Seasonal and Interannual Movement of the Mindanao Current Retroflection at the Pacific Entrance of the Indonesian Seas

Abstract

Abstract The seasonal and interannual variations of the Mindanao Current (MC) retroflection are studied using surface geostrophic currents of satellite altimeters covering January 1993–December 2019. The results show that the Mindanao Current mainstream retroflects back to the Pacific Ocean north of the Talaud Islands in boreal summer and intrudes into the northern Maluku Sea in boreal winter. The variation of the MC retroflection has resulted in the seasonal movement of the sea surface color fronts at the entrance of the Indonesian seas, both of which are highly correlated to the seasonal transport variations of the North Equatorial Countercurrent, lagging the latter due to the westward propagation of the seasonal Rossby waves. The MC retroflection and sea surface color fronts are found to move synchronously on interannual time scales at the Pacific entrance of the Indonesian seas, with the Niño-3.4 index lagging by about 2 months. The MC retroflection intrudes anomalously deeper than the seasonal cycle into the northern Maluku Sea in El Niño winters, while it tends to take a leaping path in La Niña winters. During El Niño summers, the leaping path of the MC is changed into a penetrating path sometimes. Significance Statement The Mindanao Current is the western boundary current (WBC) of the North Pacific Ocean tropical gyre and is much stronger than the ocean interior gyre circulation. This equatorward WBC forces strong exchanges between the equatorial Pacific and the marginal seas as it turns eastward at the entrance of the Indonesian seas. The path of the Mindanao Current retroflection is very important for Rossby wave reflection and for Indo-Pacific interbasin exchange leading to global repercussions, both of which are thought to be controlled by linear dynamics in the past. Here, we disclose the seasonal and interannual movement of the retroflection path, showing strong nonlinear dynamics important for ENSO and interocean exchange.