Seasonal climate summary southern hemisphere (summer 2007-08): mature La Niña, an active MJO, strongly positive SAM, and highly anomalous sea-ice

Atmospheric and oceanic conditions in the southern hemisphere are reviewed for the austral summer (December-February) of 2007-08, with particular emphasis given to the drivers of climate variability in the Australian region and their linkages and mechanisms. In the tropical Pacific and nearby, a La Niña (cold) event dominated, peaking in about February 2008. It was the strongest La Niña since 1988 by most measures. The global tropics were also highly influenced by several strong Madden-Julian Oscillation (MJO) events, the strongest activity seen since autumn 2005. In the extratropics, the positive phase of the Southern Annular Mode (SAM) dominated many aspects of the large-scale atmospheric flow, and was the most positive occurrence of the SAM since July 1993. Each of these three large-scale drivers had a discernible influence on some aspects of the summer rainfall and/or temperature seen in regional Australia, but other aspects were apparently unrelated. For example, while the wet conditions over much of eastern and northern Australia were favoured by the La Niña, the drier than normal conditions in the centre of the continent were unlike what is usually expected during a La Niña or a highly positive SAM. For the MJO, its impacts were a strong intraseasonal modulation of rainfall in the far north of Australia and an implied role in the genesis of each of the season’s four Australian-region tropical cyclones. The wet conditions in south-central Queensland in January were, however, more a consequence of the lingering effects of an ex-tropical cyclone (Helen). Besides these known large-scale drivers of southern hemisphere climate, also of note for the summer of 2007-08 was a highly anomalous sea-ice extent around Antarctica, being the largest positive sea-ice extent anomaly since satellite records began in 1979. In December 2007 sea ice extent was 15 per cent greater than the 1979-2006 average, and in January 2008 was 33 per cent greater than average, being in stark contrast to the much-reduced sea-ice seen in the northern hemisphere in its summer of 2007.