Polar vortex helping forecasters predict New Zealand weather

Posted: March 16, 2019 by oldbrew in Forecasting, Natural Variation, weather

What is the dominant climate for Oceania, Australia, New Zealand, and Antarctica?

They refer to the ‘no-show El Niño event’ but allow that a weak version could still show up at some point this year, in theory at least.

Move over El Niño, and make room for SAM, says NZ’s Stuff website.

While attention over the summer focused on the much-promised but yet to arrive El Niño, SAM – or the Southern Annular Mode, to give it its proper name – has been working away quietly in the background determining our weather.

Forecasters are becoming increasingly enamoured with SAM for the valuable guidance it gives of likely weather conditions up to two weeks ahead.

SAM measures the strength of the “polar vortex”, the ring of westerly winds which encircle the Antarctic.

When SAM is positive – which it has been for most of the past four months except for the second half of last month – it shows the westerly wind belt is in place south of New Zealand, trapping cold Antarctic air and the stormiest southern ocean air behind it.

The positive SAM has been a factor in the extended hot, dry spells and recent record-breaking summers in many parts of the country. It typically encourages large areas of high pressure to block to the east and drags warm, sub-tropical northerly winds across both islands.

A positive SAM will also bring a mild winter that is largely free of storms and drier than average for most places, except Gisborne and Hawke’s Bay and the South Island West Coast.

But when it flips negative, take cover. The westerlies below New Zealand fall weaker than normal, allowing those storms and polar southerlies to break through the vortex and blast north on to the country.

Continued here.
– – –
Figure 1 shows the variation in pressure, compared to normal, during the positive phase of the SAM – NZ Metservice

  1. Bulaman says:

    ENSO data has fallen off a cliff in the El Nino direction very recently. It will be interesting to see what kind of winter we end up with..


  2. Mark M says:

    Q. Who remembers the 97% drought vortex?
    A. No one.

    It’s 2003, and those who claim to see 100 years in the future claim the future is here, now …

    Kevin Hennesy, CSIRO: It’s very clear from the consensus of many thousands of scientists that global warming is real, it’s actually under way now, it’s not just something that’s going to happen in the future.
    Dr David Jones, Bureau of Meteorology: We’re just going to have to accept that life in the future may not provide us with as much water as we had in the past.

  3. Mark M says:

    09 Jul 1945 – Origin of Droughts in the Southern Hemisphere.
    Climate expert Jan Smuts knew that a cold Antarctica brought more rain & snow & a warm Antarctica brought less rain and snow.


  4. ren says:

    When El Niño works, the water vapor expands in the troposphere and affects the temperature at high latitudes. The winter polar vortex works through ozone in the stratosphere. In winter, the troposphere retreats to the tropics along with water vapor. Therefore, the stratospheric polar vortex decides about circulation at high latitudes.


  5. ren says:

    Ozone in the tropopause displaces water vapor and causes a drop in temperature on the surface. You can see it now perfectly over North America.

  6. ren says:

    The total ozone column indicates the circulation in the polar vortex and its impact on the jet stream.

  7. oldbrew says:

    Of course each pole has its own polar vortex.

    A polar vortex is an upper-level low-pressure area lying near one of the Earth’s poles. There are two polar vortices in the Earth’s atmosphere, overlying the North and South Poles. Each polar vortex is a persistent, large-scale, low-pressure zone less than 1,000 kilometers (620 miles) in diameter, that rotates counter-clockwise at the North Pole (called a cyclone) and clockwise at the South Pole, i.e., both polar vortices rotate eastward around the poles. As with other cyclones, their rotation is driven by the Coriolis effect. The bases of the two polar vortices are located in the middle and upper troposphere and extend into the stratosphere. Beneath that lies a large mass of cold, dense Arctic air.

    – – –
    Deflection of an object due to the Coriolis force is called the Coriolis effect.
    . . .
    This force causes moving objects on the surface of the Earth to be deflected to the right (with respect to the direction of travel) in the Northern Hemisphere and to the left in the Southern Hemisphere. The horizontal deflection effect is greater near the poles, since the effective rotation rate about a local vertical axis is largest there, and decreases to zero at the equator.


    Source: https://slideplayer.com/slide/5897303/ [slide show, 9 mins.]

  8. poly says:

    Your observations . .
    “This is just the old zonal versus meridional pattern which the Russians have known about for at least 60 years. The NZ “scientists” have only just woken up to the obvious”.
    and . .
    “And by the way, half of all the Earth’s atmospheric [and oceanic] mass is in the tropics. Claiming that much lower atmospheric [and oceanic] mass of the SAM [or even the polar vortex] is the driving mechanism for the world’s climate is like claiming that a dog is wagged by its tail”
    . . . . are what one would expect from those [snip – mod] pontificating from the backside of the southern hemisphere.

    [mod] tone it down next time please

  9. ren says:

    The temperature is still low in the north.

  10. ren says:

    The temperature is still low in the north.

  11. ren says:

    Can you doubt the strength of the winter polar vortex (as compared to the weak El Nino)?

  12. oldbrew says:

    Australian Bureau of Meteorology climate outlook April to June
    – issued 14 March 2019

    The Bureau’s climate model, as well as the majority of other international climate models, suggest the tropical Pacific will warm to El Niño levels during autumn.

    Read more: http://www.bom.gov.au/climate/outlooks/#/overview/summary
    – – –
    Note: Southern hemisphere autumn

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