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.

Shortest day today. Coldest day still to come.

Last night was the longest of the year, and today is the shortest day.

The time from sunrise to sunset is shorter today, by a few seconds, than the surrounding days.

The winter Solstice, or that point in time at which (from our viewpoint) the sun was furthermost to the north, occurred at 5:16am New Zealand local time.

Click here to see a previous MetService blog that has a graph of the sun’s changing position from our viewpoint during the year.

In Coordinated Universal  Time (UTC), this winter solstice was at 17:16 (hours: minutes) on Tuesday 21 June.  Some calendars printed overseas have June 21 marked as the shortest day, but for us it is Wednesday 22 June 2011.

The actual date of the solstice varies a little from year to year, and  wanders towards the 22nd as we  approach a leap year.  After the leap year adjustment is made and the solstice and equinox are corrected back toward their natural date, the 21st.

There is an old saying that goes something like this: “when the days start to get longer, the cold gets stronger”.  To help appreciate what that means, look at the past data page for a site near you from our ‘Towns and Cities’ or ‘Rural’ page.    Auckland is shown below as an example:

On our website you can read individual data points by using the ‘mouse-over’ option.

Notice how the long-term average of the minimum temperature for Auckland,computed monthly, reaches its lowest value of 1.9C in July, while the long-term average of the maximum temperature , computed monthly, reaches its lowest of 17.6C in August.  The coldest sea temperatures of the year in Auckland are just below 14C and usually occur in late August.  For Auckland, the warmest air temperatures usually occur on or within a few days of Waitangi Day and the warmest sea temperatures follow around ten days later.

In New Zealand, the coldest air temperatures of the year occur between mid-July and early August, around 3 to 6 weeks after the solstice.   This seasonal lag varies around the country, and is longer in the north than in the south.  The lag reflects the time required for the sun to warm the earth’s surface. In New Zealand, because of the strong effect of ocean temperature on our climate, the lag is strongly tied to the response of the oceans to changing sunlight.  Click here to read more. However, this lag is also seen in places with a continental climate – i.e. not directly affected by sea surface temperatures – where it reflects the time it takes to warm up the ground.

The graph above shows that, in Auckland, May 2011 was warmer than normal, about as warm as an average April.  So far, June has also been warmer than normal.  This has been caused by the frequent presence of  strong low pressure systems in the Tasman Sea, which have fed persistent, moist, mild northerly winds onto NZ, with very few days of cooler southwest winds.  There is still time for this pattern to change.  The latest seasonal weather outlook for July, located here, indicates that some parts of NZ are likely to see a change in weather pattern over the next few weeks.  For them, the ‘cold will get stronger’.