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.

MetService’s role in monitoring volcanic ash

Monday night’s eruption at Mount Tongariro set in motion MetService’s volcanic ash monitoring process for the aviation industry. The Volcanic Ash Advisory process involves interaction between aircraft operators, Airways Corporation and MetService, with important volcanic information input from GNS Science.

This information is also provided to Civil Aviation (CAA) and is used by them to designate Volcanic Hazard zones around those volcanoes that are known to be Volcanic Alert Level 1 or higher.

MetService produces Volcanic Ash Advisories (VAA), volcanic ash SIGMETs (warnings of significant aviation hazards) and forecasts for the track of the ash cloud, using ash trajectory models and other supporting information, including reports of visible ash.

Airways Corporation notifies airlines about which routes and procedures will be affected by each level of volcanic activity, and aircraft require air traffic control clearance via specific request from the pilot to operate within a Volcanic Hazard zone.

MetService’s Volcanic Ash Advisory Centre (VAAC) is part of a worldwide chain of 9 such centres, all working together to pass on information about ash cloud as it moves around the globe. Only last year, MetService worked closely with Australia’s VAAC in Darwin to keep an eye of the movement of the ash from Chile’s Puyehue Volcano. You can read more about that event on our website here: http://www.metservice.com/ar-2011/case-study1.html

Please note that MetService does not provide ash forecasts direct to the public, as our monitoring is specific to aviation requirements. However, if you are interested in the direction of the ash cloud, you can view a graphic representation of the latest ash monitoring information on our Volcanic Ash Advisory Centre site here: http://vaac.metservice.com/wellington

GNS Science is the organisation responsible for issuing Volcanic Alert Bulletins (including the predicted ashfall area) and Civil Defence are also monitoring the situation and issuing updates as required.

All queries regarding commercial passenger flights should be directed to your airline, and your best source of information about the volcano itself is GNS Science’s GeoNet site: http://www.geonet.org.nz/volcano/activity/tongariro/

Unusual Wind Direction

Last Monday evening, having just arrived home after the short walk from Trentham Station, I remarked to my family that it was unusual for the wind to blow from the northeast in Upper Hutt. Five days later and it’s still blowing from there.

For over a week now, mean sea level pressure has been higher than usual south and east of New Zealand while pressures have been lower than usual over the Tasman Sea (to the northwest). This anomalous pattern is not confined to the surface – it extends up through the troposphere and further. This is known as a blocked pattern, where highs and lows become slow moving or stationary. Such slow moving frontal systems can result in heavy and sometimes intense rainfall, and we have seen some of that in the Severe Weather Warnings affecting south Canterbury, Waikato, Coromandel and Gisborne this week.

Animation of MetService MSL analyis charts at 6 hour intervals over the Tasman Sea and New Zealand area, 28 July to 3 August 2012.

The animation here shows the high pressure to the southeast as either a ridge or an anticyclone. The depression over the Tasman Sea has gone through the complete life cycle from early stages of contrasting air masses on 28 and 29 July, through development and deepening on 30 and 31 July, to mature and filling in on 1 and 2 August. Today (3 August) there is a fairly uniform air mass of rotating belts of convection.

Path of the centre of the low (the point of lowest pressure) starting from midday 28 July (top left of map) and ending 6am 3 August 2012.

This simple map shows the meandering path of the centre of the depression during this time. The track starts near the north boundary of the map and ends towards the western boundary.

The satellite image below shows the old low centre this afternoon (3 August) with bands of moderate convection spiralling out.

Visible satellite image at 3pm 3 August 2012 NZST from the MtSat-2 meteorological satellite, courtesy of Japan Meteorological Agency.

All this time, the wind flow over central and southern New Zealand has been from the northeast. For Wellington this has resulted in an unusually long period of wind from between east and northeast. Winds of this direction are fairly rare as shown in the diagram below, called a wind rose.

This wind rose was constructed from hourly wind observations. The length of the coloured bars is proportional to the hours of wind from that direction, where 360 is North. The colours further partition the wind into speed ranges. The longest bar represents 12.1%, and the range circle at half way represents 6% (of the time).

Diagrams such as these are called wind roses because, for most stations, the wind blows from many directions and the shape of the resulting diagram looks a bit like a flower. The wind rose here is for day-time in winter at Kelburn (central Wellington) and shows that the wind is mostly from the northwest to north or from between southeast and south. It very seldom blows from the east or west.

The graphic below traces the hourly wind speed and direction between midday 30 July and midday today 3 August 2012. The start point is at 030 13 km/h and the end point is at 080 22 km/h. Can you find the start and end points? :-)

Trace of wind speed vs wind direction of hourly wind recorded at Kelburn, Wellington, between midday 30 July and midday 3 August 2012.

Anyway, you can see that a lot of the time during these last four days, the wind at Kelburn (and most of Wellington) was a moderate easterly. And you don’t often get that for such a long period.