AbstractThe biological response in the western equatorial Pacific Ocean during El Niño/La Niña transitions and the underlying physical mechanisms were investigated. A chlorophyll a bloom was observed near the Gilbert Islands during the 2010 El Niño/La Niña transition, whereas no bloom was observed during the 2007 El Niño/La Niña transition. Compared to the previously observed bloom during the 1998 El Niño/La Niña transition, the 2010 bloom was weaker, lagged by 1–2 months, and was displaced eastward by ~200 km. Analysis suggested that the occurrence, magnitude, timing, and spatial pattern of the blooms were controlled by two factors: easterly winds in the western equatorial Pacific during the transition to La Niña and the associated island mass effect that enhanced vertical processes (upwelling and vertical mixing), and the preconditioning of the thermocline depth and barrier layer thickness by the preceding El Niño that regulated the efficiency of the vertical processes. Despite the similar strength of easterly winds in the western equatorial Pacific during the 1998 and 2010 transitions to La Niña, the 2009–2010 El Niño prompted a deeper thermocline and thicker barrier layer than the 1997–1998 El Niño that hampered the efficiency of the vertical processes in supplying nutrients from the thermocline to the euphotic zone, resulting in a weaker bloom.
