Seasonal Variation of Mesoscale Horizontal Stirring in the North Pacific Ocean

The mesoscale horizontal stirring (MHS) is closely linked to various mesoscale dynamical phenomena, encompassing not only eddies but also meanders, filaments, and fronts. A clear understanding of its seasonality holds the potential to enhance our understanding of horizontal mixing and material dispersion. Here, we analyze the seasonal variation of MHS in the North Pacific surface ocean using ocean reanalysis (GLORYS12) current velocity data with a horizontal resolution of 1/12° from 1993 to 2019. Based on the characteristic of stirring to separate adjacent fluid trajectories, MHS is quantified using the finite-size Lyapunov exponent (FSLE), one of the Lagrangian diagnostics. The FSLE in the North Pacific shows clear seasonality but the phases of its evolution differ regionally. We identify two major modes, which contribute to over 80% of the seasonality of FSLE in the North Pacific, through the application of empirical orthogonal function (EOF) analysis to the climatological monthly mean FSLE. The first mode (57%) exhibits a variation peaking in April within the Kuroshio Extension region and the Subtropical Countercurrent region, where baroclinic instability plays a significant role. The second mode (25%) peaks during the summer season over the Kuroshio area and coastal upwelling areas of western North America. It is found that the strong seasonality in the upwelling area is induced by the North Pacific High.