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El Niño/Southern oscillation (ENSO), diagnostic discussion, issued by Climate Prediction Center/NCEP.7 October 2010. ENSO Alert System Status: La Niña Advisory
Synopsis: La Niña is expected to last at least into the Northern Hemisphere spring 2011.
La Niña continued during September 2010 as reflected by the large expanse of below-average sea surface temperatures (SSTs) across most of the equatorial Pacific Ocean (Fig. 1). All weekly Niño SST index values were between –1.3oC and –1.8oC at the end of the month (Fig. 2). In addition, the subsurface heat content (average temperatures in the upper 300m of the ocean, Fig. 3) remained below-average, reflecting a shallower-than-average thermocline in the central and eastern Pacific (Fig. 4). Convection remained enhanced over Indonesia and suppressed over the western and central equatorial Pacific (Fig. 5). This pattern was linked to a continuation of enhanced low-level easterly trade winds and anomalous upper-level westerly winds over the western and central equatorial Pacific. Collectively, these oceanic and atmospheric anomalies reflect the ongoing La Niña.
Consistent with nearly all of the forecast models (Fig. 6), La Niña is expected to last at least into the Northern Hemisphere spring 2011. Just over half of the models, as well as the dynamical and statistical averages, predict La Niña to become a strong episode (defined by a 3-month average Niño-3.4 index of –1.5oC or colder) by the November-January season before beginning to weaken. Even though the rate of anomalous cooling temporarily abated during September, this model outcome is favored due to the historical tendency for La Niña to strengthen as winter approaches.
Likely La Niña impacts during October-December 2010 include suppressed convection over the central tropical Pacific Ocean, and enhanced convection over Indonesia. The transition into the Northern Hemisphere fall means that La Niña will begin to exert an increasing influence on the weather and climate of the United States. Expected U.S. impacts include an enhanced chance of above-average precipitation in the Pacific Northwest, and below-average precipitation across the southern tier of the country. Also, La Niña can contribute to increased Atlantic hurricane activity by decreasing the vertical wind shear over the Caribbean Sea and tropical Atlantic Ocean (see the August 5th update of the NOAA Atlantic Seasonal Hurricane Outlook). Conversely, La Niña is associated with suppressed hurricane activity across the central and eastern tropical North Pacific.
Sinopse: Prevê-se que La Niña dure até à Primavera de 2011 no Hemisfério Norte.
La Niña continuou durante o mês de Setembro de 2010 que se reflecte na ampla extensão da temperatura da superfície do mar (SSTem inglês) abaixo da normal através da maior parte do Oceano Pacífico.
Figure 1. Average sea surface temperature (SST) anomalies (ºC) for the week centered on 29 September 2010. Anomalies are computed with respect to the 1971-2000 base period weekly means (Xue at al. 2003, J. Climate, 16, 1601-1612).
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Todos os valores semanais dos índices da SST estiveram entre –1.3oC y –1.8oC até aos finais do mês de Setembro.
Figure 2. Time series of area-averaged sea surface temperature (SST) anomalies (°C) in the Niño regions [Niño-1+2 (0°-10°S, 90°W-80°W), Niño 3 (5°N-5°S, 150°W-90°W), Niño-3.4 (5°N-5°S, 170°W-120°W), Niño-4 (150ºW-160ºE and 5ºN-5ºS)]. SST anomalies are departures from the 1971-2000 base period weekly means (Xue et al. 2003, J. Climate, 16, 1601-1612).
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O conteúdo calórico da subsuperfície (temperaturas médias nos 300 m superiores do oceano, permaneceu abaixo da média, reflectindo o acontecimento no Pacífico Central e Oriental. A convecção permaneceu em crescendo sobre a Indonésia e suprimida sobre o oeste e centro do Pacífico Equatorial. A todo este cenário juntou-se o aumento nos ventos alísios de leste nos níveis baixos e anomalias nos ventos de oeste nos níveis altos sobre o oeste e centro do Pacífico Leste e Central. Colectivamente, estas anomalias oceânicas e atmosféricas reflectem as condições de La Niña.
Figure 3. Area-averaged upper-ocean heat content anomalies (°C) in the equatorial Pacific (5°N-5°S, 180º-100ºW). Heat content anomalies are computed as departures from the 1982-2004 base period pentad means.
Figure 4. Depth-longitude section of equatorial Pacific upper-ocean (0-300m) temperature anomalies (°C) centered on the week of 30 September 2010. The anomalies are averaged between 5°N-5°S. Anomalies are departures from the 1982-2004 base period pentad means.
Figure 5. Average outgoing longwave radiation (OLR) anomalies (W/m2) for the four-week period 5 – 30 September 2010. OLR anomalies are computed as departures from the 1979-1995 base period pentad means.
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Consistente com quase todos os modelos dos prognósticos, prevê-se que La Niña dure até pelo menos à Primavera de 2011 no Hemisfério Norte. Metade dos modelos, dinâmicos e estatísticos, indicam que La Niña tende para um episódio relevante para o período que medeia Novembro-Janeiro antes de começar a enfraquecer.
Esta discussão é um esforço da Administração Nacional Oceânica e Atmosférica (NOAA), o Serviço Nacional de Meteorologia de NOAA e as instituições filiadas. As condições oceânicas e atmosféricas são actualizadas semanalmente.
Figure 6. Forecasts of sea surface temperature (SST) anomalies for the Niño 3.4 region (5°N-5°S, 120°W-170°W). Figure courtesy of the International Research Institute (IRI) for Climate and Society. Figure updated 14 September 2010.
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De acordo com este relatório, as condições da La Niña poderão mesmo intensificar-se durante os próximos 4 a 6 meses. Como sabeis, o evento La Niña é caracterizado por temperaturas do oceano anormalmente frias no Pacífico Tropical Central e Oriental, o que é o oposto do evento El Niño, que se caracteriza por temperaturas do oceano anormalmente quentes. Ambos os eventos podem durar 12 meses ou mais e perturbar os padrões normais de precipitação tropical e da circulação atmosférica, tendo impactos generalizados sobre o clima em várias partes do globo.
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