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Ocean Radiant Heating in the Eastern Equatorial Pacific
A proposal to be submitted to NSF as part of the EPIC2001 program
Carter Ohlmann
Institute for Computational Earth System Science
University of California, Santa Barbara
Santa Barbara, CA 93106-3060
(858) 534-6027
cohlmann@ucsd.edu
Dave Siegel
Institute for Computational Earth System Science
University of California, Santa Barbara
Santa Barbara, CA 93106-3060
(805) 893-4547
davey@icess.ucsb.edu
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Results from Prior NSF Support
Drs. Ohlmann and Siegel have recently received NSF funding to study “Radiant
heating and water mass variability in the Western Pacific Warm Pool” (OCE-91-10556;
Siegel and Washburn PI’s) and “Quantification and parameterization of solar radiation
penetration and heating in the Pacific Warm Pool” (OCE-95-25856; Siegel and
Washburn PI’s) as part of the Tropical Ocean Global Atmosphere – Coupled Ocean
Atmosphere Response Experiment (TOGA/COARE). A primary goal of TOGA/COARE
is an improved understanding of processes that regulate heat exchange between the ocean
and atmosphere (Webster and Lukas 1992). Our programs were funded to quantify the
flux of solar radiation at the base of the upper ocean mixed layer in the western equatorial
Pacific, to examine the role of this solar penetration on upper ocean temperature and
stratification, and to develop the first physically and biologically based solar transmission
parameterizations for use in coupled ocean-atmosphere model systems.
During the TOGA/COARE field program we measured over 3000 vertical profiles of
spectral irradiance, bio-optical, CTD, and surface meteorological variables. These data
allow the direct determination of solar transmission (the fraction of incident surface
irradiance at depth) for depths beyond ~10 meters, and allow for investigation into causes
of solar transmission variability. The results of our research indicate that solar radiation
penetration beyond the base of the mixed layer is a major component of the Warm Pool
heat budget (Siegel el al. 1995), and is highly variable on synoptic time scales (Ohlmann
et al. 1998). The TOGA/COARE optics data has been used to aid development of a
chlorophyll-based solar transmission model, a significant improvement over Jerlov water
type models (Ohlmann et al. 1996). To investigate near-surface (< 10 m) transmission
we have performed a full-spectral radiative transfer modeling study that shows clouds,
solar zenith angle, and chlorophyll can all significantly influences solar transmission
within the top few meters of the ocean (Ohlmann et al. 2000). Subsequent work focuses
on the specific role of clouds (Siegel et al. 1999) and presents a physically and
biologically based full-spectral parameterization that resolves solar transmission at all
depths (Ohlmann and Siegel 2000). Numerous researchers have relied on our solar
transmission results for their work (e.g. Anderson et al. 1996, Wijesekera and Gregg
1996, Cronin and McPhaden 1997, and others). All our data is available via the internet.
Research Objectives
Radiant heating is a primary term in the upper ocean heat budget for equatorial
regions. In order to accurately determine ocean radiant heating rates, both the incident
solar radiation at the sea-surface and the manner in which the solar energy diverges with
depth must be known. Great efforts are being undertaken to determine accurate surface
heat flux values during EPIC2001. In addition, the understanding of processes that cause
variations in solar transmission through the upper ocean has recently improved.
However, our knowledge of how variations in solar transmission influence upper ocean
stratification and SST, and how changes in stratification can feedback on solar
transmission and SST, is quite limited. This is because changes in radiation-absorbing
particles (primarily phytoplankton biomass in the open ocean) are not usually observed in
air-sea interaction studies and cannot be accurately modeled. We thus propose a multiscale approach combining in-situ and satellite data sets to investigate variations in solar
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transmission, their causes, and their influence on upper ocean evolution during
EPIC2001. Our research objectives are:
 make in-situ measurements (profiles) of spectral irradiance for direct calculation of
solar transmission on mixed layer depth scales.
 determine solar transmission within the top 10 meters of the ocean using an array of
near-surface temperature measurements and inverse modeling.
 use radiation profiles to validate both solar transmission models and remotely sensed
ocean color data from SeaWiFS and MODIS.
 employ solar transmission models and remotely sensed chlorophyll data to determine
spatial fields of solar transmission with error bars.
These objectives lead to the ultimate goal of improving coupled ocean-atmosphere
models through a better understanding of how solar transmission variations influence
upper ocean evolution, and how ocean models should resolve these variations.
Proposed Work
1. Basic data
Our proposed work requires direct measurement of in-water spectral
irradiance/radiance profiles and upper ocean bio-optical properties. We will collect
profiles of downwelling irradiance and upwelling radiance in 11 spectral bands (340 to
685 nm) using a Satlantic SeaWiFS Profiling Multichannel Radiometer (SPMR). The
SPMR is a long (122 cm) slender (9 cm in diameter) hand-deployed freefalling
instrument (retrieved using a small winch) that eludes ship motion and shadow.
