Period-Lengthening of the Mindanao Current Variability From the Long-Term Tide Gauge Sea Level Measurements

Long-term tide gauge sea level data from 1969 to 2014 at Davao (7.08 degrees N, 125.63 degrees E) and Malakal (7. 33 degrees N, 134.46 degrees E) are analyzed to examine the decadal frequency modulations embedded in the Mindanao Current (MC) variability. The MC variability inferred from the Davao-Malakal sea level was predominantly biennial in the 1970s. This prevailing period switched to interannual in the 1980s and lengthened to decadal during the last two decades. With the aid of the basin-scale sea level information from satellite altimeter measurements, it is found that the sea level-inferred MC variability represents the coherent changes of the wind-driven tropical gyre in the western North Pacific. An investigation into the long-term wind stress curl data reveals that its prevailing period underwent similar biennial -> interannual -> decadal transitions in the western tropical Pacific, implying the forced nature of the period-lengthening of the MC variability during the past half-a-century. While the sign of the MC variability is largely determined by the Malakal sea level signals on the interannual and decadal time scales, the Davao sea level change becomes important when the time scale extends to multi-decades. Plain Language Summary Flowing southward along the Philippine coast from 13 degrees N to 5 degrees N, the Mindanao Current (MC) is a western boundary current and serves to close the wind-driven tropical gyre in the North Pacific Ocean. Climatically, the MC plays a critical role by transporting the mid-latitude North Pacific water equatorward with a portion of it entering the eastern equatorial Pacific Ocean and contributing to the upper ocean heat budget in the cold-tongue region. The other portion of the MC permeates into the Eastern Indian Ocean via the Indonesian Throughflow and facilitates the water mass/ heat exchange between the Pacific and Indian Oceans. Despite its importance, observations of the MC transport have been rare and our understanding of its long-term variability is lacking. By utilizing the available tide gauge data at Davao and Malakal across the MC, we explored the MC transport variability of the past half-a-century. It is found that the timescale of the MC transport change has lengthened since 1969. While 2-year-period oscillations dominated in the 1970s, 4 similar to 6-year-period fluctuations took over during the 1980s, and the decadal variability became pre-dominant after 1990s. The cause behind these changes in timescale is due to the similar changes in the tropical Pacific surface wind system that ultimately drives the MC.