Heating Rate within the Upper Ocean in Relation to its Bio–optical State

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  • 1 Laboratoire de Physique et Chimie Marines, Université Pierre et Marie Curie et CNRS, Villefranche sur Mer, France
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Abstract

Solar radiation absorption and local heating within the upper layers of the open ocean are strongly influenced by the abundance of phytoplankton as depicted by the chlorophyll concentration. According to whether this concentration is high or low, the heat deposition occurs within a layer that may vary in thickness from low than 10 m to more than 100 m. A simple parameterization, accounting for this dependence, is developed. It allows the vertical profiles of heating rate to be predicted from the phytoplanktonic pigment concentration, as it can (and will) be remotely detected from space, by using ocean color sensors. This computationally efficient parameterization has been validated in reference to the results of a full spectral model. In the simplified computation, the solar spectrum is partitioned into two domains, below and above the wavelength 0.75 µm. For the infrared waveband, not influenced by biological materials the irradiance profile is described by a single exponential function. For the ultraviolet and visible (<0.75 µm) band, a bimodal exponential form is adopted. The weights associated with each of these exponential functions, as well as their specific attenuation lengths, are dependent upon pigment concentration. These dependences are explicated through polynomial formulas. The remotely sensed pigment values can thus be readily introduced in numerical models of the mixed layer and of regional upper ocean dynamics or general circulation.

Abstract

Solar radiation absorption and local heating within the upper layers of the open ocean are strongly influenced by the abundance of phytoplankton as depicted by the chlorophyll concentration. According to whether this concentration is high or low, the heat deposition occurs within a layer that may vary in thickness from low than 10 m to more than 100 m. A simple parameterization, accounting for this dependence, is developed. It allows the vertical profiles of heating rate to be predicted from the phytoplanktonic pigment concentration, as it can (and will) be remotely detected from space, by using ocean color sensors. This computationally efficient parameterization has been validated in reference to the results of a full spectral model. In the simplified computation, the solar spectrum is partitioned into two domains, below and above the wavelength 0.75 µm. For the infrared waveband, not influenced by biological materials the irradiance profile is described by a single exponential function. For the ultraviolet and visible (<0.75 µm) band, a bimodal exponential form is adopted. The weights associated with each of these exponential functions, as well as their specific attenuation lengths, are dependent upon pigment concentration. These dependences are explicated through polynomial formulas. The remotely sensed pigment values can thus be readily introduced in numerical models of the mixed layer and of regional upper ocean dynamics or general circulation.

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