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  • Author or Editor: J. Carter Ohlmann x
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J. Carter Ohlmann

Abstract

A computationally simple, double exponential, chlorophyll-dependent solar transmission parameterization for ocean general circulation models used in climate studies is presented. The transmission parameterization comes from empirical fits to a set of in-water solar flux profiles calculated with an atmosphere–ocean radiative transfer model system, run with chlorophyll concentration values over the range observed in oligotrophic, open ocean waters. Transmission parameters are available from a lookup table, or can be written as logarithmic and square root functions of chlorophyll concentration, available globally from remotely sensed ocean color data. The rms and maximum errors introduced by curve fitting are less than 3 × 10−3 and 1.5 × 10−2, respectively. Error associated with neglect of second-order cloud and solar zenith angle influences is mostly a few percent. An extension to account for second-order processes in cases where they are large (>10%) is given. The double exponential form enables solar transmission to be resolved at depths beyond 2 m. Only the first exponential term need be considered to accurately determine transmission at depths greater than 8 m. The transmission parameterization is validated with in situ optical and biological data collected in the eastern equatorial Pacific during the Eastern Pacific Investigation of Climate Processes in the Coupled Ocean–Atmosphere System (EPIC) field program, and in the western equatorial Pacific during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The rms (maximum) errors between parameterized transmission and the mean transmission profile computed from in situ values are 0.5 (1.5) and 1.9 (6.6) W m−2, for the eastern and western equatorial Pacific regions, respectively. For comparison, rms (maximum) errors between transmission from a commonly used Jerlov water type–based parameterization and mean measured values are 7.3 (26.7) and 5.0 (8.8) W m−2 for the eastern and western Pacific, respectively (both cases assume a climatological surface flux of 200 W m−2). Proper use of the solar transmission parameterization should increase the accuracy of modeled SST and upper ocean stratification. The parameterization allows ocean radiant heating in climate models to be discussed in terms of chlorophyll concentration, the physical parameter on which solar transmission most heavily depends.

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J. Carter Ohlmann, David A. Siegel, and Catherine Gautier

Abstract

A hybrid parameterization for the determination of in-water solar fluxes is developed and applied to compute the flux of solar radiation that penetrates beyond the upper-ocean mixed layer into permanent pycnocline waters on global space and climatological timescales. The net flux of solar radiation at depth is modeled using values of the solar flux incident at the sea surface, derived from the International Satellite Cloud Climatology Project dataset, and in-water attenuation coefficients, determined using upper ocean chlorophyll concentration supplied by Coastal Zone Color Scanner imagery. Solar radiation penetration can be a significant term (20 W m−2) in the mixed layer heat budget for tropical regions. In mid- and high-latitude regions, the annual solar flux entering permanent pycnocline waters is small (<5 W m−2). However, solar penetration in these regions is important on seasonal timescales since annual cycles in incident solar flux, upper-ocean chlorophyll concentration, and mixed layer depth cause trapping of penetrating solar energy of O(10 W m−2) within the seasonal pyonocline. This trapped thermal energy is unavailable for atmospheric exchange until winter—a period as long as nine months. A nondimensional parameter is introduced that quantifies the fraction of incident solar radiation contributing to mixed layer radiant heating. This parameter can be used to characterize the relative importance of solar penetration to ocean mixed layer thermal climate.

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David A. Siegel, Toby K. Westberry, and J. Carter Ohlmann

Abstract

It is well recognized that clouds regulate the flux of solar radiation reaching the sea surface. Clouds also affect the spectral distribution of incident irradiance. Observations of spectral and total incident solar irradiance made from the western equatorial Pacific Ocean are used to investigate the “color” of clouds and to evaluate its role in upper-ocean radiant heating. Under a cloudy sky, values of the near-ultraviolet to green spectral irradiance are a significantly larger fraction of their clear-sky flux than are corresponding clear-sky fractions calculated for the total solar flux. For example, when the total solar flux is reduced by clouds to one-half of that for a clear sky, the near-ultraviolet spectral flux is only reduced ∼35% from its clear-sky value. An empirical parameterization of the spectral cloud index is developed from field observations and is verified using a plane-parallel, cloudy-sky radiative transfer model. The implications of cloud color on the determination of ocean radiant heating rates and solar radiation transmission are assessed using both model results and field determinations. The radiant heating rate of the upper 10 cm of the ocean (normalized to the climatological incident solar flux) may be reduced by a factor of 2 in the presence of clouds. This occurs because the near-infrared wavelengths of solar radiation, which are preferentially attenuated by clouds, are absorbed within the upper 10 cm or so of the ocean while the near-ultraviolet and blue spectral bands propagate farther within the water column. The transmission of the solar radiative flux to depth is found to increase under a cloudy sky. The results of this study strongly indicate that clouds must be included in the specification of ocean radiant heating rates for air–sea interaction studies.

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