Applying CMEMS ocean model data to monitor seasonal variability of temperature and salinity profiles in the central Mediterranean sea

Abstract

Stratification of the Mediterranean Sea can be characterized by vertical temperature and salinity profiles which highlight regions of rapid change with depth as thermocline and halocline layers, respectively. Thermoclines and haloclines have been observed to form seasonally in many regions across the ocean. The specific behavior of these layers can be unique to a given location. Understanding how these layers change over time builds a foundation for studies focusing on localized primary productivity, nutrient distribution in the water column and greater Mediterranean Sea circulation patterns. Extensive temperature and salinity profiling had not yet been completed in the central Mediterranean Sea, therefore little was known about thermocline and halocline development in the regions surrounding the Maltese Archipelago. To address this, the main goal of this study was to apply CMEMS Mediterranean Sea Physics Reanalysis Product data to investigate the seasonal variability of temperature and salinity profiles in the central Mediterranean Sea at 20 locations across two transects, one between Malta and Sicily, and the other extending from Malta towards Africa. The transects cover regions with different physical features and dynamics, such that their results could be compared. The region to the north of Malta lies on a continental shelf, while the region to the south does not. Scripts were written in Matlab to plot the temperature and salinity profiles at each point from 2020 to 2022, and to identify and display the thermocline and halocline layers using the threshold method. The layers were identified using threshold values of 0.2°C/m and 0.02 PSU/m to detect the thermocline and halocline boundaries, respectively. The resulting time series heat maps showed that a seasonal thermocline formed at every point each year in early July and dissipated by December. At the same time of the year, a weak halocline formed at nearly all points, though it was stronger in the lower latitudes. The thickness of both layers changed from the time they developed to when they dissipated each year. The thermocline reached greater depths in the south than in the north. Interestingly, a permanent halocline was observed at depth of ∼150 m in both the north and south regions, but was more stable and thicker in the south where the depth was greater. This result agreed with prevailing Mediterranean Sea circulation theories. It was determined that CMEMS Mediterranean Sea Physics Reanalysis Product data can be applied to monitor variations in temperature and salinity and that seasonal changes are best visualized in the form of a time series heat map. These findings show the manner in which two physically and biologically significant parameters of sea water change seasonally throughout the water column in the central Mediterranean Sea.