Variability of Ocean Radiant Heating in the Eastern Tropical Pacific

Carter Ohlmann, David Siegel, Institute for Computational Earth System Science,

University of California, Santa Barbara, CA, 93106 USA

carter@icess.ucsb.edu, davey@icess.ucsb.edu
Scott Doney, WHOI, Woods Hole, MA, 02543 USA

sdoney@whoi.edu
Chuck McClain, Goddard Space Flight Center, NASA, Greenbelt, MD, 20771 USA
chuck@seawifs.gsfc.nasa.gov

The role of solar transmission and in-water solar flux variations on ocean radiant heating rates in the eastern tropical Pacific Ocean are assessed using in-situ data, satellite observations, and numerical model results. Data collected aboard the R/V Ron Brown at the 10° N, 95° W EPIC site from 12 September to 4 October 2001 indicate that mixed layer chlorophyll concentration varied from less than 0.15 to more than 0.35 mg m^-3 and solar transmission at 20 m ranged from more than 0.18 (18% of the solar energy reaching the ocean surface) to less than 0.09 on daily time scales. The cruise mean solar transmission profile computed as an exponential fit to the 143 observed profiles is TR=0.525exp(-0.075z) where z is depth. The observed transmission is generally between 50 and 100% greater than transmission values computed assuming Jerlov type II water, the characteristic Jerlov water-type for the region. The transmission difference corresponds to an absolute solar flux value near 20 W m^-2, and a daily heating rate discrepancy of more than 0.04 C day^-1 for the 20 m layer. Observed radiant heating rate variations were smaller during the observation period due to reduced incident solar flux values compared with climatology. During one particularly rainy period the daily average solar flux incident at the sea surface was less than 10 W m^-2, and daily mean values were often less than 50 W m^-2.

In-water solar flux values that penetrate beyond the upper-ocean mixed layer are calculated along 95° W for a 5-year period using a recently developed parameterization (Ohlmann 2003). The solar flux parameterization is forced with remotely sensed incident solar flux and upper layer chlorophyll distributions from the Sea-viewing Wide-Field of view Sensor (SeaWiFS) ocean color satellite. Daily mean incident fluxes vary by more than a 100 W m^-2 at the EPIC site (10° N, 95° W) mostly on seasonal time scales. Upper ocean chlorophyll concentrations vary by more than a factor of 10, mostly from reduced biomass associated with the 1997/98 El Niño event. The solar flux at 20 m ranges from less than 10 to more than 50 W m^-2 over the multi-year period. Mixed layer radiant heating rates (where mixed layer depth is from the NCAR Community Ocean Model) vary from 0.1 to more than 0.6 C per day.