Errors in Current Velocity in the Low-latitude North Pacific: Results from the Regional Ocean Modeling System

Abstract

Using the Regional Ocean Modeling System, this study investigates the simulation uncertainties in the current velocity in the low-latitude North Pacific where the Kuroshio originates [i.e., the beginning of the Kuroshio (BK)]. The results show that the simulation uncertainties largely reflect the contributions of wind stress forcing errors, especially zonal wind stress errors, rather than initial or boundary errors. Using the idea of a nonlinear forcing singular vector, two types of zonal wind stress errors (but sharing one EOF mode) are identified from error samples derived from reanalysis data as having the potential to yield large simulation uncertainties. The type-1 error possesses a pattern with positive anomalies covering the two zonal bands of 0°–15°N and 25°–40°N in the Pacific Ocean, with negative anomalies appearing between these two bands; while the type-2 error is almost opposite to the type-1 error. The simulation uncertainties induced by the type-1 and −2 errors consist of both large-scale circulation errors controlled by a mechanism similar to the Sverdrup relation and mesoscale eddy-like errors generated by baroclinic instability. The type-1 and −2 errors suggest two areas: one is located between the western boundary and the meridional 130°E along 15°–20°N, and the other is located between 140°–150°E and along 15°–20°N. The reduction of errors over these two areas can greatly improve the simulation accuracy of the current velocity at BK. These two areas represent sensitive areas for targeted observations associated with the simulation of the current velocity at BK.