Ocean Radiant Heating in Climate Models

Carter Ohlmann, Institute for Computational Earth System Science,

University of California, Santa Barbara, CA, 93106 USA

carter@icess.ucsb.edu

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 W m^-2 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 W m^-2 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.