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La Niña and Winter Weather
Frequently Asked Questions

La Niña

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 normal.

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 since 1950?

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 season.

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.

Hurricanes

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 average -- 100 percent above normal -- in 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.


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