Spatio-temporal variability of 40-Day Period Deep Currents in the Ulleung Interplain Gap

Abstract

Variability in the deep currents flowing into the Ulleung Basin of the East/Japan Sea (EJS) is a key factor in understanding the meridional overturning circulation and deep water mass transformation in the region. In particular, the Ulleung Interplain Gap (UIG), located between Dokdo and Ulleungdo, serves as a major pathway for deep water transport along the meridional circulation route. The deep current flowing through the UIG exhibits an asymmetric pattern: a northward flow is observed at mooring stations U4 and U5 known as the Dokdo Abyssal Current (DAC), while a southward flow appears at U1 through U3. While the DAC is known to exhibit variability on timescales of 5 to 60 days due to the influence of topographic Rossby waves, the spatial distribution and underlying causes of the prominent 40-day variability across the UIG have not been clearly investigated. Using current meter data from five mooring lines (U1, U2, EC1, U4, and U5) deployed across the UIG from December 2002 to April 2004, Temporal variability in both the upper and deep layers was examined. In the upper layer (200 m), a noticeable enhancement of 40-day current variability was detected at EC1 between September 2003 and February 2004. In the deep layer, enhanced 40-day variability during winter (January–April 2003 and December 2003–March 2004) was observed at four stations (U2, EC1, U4, U5), excluding U1. The increase in variability at EC1 in the upper layer was attributed to the influence of a warm eddy. In the two periods of enhanced deep-layer variability, the first period, the flow exhibited vertically barotropic characteristics, with strong relative vorticity observed at U3 and U4. the second period was marked by an increase in the standard deviation (STD) at U2, which exceeded that at EC1, accompanied by negative relative vorticity between U2 and EC1. These findings suggest the possibility of the existence of a small scale deep circulation within the UIG that are distinct from the general slope-following deep circulation. By analyzing the spatial distribution of the 40-day current variability in the UIG and its relationship with sea level anomaly (SLA) and wind stress variability, this study enhances our understanding of the asymmetry and spatial characteristics of deep currents in the region.