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LA NIÑA AND WINTER WEATHER
FREQUENTLY ASKED QUESTIONS
Q. What is meant by
a La Niña?
La Niña refers to a period when ocean temperatures across
the central and eastern tropical Pacific Ocean are cooler than
Q. Why does La Niña impact the weather patterns?
The cooling of the ocean waters leads to changes in the patterns
of tropical rainfall from Indonesia to South American (a distance
of more than one-half the circumference of the earth), which
significantly affects the strength and location of the atmospheric
jet stream over the eastern North Pacific and North America.
These changes in the jet stream alter the weather patterns.
Q. Last winter was a La Niña winter. Does
that mean that this winter will be just like the last one?
There will likely be many similarities between the two winters,
and there will also be some differences. However, our weather
patterns are controlled to a large extent by two distinct jet
streams (one over the North Pacific extending into the western
U.S. and one extending from the eastern U.S. to Europe). La Niña
affects the Pacific jet stream, and therefore affects only a
portion of the flow that actually determines our weather patterns.
Q. What is meant by a "moderate" La Niña?
The La Niña's can be categorized according to the combined
strength, location and coverage of the cold ocean waters. Moderate-to-strong
La Niña's impact the weather patterns in a very similar
manner, and thus, this distinction is thought by many to have
little meaning. The weather patterns with a weak La Niña
tend to be somewhere in between those of a stronger episode and
those of near-neutral conditions.
Q. Are back-to-back La Niña's rare?
No, The historical record reveals many back-to-back events. Some
prolonged cold episodes during the past 50 years are: 1983-85,
1973-76, 1954-56 and 1949-51.
Q. How many La Niña winters are there been
14. The last occurred in 1998/99, preceded by 1995/96 and 1988/89.
Q. Is there something unique about this particular
La Niña that makes it different?
No, it is very typical to other La Niña episodes that
we have observed during the past 50 years.
North Atlantic Oscillation
Q. There are so-called "wild cards" that
affect winter weather patterns. What are they?
The "wild cards" are the naturally occurring global-scale
circulation patterns that are a prominent aspect of our climate
and that are independent of the La Niña influence.
Q. Is the North Atlantic Oscillation (NAO)
one of these wild cards?
Yes, it is a major wild card. The NAO affects the weather patterns
across central and eastern North America, and it is essentially
independent of the La Niña.
Q. Tell me about the NAO.
The NAO reflects large-scale changes in the portion of the jet
stream which extends from the eastern U.S. to Europe. It exhibits
an enormous degree of variability, in that it can persist for
many years in a particular state, and than transition to a completely
different phase the next winter. It can also vary substantially
within a given year. This variability, as well as the El
Niño-La Niña cycle, are major contributors
to the differences in weather patterns from one winter to the
next, and to the variability in weather within a given winter
Q. Can we predict the NAO at extended ranges?
No. Scientists have not yet identified a clear climate signal
that will give us a meaningful hint as to the state of the NAO
during the upcoming winter.
Q. Has the NAO affected the La Niña signal
in the past?
Yes. Two recent examples for the eastern U.S. are the cold and
snowy 1995/96 La Niña winter across the eastern U.S.,
compared with the very mild 1998/99 winter. Opposite phases of
the NAO during these two winters were a major contributing factor
to these differences. Therefore, caution must be exercised in
making comparison of one La Niña to the next.
Q. How does La Niña influence hurricane season?
There's no question that La Niña continues to contribute
to the tropical Atlantic weather that we are experiencing. But
remember, La Niña is only one of the climate elements
that can produce an above average tropical storm and hurricane
season in the North Atlantic.
The ongoing pattern of tropical
rainfall features above-normal rains across Indonesia and the
eastern Indian Ocean, and a near-absence of rainfall across the
central and eastern equatorial Pacific. These conditions are
consistent with the continuing pattern of below-normal sea-surface
temperatures over the central and eastern tropical Pacific (La
Niña), and strongly influence the atmospheric circulation
throughout the global tropics and subtropics. Over the tropical
Atlantic, the impact of these conditions is often a large-scale
and persistent pattern of atmospheric wind and pressure that
is conducive to an active hurricane season.
Q. NOAA predicted a more active (1999) Atlantic
Hurricane Season. How are we doing so far?
So far, with 34 days remaining in the season, we've had an "average,"or
"slightly above average" season. An "average season"
would be about 9 (9.7) named tropical storms, and with about
5 (5.7) hurricanes. As of October 25, 1999 we've had 10 tropical
storms with 7 of them developing into hurricanes. There have
been two or three occasions when a developing storm could have
become a "Named Storm" (at 39 mph), but aerial reconnaissance
could not be completed before the storm made land fall.
We are way above average100
percent above normalin the area of "Intense Hurricane"
(Category 3 or higher, winds in excess of 111 mph), with four
intense hurricanes: [BRET (Aug 18-24)(winds 140 mph)], [CINDY
(Aug19-31)(140)], [FLOYD (Sep 7-17)(155)], [GERT (Sep 11-23)(150).
A "normal season" would have only two intense hurricanes.
Q. What can we expect between now and when the
hurricane season ends?
One additional hurricane would be a reasonable expectation, but
two would not be a surprise.
posted October 27, 1999