These global-scale, false-color images represent data collected by
the NOAA Advanced Very High Resolution Radiometer (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 Niño 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
In contrast, April 1989 was a La Niña 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.