Weather Eye with John Maunder

New Zealand's NIWA has an informative review of the Southern Annular Mode (SAM) written by Jim Renwick and David Thompson.

The Southern Annular Mode (or SAM) is a ring of climate variability that encircles the South Pole and extends out to the latitudes of New Zealand. (Its counterpart, the NAM, centres on the North Pole and affects climate in the Northern Hemisphere.)

The SAM involves alternating changes in windiness and storm activity between the middle latitudes, where New Zealand lies (40-50°S), and higher latitudes, over the southern oceans and Antarctic sea ice zone (50-70°S).

For further Infomation about a wide range of weather/climate matters see my new book "Fifteen shades of climate... the fall of the weather dice and the butterfly effect".

The extract below on "Southern Annular Mode (SAM) " is from pages 163-166 of my book.

SAM in its positive phase is associated with relatively light winds and more settled weather over New Zealand latitudes, together with enhanced westerly winds over the southern oceans. In the opposite (negative) phase, the westerlies increase over New Zealand, with more unsettled weather, while windiness and storm activity ease over the southern oceans.

Identifying the SAM

The SAM was first identified in the 1970s. On a week-to-week basis, it flips between states – causing either windier or calmer weather over New Zealand latitudes – in an unpredictable way, apparently at random. Though these phase changes of the SAM cannot be predicted more than a few days in advance, once changed, the phases tend to persist for several weeks. In recent years, scientists have noticed a trend in the SAM towards more periods of the positive phase, with a tendency towards strong westerlies over the southern oceans and lighter winds over the middle latitudes. The trend appears to be related to the Antarctic ozone hole, and the influence of the stratosphere on the weather lower down. The question is: what effect is this having on weather and climate in New Zealand?

During the summer of December 2005-February 2006, Renwick and Thompson of NIWA examined rainfall and temperature data from stations around New Zealand, looking for days when the SAM was strongly positive or strongly negative. Then they compared daily climate statistics over New Zealand for the two extremes of the SAM.

Initial results suggested some striking effects on New Zealand climate, consistent with the kinds of wind and pressure changes outlined above. The data revealed lower than normal rainfall and higher than normal temperatures throughout western parts of the North and South Islands; these rainfalls and temperatures are consistent with the weaker than normal westerly winds in those regions. Precipitation was as much as 5 mm less per day over the west coast of the South Island, and temperatures were at least 0.5°C higher in a broad region stretching from Auckland to the Fiordland. The negative SAM phase showed the opposite picture, with cooler and wetter conditions in the west of both islands. These temperature and precipitation anomalies in the western half of the country are statistically significant, but the effects along the east coast of the country are weak and are not statistically significant.

Each positive or negative SAM event tends to last for around one to two weeks, though longer periods may also occur. The time frame between positive and negative events apears to be quite random, but typically in the range of a week to a few months. The effect that the SAM has on rainfall varies greatly depending on season and region. In the Australian region (www.bom.gov.au) SAM has three phases: neutral, positive and negative.

In a negative SAM phase, the belt of westerly winds expands and is positioned more northwards towards the equator and Australia. This results in stronger than normal westerly winds, lower atmospheric pressure, more cold fronts and more storm systems over southern Australia.

Typically this means that there are more rain events in winter for southern Australia. However, in eastern Australia, the northward displacement of the westerly winds means less moist onshore flow from the east, and thus decreases rainfall for eastern Australia. New Zealand lies mid-way between the tropics and the Southern Ocean, and our day-to-day weather can arrive from either direction. The following are abstracts relating to SAM and ENSO are from the website of the New Zealand Meteorological Service.

On a longer time scale (weeks to months), tropical weather is influenced by the state of the El Niño-Southern Oscillation (ENSO). The ENSO climate pattern affects wind flows, rainfall distribution and sea temperatures right across the tropics, from equatorial South America to Indonesia. It also has some effect on New Zealand's climate (winds, temperatures and rainfall patterns), although generally to a lesser degree than in the tropics. This is because weather patterns to the south of the country also play their part. The word ‘annular' means ‘ring-like', and that is exactly what the SAM does – it controls the ring of westerly winds that circle the South Pole.

The Annual average SAM index from 1887-2016. published by the NZ Department of Statistics is shown below.

The Annual average SAM index from 1887-2016. published by the NZ Department of Statistics is shown below.

Source:GEOMAR, Helmholtz Centre for Ocean Research Kiel (Visbeck LDEO) ,and National Weather Service Climate Prediction Center (CPC).

The SAM produces alternating bands of windiness and storminess between the higher latitudes (the Southern Ocean and Antarctica) and the mid-latitudes (e.g. the New Zealand region). When the SAM is positive, pressures are lower than normal over Antarctica, and westerly winds are stronger than normal over the Southern Ocean. Over New Zealand itself, pressures are higher than usual, particularly over the South Island. Relatively light winds (weaker westerlies) and settled weather prevail over much of New Zealand during the positive phase of the SAM.

Daily values of the Southern Annular Mode, as observed between 1 September 2014 and 1 February 2015 show there was a sustained period of negative SAM in late October and through November 2014, and the strongly positive SAM phase during December 2014 and most of January 2015.

In contrast, during the negative phase of the SAM, pressures are higher than usual over Antarctica, and westerly winds are weaker than normal over the Southern Ocean. Lower pressures dominate over the South Island, and stronger, stormy westerlies affect most of New Zealand. Evidence of the SAM in New Zealand can be seen in the seasonal rainfall and temperature. The response in the positive phase is roughly the reverse of that seen in the negative phase. However, the effects do vary by season and by region, and typically the most significant changes occur in western regions of both Islands. It is, on average, warmer and drier than usual during positive phases of the SAM, and wetter and cooler than usual in the negative phase, for western regions of the country. The response in eastern parts of the country is much less clear-cut. Responses tend to be small, or weakly opposite that seen in the west of the country. Some details are given for the positive phase of the SAM in the text below.

During the positive phase of the Southern Annular Mode (SAM) the following climatic factors prevail:

• Ring of westerly winds contracts towards Antarctica.

• Stormy Southern Ocean.

• Higher pressures over the South Island.

• Warmer summer days for western and inland parts of both Islands, also Southland and Otago; cooler summers elsewhere along the eastern coastline of both Islands.

• Warmer winters right across the country.

• Drier than normal summers for western and inland parts of both Islands, also the eastern South Island south of about Banks Peninsula. • Risk of wetter than usual summers for Northland.

• No clear signal in summer rainfall for Nelson, Marlborough, Bay of Plenty, or the east of the North Island

. • Drier than normal winters for most regions, excepting Westland and Fiordland (wetter), and Nelson, Bay of Plenty, Gisborne and Hawkes Bay (no clear signal or risk wetter).

There are therefore links between the state of the SAM, and monthly or seasonal pressure, rainfall and temperature patterns in the New Zealand region. An important question is; can the state of the SAM be used to forecast our likely climate variations over the next few weeks? The answer is ‘almost'. The phase of the SAM can, and does, flip-flop week to week. But usually, once the SAM has changed phase, the phase tends to last several weeks. There are already operational forecasts of the SAM phase in use, which extend out for the next fortnight.