What an odd year
The weather so far in 2017 has been unusually volatile – and very wet for most of the country (see Figures 1 and 2).
MetService forecasters regularly analyse weather map characteristics at the weekly to monthly time scale, to identify why certain regions of the country have run wetter or warmer, cooler or drier, than is normal. Filtering out the detail, also known as the ‘weather noise,’ means we zoom out from individual fronts, lows and highs, and instead look at the big picture to explain what is going on. We also use this technique to forecast what is coming next, at the intra-seasonal time scale (for the coming few weeks to months).
When we look at the ‘average’ weather map so far in 2017 (not shown), we see the year-so-far has been characterised by low pressures over the country, overall. This will come as no surprise to the growing community. January 2017 was particularly stormy across the entire country (excluding Northland), with wet, cold and very windy weather experienced in most regions. However, the door to the tropics soon opened once we moved into March and April. Sub-tropical, northerly airstreams produced intermittent but damaging downpours over the north and east of both Islands during autumn, with a late Cyclone
Season to boot. In contrast, High pressure favoured the southern South Island during autumn. Recently, we’ve seen the atmosphere revert to a more normal progression of westerly fronts and mobile Highs across the country.
THE SOUTHERN OCEAN
The Southern Ocean is often the lead influence on New Zealand weather maps, on time scales weeks to months. The other main drivers of our weather patterns are the Tasman Sea, and the tropics. So far in 2017, we’ve seen the Southern Ocean flip-flop with reasonable speed between its northerly and southerly phases (Figure 1) – in stark contrast to 2016, when the atmosphere became ‘stuck’ for extended periods in one gear, or the other.
The number one influence on early winter weather patterns for New Zealand will be whether the Southern Ocean continues to flip-flop regularly between phases. If it does, farmers can expect the full range of weather maps that New Zealand can offer over winter. Expect changeable, and mobile, weather systems in the New Zealand region, with the Tasman Sea and Southern Ocean both contributing at times. On the farm, expect both easterly winds and the more usual westerly regimes, rather than an extended period of any one thing. Basically, things look likely to be a real mixed-bag to start winter. >
Looking further ahead, the tropical Pacific remains in the neutral El Nino-Southern Oscillation (ENSO) range. Despite equatorial sea surface temperatures warming since the start of 2017, they remain below El Nino thresholds. The most recent model predictions have pulled back from developing a strong El Nino event. Around half of the international climate models develop an El Nino event in the second half of the year. From a New Zealand perspective, even if an El Nino were to develop towards spring, it won’t be a major player for us over the winter period. However, it will probably pay to remember one thing: the usual New Zealand response to moderate or strong El Nino events is a cold spring.
You can catch our latest thinking about future New Zealand weather patterns at http://www.metservice.com/rural/ monthly-outlook, including monthly forecasts of regional rainfall and temperature. MetService Meteorologists are also happy to answer farming questions on Twitter and Facebook. You can find us at MetService New Zealand on Facebook and @metservice on Twitter.
▶ Figure 1: Rainfall accumulation plots for Kerikeri (top map) and Te Puke (bottom map). The average annual rainfall accumulation is shown in black, the total for this year shown in red (data 1 January – 20 May 2017).
▶ Figure 2: Rainfall accumulation plots for Blenheim (bottom map) and Christchurch (top map). The average annual rainfall accumulation is shown in black, the total for this year shown in red (data 1 January – 20 May 2017).
Figure 3: A plot of the Southern Ocean storminess (also known as SAM) for 1 January 2016 – 20 May 2017. Note the extended positive phase of the SAM for the first four months of 2016, versus the sustained negative phase November 2016 – January 2017. The weather maps in the first few months of 2016 were dominated by High pressure over and to the east of New Zealand (circled in green). 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 absolute contrast, the weather maps in late 2016/early 2017 (circled in purple) were frequented by active fronts and stormy southwesterlies, which produced wet and cold weather for most of New Zealand. Notice that the Southern Ocean has been regularly switching phases since February 2017.