The Old Man Southerly

A look at classical Christchurch and Wellington southerly blasts.

By Bob McDavitt, MetService Weather Ambassador

In Athens there’s a clock tower with sculpted figures depicting things which the ancient Greeks associated with the cardinal winds. When the wind blows from a certain direction it takes on certain characteristics, and some winds have seasonal components. In New Zealand this is also the case and in this article I will look at the attributes of one of our own four main wind directions: the southerly.

To understand the peculiarities of the southerly, we must first understand the rhythms of the sub-tropical ridge.

The subtropical ridge

New Zealand is located midway between equator and pole. Although our prevailing winds are westerly, we occasionally get winds from the chilly southern ocean or the warm moist tropics. Because these masses of air are of radically different density, when they encounter each other they clash rather than mix, forming cloud bands known as cold fronts. Planetary spin causes these zones of clashing winds to rotate, spiraling into large circular areas of lowered air pressure (called lows, cyclones or depressions) marked by closed isobars on weather maps. Such lows have a lifetime of only about three days. Most form and stay over the Southern Ocean where they can deepen to below 950 hPa, making the westerly winds of the “Roaring 40s�.

New Zealand’s seasonal weather is molded by the subtropical ridge, a belt of high pressure found around the globe at about 30° south, formed by the sinking of air that has risen in thunderstorms around the equator. This overturning of the atmosphere is powered by the heat of the sun, in a chaotic fashion because of clouds intermittently cutting out sunshine. You can find it on a weather map by drawing a line along the main ridges, connecting the centres of the main highs. The subtropical ridge gets only passing mention in meteorology textbooks, as the zone that divides the trade winds from the ‘roaring 40s. But in the New Zealand weather kitchen the subtropical ridge is the top chef. If you understand its rhythms you’ll never get caught out by weather change.

Above:Typical weather map near mid-winter when the subtropical ridge is northern-most

As the sun appears to go north in our winter and south in our summer, the subtropical ridge follows it. This explains our seasons. The roaring 40s get their strength from the temperature difference between tropics and pole. This difference is strongest at the spring equinox (late September), when Antarctica is at its coldest and the flywheel of the roaring 40s extends covers New Zealand, causing our ‘equinoctial gales’. At the autumn equinox (late March), the subtropical ridge is normally over central New Zealand, giving long periods of light winds and settled weather which some call ‘Indian Summer’.

El Niño weakens the subtropical ridge and tugs it north, giving us more southwesterly weather. La Niña strengthens the subtropical ridge and keeps it in the south, bringing us less southwesterly and more northeasterly weather.

Many Kiwis take it for granted that anticyclones move along the subtropical ridge from west to east. This ain’t necessarily so. Anticyclones in most parts of the world stay more or less stationary. Our anticyclones can also sometimes linger in much the same place if they’re held by static upper airflow patterns. These “blocking anticyclones� distort the subtropical ridge.

The subtropical ridge hugs the interior of Australia in winter, is pushed off to the Australian Bight in summer and skirts the Australian south coast in spring and autumn. Its relationship with New Zealand is much less well defined, and it sometimes weakens so much that lows can form and deepen over the Chatham Islands.

So the subtropical ridge, powered by the sun in a chaotic way, has a seasonal rhythm, an El Niño or La Niño wobble, and can produce either migratory or blocked anticyclones. And when it collapses just east of New Zealand, we get the Old Man Southerly.

When does winter start?

There is no official start to the season of winter. For practical reasons the three coldest months of the year are designated as the winter months: June, July, and August. Looking at it logically, the seasons are passages of time marking our orbit around the sun, and this can be conveniently split into four pats using the solstices and equinoxes as a guide. Seen from earth, the sun is directly overhead a different latitude each day, and the latitude of the over-head sun (also called the sun’s declination) gets as far south as 23.5 degrees south - the tropic of Capricorn on around December the 21st, our longest day - then, six months later it gets to 23.5 degrees north - the tropic of Cancer, our shortest day. It is logical to use these days last the start of summer and winter-and the equinoxes in-between, when the sun is directly over the equator and thus rises due east everywhere and sets due west everywhere, as the start of spring and autumn.
In New Zealand the frostiest mornings of the year are usually in early July, the coldest days in late July to early August, and the hottest days are usually around the first week of February. See weather climate extremes.
These mid-summer and mid-winter events fit in more closely with the logical definition for nominating the start of the seasons rather than using calendar months. There are other ways of marking the start of winter, such as using the first frost or a similar threshold. This has the advantage of allowing us to compare years with varying seasons. The attached graphic shows the latitude of the overhead sun (its declination) for each month of an average year:

Old Man Southerly

Early settler in Wellington coined this apt name “Old Man� to refer to the southerlies that roar across the city and usually hang around for three days. This term is not meteorological but has settled as part of Kiwi folklore. Several times a year it reaches gale to storm force through Cook Strait. I’m using this name here in a generic way for all the southerlies that form over eastern New Zealand when a low pressure system develops or deepens in the Chatham Island area.

