Global distribution and variability of subsurface chlorophyll a concentration.

Chlorophyll a (Chl-푎) often retains its maximum concentration not at the surface but in the subsurface layer. The depth of the Chl-푎 maximum primarily depends on the balance between light penetration from the surface and nutrient supply from the deep ocean. However, a global map of subsurface Chl-푎 concentrations based on observations has not been presented yet. In this study, we integrate Chl-푎 concentration data not only from recent biogeochemical floats but also from historical ship-based and other observations, and present global maps of subsurface Chl-푎 concentration with related variables. The subsurface Chl-푎 maximum deeper than the mixed layer depth was stably observed in the subtropics and tropics (30°S to 30°N), only in summer in midlatitudes (30–40°N/S), and rarely at 45–60°S of the Southern Ocean and in the northern North Atlantic (north of 45°N). The depths of the subsurface Chl-푎 maxima are deeper than those of the euphotic layer in the subtropics and shallower in the tropics and midlatitudes. In the subtropics, seasonal oxygen increases below the mixed layer implied substantial biological new production, which corresponds to 10% of the net primary production there. During El Niño, the subsurface Chl-푎 concentration in the equatorial Pacific is higher in the middle to the east and lower in the west than that during La Niña, which is opposite that on the surface. The spatiotemporal variability of the Chl-a concentration described here would be suggestive results not only for the biogeochemical cycle in the ocean but also for the thermal structure and the dynamics of the ocean via the absorption of shortwave radiation. [ABSTRACT FROM AUTHOR]