Local Monsoon Drives Seasonal Transition of Equilibrium States in the Mindanao-New Guinea Confluence

Within the Mindanao-New Guinea Confluence, the penetrating and choking states are the predominant dynamic characteristics, and the transition between the two equilibrium states plays an important role in the material transport and energy transfer between the Pacific and Indian Oceans. To uncover the dynamical mechanism underlying the seasonal state transition, we conducted a series of sensitivity experiments with and without the monsoon using a well-established regional ocean model. It is revealed that the seasonal transition of the local monsoon in April and October results in opposite wind stress curl anomalies, altering the relative strengths of the Mindanao Current (MC) and the New Guinea Coastal Current/Undercurrent. This dynamic process, in turn, prompts a meridional shift in the retroflection position of the confluent flow formed by two western boundary currents, as indicated by the sea surface height and relative vorticity fields, and finally leads to a state transition between the penetrating and choking states. Furthermore, the state transition is a primary driver of seasonal variations in the Indonesian Throughflow (ITF) transport, by modulating the leakage and retroflection transports of the MC. In addition, an eddy-shedding state can occur sporadically under stronger or equatorward shifting wind conditions, mainly during the winter and early spring. The eddy-shedding state sometimes appears to overwhelm the seasonal transition between penetrating and choking states. This study provides new insight into the atmospheric effect on a nonlinear dynamic system and suggests a mechanism for the seasonal variation of the upper-layer ITF under the influence of the equilibrium state transition.