Two opposing patterns predicted for summer
It might pay to standby for SAM.
Lately, there’s been a lot of talk about El Nino and a lot of air time around a hot November. Instead, as MetService expected, November temperatures swung sharply during the month.
MetService highlighted that November temperatures were likely to be extremely volatile, bouncing from unusually hot to extremely cold. Have a look at the weekly temperature anomaly charts that were issued to our horticultural clients on November 5 as seen in Figure 1.
These plots show the deviation from the normal temperature for the time of year. The strength of the colder-than-usual signal in week three is abnormally large for three weeks out, and we noted this in our commentary. This was the week of the unseasonal snow down south.
And while these weekly temperature forecasts aren’t perfect – and don’t show day-to-day detail - they do illustrate the power of ensemble-based forecasting, which you can find out more about at https://blog.metservice.com/ EnsembleForecasting].
Looking ahead to summer
With respect to summer weather from December to February, two opposing weather drivers may well be in play.
MetService monitors both the tropics, the centre of action for El Nino/La Nina, and the Southern Ocean. This is critical, given that New Zealand lies halfway between the tropics and the pole. Both drivers can contribute strongly to our weather patterns.
All the ensemble-based guidance that MetService monitors is currently indicating a relatively quiet Southern Ocean during summer. This is known as a positive Southern Annular Mode (SAM). This pattern favours northerlies across NZ, persistent highs near the Chatham Islands, and an elevated chance of a wetter-than-normal summer in northeastern regions of the country,as seen in Figure 2.
If El Nino forms this summer, and this is not yet a certainty at the time of writing, then it looks likely that we see two opposing weather patterns. Farmers and growers should not expect to see a typical El Nino summer, as seen in Figure 3, given that the Southern Ocean is forecast to act in the opposing direction during the summer period.
Bottom line - make informed decisions
Given the expected battle for the NZ weather map this summer it will pay to keep up to date with the longer-range forecasts and make informed decisions. You can sign up for free to get the Monthly Outlook emails straight to your inbox at www.metservice.com/emails. These emails include some ensemble-based forecast maps. But more importantly, they also contain the latest forecaster commentary. Even if the ensembles weren’t so handy, this alone can be worth signing up for.
▴ Figure 2: Typical summer rainfall anomaly, during moderate to strong positive phases of the Southern Annular Mode (SAM).
▴ Figure 3: Regions with an elevated chance of below normal summer rainfall during moderate to strong El Nino events (shaded orange).