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WHAT'S AHEAD – WEATHERWISE WHAT MAKES A FICKLE SUMMER? BY DANIEL CORBETT, METEOROLOGIST T his summer has been quite different to the summer of 2010-11, in some cases you might say it has been rather fickle. It was wetter and cloudier than normal in many places and for many holidaymakers it was a summer they would rather forget, too! SO WHY WAS THE SUMMER SO FICKLE? Well there are several things to look at: • The southwest Pacific experienced weak to moderate La Nina conditions, in a broad sense the sea temperatures along the equator help to drive the weather engine. In a La Nina, slightly cooler than normal water near the dateline at the equator causes the sub-tropical water to migrate further south than normal, which in turn spreads northeast winds on to northern New Zealand. This wind direction picks up moisture as it moves across the tropics on its way to New Zealand. • One reason the weather got "stuck" over the New Year period and early in February was because of something known as atmospheric blocking. Imagine if you wedged a large boulder into a stream. The water in the stream is diverted around the boulder. This is not a damming of the water but just a diversion. The air flows like a fluid and is sometimes diverted around the feature in the upper atmosphere. This causes surface highs and lows to stall and become blocked. When the atmosphere gets blocked you end up getting more precipitation in one area and drier conditions in another. • Another piece of the fickle summer was the strength of the prevailing westerly winds in the Southern Ocean. The polar jet stream, the river of air in the upper atmosphere that drives and develops weather systems, has been more active than usual for the time of year. One way we measure how active it is, is by using an index called Southern Annular Mode (SAM). The index is based on pressure readings from points around the southern hemisphere. A positive index would indicate a more westerly flow, and a negative index would mean less and perhaps favour some sort of polar outbreak. Over the past month or so the SAM index has remained mostly positive, so this has been another major contributor to our fickle summer. Note the blue (cool) zone near the dateline and the equator – that's La Nina in the ocean LA NINA COMES TO AN END A La Nina episode in the tropics tends to encourage high- pressure systems to track a little further south than normal around New Zealand. This is consistent with the large highs that blocked well east of New Zealand. The relaxing of the current La Nina over the next few weeks should allow the anticyclones to take a path across central and northern New Zealand rather than southern New Zealand. If this trend continues then we are likely to see west and northwesterly airflows blow from Australia to parts of New Zealand, bringing episodes of warm, dry air. Between the anticyclones, fronts and troughs are expected to cross the country, increasing the rainfall in southwestern areas. The strengthening westerlies could make some of the approaching fronts quite active with heavy rain and strong winds, followed by cooler, southerly winds. There are other factors at work as well, and these can add a bit of variety and some extremes to the weather patterns over the next few months. In summary it looks like we can expect an active autumn this year. Graph of the Antarctic Oscillation (A proxy for the Southern Annular Mode – SAM) for the last 120 days from the Climate Prediction Centre (NOAA – USA) Notice the blue dips in early November, this was associated with our snow around Guy Fawkes Day. The dips in late February/ early March were associated with the active troughs that brought heavy rain and severe gales Daniel Corbett trained in the United States as an aviation and industrial forecaster. He worked for the British Met Office and the BBC for many years before joining the New Zealand MetService in mid-2011. May/June 2012 Professional Skipper 59 So the summer of 2011-12 was fickle in nature but what is in store as we head into autumn? First, we take a look at a couple of different background parameters. THE OCEAN The moderate strength La Nina is still showing in the ocean as cooler-than-normal sea across the equatorial Pacific but recently these waters have begun to warm across the eastern equatorial Pacific; the usual indicator that a La Nina has passed its sell by date. The warmer zone of water that stretches from the Coral Sea down to the Kermadecs is still showing but has eased recently and the waters around New Zealand have warmed in places slightly, too. Global models are picking this La Nina episode to end in the next couple of months.