results of the MEDSEA_MULTIYEAR_PHY_006_004 (Escudier et al., 2020) and is evaluated as the average temperature of a given year at a specific depth minus the reference temperature at the same depth, considering as a reference the time-averaged temperature computed from 1993 to 2014 (using the same reanalysis results). A positive anomaly means that the sub-surface temperature is warmer than the reference, whereas a negative anomaly indicates a colder sub-surface temperature with respect to the reference one.

'''CONTEXT''' Subsurface temperature is a key Essential Ocean Variable from which the ocean heat storage and heat transport can be deduced. Large-scale temperature variations in the upper layers are mainly related to the heat exchange with the atmosphere and surrounding oceanic regions, while the deeper ocean temperature in the main thermocline and below varies due to many dynamical forcing mechanisms, including climate change (e.g. Forget and Wunsch 2007; Roemmich et al. 2015; Riser et al. 2016). The Mediterranean Sea is a semi-enclosed sea characterized by an annual average surface temperature which varies horizontally from ~14oC in the North and Western part of the basin to ~23oC in the Sothern and Eastern areas during winter. The basin is characterized by seasonal variations of the upper-ocean temperature stratification with highly stratified waters in the first 50-100 m depth during summer and early fall, while during winter the water column is unstratified due to intense vertical mixing processes. In recent decades the Mediterranean Sea shows a trend of increasing temperatures in the whole water column. This OMI is used to show the temporal and vertical evolution of the temperature anomaly of the basin. The scientific context of this OMI is proposed in Mulet at al. (2018) in CMEMS OSR2 (von Schuckmann et al., 2018).

'''CMEMS KEY FINDINGS''' The Mediterranean Sea reanalysis shows the increase of the Mediterranean Sea sub-surface temperature anomaly ranging from ~-0.65 to ~0.65 oC over two and a half decades. Negative values are found mostly before year 2005 and positive values, appearing initially in 2002-2003, start to continuously increase in time after 2005 with larger values achieved in the last years. In general, larger anomalies are located at surface layers while the lower layers temperature is closer to the reference climatology (1993-2014).