Numerical study of the dynamics of the Ryukyu Current system

Abstract

Recent observations indicate that the Ryukyu Current system, part of the western boundary current of the North Pacific subtropical gyre on the eastern slope of the Ryukyu Islands, Japan, exhibits bottom intensification to the north of Okinawa Island and a nearly barotropic current to the south. We performed numerical experiments of this system using two primitive equation models that incorporate realistic and idealized topography, respectively, to identify the processes that act to maintain the Ryukyu Current system. The dynamics responsible for baroclinic structures of the Ryukyu Current system are examined in detail by diagnosing numerical solutions based on a characteristic equation derived from two‐layer planetary geostrophic equations. The diagnoses show that bottom intensification of the Ryukyu Current system is primarily formed along the characteristics due to the first baroclinic mode topographic Rossby wave that emanates from the Kuroshio in the Tokara Strait. This topographic wave depresses the main thermocline along the inshore slope area east of the Ryukyu Islands such that the passage of the wave over the barotropic northward flow along the slope establishes a state of no motion in the upper layer and bottom intensification of the northward current in the lower layer. The diagnoses also suggest that the characteristics that result from this topographic wave are expelled in the slope area south of Okinawa Island by the first baroclinic mode planetary Rossby wave that propagates westward from the North Pacific interior region, resulting in bottom intensification only in the northern slope area.