Tweets -

I am posting here an FAQ of El Nino information compiled by Eric Taylor of
Environment Canada's Atmospheric Environment Branch in Vancouver, BC. It is based
on work done by climatologists in Montreal, PQ. It begins with a summary of El
Nino and then discusses some of its effects on weather. Any comments of my own are
in parentheses.

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EL Nino FAQ

1) What is El Nino?

El Nino is part of a natural cycle of the ocean and atmosphere (the whole cycle's
full name is the El Nino-Southern Oscillation or ENSO for short). Every two to
seven years it appears in the tropical Pacific Ocean. During an El Nino, the
tropical sea surface temperatures in the eastern and central Pacific are warmer
than usual. This warm water adds considerable amounts of additional heat and
moisture to the tropical atmosphere, resulting in large scale changes to the
earth's atmospheric circulation, both at the tropics and elsewhere around the
world (remember that the part of the atmosphere in which weather occurs is, in
relative size, like the skin of an onion around the Earth).

2) Where did this phenomenon get its name?

The phenomenon known as El Nino was observed as early as the 1600s off the coast
of Peru. At varying intervals, warm waters off the Peruvian coast appeared around
Christmas. This warming, dubbed El Nino for the Christ child by Peruvian (i.e.
non-native, conquering, Spanish) fishers, caused the fishery to decline
significantly and sometimes collapse completely (the cultural bias in this bit of
scientific history is pathetic in my opinion; doubtless the native Peruvians
before the 1600s had other names for this event).

3) What causes El Nino?

Normally, trade winds at the equator blow from east to west, pushing vast amounts
of warm Pacific Ocean water westwards towards Indonesia (Actually, the trade winds
blow from the northeast to the north of the equator, and from the southeast to the
south of the equator. This leads to the surface winds "piling up" or converging
near the equator in what is known as the InterTropical Convergence Zone or ITCZ
for short. If you look at a whole globe satellite shot of the Earth from one of
the geostationary satellites, this can easily be seen as a ring of thunderstorms
near the equator over the oceans as the air is forced to rise. In El Nino years
the ITCZ is disrupted because of the weaker trades). Cool water in the deep ocean
along the coast of Peru rises to the surface to replace this westward-flowing
warmer surface water, resulting in relatively cool water in the eastern tropical
Pacific.

During El Nino years, this normal pattern of atmospheric and oceanic circulation
becomes disrupted. The trade winds diminish or sometimes reverse direction. This
allows very warm water from the western tropical Pacific to flow back eastward
toward the central and eastern Pacific over the span of several months. Also, the
upwelling of cold oceanic water along the coast of Peru diminishes or stops
altogether (Another consequence of the normal upwelling cold water along the
western S. American coast is that convective cloud buildups are suppressed and
this is why this area is usually so dry. This mechanism is shutoff during El Nino
years often resulting in copious precipitation, such as in the current El Nino).
The result is that sea surface temperatures throughout the central and eastern
tropical Pacific rise substantially, sometimes as much as 5 C degrees or more (try
>10 C degrees currently).

4) How does El Nino affect weather patterns in the northern Pacific?

The warmer sea surface temperatures in the tropical Pacific tend to strengthen the
upper winds over the northern Pacific Ocean. This leads to weather systems in the
Gulf of Alaska that are more vigorous than normal during the winter months. These
systems pump abnormally warm air into western Canada and southern Alaska (air
warmed by abnormally warm sea water temperatures).

5) What are the impacts of El Nino on BC and Yukon on temperature?

During El Nino events, the winter temperatures in most of BC and southern Yukon
are usually milder than normal. For example, temperatures from November through
March during five of the six strongest El Nino events of the last 50 years have
been above average at both Vancouver Airport (up to 2.1 C degrees) and at Prince
George (up to 5.7 C degrees). During other season, there appears to be little
significant impact of El Nino on temperature in BC and Yukon (the greatest impact
is on overnight minimum temperatures since the increased cloud cover acts as an
insulating blanket; this is especially evident in continental climates such as
Prince George).

6) What are the impacts of El Nino on BC and Yukon snowfall?

During El Nino events, the amount of snow is usually lower than normal in BC and
Yukon. For example, during four of the six strongest El Nino events of the last 50
years, Vancouver and Victoria have had little snow. Normally about 50 cm falls in
these cities each winter. In the southern interior of BC, Summerland has averaged
half its normal winter snowfall during five of the last six strongest El Ninos. In
Yukon, Whitehorse has had about 70% of its normal snowfall in four of these six El
Ninos (this only tells part of the story: does this mean that the ratio of
rain/snow was increased during El Nino years - which would be expected given the
influx of warm, moist air - or was *total* precipitation lower than normal. See
the next question!).

7) What are the impacts of El Nino on total precipitation (rain plus snow) in BC
and Yukon?

The amount of winter precipitation during El Nino events varies considerably
across BC and Yukon. During the last 50 years, the souther interior of BC and the
southern Yukon have been slightly drier than normal during strong El Ninos. For
example, the total precipitation in Summerland during five of the last six
strongest El Nino events has averaged about 25% below normal. In Whitehorse, there
is a similar but weaker connection between total precipitation and El Nino. On the
BC coast, El Ninos tend to produce normal amounts of winter precipitation in some
areas and slightly drier conditions in others. For example, both Vancouver and
Prince Rupert, BC have had near normal precipitation for four of the last six El
Nino winters. Victoria has had near normal precipitation for three of the same six
events, while the other three years were slightly drier by about 15% on average
(Victoria is in the Olympic Peninsula rainshadow. Since El Nino years have
stronger jetstreams, it would be reasonable to assume that the rainshadow effect
would be stronger there in El Nino years which would contribute to lower
precipitation accumulations).

8) What are the effects on El Nino in other areas of the world?

The following is a brief list of some of the usual impacts of El Nino events from
November through March outside BC and Yukon. Strong relationships between El Nino
and any areas not mentioned below either do not exist or have not yet been
determined (highly likely the latter). The information on Candian locations has
been provided by Envirnoment Canada scientists:

- Canadian prairies: milder and somewhat drier than normal.
- Ontario, Quebec, and Atlantic Canada: usually milder than normal.
- Southern Northwest Territories: milder, more precipitation.
- Southeastern United States, northeastern Mexico: wetter and cooler than normal.
- Northwestern South America: wetter than normal.
- Hawaii: drier than normal.
- Indonesia, southeast Asia and northern Australia: drier than normal.
- Japan, southeast Asia, India, southern China: warmer than normal.
- South Africa: drier and warmer than normal.

9) What is La Nina?

While El Nino is the warm phase of the trpoical Pacific oscillation, La Nina is
the cold phase. The central and eastern tropical Pacific Ocean becomes cooler than
normal during La Nina conditions. Weather conditions around the world are in
general opposite to those occurring during El Nino conditions.

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Much yet remains to be learned of this phenomenon, especially what triggers the
different phases of the ENSO.

- Jack

Jack Bowling
Prince George, BC
jcbowling at mindlink.bc.ca