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At this time of year, the Southern Ocean becomes the key driver of our temperatures. When the southern storms frequently wash up and across New Zealand, extended periods of unsettled southwesterlies produce cooler than average temperatures across the country.
The reverse situation is characterised by mostly settled weather, with Southern Ocean lows and fronts displaced well to the south of the country.
The Southern Ocean storminess is so influential on what sort of weather maps we see in the New Zealand region, that it is monitored by MetService meteorologists. We use an index called the Southern Annular Mode (also known as SAM). When the SAM index (Figure 1) remains negative over a few weeks, this usually spells trouble – expect cold, stormy, and wetter weather to prevail.
If the SAM maintains in its positive phase for several weeks, expect relatively ‘quiet’ weather. Highs often sit to the east and south of the country. Northerly flows are common during positive SAM phases, as are above average temperatures. For those regions exposed to northerly rain, an extended spell of positive SAM weather maps may not mean drier conditions (for example, in Nelson or the Bay of Plenty). Wintry temperatures
The expectation for a negative SAM (stormy Southern Ocean) during the second half of May contributed to the MetService prediction of wintry temperatures, and a much colder than usual fortnight. This was a brave call in the face of the persistent warmth that had been observed for much of the first half of May!
You can see our forecast weekly temperature deviations, issued midMay (Figure 2). Much of the South Island was predicted to run unusually cold during the week 21 – 27 May. Weekly temperatures were forecast at between 2 and 3 degrees below the late May average. (Note that this is a very large weekly deviation!) For the week 28 May – 3 June, a colder than usual week was forecast across most of the country.
These forecast maps use ensemble data. This is just a fancy way of saying we use a group of weather models
(51 in fact), and run them as a pack out through time. The average outcome of the ensemble group often validates well, even when run out for three or four weeks. This is especially true for temperature forecasts, and for very ‘strong’ climate signals (e.g. strong Highs or deep Lows).
While you can’t find ‘within-week’ information (such as what might happen on a particular day) using weekly ensemble data, you can see the big picture. In this case, the ensembles are strongly signalling an unusually cold fortnight for late May/early June.
You can find out more at https://blog.metservice.com/EnsembleForecasting.
Going forward through winter, the ongoing state of the SAM will be key to our temperatures, and weather maps. In addition, some of the global climate models signal an El Nino in the wings for spring. However, this is not yet certain, as it depends on the ocean and the atmosphere working in tandem. You can catch our latest thinking at www.metservice.com/ rural/monthly-outlook, or sign up to free long-range information and the weekly ensemble forecast maps at www.metservice.com/emails.
▶ Figure 1: A plot of the SAM for 1 January 2017 – 28 May 2018* The sustained positive phase earlier in 2018 has been replaced by intermittent negative forays during autumn. *Forecast SAM data were used in the period 15-28 May.