Quarterdeck 3.3
Quarterdeck 3.3
by Niall C. Slowey and Thomas
J. Crowley
. . .Continued from Part 3
To study the growth rate of a coral, divers first collect cores of skeletal
material using a drill (see cover). We then cut slabs from the cores in
the laboratory and x-ray them so that the coral's density bands can be studied
and the material can be sub-sampled for stable isotope analysis.
The growth rates of sixteen coral colonies of Montastrea annularis living
at the Flower Gardens were reported by Harold Hudson and Daniel Robbin and
by Kenneth Deslarzes. They found that average annual growth rates of these
corals display significant decadal-scale variations. The growth rate was
7.9 millimeters per year (mm/yr) from 1888 to 1907, it increased to 8.9
mm/yr from 1907 to 1957 and then declined to 7.2 mm/yr from 1957 to
1979. The growth rate increased again to 9.0 mm/yr in 1989, the last
year for which data exists. Richard Dodge and Judy Lang suggested the abrupt,
enigmatic decline in coral growth that occurred during 1957 was due to local
variations in water temperature or outflow from the Atchafalaya River. We
favor the temp-erature explanation because it fits well into the context
of regional and global-scale climate change.
[81K] Interdecadal changes in
the average growth rates of Montastrea annularis coral at the Flower Gardens
during the past century correspond closely with changes in minimum winter
air temperature and the average winter air temperature (not shown) at New
Orleans. Data sources: Hudson and Robin 1980, Deslarzes 1992, U.S. Weather
Bureau and the Department of Commerce.
[46K] Effects of the late 1950s shift
in Northern Hemisphere circulation from a more zonal to a more meridional
pattern are evident in the growth rates of Montastrea annularis coral at
the Flower Gardens, the average winter air temperatures at New Orleans,
and the average winter PNA pattern index. White lines show average values
for indicated time span. PNA index data exist only after 1946. The index
is calculated from monthly average 500-millibar (a unit of atmospheric pressure)
heights measured over the the southeast United States, the Rocky Mountains
and the central North Pacific.
Changes in coral growth rate correspond closely to changes in winter air
temperatures along the Gulf of Mexico coast. What is the relationship between
the two? Water temperatures less than 18°C generally limit the development
of tropical coral reefs and temperatures less than 16°C are often lethal.
Temperatures at the Flower Gardens approach these limits during winter,
so the physiology and growth of corals there should be quite sensitive to
variations in winter temperature. The passage of fronts during winter can
bring cold, dry Arctic air and low air temperatures to the region. These
fronts cool the waters of the northern Gulf of Mexico and we believe this
stresses the corals and stunts their winter growth.
In the late 1950s the winter climate shifted significantly toward colder
winters. This shift is displayed by the PNA pattern index and is clearly
reflected by the abrupt decline in coral growth at the Flower Gardens and
the presence of stress bands in the corals. The wintertime PNA index shifted
from negative to positive values at this time. Jeffrey Rogers and Robert
Rohli observed that no major Florida citrus freezes occurred during the
decade prior to winter of 1956/57, while they occurred often from 1957/58
to the present.
The shift from negative to positive values of the PNA index corresponds
to a shift from a zonal (west-east) orientation of the jet stream and air
flow over North America to a more meridional (north-south) orientation.
The PNA pattern is strongest during winter so a meridional orientation allows
cold, dry Arctic air to reach south to the Gulf of Mexico. Thus, climatic
changes in the southeast United States are related to large-scale changes
in the extratropical Northern Hemisphere circulation associated with the
PNA pattern.
Data for calculating the PNA pattern index comes from a global-scale data
collection network which has existed only since 1946. We believe that coral
growth records from the Flower Gardens can provide valuable information
about fluctuations in the PNA pattern that occurred prior to the development
of this network. For example, existing coral and temperature records extend
into the 1800s. Both coral growth and winter temperatures were relatively
low from the late 1880s to the first decade of the twentieth century, suggesting
that the PNA pattern and the climate during that period were like those
which prevailed from the 1960s to the present. Other evidence supports this
interpretation, including low winter temperatures throughout the southeast
United States during both periods, as well as the tracks of winter storms
across North America and documented freeze damage to citrus crops and other
agricultural commodities.
Working with members of National Oceanic and Atmospheric Association's Flower Garden Banks National Marine Sanctuary program lead by Stephen Gittings, we recently collected cores of skeletal material from several large Montastrea and Siderastrea corals that potentially preserve climate records nearly five centuries long! We are in the process of measuring the growth rates and stable isotopic composition of these corals to obtain long, high-resolution estimates of how environmental conditions at the Flower Gardens changed through time. This information will allow us to describe the temporal nature of climate variability in the northwestern Gulf of Mexico and the PNA pattern to an extent not otherwise possible, and to examine its relationship to other climate phenomena. Y
Slowey, Niall and Thomas Crowley, 1995: Interdecadal variability of Northern
Hemisphere circulation recorded by Gulf of Mexico corals. Geophysical
Research Letters. 22, 2345-2348.
Oceanography, Texas A&M
University
rshatto@ocean.tamu.edu
URL=http://oceanography.tamu.edu/Quarterdeck/QD3.3/Slowey/slowey-d.html
Updated January 8, 1996