Multi-time scale variability of surface currents in the Sunda Strait observed by high-frequency radar

An in-depth analysis of the variability of surface currents in the Sunda Strait has been undertaken using high-frequency (HF) radar velocity surface currents. On the daily time scale, in the northern part, tidal currents were dominated by K1 constituent, followed by O1 and M2. A counterclockwise rotating tidal current dominated the strait except for the narrowest area and the areas more than 90-m deep, with a magnitude of 0.41 m/s. Seasonally, surface currents are strongly associated with monsoon winds through Ekman’s veering and modulated by vortexes in the southeast Indian Ocean near the Sunda Strait
during northwest monsoon, particularly December-January–February. The strong association is indicated by a moderate to strong absolute complex correlation (0.5–0.8) and angle 60–114° of HF radar-derived current vector counterclockwise to the wind. The circulation is primarily directed from the Java Sea toward the Indian Ocean throughout the year. The area with the highest mean currents is mainly in the center and northeast. The highest and the lowest mean current were 0.68 m/s in July 2020 and 0.25 m/s in April 2021, respectively. Intraseasonally, the currents are influenced by the propagation of
Kelvin waves, which are detected by HF radar along-strait velocity and confirmed by sea level variations (tides observation and SLA). The effect includes an enhancement of magnitude (positive and negative anomalies) and a reversing direction (positive anomalies) in the transition period, usually occurring in November, April, May, and June. The fastest phase speed of positive anomalies is 4 m/s within 16 days of duration