A study on low-frequency variability in current and sea level in the Lombok Strait and adjacent region

Current meter data in the mid-Lombok Strait region in 1985 show that 42% of the variance in the currents is within the intraseasonal frequency band (10-100 day-1). It is about two times the variance of the tidal currents. This intraseasonal variability appeared as episodic northward flows of 20 to 80 cm/s recurring every 20 to 60 days. Our data indicate these northward flows are driven by episodes of sea-level rise south of the Lombok Strait which are controlled by large scale westerly wind events in the Southeastern Indian Ocean. These northward flow events occur almost simultaneously with either a cyclone or a typhoon generated in south or north monsoon trough, respectively. The occurrence of these northward flows and cyclone/typhoon events follow 5-20 days after the appearance of an atmospheric pressure trough system over the Eastern Indian Ocean - Western Pacific region. Sea-level data from 1984-1990 in the Eastern Indian Ocean and Western Pacific region show that the intraseasonal variability is a persistent phenomena along the southern coast of the Indonesian Archipelago. The energy of this variability is concentrated at a 40-60 day period band. The magnitude of this variability suggests that the intraseasonal variability can serve as a significant modulator of the Indonesian throughflow. The surface currents in the Lombok Strait reach 150 cm/s in the mid Strait region and 300 cm/s in the sill region. The monthly mean flow varies from 20 cm/s northward into the Flores Sea to 60 cm/s southward into the Indian Ocean. Within the Lombok Strait, the geostrophic approximation computed from cross-strait sea-level differences explains most the sub-inertial along-strait flow less than 60 cm/s. The sea-level difference outside the entrances of the Strait suggests the flow through the Strait is a frictional balance with a Rayleigh friction coefficient estimated 4.6 × 10-5 sec-1.