Variability of chlorophyll associated with El Niño–Southern Oscillation and its possible biological feedback in the equatorial Pacific

The relationship between oceanic phytoplankton and climate variability has been given increasing attention with the accumulation of satellite-derived chlorophyll data over the past decade. Here we examine the dominant variability of phytoplankton and its associated tropical climate systems; in particular, the El Nino-Southern Oscillation (ENSO). The analysis, using 148 months 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. It is also found that when El Nino events occur, the reduced surface solar radiation (enhanced convective activity), as well as the insufficient nutrient supply (suppressed equatorial upwelling), can also play a significant role in reducing chlorophyll concentration. The effect of reduced surface solar radiation on chlorophyll is larger in the central Pacific than in the eastern and western Pacific, and this regional difference of the impact induces a distinctly asymmetric response of ocean chlorophyll to El Nino and La Nina in the central Pacific. A linear statistical analysis shows that the dominant variability of chlorophyll associated with ENSO contributes radiant feedback to the equatorial Pacific by altering the surface shortwave albedo. The decreased chlorophyll concentration during El Nino tends to induce radiant cooling at the ocean surface.