Variability of chlorophyll associated with ENSO and its biological feedback in the Equatorial Pacific

The relationship between oceanic phytoplankton and climate variability has attracted increasing attention with the accumulation of satellite-derived chlorophyll data over a decade. Here, we examine the dominant variability of phytoplankton and its associated tropical climate variability, in particular, El Niño and Southern Oscillation (ENSO). The analysis using 148-month length of chlorophyll data reveals that the first two leading modes of tropical chlorophyll anomalies are linked to the mature phase and the decaying phase of the ENSO cycle. When El Niño events occur, both effects of the insufficient nutrient supply (suppressed equatorial upwelling) and the reduced surface solar radiation (enhanced convective activity) can cause the decrease of chlorophyll. However, the effect of reduced surface solar radiation on the chlorophyll concentration becomes stronger in the western Pacific than in the eastern Pacific. This regional difference induces the more asymmetric response of ocean chlorophyll to the El Niño and La Niña in the western Pacific. A linear statistical analysis shows that the dominant variability of chlorophyll associated with ENSO gives a radiant feedback to the equatorial Pacific by altering the surface shortwave albedo. The decreased chlorophyll concentration during El Niño tends to induce the radiant cooling at the surface.