Measurements will be taken from the R/V Brown. Coincident measurements of
downwelling spectral irradiance incident at the surface will be made using an identical
Satlantic radiometer mounted to the mast of the R/V Brown. Water samples will be
collected at 10 discrete depths within the upper 200 meters using the R/V Brown’s
CTD/Rosette system.
2. Sampling scheme
The proposed sampling includes a component at the 10N, 95W ITCZ station, and a
component during the transects along 95W. While stationed at 10N, 95W, an optics
profile to 100 meters will be made every hour during the daytime. This sampling
frequency will enable transmission variations due to changes in chlorophyll biomass,
cloud amount, and solar zenith angle to be resolved. Water samples will be collected
near local noon daily and analyzed for chlorophyll and nutrient concentrations to monitor
synoptic scale evolution and to validate satellite data. To directly monitor diel variations
in biomass, we will collect water samples every 4-hours only in the first and last 24-hour
periods on station to avoid a major disruption of other time-series measurements. During
the transects along 95W a CTD/Rosette cast to 200 m and an optics cast to 100 m
(daytime stations) will be made every 1/2 degree from 10N to 10 S. Stations will be
made at each of the TOA moorings. The transect sampling will allow upper ocean
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optical properties to be examined in and out of the cold tongue region, provide
supplemental information at each of the TAO moorings, and should insure data collection
under clear skies (outside the ITCZ). Most importantly the spatial data will help place
error bars on remotely sensed chlorophyll data required by ocean models to accurately
parameterize solar transmission.
3. Data analysis
The surface and in-water irradiance data will be combined for direct calculation of
solar transmission profiles on mixed layer depth scales (> ~10 m). Accurate solar
transmission profiles will be coincident with surface heat flux data recorded aboard the
R/V Brown (by Chris Fairall, NOAA-ETL) and at the TAO moorings (by Meghan
Cronin, NOAA-PMEL). Solar insolation combined with transmission profiles gives inwater solar flux profiles. Knowledge of in-water solar fluxes is necessary information for
EPIC2001 investigators examining mixed layer heat budgets. In the western Pacific
Warm Pool, solar penetration is the second largest term in the upper ocean heat budget
(41 W m-2; 75% of the net surface flux; Wijesekera and Gregg 1996). We believe that
solar penetration is even more important in the eastern Pacific due to the shallow mixed
layer and thermocline there. A shallower mixed layer necessarily causes an increased
solar flux at its base (for a constant chlorophyll value).
Figure 1. Daily averaged solar transmission calculated from in-situ solar flux data
recorded in the western equatorial Pacific during TOGA-COARE.
In the western equatorial Pacific, solar penetration is highly variable on synoptic time
scales (Figure 1). We believe that synoptic variations are even greater in the eastern
equatorial Pacific. Climatological monthly mixed layer depths at 10N, 95W computed
with T-S profiles from the NOAA World Ocean Atlas (1994) following Sprintall and
Tomczak (1992) range from 16 m for February to only 9 m for September. Chlorophyll
concentrations at 10N, 95W from SeaWiFS vary from ~0.03 to 0.12 mg m-3 (Figure 2).
These values suggest that solar transmission at the mixed layer base can vary from 0.08
to 0.15, corresponding to an absolute solar flux range of ~15 W m-2 (based on a mean
surface irradiance of 200 W m-2; Ohlmann et al. 1996). Our data will allow accurate
quantification of such variations
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4. The upper 10 meters
The attenuation of solar energy is a strong function of wavelength. Solar energy in
the visible wavebands can penetrate to depths beyond 100 m, whereas energy outside the
visible spectral range is completely attenuated within ~10 meters of the surface. By
measuring only in the visible (~350-700nm; what instrumentation currently allows) our
data resolves the total irradiance for depths greater than ~10 meters, but not depths
shallower. We propose an inverse modeling scheme to determine solar transmission
within the upper few meters of the ocean. An array of 12 thermisters positioned
logarithmically with depth from one to 10 meters will be attached to the TAO mooring at
10 N, 95W. Using the temperature and accompanying air-sea heat flux data, the onedimensional heat equation will be inverted and solved for the solar flux profile and the
vertical eddy diffusivity within the near-surface layer. The computed transmission values
will be compared with near-surface estimates of total solar transmission from numerical
solutions to the in-water radiative transfer equation and from the Ohlmann and Siegel
(2000) full spectral solar transmission parameterization. Estimates of the vertical eddy
diffusivity will be compared to those measured by Mike Gregg (University of
Washington) aboard the R/V Brown. This attempt at considering radiant heating within
the top few meters of the ocean from in-situ data is an important step for determining
differences between ocean temperatures precisely at the air-sea interface and “bulk
surface temperature”. Near-surface solar transmission values will also be combined with
near-surface velocity profiles, to be measured concurrently by Rob Pinkel (Scripps), to
investigate relationships between near-surface solar transmission (stratification) and the
vertical mixing of momentum. To our knowledge, this will be the first in-situ study of
ocean radiant heating focused specifically on the upper few meters.
