Biophysical responses near equatorial islands in the Western Pacific Ocean during El Ni?o/La Ni?a transitions

摘要

The biological response in the western equatorial Pacific Ocean during El Ni?o/La Ni?a transitions and the underlying physical mechanisms were investigated. A chlorophyll a bloom was observed near the Gilbert Islands during the 2010 El Ni?o/La Ni?a transition, whereas no bloom was observed during the 2007 El Ni?o/La Ni?a transition. Compared to the previously observed bloom during the 1998 El Ni?o/La Ni?a 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 Ni?a 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 Ni?o 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 Ni?a, the 2009-2010 El Ni?o prompted a deeper thermocline and thicker barrier layer than the 1997-1998 El Ni?o that hampered the efficiency of the vertical processes in supplying nutrients from the thermocline to the euphotic zone, resulting in a weaker bloom. Key Points Chl a blooms near the Gilbert Islands during some El Nino/La Nina transitions Distinct physical conditions are necessary for blooms to occur in this region Ocean preconditioning from El Nino and wind forcing from La Nina control blooms