Effect of Mesoscale Eddies on Subtropical Mode Water Variability from the Kuroshio Extension System Study (KESS)

Abstract

Abstract Forty-eight profiling floats have been deployed in the Kuroshio Extension (KE) region since May 2004 as part of the Kuroshio Extension System Study (KESS) project. By combining the float temperature–salinity measurements with satellite altimetry data, this study investigates the role played by mesoscale eddies in controlling the property changes in North Pacific Subtropical Mode Water (STMW). Following a 3-yr period of low eddy activity in 2002–04, the KE showed a transition to a high eddy kinetic energy state in 2005. This transition is the result of delayed oceanic response to the 2002 shift in the basin-scale surface wind forcing in connection with the Pacific decadal oscillation. The high eddy kinetic energy state of the KE is characterized by successive shedding of strong cold-core rings into the recirculation gyre, resulting from the interaction of the KE jet with the Shatsky Rise or the preexisting cutoff rings. By transporting northern-origin, high-potential-vorticity (PV) KE water into the recirculation gyre, the enhanced eddy activity affects STMW in two ways: first, it hinders the formation of deep winter mixed layer (hence the source for STMW) by modifying the upper-ocean stratification and, second, it provides a direct high-PV source to mix with the surrounding low-PV STMW. The eddies’ influence upon STMW is observed to be both significant in magnitude and efficient in time. Relative to 2004, the PV signal in the core of STMW was reduced by one-half in 2005, and this weakening of STMW’s intensity occurred within a period of less than 7 months. This result supports recent findings by the authors based on historical temperature data that the variability in STMW formation depends more sensitively on the dynamic state of the KE than on the overlying atmospheric conditions.