Mirror patterns of physical variables in the ocean

Temperature and salinity are independent thermodynamic variables of seawater. However, it was determined from monthly Argo data that the correlation coefficient between layer-averaged temperature and salinity (T–S) time series peaks at a depth of approximately 300 m. Meanwhile, T–S patterns around that depth are consistent. Therefore, this layer is named “T–S mirror layer”. Since the major feature of the T–S mirror layer is the high similarity of temperature and salinity patterns in it, spatial correlation should be a more direct way to determine it. Following this idea we introduce another kind of “T–S mirror layer”, which resides at 100–200 m in four sets of objectively analyzed ocean data products (WOA13, EN4, Ishii and Argo). The “T–S mirror layer” derived in this way couples with the quasi-linear “slender waist” in T–S scatter diagrams; and this coupling is typical in mid- and low-latitude oceans. The slender waist in T–S scatter diagram implies that the profiles of temperature and salinity are in phase around it. We can then give a first guess for the depth of mirror patterns of two variables just according to their local profiles. Hereby, we find mirror patterns between other variable pair (e.g., temperature and sound velocity), inverse mirror patterns (i.e. the highest spatial correlation between variable pair, e.g., temperature and density, are negative), and malposed mirror patterns (spatial correlation peaks for variable patterns in different layers). This work reveals a phenomenon that bridges horizontal patterns of variable pairs in the ocean to their vertical profiles.