Spiciness and potential-density anomalies as useful variables to assess the contribution of diapycnal and isopynal processes to water mixing

Abstract

Temperature-salinity (T-S) diagrams are widely used to spot the temperature (T) and salinity (S) relations at different places and times. They also help visualize how water masses with different T and S values mix up as they propagate away from the formation regions. Potential density (rho) is typically included in these diagrams to emphasize the vertical stability of a given hydrographic cast or to illustrate how two water masses remain at different density levels. Alternatively, we may introduce a new variable that sets a surrogate orthonormal space, with rho on one axis and spiciness (pi) on the other (Flament, 2002; but see Veronis, 1972, and Jackett and McDougall, 1985). Here we discuss how the rho and pi anomalies in this space may be interpreted as latent diapycnal and epipycnal mixing, or mixing yet feasible to take place, between two water masses. We illustrate these concepts with casts that sample the Mediterranean Outflow (MO) running under the North Atlantic Central Waters (NACW) stratum in the eastern Gulf of Cádiz (Fig. 1)