6. Data from the equator
During El Niño, the trade winds relax in the central and western
Pacific leading to a depression of the thermocline in the eastern Pacific,
and an elevation of the thermocline in the west. This reduces the efficiency
of upwelling to cool the surface water and cuts off the supply of nutrient-rich
thermocline water to the surface zone. The result is a rise in sea surface
temperature and a drastic decline in primary productivity, the latter of
which adversely affects higher levels of the food chain, including commercial
fisheries in the region.
a.) The diagram below shows a model of surface temperatures, winds,
areas of rising air, and the thermocline (blue surface) along the equator
during normal and El Niño conditions.
Briefly describe the differences in:
i. sea-surface temperatures (SSTs)
iii. areas of rising air
b.) The images below show other views of normal and El Niño conditions.
The first diagram shows a map view of normal conditions at the top
and El Niño conditions on the bottom (red is warm water, pink arrows
are winds, and white lines indicated clouds-rising air).
The second diagram shows a profile view of the two climatic situations
along the equator.
Red indicates warm water and blue indicates cold water.
c.) The graphs below show changes in wind direction and in ocean
temperatures from 0-200 meters depth for a two-year period in the eastern
Pacific. These data were collected by the buoys along the equator viewed
in part 5. In the upper diagram, red lines pointing upward indicate eastward-moving
winds; lines pointing downward indicate westward-flowing winds.
Where in the Pacific Ocean were these data collected? (Look at
a world map at the appropriate latitude and longitude.)
Based on the equator.model in part a.), during which months would you say
that El Niño conditions were strongest during the 2-year period shown
in the diagram above?
d.) The diagrams below also show changes in sea-surface temperatures
and winds along the equator. Whereas the image in part b.) shows
variations through time (1982-83) at one location along the equator, these
images show variations along the equator at two specific times. The two
times illustrate normal (1991) and El Niño (1992) conditions.
Based on what you have learned above, explain the changes between
normal and El Niño years that are illustrated by these data (collected
by the NOAA buoys). Keep in mind that these data show only surface conditions
(not conditions at depth). Arrows showing winds are vectors; that is, the
longer the arrow, the faster the wind.
e.) The next image is the same as those in part c.), but shows
data from 1 December 1996.
Based on the sea-surface temperatures and winds shown in this diagram, do
you think that this is a year of normal or El Niño conditions?
f.) The next image, also based on buoy data, shows both variations in
time (y axis) and in space along the equator (x axis). The left panel shows
SST at the Equator in the Pacific Ocean (Indonesia is towards the left,
South America is towards the right). Note the blue "scallops"
on the right of the plot, in the eastern Pacific. These indicate the cool
water typically observed in the eastern Pacific (called the "cold tongue").
Cold tongue temperatures vary seasonally, being warmest in the northern
hemisphere springtime and coolest in the northern hemisphere fall. The red
color on the left is the warm pool of water typically observed in the western
Pacific. El Niño is an exaggeration of the usual seasonal cycle,
where warm water (red) penetrates further eastward than normal. The right
panel (anomalies) shows how much SST is different from the usual value for
each month. Water temperatures significantly warmer than normal are shown
in red, and temperatures cooler than normal are shown in blue.
Based on the data in these graphs, which years from 1986 to 1994 were
El Niño years?