The gales that battered Wellington on 10 April 1968 and led to the sinking of the WAHINE were an extreme example of an ‘old man southerly’

The BLACK FRIDAY Southerly - 13 Oct 2000

A memorable ‘old man’ was on “Black Friday� storm of Friday 13 October 2000. This particular event produced a narrow jet of southerly wind that brought severe gales to Lyttelton and devastated the marina. The weather map of this event shows all the main features of a full-on ‘old man’:

1. A strong high-pressure system wandering way off to the east south of Tahiti.
2. A high pressure system in the Tasman Sea.
3. A weakness in the sub-tropical ridge just east of New Zealand.
4. A low deepening between Cook Strait and the Chatham Islands.

On Wed 11 October 2000 a cold front brought heavy rain and snow to Fiordland The next day as this system moved north it encountered a warm front that had moved towards Nelson from Australia. This clash of cold air with moist air occurred underneath an upper jet stream was able to move the air off faster than it could rise, thus producing a rapid fall in surface pressure. In this case the central low pressure dropped from 1002 to 980 hPa, mostly during a 24 hour period. When this weather bomb formed, isobars were squashed between the low centre and Banks Peninsula. Banks Peninsula bore the brunt of the storm. The weather station at Le Bons Bay on its eastern side recorded a steady 60kt (111km/hr) of wind for all of the daylight hours on the 12th with a rain rate of 5mm/hr and a temperature of 6ºC. The Lyttelton harbour anemometer measured a steady 50kt (93km/hr) for several hours during the period when the marina.

A full account of this storm is available from our archived weather highlights web page at http://www.metservice.co.nz/default/index.php?alias=2000spring2194257 .

Manawatu Floods February 2004:

Sometimes the Tasman high is near Tasmania and extends a ridge to Antarctica. This helps to produce a southerly flow that shovels polar chilled air out of the Southern Ocean and onto New Zealand - a polar outbreak. Normally the air around Antarctica hugs and circulates in its own space, held by winds called the polar vortex; but occasionally these break loose and polar air spreads north like a dam-burst. There was a polar outbreak into the central Tasman Sea on another black Friday - 13 February 2004… this produced a low that spiraled cold air northwards and tropically-laden moist air southwards so that they clashed together over the lower North Island during the following weekend, producing the Manawatu Floods.

Two recent examples of the ‘Old Man’

On the 29 June and 5 July 2008 we had two ‘Old Man Southerlies’ - one weekend after another as we moved into mid winter. They behaved slightly differently because of the difference in the high south of Tahiti.

On the 29th, winds gusted between 110 and 117 km/hr at Sugarloaf in Christchurch and between 130 and 150 km/hr at Mt. Kaukau in Wellington for five hours. Slips closed the roads around Hurunui where 56 mm of rain fell in 15 hrs. Lower Hutt reported 99 mm of rain in 24 hours. Winds closed light aircraft operations at Wellington airport and swell cancelled the Cook Strait ferries.

On the 5th, snow and ice closed many alpine roads in both the North and the South Islands and a few sailings of the ferry were cancelled. Sugarloaf had wind gusts to 110 km/hr and Mt. Kaukau between 110 and 130 km/hr for 12 hours. For many places it was the COLDEST DAY OF THE YEAR SO FAR (and will be hard to beat). For the All Blacks test against South Africa it was a southerly gale and 7C, in the wind it felt as cold as -1C.

IRISH TEST IN WELLINGTON 7 JUNE 2008

This wasn’t a classic Old Man Southerly--- the low was in the wrong place. The southerly front that brought a southerly gale to Wellington for the All Blacks test against the Irish in Wellington brought gusts to 110 km/hr at Mt. Kaukau. The westerly winds ahead of this front tossed trees over in Central Otago and brought wind gusts to 170 km/hr at Castlepoint. Snow briefly closed Christchurch airport, and trapped cars at Porters Pass. The winds were strong during the game but not gale force. It was 7C, and in the wind it felt as cold as 0C. Because there was no stalling low this southerly breezed by and was effectively gone by dawn.
Right: Weather map for the Irish Test-NOT an old man southerly.

If we had a wind tower like the one in Athens, then to represent our southerly I would suggest a staunch old man who doesn’t want to go away. As a result of our recent switch from LA NINA to ENSO Neutral condition, the subtropical ridge north of us is weakening and pulling off to the north, and this puts more Old Man Southerly events on the menu for our seasonal forecast. See http://www.metservice.com/default/index.php?alias=farmniseasonal