Sometimes, we see the atmosphere seem to repeat itself. Weather maps can have the same general ‘look’ for a couple of weeks running.
By that I mean, sometimes we see Lows for each weekend, followed by a pattern of High pressure mid-week. This is the somewhat infamous “seven day cycle” that kiwi farmers (and sailors, and surfers, and sports coaches everywhere….) like to comment on, usually over a beer! Or perhaps we see a series of cold fronts moving over the Tasman Sea, with each of them standing up and developing a wave (small low) in roughly the same place, one after another. Repeat patterns often mean a run of poor weather in a particular farming region, and an extended spell of settled weather in another (usually the district over the hill). This is because New Zealand is a land of hills and mountains, with windward rain and lee sheltering.
There are physical mechanisms that cause repeating weather maps – usually the jet streams over the Tasman Sea, which are zones of extremely strong winds high up in the atmosphere. Figure 1. A plot of the Southern Ocean storminess (also known as SAM) for 1 January 2016 – 27 March 2017. Note the extended positive phase of the SAM for the first four months of 2016 (circled in green), versus the sustained negative phase November 2016 – January 2017 (circled in purple). The weather maps in the first few months of 2016 were dominated by High pressure over and to the east of New Zealand. Much of the country ran very dry over this period under the blocking High, and it was very hot, due to frequent northerly winds. In comparison, the weather maps during summer 2016/2017 were frequented by active fronts and stormy southwesterlies, which produced wet and cold weather for most of New Zealand.
These jet streams cause certain locations to favour developing Low pressure, and other areas to favour High pressure. Another major driver of the weather map is the Southern Ocean, with its prevailing storms blowing their way around the southern hemisphere. Sometimes these storms get an oscillation going (rather like a swimming pool when the kids decide to start a whirlpool), with the storms tracks sometimes extending further north, washing up and over New Zealand, and sometimes staying away well to the south of the country. Lastly, unusual sea surface temperatures, sometimes in quite localised regions, can affect weather system intensity, too.
The MetService long-range forecasters use understanding of these mechanisms, and a whole lot of computer modelling, to forecast the next few weeks for our rural customers. The most important thing of all is to spot when the pattern change will occur. One of the most useful indicators of all is the Southern Ocean (see Figure 1).
During March 2017, the weather maps in the New Zealand region exhibited a repetitive pattern. Several slow-moving, complex (multi-centred) Lows sat over the north Tasman Sea, sending multiple humid, northeasterly rain bands across the upper North Island, and eventually, across the South Island, too. Ridging, or High pressure areas, tended to favour southern latitudes (in the vicinity of Stewart Island).
As we move into April, the pattern of the Highs favouring southern latitudes, and low pressure being partial to northern New Zealand, looks likely to stay in place for the short termm. However, at some point mid to late April, expect a sharp pattern change to the more usual circulation pattern of westerly winds firing up over the South Island.
You can catch our latest thinking about coming weather patterns at www.metservice.com/rural/monthly-outlook, including monthly forecasts of regional rainfall and temperature. If you sign up to the Monthly Outlook at www.metservice.com/emails, you will receive FREE longrange intel and forecast maps. MetService Meteorologists are also happy to answer horticultural questions on Twitter and Facebook.You can find us at MetService New Zealand on Facebook and @metservice on Twitter.
Figure 2. Mean sea level analysis for a) Left: midday 7 March 2017, b) Centre: midday 10 March 2017, and c) Right: midday 26 March 2017. In all three situations, the ridge or High pressure lies over the central South Tasman Sea, and a complex Low lies...