Spatial Distribution of 40-Day Period Deep Currents in the Ulleung Interplain Gap

The variability of deep currents flowing into the Ulleung Basin through the Ulleung Interplain Gap, located between Ulleungdo and Dokdo, is critical for understanding the meridional circulation of the East Sea and the changes in the deep waters of the Ulleung Basin. The deep currents in this region exhibit asymmetry due to the influence of the Dokdo Abyssal Current and are known to show variations with periods of 5–40 days, attributed to topographic Rossby waves. However, studies on the spatial distribution and causes of the 40-day period variability within the Ulleung Interplain Gap remain unknown. This study aims to address this gap. Deep currents were observed using five mooring lines (U1, U2, U3 or EC1, U4, U5) deployed across the Ulleung Interplain Gap from 2002 to 2004. In the upper layer (200 m), an increase in the 40-day period current variability was observed at U3 between October 2003 and March 2004. In the deep layer (from 1000 m to 2240 m), enhanced 40-day period variability of current occurred during the winter seasons (January–April 2003 and December 2003–March 2004) at four stations (U2, U3, U4, and U5), excluding U1. The first major fluctuation was observed at U3 and U5, while the second was observed at U2 and U4, showing increased amplitudes. This suggests that the cyclonic/anti-cyclonic deep currents within the Ulleung Interplain Gap are associated with changes in their radius or location. Furthermore, these cyclonic/anti-cyclonic deep currents exhibit characteristics distinct from the patterns reported in previous studies, which described deep currents flowing along the continental slope around the Ulleung Basin. This study aims to define the 40-day period deep current patterns and identify their causes by analyzing their relationship with variability in the upper layers.