The Solar Radiation Environment in the Eastern Tropical North Pacific Ocean
Solar radiation measurements were made in the DOMES area of the eastern tropical Pacific Ocean, between 5° and 18° North and 126° and 151° West, during the late summer 1975 and winter of 1976, aboard the NOAA ship OSS OCEANOGRAPHER. These were of the net photon irradiance above the surface, and the downward photon irradiance to a depth of 150 m. Measurements were of the broad waveband between 400 and 700 nm, i. e., the photosynthetically active radiation.
Results show an average net irradiation of 39 (E/m2-day) during the summer, and a 16% greater amount in winter, i. e, 45 (E/m2-day), due to reduced cloudiness. During the same interval, biological primary productivity increased by 20%. Clouds were responsible also for: the unequal partitioning of radiation with respect to local apparent noon; a daily photon north-south minimum at 12°N; and an east-west maximum of daily averages in the central part of the region during both seasons.
Submarine measurements show the water to be a clean open-ocean type with marked, alternating, horizontal stratifications, fewer in number during the winter. Characteristic features include: low transmittance in a shallow layer near the surface; an increase of transmittance in the layer below to the mixed layer depth, MLD; a decrease of transmittance in the stable upper thermocline; and a general increase below. Several deviations existed. Diagnoses indicate clearer water toward the north, and toward the west away from Central America, in general. The MLD also increased toward the west. Axes of maximum values of phytoplankton parameters were generally found between the levels of 1 to 10% of near-surface values, and coincided with minimum-transmittance layers. In the summer, these biological maxima were always below the MLD (the 10–25% light level); in the winter they were above and below the deeper MLD (the 1–10% light level). In the latter instance, they could be seen in the photon signature only in the eastern transect. With the deeper winter MLD, the near-surface transmittance was higher than with the summer situation. This was not the case near 126°N.
KeywordsVertical Distribution Photosynthetically Active Radiation Late Winter Turbid Water Euphotic Zone
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