Statistical analysis of mesoscale eddy propagation velocity in the South China Sea deep basin

Using mesoscale eddy trajectory product derived from satellite altimetry data from 1993 to 2017, this study analyzes the statistical characteristics of spatiotemporal distribution of mesoscale eddy propagation velocities (C) in the South China Sea (SCS) deep basin with depths >1 000 m. Climatologically, the zonal propagation velocities (c(x)) are westwards in the whole basin, and the meridional velocities (c(y)) are southwards in the northwestern basin, and northwards in the southeastern basin. The variation of c(y) with longitude is consistent with that of the background meridional currents with correlation coefficient R-2 of 0.96, while the variation of c(x) is related both to the background zonal currents and beta effect. The propagation velocities characterize significant seasonality with the minimum magnitude occurring in summer and the maximum in winter for c(x) and C. Interannually, larger values of c(x) and c(y) mostly occurred in La Nina years in the negative phase of the Pacific Decadal Oscillation (PDO). Mesoscale eddies move fast at the beginning and end of their life span, i.e., at their growth and dissipation periods, and slowly during their stable "midlife" period. This trend is negatively correlated with the rotating tangential velocity with R-2 of -0.93. Eddies with extreme propagation velocities are defined, which are slower (faster) than 1.5 cm/s (15.4 cm/s) and take 1.5% (1.9%) of the total eddies. The extremely slow-moving (fast-moving) eddies tend to appear in the middle (on the edge) of the basin, and mostly occur in summer (winter). The mechanism analysis reveals that the spatiotemporal distributions of the propagation velocities of mesoscale eddies in the SCS are modulated by the basin-scale background circulation.