Special Topics in Oceanography
Antarctic Ozone Hole
From September through November, atmospheric ozone is
destroyed due to
chemical processes in the stratosphere over Antarctica, resulting as the
Antarctic Ozone hole. While the details are not fully understood, the
ozone loss is known to be the outcome of chemical reactions with compounds
containing chlorine, which comes from man-made chemicals, such as
chlorofluorocarbons (CFCs) that are used as refrigerants and other
industrial means.
The rapid ozone destruction is confined to the Antarctic because of its
unique meteorological conditions in the stratosphere during spring
(September through November). The extremely cold temperatures of the
Antarctic stratosphere (less than 190 K) cause the formation of Polar
Stratospheric Clouds (PSCs) which are composed of ice and nitric acid.
Elsewhere, the stratosphere is too warm and too dry for clouds to form, so
these clouds are unique to the polar region. The cloud provides the
surface for a multitude of reactions, many of which speed the degradation
of ozone molecules but which also prevents ozone depletion by removing
reactive nitrogen compounds from the stratosphere that would otherwise
react with chlorine. The net result is the rapid destruction of ozone in
the lower stratosphere.
Ozone hole position over the
Antarctic continent for 13 and 29 October 1990. The position of the pack
ice (white area) and the mean position on the Antarctic convergence (outer
edge of the darker blue) are also shown. The average size of the ozone
hole in 1990 was 18 x 106 km2
Image permitted and copyrighted, "An Introduction to the World's Oceans",
Duxbury, 1997, Wm. C. Brown Publishers
Clouds do form during the
winter, but many of the chemical
reactions
require sunlight, so the ozone destruction does not begin until the polar
night ends in the late winter or early spring. The northern hemisphere is
warmer than the southern hemisphere, and it warms up earlier in the spring
as a result of the differing weather patterns in the two hemispheres.
Thus, by the time there is sunlight available, the clouds have already
disappeared. This appears to explain why there is no Arctic ozone hole
(yet). Recent observations show that the Arctic stratosphere is
significantly chemically perturbed, however.
Later in the spring, as the stratosphere warms, the clouds evaporate and
the ozone destruction ceases. At this time the circulation also undergoes
major changes, and region of low ozone, which is confined near the pole,
is mixed with air from lower latitudes. This is largely a transport
process, not a chemical one. Finally, ozone levels gradually recover
during the summer, setting the stage for the process to repeat itself the
following spring.
Yearly comparison of ozone levels in Antarctic. NASA and NOAA instruments
have been measuring Antarctic ozone levels since the early 1970s. Large
regions of depleted ozone began to develop over Antarctica in the early
1980s. Though ozone "holes" of substantial size and depth are likely to
continue to form during the next few years, scientists expect to see a
reduction in ozone losses as levels of ozone destroying CFCs are gradually
reduced.
Global ozone levels are measured by the Total Ozone Mapping Spectometer
(TOMS).
Image of the record-size
ozone hole taken by NASA satellites on September 9, 2000. Blue
denotes low ozone concentrations and yellow and red denote higher
levels of ozone. Notice the "croissant" of high ozone
concentrations formed when the Antarctic vortex blocks the
southerly migration of ozone formed in the tropics.
Harmful Algal Blooms
The occurrence of harmful algal blooms (HABs) is increasing in frequency
and severity in many US coastal environments as well as worldwide. HABs
contain high densities of unicellular algae that can cause mass
mortalities of marine organisms and disrupt ecosystem links and dynamics.
The algae can also cause a variety of illnesses such as PSP (paralytic
shellfish poisoning), NSP (neurotoxic shellfish poisoning), or DSP
(diarrhetic shellfish poisoning). The incidence of these blooms may be a
marker for changes in the global environment due to the result of human
activities (e.g. high nutrient input into coastal waters) or cyclical
changes in global climate. In addition to risks to human health and
environmental impact, significant economic losses occur due to closure of
aquaculture businesses, fisheries, and tourism. It has been estimated
that blooms pose a potential threat to every coastal state and involve a
multitude of different species.
HABs often occur in sufficient densities to discolor the seawater, contain
a single species, and are spatially extensive and persist for periods of
weeks. Many species of harmful algae have distinguishing pigments that
may make them recognizable in water-leaving radiances that are captured by
ocean color satellites such as SeaWiFS
and MODIS. The monitoring of
broad
areas of the blooms provides information about the spatial extent,
temporal dynamics of the bloom, and it also contributes to the
understanding of the role of phytoplankton in the carbon cycle and
its impact on society and the overall ecosystem.
Global distribution of
paralytic shellfish poisoning events observed in 1970 and in 1990.
Eighteen events were recorded in 1970 versus forty-four
1990.
Image permitted and copyrighted, "An Introduction to the World's Oceans",
Duxbury, 1997, Wm. C. Brown Publishers
