Not all El Ninos are the same

The word you probably heard a lot during last Summer was La Nina. As we head into the spring you might hear another term quite a bit and that is El Nino. We all know that El Ninos bring different types of weather to New Zealand compared to La Ninas but what is an El Nino? Does it always mean the same sort of weather for New Zealand? Not necessarily - this season we might not have the typical sort of El Nino weather.

So what is a typical El Nino?

It all boils down to the distribution of warm and cold water across the Pacific Ocean and this has an effect on the way that weather systems develop and move across the Pacific including New Zealand. A typical El Nino pattern would have warmer than usual water across central and eastern parts of the equatorial Pacific. The chart below shows this distribution of warm water (sea-surface temperature in degrees C.).

Image courtesy of NOAA

The anticyclonic ridge axis would also not extend as far south in an El Nino pattern. In a typical summer the anticyclonic ridge axis would be draped across New Zealand, bringing us settled spells interspersed with troughs and lows that brought unsettled weather.

A typical flow pattern in an El Nino would feature an anticyclone just to the northwest of New Zealand and an area of low pressure to the southeast. This in turn would bring enhanced westerlies across New Zealand as shown below. The enhanced westerlies would bring normal to above normal rainfall to western areas and below normal rainfall to eastern areas. Troughs would frequently cross the country in the westerlies, bringing changeable and unsettled weather.

A typical El Nino Flow pattern

What will be different about this spring?

The El Nino that will be here for spring might be slightly different than the typical El Nino because of some other factors at play.

Firstly, there have still been some hints of La Nina (the opposite of El Nino), as well as the neutral phase, in our weather patterns recently. The equatorial sea surface temperatures have continued to warm over the past several weeks. The area of above average sea surface temperatures over the eastern Pacific continues its slow progression to the west. The subsurface waters of the eastern equatorial Pacific have also shown signs of warming in the past few weeks. Global models are picking the El Nino conditions to develop within the next couple of months.

For New Zealand, an El Nino usually means that the disturbed westerly flow of the roaring forties is displaced northwards, but a fair amount of variability can be expected in the shorter term, especially until the intensity of the El Nino is better understood. As a result, the typical wind flows normally associated with El Nino patterns may not occur all the time.

Atmospheric weather patterns have recently been bucking the El Nino trend that has been set by the ocean. The Southern Oscillation index (SOI), which gives a snapshot of the weather patterns between Tahiti and Darwin, has been drifting the other way and ended July slightly above zero.

This shows that the atmosphere is currently not following the lead given by the oceans.  This is probably a temporary situation and we are likely to see a drop in the SOI during spring.

Back in June we saw a sharp early dive into wintry temperatures fed by southerly winds around anticyclones that lingered near Tasmania – see Chris Webster’s blog ‘The structure of highs’ for more about how highs work. In July this pattern was replaced by milder air and east to northeast wind flows around anticyclones to the southeast.

The weather over the next several weeks may still have some hints of variability before we ease into a more El Nino like pattern in the months to follow. The peak intensity of the El Nino will also then determine the flavour of our weather patterns through the late spring and into summer.

However, weather patterns in August and September may not be typical of a late winter/early spring El Nino transition, with temperatures and rainfall likely to be near normal in all regions, except in the north and east where rainfall could be slightly above normal. You can read more about this in our seasonal weather outlook for the next few weeks: http://metservice.com/rural/seasonal-forecast-north-island, or in the latest seasonal outlook for the next three months from NIWA: http://www.niwa.co.nz/climate/sco/seasonal-climate-outlook-august-october-2012

In a typical period of transition into El Nino-like weather patterns, there should be enhanced westerly winds – but this may not be the case with this El Nino.

Settled periods are expected, with clear sunny days and frosty or foggy mornings associated with passing anticyclones.  Fronts and troughs rolling in from the Tasman Sea may be followed by episodes of cool southwest winds lasting several days across the whole country.  Occasionally a low pressure centre may move onto the country from the north, preceded by an easterly flow with some heavy rain for north-eastern areas. Keep an eye out for any blocking pattern over the next few weeks (where the weather patterns get stuck – see our recent blog ‘Unusual wind direction’), because anomalies can sometimes occur as a result.

If you’d like to read more about El Nino and the Southern Oscillation, we recommend Erick Brenstrum’s ‘The New Zealand Weather Book’ published by Craig Potton Publishing. Although now out of print, your local bookseller may have copies in-store.

2011 – The Weather in Review

 

The temperature trend in New Zealand during the past year can be read from the graph below and tells a story. Last summer was significantly hotter than normal, and several tropical weather systems visited our northern parts in January.

Data taken from NIWA’s monthly climate summaries.

Autumn started normally enough in March and April.  Then blocking led to the dominance of north and northeasterly wind patterns, delaying the onset of winter (and the opening of the ski season) by about three weeks.  This all changed in July – click here for a more detailed explanation from our Chief Forecaster, Peter Kreft - when we experienced the coldest time of the year, with polar outbreaks on 25 July and 14 August to 16 August. Spring started a few weeks late but managed to finish on schedule.  And, of course, there was the deluge that struck Nelson in mid December.

Global temperature measurements currently score 2011 as the tenth highest on record. That data is from the global mean surface land-ocean temperature index. 2011 brought its fair share of global meteorological mayhem. In January Brisbane had its worst floods since 1974, and Brazilian landslides killed 900 people in the mountains just north of Rio de Janeiro. Rain in October eased the drought in the Horn of Africa that had been affecting around 12 million people, but the rain was so intense that it brought crop damage to Kenya. The Monsoon stalled over Thailand in October, taking the lives of 650 people in floods, notably around Bangkok.  On Sunday 18th December, Tropical Cyclone Washi became the deadliest storm of the year with landslides taking more than 1000 lives in the Philippines.

