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These
global-scale, false-color images represent data collected
by the NOAA AVHRR sensor and composited for the months of
April 1983 and April 1989, respectively. Over the oceans,
the colors represent sea surface temperature anomaly--reds
show higher than average temperatures, blues are lower than
average, and white is average. On land, the colors represent
anomalously high or low production of green foliage (also
known as vegetation index)--greens represent anomalously high
productivity and browns show areas of anomalously low productivity.
These images demonstrate that there is a relationship between
sea surface temperature and vegetation productivity.
Note
that in April 1983, there was a large El Nino in the equatorial
Pacific Ocean, whereas the Southern Atlantic was cooler than
average. Much of the continent of South America was experiencing
drought or drier than normal conditions, reflected by the
low vegetation index values (brown pixels) at that time. Along
the equator, winds tend to blow from east to west, helping
transport moisture and rain clouds inland. Yet, because water
evaporates much less readily from a cooler sea surface than
from a warmer one, the trade winds brought very little precipitation
from the Southern Atlantic to South America at this time.
Conversely, at higher latitudes in both the Northern and Southern
Hemispheres, winds tend to blow from west to east. Notice
the effects of the anomalously warm sea surface on both North
America and Australia during this month. The large pocket
of warm water off the west coast of North America contributed
to unusually high rainfall over the arid regions of Southern
California and New Mexico, helping to spawn temporary grasslands
where there is typically desert. Notice also how the large
region of anomalously warm water in the Indian Ocean contributed
to extensive greening across the typically dry Australian
outback.
In
contrast, April 1989 was a La Nina year in the equatorial
Pacific and the Southern Atlantic was warmer than average.
Many of the signals described in the previous paragraph appear
to be reversed in this later image. For instance, the warm
Atlantic waters are feeding more moisture than usual into
the overlying atmosphere, which was eventually precipitated
across South America, hence the greener than average landscape.
However, the large pocket of cool water off the west coast
of North America contributed to drought in the Great Plains
region. There was also extensive drying across northern Australia
as much of the Indian Ocean was cooler than normal.
Images
courtesy NASA/GSFC/Sietse Los, James Collatz, and Jim Tucker
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