The Role of Tide-induced Vertical Mixing in Modulating the Decadal Change of El Nino

Abstract:

The El Nino-South Oscillation (ENSO) is an important phenomenon occurring every 3-7 years in the tropical Pacific Ocean and plays a role in the regional and global climate changeThe conventional El Nino is characterized by the maximum sea surface temperature (SST) anomalies located at the eastern tropical Pacific. Recent studies have suggested that a different type of El Nino, characterized by a westward shift of the maximum SST anomalies to the central Pacific and thus called central Pacific (CP) El Nino, has occurred more frequently since 1990s and has different weather /climate impacts on the countries surrounding the Pacific However, the mechanism of this decadal change of El Nino is still elusive. In this study, we first report evidence that the tide energy has decreased over the past several decades as observed by the tidal stations at western Pacific, which is consistent with the weakening of the lunar tidal forcing during the same period as determined by the relative position of the moon, the earth and the sun. We then hypothesize that the decrease of tidal energy is responsible for the more frequent occurrence of CP El Nino through the role of tidal mixing in the ocean during the past several decades. The GFDL coupled model is employed to test this hypothesis through sensitivity experiments on the change of tidal mixing. The experimental results as well as the corresponding analysis demonstrate that the weakening of the tidal mixing does lead to the more frequent occurrence of CP El Nino. This finding makes a significant contribution to the understanding of the mechanism of decadal variability of El Nino observed in recent decades and the future projection of the decadal change of El Nino.