Ciao a tutti,
i climatologi danno finalmente buone notizie per il clima in Polinesia per i prossimi mesi: l'anno scorso le precipitazioni eccessive dovute al fenomeno ciclico del Nino hanno rovinato pił di qualche giorno di viaggio a molti, ma per fortuna ora dovrebbe arrivare il periodo della Nina, che per la Polinesia vuol dire maggior stabilitą climatica e meno precipitazioni. Qui i testo integrale in inglese da Tahiti News:
Science : 04/03/2007 at 10:49AM
El Nino is finished, El Nina may be on the way
(Tahitipresse) - The World Meteorological Organization (WMO) has made it official: El Nino, which has been blamed, along with climate warming, for weather disturbances throughout the globe since the second half of last year, "has now ended".
The immediate results, the WMO reported at the end of March, are neutral conditions throughout the Pacific Basin. And El Nina may be on the way, but the WMO is not yet betting the weather bureau on such a result.
The WMO is hedging on its predictions for one simple reason: "Forecasts made at this time of year notoriously lack skill, and the period March-May is often referred to as the 'spring barrier' in the predictability of El Nino and La Nina."
For those who may have forgotten, El Nino occurs when sea temperatures at the surface of the central and eastern tropical Pacific Ocean become substantially higher than normal. But La Nina occurs when sea surface temperatures in the same regions become lower than normal.
Simply put, an El Nino could mean the eastern Pacific, which includes French Polynesia, could have more tropical storms and cyclones, while the Atlantic, Gulf of Mexico and the Caribbean Sea could have fewer hurricanes. La Nina could mean the opposite effects - fewer tropical storms and cyclones in the eastern Pacific and more hurricanes than normal in the Atlantic, Gulf of Mexico and Caribbean.
So if a La Nina period is on the way that is good news for the eastern Pacific. But to be classified as a full-fledged La Nina episode, a threshold of a negative Oceanic Nino Index (ONI) of "less than or equal to -0.5 degrees Celsius . . . must be exceeded for a period of at least five consecutive months", according to the National Oceanic and Atmospheric Administration (NOAA) in the U.S.
Meteorologists throughout the world closely follow such events because those temperature changes are strongly linked to major climate fluctuations around the globe. Once initiated, such events can last for 12 months or more. For example, the strong El Nino event of 1997-1998 was followed by a prolonged La Nina phase that extended from mid-1998 to early 2001.
The WMO notes that the El Nino event that prevailed during the second half of last year exerted "substantial influence on climate patterns during that period". Sea-surface temperatures were 2 degrees Celsius warmer than normal in December in the central and eastern Equatorial Pacific. That was one of several months that climate patterns displayed many characteristics usually associated with El Nino.
They included "drier than normal conditions across many parts of Australia, Indonesia and Fiji and unusually heavy rains and flooding across parts of eastern Africa and extended dry spells across many southwestern parts of southern Africa", the WMO reported.
El Nino rapidly dissipated during the first two months of 2007, "especially during February". That led to neutral conditions in the Equatorial Pacific. "Indeed, sea-surface temperatures had already become cooler than normal in the central and eastern Equatorial Pacific by the end of February."
Despite the unpredictability of developments in the tropical Pacific at this time of year, the WMO reported, "there are indications that cooler than normal waters may prevail over the next several weeks in the central and eastern Equatorial Pacific." If that proves to be accurate, a La Nina event would be confirmed, which would likely mean La Nina "would persist for much of the remainder of the year".
However, just to make sure, WMO said it plans a further update in the next two to three months (May to June), "given the uncertainty that currently exists on the expected conditions in the tropical Pacific.
Meanwhile, a tropical rainfall outlook for March to May predicts near or above average rainfall for the Tuamotu Archipelago and the Society Islands (the Leeward and Windward Islands). Near or below average rainfall is predicted for the Marquesas Islands, which experienced well below normal rainfall during February, according to the Island Climate Update prepared by NIWA Science, which is part of the National Institute of Water and Atmospheric Research (NIWA) operation in New Zealand.
