Seasonal climate summary southern hemisphere (summer 2011-12): a mature La Niña, strongly positive SAM and active MJO

Southern Oscillation Index The Troup Southern Oscillation Index1 (SOI) is based on the mean sea-level pressure (MSLP) difference between Tahiti and Darwin. Sustained negative SOI values below around –8 generally indicate an El Niño event, while sustained positive values above around +8 are associated with La Niña periods. Fig. 1 shows SOI values from January 2008 to February 2012. The SOI shifted from negative to positive in April 2010 following the rapid decay of the 2009–10 El Niño (Campbell 2011). Record or near-record high SOI values were attained in several months between September 2010 and April 2011 (Lovitt 2011, Imielska 2011, Tobin and Skinner 2012), coinciding with one of the strongest La Niña events on record (Beard et al. 2011). More neutral values in winter 2011 increased significantly in spring as another La Niña developed (Cottrill 2012). This La Niña peaked during summer 2011–12, with SOI values of +23.0, +9.4 and +2.5 for December, January and February respectively, and an average summer value of +11.6. The December SOI of +23.0 was exceptional for December, exceeded only in December 2010 (+27.1) and ranking equal second alongside December 1950 (records began in 1876). The individual pressure anomalies at Darwin and Tahiti contributed relatively equally to the SOI. MSLP was well below average in Darwin during December (anomaly –2.4 hPa) but gradually moved closer to average with anomalies of –0.4 hPa in January and +0.7 hPa in February. A strongly 1The Troup Southern Oscillation Index (Troup 1965) used in this article is ten times the standardised monthly anomaly of the difference in MSLP between Tahiti and Darwin, based on a sixty-year climatology (1933– 1992). Darwin MSLP is from the Bureau of Meteorology, and Tahiti MSLP is from Météo France inter-regional direction for French Polynesia. All data are available at http://www.bom.gov.au/climate/current/soihtm1. shtml

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