A global, 13-year record of photo-synthetically available radiation (PAR) at the ocean surface (9-km resolution) has
been generated from SeaWiFS, MODIS-Aqua, and MODIS-Terra data. The PAR values are essentially obtained by
subtracting from the solar irradiance at the top of the atmosphere (known) the solar energy reflected by the oceanatmosphere
system (satellite-derived) and absorbed by the atmosphere (modeled). Observations by individual
instruments, combinations of two instruments, and three instruments are considered in the calculations. Spatial and
temporal biases between estimates from one, two, or three instruments are determined and corrected, resulting in a
consistent time series for variability studies. Uncertainties are quantified on daily, weekly, and monthly time scales
for the various instrument combinations from comparisons with in situ measurements. The correlative behavior of
PAR, sea surface temperature, and chlorophyll concentration in the Equatorial Pacific is examined. PAR monitoring
will continue with current and future satellite ocean-color sensors, in particular VIIRS, and the methodology will be
extended to generating UV-A and UV-B irradiance.
Numerical experiments conducted with an ocean general ocean circulation model reveal the potential influence of solar radiation absorbed by phytoplankton on the thermal structure and currents of the Tropical Atlantic Ocean. In the model, solar radiation penetration is parameterized explicitly as a function of chlorophyll-a concentration, the major variable affecting water turbidity in the open ocean. Two types of runs are performed, a clear water (control) run with a constant minimum chlorophyll-a concentration of 0.02 mgm<sup>-3</sup>, and a turbid water (chlorophyll) run with space- and time-varying chlorophyll-a concentration from satellite data. The difference between results from the two runs yields the biological effects. In the chlorophyll run, nutrients and biology production are implicitly taken into account, even though biogeochemical processes are not explicitly included, since phytoplankton distribution, prescribed from observations, is the result of those processes. Due to phytoplankton-radiation forcing, the surface temperature is higher by 1-2 K on average annually in the region of the North Equatorial current, the Northern part of the South Equatorial current, and the Caribbean system, and by 3-4 K in the region of the Guinea current. In this region, upwelling is reduced, and heat trapped in the surface layers by phytoplankton is not easily removed. The surface temperature is lower by 1 K in the Northern region of the Benguela current, due to increased upwelling. At depth, the equatorial Atlantic is generally cooler, as well as the eastern part of the tropical basin (excluding the region of the sub-tropical gyres). The North and South equatorial currents, as well as the Equatorial undercurrent, are enhanced by as much as 3-4 cms<sup>-1</sup>, and the circulation of the subtropical gyres is increased. Pole-ward heat transport is slightly reduced North of 35°N, suggesting that phytoplankton, by increasing the horizontal return flow in the subtropical region, may exert a cooling influence on higher latitude regions. The findings indicate that biology-induced buoyancy plays a significant role, in an indirect if not direct way, in the variability of the Tropical Atlantic Ocean, with consequences on atmospheric circulation and climate.
Sensitivity experiments conducted with the MIT ocean general circulation model reveal the potential influence of solar radiation absorbed by phytoplankton on the thermal structure and currents of the equatorial Pacific Ocean. In the model, vertical attenuation of solar radiation is parameterized as a function of chlorophyll pigment concentration, the major variable affecting turbidity in the euphotic zone. To isolate turbidity effects, the model is run from 1948 to 2001 with either a constant minimum pigment concentration of 0.02 mgm^-3 during the entire period or spatially and temporally varying pigment concentration from the Sea-viewing Wide Field-of-view Sensor during 1997-2001. The two model runs are compared for 2001, a relatively normal year following the strong 1997-1998 El Nino and subsequent La Nina. Due to phytoplankton-radiation forcing, equatorial sea surface temperature is decreased by 0.3K on average annually between 100W and 160W, but the negative temperature change is more pronounced in sub-surface layers, reaching -1.5K at 110W. In that region, heat trapping by phytoplankton causes the mixed layer to shallow and isotherms to shoal toward the equator, generating geostrophic currents that enhance the south equatorial current. These surface currents diverge north and south of the equator as they progress westward, creating equatorial divergence, convergence at the level of the equatorial undercurrent, and upwelling, explaining the change in thermal structure. The equatorial undercurrent is strengthened by as much as 4 cms<sup>-1</sup> at its core. The findings support previous results obtained with the MHI Ocean isoPYCnal general circulation model and pigment concentration from the Coastal Zone Color Scanner. They indicate that biology-induced buoyancy my play a significant role in the equatorial Pacific Ocean circulation and suggest the existence of a biophysical feedback mechanism that contributes to maintaining the cold tongue in the eastern equatorial Pacific Ocean, with implications for inter-annual variability associated with El Nino.