5. Ocean color remote sensing
SeaWiFS and a subsequent suite of ocean color sensors (OCTS, MODIS) will give
continual near real-time global coverage of upper ocean chlorophyll concentration well
into the future. A time-latitude plot of chlorophyll (Figure 2) shows the sort of variations
in upper ocean biomass concentration that exist in the eastern equatorial Pacific. Such
chlorophyll variations are the primary mechanism responsible for changes in solar
transmission on mixed layer depth scales. We believe that upper ocean and air-sea
interaction modeling efforts can be greatly improved by accurately resolving spatial and
temporal variations in solar transmission (e.g. Cohen-Solal and LeTreut 1996, Ohlmann
et al. 1998, Schneider and Zhu 1998). Moreover, existing remote sensing techniques and
transmission parameterizations allow the variations to be resolved.
We propose to use our EPIC in-situ transmission data to validate the performance of
the Ohlmann et al (1996) and Ohlmann and Siegel (2000b) chlorophyll based solar
transmission models forced with SeaWiFS remotely sensed chlorophyll data. This will
give necessary information regarding the accuracy of the transmission models. One way
we will address transmission model error is by using our in-situ chlorophyll data to
validate the SeaWiFS remotely sensed data. The in-situ values will allow us to
investigate the sorts of high frequency and depth variations in chlorophyll that possibly
exist. In addition, we will likely measure chlorophyll concentrations in the presence of
clouds. Such information is not available with remotely sensed data alone. Once the
validation process is complete and error bars are in place we can reach our final goal. We
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will ultimately provide a chlorophyll-based solar transmission parameterization and fields
of chlorophyll from SeaWiFS ocean color data to the ocean and air-sea interaction
modelers.
The modeling studies of others will have the necessary information to answer
questions such as:
 What is the influence of temporal and spatial variations in solar transmission on upper
ocean evolution over synoptic, seasonal, and inter-annual time scales (previous studies
have addressed only the idealized case of chlorophyll vs. no chlorophyll)?
 Is there a negative feedback whereby increased solar fluxes penetrating the mixed layer
leads to destabilization and entrainment of nutrient-rich thermocline waters, thus
increased chlorophyll and decreased transmission, promoting stratification, a decrease in
mixed layer depth, and thus increased solar fluxes penetrating the mixed layer?
 What are the changes in SST and heat loss to the atmosphere that accompany variations
in the solar flux divergence within the mixed layer?
Figure 2. Chlorophyll
concentration (mg m-3)
along 95 W determined
from SeaWiFS ocean
color imagery.
6. Summary
The proposed work
will provide information
necessary for all EPIC
scientists addressing
upper ocean temperature,
stratification, and air-sea
heat exchange questions.
One-dimensional in-situ
data is imperative for EPIC researchers concerned with closing heat budgets and
understanding upper ocean thermal processes. The in-situ data is also useful for
validating ocean color and solar transmission algorithms, and addressing high frequency
variations in transmission that are not resolved with remote sensing techniques. The
near-surface heat budget analysis is required for validation of near-surface radiant heating
parameterizations used in mixed layer models and for investigation of surface and bulk
temperature differences. In-situ studies of ocean radiant heating on meter depth scales
have yet to be conducted. Finally, we will provide a state-of-the art solar transmission
parameterizations and the necessary chlorophyll fields (including time-space variability)
to the oceanographic community. Air-sea interaction modeling studies will then be
armed with the necessary information to answer questions related to the influence of solar
transmission on upper ocean evolution and subsequent air-sea heat exchange processes.
This work is clearly required for a better understanding of upper ocean thermal processes
and their variations known to influence the eastern Pacific coupled ocean-atmospheric
system and climate.
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References
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layer of the western Pacific warm pool: Observations and 1-D model results, Journal
of Climate, 9, 3056-3085, 1996.
Cohen-Solal, E. and H. LeTreut, Impacts of ocean optical properties on seasonal SST:
results with a surface ocean model coupled to the LMD AGCM, Climate Dyn., 12,
417-433, 1996.
Cronin, M. F., and M. J. McPhaden, The upper ocean heat balance in the western
equatorial Pacific warm pool during September-December 1992. J. Geophys. Res.,
102, 8533-8553, 1997.
Ohlmann, J. C., and D. A. Siegel, Ocean radiant heating: 2. Parameterizing solar
radiation transmission through the upper ocean, Journal of Physical Oceanography,
in press.
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heating: A global analysis, Journal of Climate, 9, 2265–2280, 1996.
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influences, Journal of Physical Oceanography, in press.
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1998.
Schneider, E. K., and Z. Zhu, Sensitivity of the simulated annual cycle of sea surface
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