Previsão sazonal - Inverno 2007/2008

[Vince]

Do MetOffice, a propósito da Oscilação do Atlântico Norte (NAO).

Early indications for Winter 2007/8 (December, January and February)

Temperature
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.

Rainfall
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.

Fonte:

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
SEASONS.

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
(CFS).

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
FORECAST TOOLS.



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.

Continua:
http://www.cpc.ncep.noaa.gov/products/predictions/90day/fxus05.html

 

[Vince]

Da UCL (University College London), também sobre o NAO

North Atlantic Oscillation Forecast for Winter 2007/8

The North Atlantic Oscillation is forecast to be slightly below-norm during the
coming winter (1st December 2007 - 29th February 2008).
The North Atlantic Oscillation (NAO) is the dominant influence on winter climate variability
over the North Atlantic, Europe and eastern North America. Our NAO forecast for winter
2007/8 indicates a slightly below-norm NAO. Historically such an NAO would give rise to
slightly below-norm temperature and windspeed and near-norm precipitation over Northwest
Europe. The forecast spans the period from 1st December 2007 to 29th February 2008. The
prediction is made using an ensemble of two separate forecasts made with prior June/July
northern hemisphere subpolar surface air temperatures and prior June/July northern
hemisphere snow cover (Saunders et al., 2003; Fletcher and Saunders, 2006 ; Saunders and
Lea, 2006 ).
This multi-model anticipates whether the winter NAO will be above or below
median - for a range of NAO indices - in 66%-83% of the winters during the 1972/3-2006/7
period of reliable snow cover monitoring. Deterministic and tercile probability forecasts for
three leading NAO indices for winter 2007/8 are given below.


What Does the NAO Forecast Imply for Winter Temperature, Precipitation
and Windspeed across Europe?
The implications of our winter 2007/8 NAO forecast for potential temperature, accumulated
precipitation and windspeed anomalies across Europe is examined by probability weighting
each winter from 1950/1 to 2006/7 based on the NAO Index 1 value occurring in that winter
and our forecast probability of that same NAO index value occurring in winter 2007/8. The
distribution of NAO Index 1 values is taken to be normal. The implied forecast anomaly in
each weather parameter by grid point follows by taking the mean of the individual probability
weighted yearly values for that particular weather parameter at each grid point.
Figure 2 displays the resulting most likely anomalies in temperature, precipitation and windspeed
for winter 2007/8. These show for Northwest Europe that anomalies in temperature are
typically 0.0°C to 0.5°C below norm, precipitation is close to norm, and windspeed
is 10-20% below-norm. Over the central and eastern Mediterranean and southeast Europe precipitation
is 10-30% above norm. Although these are the most likely projections, the uncertainties are
high.

Fonte: UCL - North Atlantic Oscillation Forecast for Winter 2007/8 (PDF)

 

[Vince]

Do Jeff Masters a propósito do degelo no Ártico e de como pode influenciar o Inverno.

With one third of the Arctic ice cap already gone, and another month of melting to go, we need to consider what effect this will have on weather, climate, and sea level rise. Well, we don't need to worry about sea level rise, since the polar sea ice is already in the ocean, and won't appreciably change sea level when it melts. However, the remarkable melting of the ice cap will likely lead to unusual weather patterns this fall and winter. The lack of sea ice will put much more heat and moisture into the polar atmosphere, affecting the path of the jet stream and the resultant storm tracks. Expect a much-delayed arrival of winter to the Northern Hemisphere again this year, which may lead to further accelerated melting of the ice cap in future years.
http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=764&tstamp=200708

 

[Vince]

E a previsão do CPC Noaa

http://www.cpc.ncep.noaa.gov/products/people/wwang/cfs_fcst/

 

[Aurélio]

Tenho um bocado de dificuldade em perceber todos os mapas que foram aí colocados por ti, mas os gráficos respeitantes ao NAO parecem mostrar que o mediterranio central e Oriental, e ligeiramente o Sul de Portugal e Espanha deverá ser ligeiramente mais chuvoso do que o normal.
Vince tb partilhas da opinião sugerida pelo modelo do NAO ou achas que os modelos do CPC Noaa é mais valiosa no que á precipitação diz respeito!!

 

[Seavoices]

Tenho um bocado de dificuldade em perceber todos os mapas que foram aí colocados por ti, mas os gráficos respeitantes ao NAO parecem mostrar que o mediterranio central e Oriental, e ligeiramente o Sul de Portugal e Espanha deverá ser ligeiramente mais chuvoso do que o normal.
Vince tb partilhas da opinião sugerida pelo modelo do NAO ou achas que os modelos do CPC Noaa é mais valiosa no que á precipitação diz respeito!!

O mapa que vÊs acima indica uma estimativca a longo prazo, por período mensal, do valor médio de temperatura para a época (entre 1982-2003).

O que o mapa mostra é que na zona ocidental e mediterrânica, a média das temperaturas será cerca de 0,5 graus mais baixo que naquele período de referencia.

Ora, neste ponto apenas que poderiamos esperar um inverno um pouco mais frio. Não quer dizer que vamos ter muitos dias de neve e que não quer dizer que não iremos ter dias de 'pouco inverno' durante os próximos meses já que os fenómenos em Portugal dependem muito de outras variáveis (AA Açores, JetStream, etc)

Mas para já dá para ter uma ideia breve do que nos espera!