tral shape of the transmitted irradiance is charac-
teristic of ice with biogenic material. Algae and
10 1
detritus levels continued to increase through 5
a
December, causing a further reduction in trans-
b
mittance.
A model was recently developed to examine
radiative transfer in a coupled atmosphereice
c
ocean system (Jin et al. 1994). The model is a
multilayer and multistream formulation based on
10 2
the discrete ordinates method. Radiative transfer
within the entire atmosphereiceocean system is
determined based on a description of physical
properties of the atmosphere, ice and ocean from
which the optical properties are derived. The
model computes the distribution and absorption
10 3
of solar radiation in the atmosphere, ice and ocean.
Results indicate that sea ice has a strong influ-
ence on the distribution of solar radiation in the
system (Jin et al. 1994). Such models provide a
promising tool for investigating atmosphereice
ocean radiative feedbacks.
10 4
400
500
600
700
SUMMARY AND CURRENT
Figure 19. Seasonal changes in underice spectral irra-
AREAS OF INTEREST
diance calculated using a bio-optical model (Arrigo et
al. 1991). Curves are predicted spectral transmitted
By now the reader is no doubt aware that the
irradiance at noon on a) 7 October 1984, b) 13 Novem-
optical properties of sea ice are variable and com-
ber 1984 and c) 5 December 1984.
plex. The reader is also aware that much of this
complexity is comprehensible. While many of the
ice cooler than the eutectic point, and as a func-
details still need to be determined, we do have a
tion of brine volume. Most importantly, they also
qualitative understanding of sea ice optical prop-
derived relationships for the extinction contribu-
erties and their variability. This understanding is
tions from absorption due to microalgae and de-
based on a few fundamental principles. Changes
tritus. With this model it is possible to examine
in such optical properties as the albedo, reflec-
the impact of biogenic material on transmitted
tance, transmittance, and extinction coefficient are
spectral irradiance and to investigate temporal
directly tied to changes in the state and structure
changes.
of the ice. Physical changes in the ice which en-
Combining field observations with model cal-
hance scattering, such as the formation of air
culations, Arrigo et al. (1991) were able to com-
bubbles due to brine drainage, result in larger
pute a time series of transmitted spectral irradi-
albedos and extinction coefficients. Radiative
ances (Fig. 19). The calculations were done for
transfer in sea ice is a combination of absorption
bare ice in McMurdo Sound, Antarctica, roughly
and scattering. Differences in the magnitude of
1.71.8 m thick, between 7 October and 5 Decem-
these optical properties are due primarily to dif-
ber. During this period there was a constant in-
ferences in scattering. Spectral variations are
crease in the amount of microalgae and detritus.
mainly a result of absorption.
On 7 October, levels of microalgae were low and
In addition to these general principles, there
there was no detritus present, so light losses were
are also several specific comments that can be
primarily due to extinction by the sea ice. By 13
made regarding the sea ice optical properties.
November the spring bloom had produced sig-
Spectral absorption coefficients for ice are well
nificant amounts of algae and detritus. The pres-
known, however, representative values for brine
ence of this biogenic material resulted in an over-
are less certain. Absorption by algae and particu-
all reduction, and a change in the spectral shape,
lates is also important and needs further investi-
of the transmitted irradiance. The distinct spec-
gation. More work, both experimental and theo-
19