Do MetOffice, a propósito da Oscilação do Atlântico Norte (NAO).
Sobre a previsão sazonal da NOAA CPC:
Early indications for Winter 2007/8 (December, January and February)
Last winter was exceptionally warm over much of Europe and the second warmest on record for the UK. The signal from the statistical method suggests Winter 2007/8 is likely to be less mild for Europe as a whole than 2006/7. For western Europe, including the UK, indications favour temperatures less mild than last year, but still above the 1971-2000 normal.
Above-normal precipitation was experienced over the UK last winter with 130% of the long-term UK average. Early indications suggest that Winter 2007/8 is likely to be closer to normal.
The North Atlantic Oscillation (NAO)
The North Atlantic Oscillation (NAO) is a phenomenon associated with winter fluctuations in temperatures, rainfall and storminess over much of Europe. When the NAO is in a positive phase westerly winds are stronger or more persistent, northern Europe tends to be warmer and wetter than average and southern Europe colder and drier. When the NAO is in a negative phase westerly winds are weaker or less persistent, northern Europe tends to be colder and drier and southern Europe warmer and wetter than average.
The phase and amplitude of the NAO is often described using an index. One of the simplest NAO indices is the winter difference in pressure at sea level between the Azores and Iceland. The index used here is somewhat different but it essentially captures the same phenomenon.
Winter 2007/8 NAO prediction
The figure below shows that the predicted winter NAO index for 2007/8 is weakly negative at -0.34 with a standard error of ±1.0. The relatively small amplitude of the predicted index relative to the error bar means there is little associated signal for below- or above-normal European winter temperatures or precipitation. However, the prediction is consistent with a cooler, drier winter over northern Europe as a whole than experienced in winter 2006/7, when the observed index was +1.1.
Sobre a previsão sazonal da NOAA CPC:
SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS
THE MAIN FACTORS WHICH USUALLY INFLUENCE THE SEASONAL CLIMATE OUTLOOK INCLUDE:
1) EL NINO AND LA NINA - WHICH COMPRISE EL NINO/SOUTHERN OSCILLATION OR ENSO.
2) TRENDS - APPROXIMATED BY THE DIFFERENCE BETWEEN THE MOST RECENT 10-YEAR MEAN
OF TEMPERATURE OR 15-YEAR MEAN OF PRECIPITATION FOR A GIVEN LOCATION AND TIME
OF YEAR AND THE 30-YEAR CLIMATOLOGY PERIOD (CURRENTLY 1971-2000). THE OCN TOOL
REPRESENTS THE TREND.
3) THE TROPICAL 30-60 DAY OSCILLATION - AFFECTS CLIMATE VARIABILITY WITHIN
4) THE NORTH ATLANTIC OSCILLATION (NAO) AND THE PACIFIC NORTH AMERICAN (PNA)
PATTERNS - WHICH AFFECT THE TEMPERATURE ANOMALY PATTERN ESPECIALLY DURING THE
COLD SEASONS. THESE PHENOMENA ARE CURRENTLY KNOWN TO BE PREDICTABLE ONLY OVER A
WEEK OR SO.
5) PERSISTENTLY DRY OR WET SOILS IN THE SUMMER AND SNOW AND ICE COVER ANOMALIES
IN THE WINTER. THESE FACTORS TEND TO PERSIST FOR LONG PERIODS AND ACT AS A KIND
OF MEMORY IN THE CLIMATE SYSTEM.
6) STATISTICAL FORECAST TOOLS - CANONICAL CORRELATION ANALYSIS (CCA)... AND
SCREENING MULTIPLE LINEAR REGRESSION (SMLR).
7) DYNAMICAL FORECAST MODELS... INCLUDING THE NCEP CLIMATE FORECAST SYSTEM
8) AN OBJECTIVE CONSOLIDATION (ABBREVIATED CON IN THE TEXT) OF THE OCN...
CCA... SMLR AND CFS FORECASTS IS USED AS A FIRST GUESS IN PREPARING THE
FORECAST MAPS. THIS TECHNIQUE MAKES OPTIMUM USE OF THE KNOWN SKILL OF OUR
EQUATORIAL PACIFIC SEA SURFACE TEMPERATURES (SSTS) ARE CURRENTLY SLIGHTLY ABOVE
NORMAL IN THE WESTERN PACIFIC AND BELOW NORMAL EAST OF THE DATE LINE. SSTS IN
THE NINO 3.4 REGION (FROM 5N TO 5S AND 170W TO 120W) ARE CONSIDERED A GOOD
INDICATOR OF THE STRENGTH OF EL NINO (WARMER THAN NORMAL SSTS) OR LA NINA
(COLDER THAN NORMAL SSTS). SSTS IN THE NINO 3.4 REGION HAVE BEEN ABOUT ONE-HALF
OF ONE DEGREE C BELOW THE LONG-TERM AVERAGE DURING THE LAST TWO WEEKS
SUGGESTING THE POSSIBILITY OF A TRANSITION FROM CURRENT ENSO-NEUTRAL CONDITIONS
TO LA NINA CONDITIONS DURING THE NEXT TWO MONTHS. THE NOAA THRESHOLD FOR LA
NINA ONSET IS WHEN THE THREE-MONTH MEAN SSTS FOR THE NINO 3.4 REGION ARE MORE
THAN ONE-HALF OF ONE DEGREE BELOW THE LONG-TERM AVERAGE. THERE IS A SIGNIFICANT
CHANCE THAT THE THRESHOLD FOR LA NINA WILL BE SURPASSED FOR THE
SEPTEMBER-OCTOBER-NOVEMBER (SON) SEASON.
THOUGH A RAPID TRANSITION OF THE NINO 3.4 REGION SSTS FROM EL NINO TO NEUTRAL
CONDITIONS OCCURRED EARLY IN 2007... AND DYNAMICAL MODEL PREDICTIONS AT THAT
TIME CALLED FOR AN ONSET OF LA NINA BY THE BEGINNING OF SUMMER 2007... SSTS DID
NOT DECREASE AS EARLY OR AS RAPIDLY IN THE CENTRAL PACIFIC AS FORECAST... DUE
IN PART TO INTRASEASONAL VARIABILITY. STATISTICAL MODELS... WHICH EARLY IN 2007
HAD FORECAST A CONTINUATION OF ENSO-NEUTRAL SSTS IN THE CENTRAL PACIFIC OR A
SLOW DECREASE... ARE NOW IN GREATER AGREEMENT WITH THE DYNAMICAL MODELS
ALTHOUGH STILL FORECASTING WARMER CENTRAL PACIFIC SSTS THAN MOST DYNAMICAL
MODEL FORECASTS. THREE-MONTH MEAN SSTS IN THE CENTRAL PACIFIC ARE EXPECTED TO
BE NEAR OR BELOW THE THRESHOLD FOR LA NINA THROUGH THE
DECEMBER-JANUARY-FEBRUARY (DJF) SEASON... AND LA NINA CONDITIONS COULD HAVE
SOME INFLUENCE ON THE SEASONAL OUTLOOK INTO THE EARLY PART OF 2008. UNCERTAINTY
IN THE PROJECTED PACIFIC SSTS INCREASES BY THE SPRING OF 2008.