Low-Frequency Variability of the Western Boundary Currents at the Gappy Western Pacific Using a Shallow-Water Equation Model : Basic Bifurcation Diagram

Abstract:

For a long time, observations have indicated that the Kuroshio displays multiple steady states (loop, leap or direct intrusion) at Luzon Strait. Similarly, Low-latitude western boundary currents of the Pacific - Mindanao Current and South Equatorial Current also show some bimodal behavior (intrusion or choke) east of the Sulawesi Sea. It is still not well known whether these low frequency variabilities arise from external forcing or internal mechanism. In this paper we explore how temporal variability arises through successive bifurcations of the wind-driven circulation as lateral friction is decreased in a 1.5-layer shallow-water equation models with real continental geometry boundary of the western Pacific. Multiple equilibria of the intrusion of the western boundary currents are found. The origin of these equilibria is investigated by determining the structure of steady solutions by using continuation techniques. In both single-gyre model (analog to the Kuroshio at Luzon Strait)and double-gyre model (analog to the Mindanao Current and South equatorial Current at the source region of the Indonesian Throughflow), the western boundary currents become unstable as the lateral friction decreases. These solutions subsequently destabilize to two sets of barotropic oscillatory modes, introducing an annual and monthly time scale.