Despite observations for decadal and long-term changes in the Equatorial Pacific Climate Mean State and El Niño Southern Oscillation (ENSO) behaviors over the past century, the drivers for these changes are debated. Recent studies suggest that the connections between the subtropics and the tropics through the western boundary route of the South Pacific likely play an important role for these changes. Using nitrogen isotopes recorded by porites coral skeleton obtained from the Solomon Island in the Western Equatorial South Pacific, we find that ¹⁵N/¹⁴N variabilities in the upper Solomon Sea are primarily influenced by (1) changes in the equatorial upwelling intensity; and (2) strength of the equatorward flow through the Solomon Sea. Both of these processes are closely related to changes in the Pacific Mean State. The ¹⁵N/¹⁴N decreases during basin-wide ENSO events between 1919 and 2012, consistent with reduced upwelling in the central and eastern equatorial Pacific, as well as coincidental increases in the equatorward transport of the subtropical water masses during ENSO events. The decadal variabilities in the ¹⁵N/¹⁴N are strongly correlated with Pacific Decadal Oscillation (PDO), with a clear phase shift occurred at early 1980s when ¹⁵N/¹⁴N showed a step-like decrease following the observed weakening in the trade wind and warming in the topical Pacific. This is in line with theoretical works suggesting extratropical controls on the decadal changes in the tropical climate. Finally, the interannual and decadal changes in the ¹⁵N/¹⁴N are likely superimposed on a long-term declining trend, associated with the observed weakening of the walker circulation in the equatorial Pacific. Longer record is called for to validate this trend and evaluate whether this is related to the anthropogenic warming.