Biological heating in the equatorial Pacific: observed variability and potential for real-time calculation

Changes in phytoplankton concentration, mixed layer depth, and incident radiation strongly modify the upper- ocean heat budget. An extreme example occurred during the 1997/98 El Niño–La Niña. In the central equatorial Pacific, biological heating of the mixed layer increased from ∼0.1°C month⁻¹ in December 1997 (El Niño) to ∼1.0°C month⁻¹ in July 1998 (La Niña). This change was due to 1) shoaling of the mixed layer from ∼100 to ∼20 m (∼56% of the 0.9°C month⁻¹ increase); 2) a twentyfold increase in surface chlorophyll concentrations (∼29% of the increase), coincident with a shoaling of the subsurface chlorophyll maximum from ∼100 to ∼50 m; and 3) an increase in incident shortwave radiation from ∼175 to 275 W m⁻² (∼15% of the increase). The observed range of heating rates (0.1°–1.0°C month⁻¹) corresponds closely to the mean condition of the western (oligotrophic) and eastern (mesotrophic) equatorial Pacific, respectively. Increased phytoplankton concentrations act to retain heat near the surface and should result in shallower mixed layer depths. The influence of decadal changes in chlorophyll concentrations on heat storage was also quantified. The observed chlorophyll variability leads to interannual changes in penetrative heat flux (E_d,SW,PEN, the irradiance flux out of the bottom of the mixed layer) of the order of 5 W m⁻², or from 65% to 170% of the mean. This variability is significant when compared with recent work that describes couplings between tropical and global ocean temperature dynamics. The analyses presented here show that satellite and buoy data can be used to accurately and simply estimate the biological contribution to heating for basin-scale studies, and possibly for future improvement of ocean circulation models.