Vertical structure of North Pacific mode waters

Abstract This study examined the vertical structure of mode waters in the North Pacific using Argo data. Thick low-potential vorticity (Q) pycnostads were detected. These contain well-known water masses such as Subtropical Mode Water (STMW), Eastern STMW (ESTMW), Central Mode Water (CMW), and Transition Region Mode Water (TRMW). To examine differences in the vertical structures of these mode waters, a distribution area was defined for each. Near the Q-minimum core in the STMW area, vertical gradients of both potential temperature (θ ) and salinity (S) were shown to be very small, indicating high vertical homogeneity. Conversely, the vertical gradients of both θ and S are relatively large near the core of pycnostads in the CMW, TRMW, and ESTMW areas, indicating that density-compensating stratification of θ and S occurs within these mode waters. Histograms of θ and S anomalies relative to the core properties inside pycnostads in the STMW area show sharp peaks in θ and S at the core, demonstrating high homogeneity throughout the pycnostad of STMW. However, such histograms for CMW show a flat distribution of water properties inside the pycnostads, indicating relative inhomogeneity of this mode water. Histograms in the ESTMW (TRMW) area show sharp peaks at the core, as was the case for STMW, but are negatively (positively) skewed, suggesting relative inhomogeneity only at the colder (warmer) and fresher (saltier) side of pycnostads. Because these waters are to some extent homogeneous with respect to their potential density, their spiciness is high, implying that they mix with surrounding cold (warm) and fresh (saline) water by double diffusive convection. The Turner angle (Tu) was calculated to examine the potential of double diffusive convection. Tu values near the core of pycnostads in the STMW and CMW areas are 50°–60° and 60°–70°, respectively, indicating that salt fingering is weakly possible in both STMW and CMW. In the TRMW (ESTMW) area, pycnostads show Tu values greater than 70° on the upper (lower) side of the core, indicating that the water properties of TRMW (ESTMW) are probably modified by strong salt fingering with warm (cold) and saline (fresh) water on the upper (lower) side of the pycnostads. These results support an idea previously proposed based on an analysis of climatological data: each mode water has its own vertical structure, presumably owing to differences in its formation and/or modification mechanisms. Density compensating θ and S gradients and the resulting salt-finger-type convection are suggested to be important in mode water processes from their formation through their decay.