The World Meteorological Organization reports that 14 weather events, each with upwards of US $1 billion of damage have occured in North America during 2011:  heavy snow in February, the most active tornado season on record in April and May, a drought and wild fires in Texas, and the wettest year on record in the east along the Canadian/US Border, culminating in the late August with severe flooding from Hurricane Irene and Tropical Storm Lee.

 In New Zealand, 2011 started with an intense La Nina episode and large anticyclones over Chatham Islands. In the days between Christmas and New Year, an intense storm passed over the country and brought gale force winds to many areas and flooding to the Rakaia area, Pelorus Bridge camp site, Rai Valley, and Aorere River in Golden Bay.

In January there were many episodes of strong easterly winds as three lows of tropical origin brought torrential rain and gales. Former tropical cyclones Vania and Zelia produced heavy rain on the 18th on the West Coast, resulting in the Fox River bursting its banks. On 22nd/23rd, a low which formed near New Caledonia moved towards NZ producing rain that caused slips and road closures over much of the North Island, along with a significant storm surge for Auckland.  Insurance payouts for this flooding were $7NZ million. Also, Tropical Cyclone Wilma moved rapidly across the northeastern North Island on the 28th/29th, and brought severe flooding and slips to northeastern regions of the North Island. Insurance payouts for this were $20NZ million, making it the most expensive weather event in NZ in the past year (insurance payout figures from Insurance Council of NZ: http://www.icnz.org.nz/current/weather/).

MetService Weather Map showing Tropical Cyclone WILMA on 28 January 2011. This was our most damaging storm in the past year.

In February, weather conditions were generally settled.  There was a record-breaking heat wave on Waitangi Day, with Timaru Garden’s 41.3 degrees centigrade topping the list of available temperatures.  It was an extremely dry month in the North Island – the driest February in Dannevirke since records began there in 1951. By way of contrast, central Otago had twice its normal February rainfall.

March was very wet over the North Island and parts of the South. After crossing Raoul Island, Tropical Ccyclone Bune wound down as it passed east of East Cape at the end of the month.

April brought more southeast winds than usual, with wet conditions for eastern areas.  The flooding and slips in Hawke’s Bay on 26-27th April triggered evacuations and resulted in insurance payouts of $6.4million.

During May and most of June north or northeast flow patterns dominated, bringing record-breaking warmth.  Ash clouds from a South American volcano also challenged air travel. May will be long remembered for the tornado that struck Albany on the 3rd, taking the life of a worker.  Insurance payouts for this event were $6 million. On 18th June a landslide took the life of a teenager at Ohope. On 19th June tornadoes hit Taranaki, triggering an insurance payout of around $2 million.

During July and August stormy west, southwest, and southerly flow patterns dominated; click here for a more detailed explanation from our Chief Forecaster, Peter Kreft.  This was the coldest time of the year, with polar outbreaks on 25 July and 14 August to 16 August.  Snow fell across the lower North Island, with flurries in locations well north in the country.

 

MetService Weather map at midnight start of Monday 15 Aug showed a blast of southerly air from Antarctica to New Zealand.

September was characterized by southwest flows and October by northeast flows, with winds less than normal and generally settled weather because of dominant anticyclones.  Most of the Rugby World Cup games were not directly affected by the weather.  It is interesting to note that on 11th September, just 36 hours after calm clear weather for the opening ceremony, a tornado hit within 10 km of Eden Park. Then on 13th September Wellington was coated with hail (MetService tending on Twitter, such was the intensity of the event), and on 14th September winds caused damage in Auckland, Waikato and Bay of Plenty.  On 18-19th October, Dunedin andChristchurch had their wettest day in 18 months.  On Labour Day (24 Oct) wind also caused damage in Southland and central Otago.

During November,anticyclones came south earlier than normal but the frontal systems of spring remained steadfast. As a result, isobars over central New Zealand were squashed closer together than normal, causing many bouts of wind.  This produced drier than normal conditions in the northeast, and cooler and wetter than normal conditions in the southwest. The moisture causing the rainfall that flooded the Grey River in Westland on 21st came from Australia.

In mid-December a major trough stalled for about two days between a low in the Tasman Sea and a large blocking high east of New Zealand.  This brought a deluge of rain to Nelson.

 

MetService weather map for 15th December 2011 showing a front stalled over Nelson.

The most significant falls of rain occurred in the hills behind Nelson and in the lowland areas of Takaka and Richmond. The rain gauge trace seen at Kotinga in the Takaka township measured 423 mm/24 hours, a new record for this site – the previous record of 256mm/24hr was measured in August 1990.  The New Zealand record for 24 hour rainfall is held by CroppRiver at its waterfall (inland from Hokitika), at 758 mm in 24 hours ending 0620 hours on 27 Dec 1989.

 

Rainfall accumulation graph for Takaka (TAN), Kotinga (TSR) and Nelson Airport (NSA). Accumulation of rain is in mm, and timestamps are in UTC (add 13 hours to get NZDT).

 
 

The total rainfall over the country from this trough can be seen in the rain map for the 7 days ending 18th Dec 2011.

MetService rainfall accumulation map for the 7 day period ending 9am on 18th Dec 2011.

The purple areas received more rain than desired, but other parts of the country received enough rain to be useful for filling up water tanks and replenishing pastures and gardens before the holiday break.

In summary, NZ’s weather in 2011 had its ups and downs, and produced some interesting events.

Everyone at MetService takes this opportunity to wish our blog readers a refreshing break at the end of the year, and we look forward to sharing with you our insights into the interesting weather events of 2012.