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

Abstract

AbstractLong‐term tide gauge sea level data from 1969 to 2014 at Davao (7.08°N, 125.63°E) and Malakal (7. 33°N, 134.46°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.