El Niño is the name for a period of anomalous climatic conditions that repeats on
intervals of 2 to 7 years. It is centered in the tropical Pacific, but its effects can actually
be observed all around the globe. Some of the more significant global consequences
attributed to El Nino include South American floods, North American rains, and African
droughts. The phenomenon from which El Nino gets its name is perhaps its most
consistent consequence- the presence of unusually warm water off the coast of Peru.
Typically, the Peruvian coastline is a rich fishery due to strong upwelling at the
continental boundary, which lifts cold, nutrient-rich water to the surface and stimulates
biological productivity. However, during El Nino events, this upwelling is cut off, a
warm, southward flowing current flows along the coast, and the fish move further
offshore. Peruvian fishermen named this phenomenon El Nino, in reference to the Christ
Child, since the warm current typically appeared around Christmastime. Since then, the
name has been given to the overall climate fluctuation that causes this anomaly.
While El Nino events remain very difficult to predict, the driving force in the
atmosphere is known to be the pressure difference between the South Pacific high (in the
east) and the Indonesian Low. Wind circulation in the southern tropics is dominated by
the South-East Trades, which travel westwards north of the South Pacific Low and south
of the ITCZ. When air arrives in Indonesia, it is warm and moist, and it rises, causing a
lot of rain. This air travels back eastward at altitude, becoming cooler and it sinks around
the coast of South America, maintaining the South Pacific High. The strength of these
trade winds is proportional to the difference in surface pressure between the two pressure
systems on either side of the Pacific.
During El Nino, the Indonesian Low develops anomalously high pressure and the
South Pacific High develops anomalously low pressure, greatly reducing the strength of
the trade winds. As a result, Peruvian upwelling is cut off or greatly reduced, and the
typical patterns of precipitation are changed, not only in the tropical Pacific, but also
around the globe. Teleconnections, by which anomalous weather patterns are linked
casually to an El Nino event, are the result of the propagation of Rossby waves in the
Pacific. These waves can require up to a year to propagate across the entire ocean so
there can be large lags in the observance of teleconnections.
In South America, El Nino is associated with sudden flash floods due to deep
convection of hot air at the surface of the warm pool of water off the western coast.
Many of the regions which receive this sudden rain are desert regions during most of the
year; since desert soils are ill-suited for such sudden, intense rainfall, lots of erosion can
occur and rivers can flood, leaving thousands of people homeless. In August of 1997, as
a result of a particularly strong El Nino, 20,000 Brazilians were left without homes due to
flooding, and some rivers crested up to 13 meters above normal. Floods that seem to
correlate with El Nino are also observed in distant regions such as central Europe and
Kenya and Madagascar.
Because of the shift of the location of low-pressure air columns towards the
Americas, much of the moisture that falls on the Indian Ocean and Polynesia disappears
during El Nino phases and these regions become extremely dry. In 1997, the drought that
resulted from this particularly strong El Nino resulted in rampant forest fires in Malaysia,
Borneo, and Sumatra. These fires produced so much haze that drivers had to use their
headlights at midday. In Africa, El Nino can result in both pockets of intense rain and
prevailing drought conditions. The drought is particularly strong in the East, which can
put heavy stress on subsistence farming in Ethiopia and Somalia and surrounding regions.
It is also very strong in the South, due to cooling of the southwestern Indian Ocean and a
strengthening high-pressure area that keeps rainfall away.
North American effects of ENSO are a little more subtle. A ridge of high
pressure over North America's west coast during El Nino winters keeps temperatures
normal in the coastal western US and steers storms that would otherwise pass through
Washington and Oregon northward toward the coast of Alaska. El Nino also creates a
favorable environment for storms to develop in the Gulf of Mexico, bringing heavy rains
to much of the southern United States- these rains are sometimes called the North
American Monsoon.
In a recent study, Christopher Castro et al. described the effect of ENSO on the
North American Monsoon. They constructed a composite index of sea surface
temperatures, which included the Nino-3, the Central North Pacific, and the Eastern
North Pacific. They called this the Pacific index, and used it primarily to draw
correlations between sea surface temperatures and rainfall patterns in the western US.
The way they did this was to construct an index related to precipitation and compare it to
their Pacific Index. Analysis of precipitation throughout the western United States is
difficult, due to poor spatial and temporal resolution given by the available precipitation
measurement stations. For this reason, they focused on horizontal moisture fluxes in the
atmosphere. By calculating the divergence of these fluxes (i.e. the vertical transport),
they were able to determine the balance between precipitation and evaporation.
They found that ENSO-associated teleconnection patterns and climate anomalies
are most likely when El Niño (or La Niña) corresponds with its favored North Pacific
Oscillation (NPO) phase. Destructive combinations of ENSO and NPO tend to weaken
the effects of either mode of influence, resulting in a weak and incoherent climate
response. The NPO was in a high phase, on average, from the late 1970s through most of
the 1990s, coincident with an increase in frequency and severity of El Niño events.
In the study, they found that the N. Pacific and tropical Pacific sea surface
temperatures have distinct, separable effects on regional teleconnection patterns. N.
Pacific temperature has a strong influence on the northeast/southwest displacement of the
monsoon ridge or trough, while ENSO seemed to have more control over the north/south
displacement. So a combination of these influences exerts strong control over the
location of the high and low pressure systems in the area.
The direction of atmospheric circulation and the amount of moisture from both
the Pacific and the Gulf of Mexico will determine both the timing and the wetness of the
weather patterns that comprise the monsoon. Since the NPO’s influence also has a
north/south component, it can interfere with ENSO’s influence either constructively or
destructively. For destructive interference or even years when the anomalies are not very
strong, it is very difficult to predict what effect the system will have on the North
American Monsoon. Castro et al. found that for a high NPO phase and El Nino together,
moisture transport from the Gulf of Mexico would be displaced to the north and east,
whereas for La Nina and a low NPO, moisture from the Gulf would be displaced to the
southwest.
Understanding the global teleconnections of El Nino is extremely important
because El Nino oscillations have strong implications for human health. Precipitation
patterns can have strong influence on tropical diseases such as malaria. Both floods and
droughts can take lives and destroy food resources, leaving home destroyed and entire
regions malnourished. A better understanding of this system can leave people better
prepared to deal with its consequences.