During El Nino, the trade winds weaken, leading to a rise in sea surface temperature in the eastern equatorial Pacific and a reduction of upwelling off South America.
Heavy rainfall and flooding occur in Peru, with drought over Indonesia and Australia. The supplies of nutrient-rich water off the South American coast are cut off due to the reduced upwelling, adversely affecting fisheries in that region.
In the tropical South Pacific the pattern of cyclone occurrence shifts eastward, so there are more cyclones than normal in areas such as the Cook Islands and French Polynesia.
Even closer to home 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 would bring 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.
The enhanced westerlies 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.
In winter, the winds tend to be more from the south, bringing colder conditions to both the land and the surrounding ocean. In spring and autumn southwesterlies tend to be stronger or more frequent, providing a mix of the summer and winter effects.
It is also worth noting that not all El Ninos are the same and not all bring the typical wet in the west and dry in the east scenario.
The weather over the next few weeks will take on a typical late autumn/early winter theme with some subtle variations at times but will also hold a few hints of what type and strength of El Nino may unfold as the rest of the season progresses.
Other factors such as SAM (Southern Annular Mode), local sea surface temperature distribution as well as atmospheric blocking can all play a part in the make-up of our weather over the season.
What is to come?
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 northeastern areas. Keep an eye out for any blocking pattern over the next few weeks.
In a typical period of transition into El Nino-like weather patterns, there should be enhanced westerly winds -- but there can be breaks in this periodically especially when a block sets up.
This in turn can cause the weather to get stuck and remain in a particular place for several days or more. The type of weather a place receives depends on where in the block they get stuck i.e. wet or dry.
