Hi Aaron

Seasonal Outlook covering February and March 2011

Tropical Trouble

So far, this summer has been warmer than normal over northern areas and slightly cooler than normal in the south.  Several fronts and the remnants of cyclone Vania brought wetter than normal conditions to the western South Island.  The North Island has been buffeted by the remnants of cyclones Zelia and Wilma, plus a low from New Caledonia that deepened to near 985 hPa on Sun 23 Jan, as it passed just west of Auckland on a near king tide.

The Ocean

Sea temperatures in the eastern equatorial Pacific Ocean remain less than normal, indicating the oceanic impact of this La Nina episode.  There are signs now that this La Nina may have peaked, as the anomalies are starting to relax near America.  However, the Oceanic Nino Index, which is used as a comparative measure, is holding on at  -1.4.

The horseshoe of warmer-than-normal sea that surrounds this cool pool has recently become more organised. In the southern hemisphere, the warm pool is most notable stretching from the Coral Sea to the area northeast of New Zealand.

Computer models predict this La Nina episode will slowly weaken over the next few months, and there is a 50-50 chance that a weak La Nina may then continue into winter.

The Atmosphere

Southern Oscillation Index is still taking record high values for this time of the year.  The January value of 1.99 is the highest January SOI since 1974.  This is matching the abnormality seen in Australia’s recent weather.  It has relaxed from its December high of 2.7.

Seasonal Weather Outlook for late summer

This enduring La Nina is likely to continue its influence on us as we switch from summer to autumn, by shifting our normal weather zones southwards.  This provides a variety of impacts in different parts of New Zealand.

Anticyclones are likely to linger in the south Tasman Sea, bringing extended periods of warm and settled weather especially to the north and west of the North Island.  These high-pressure systems are now expected to mostly weaken as they wander off to the east.

The easterly trade winds normally found in the tropics may occasionally extend southwards onto northern and eastern parts of the North Island, bringing subtropical humidity and a few wet episodes.

During the transition from one anticyclone to the next, weather is likely to be a mixed bag of passing troughs, preceded by warm northerlies, and followed by cool southerly wind changes that may bring a few squally thunderstorms to eastern districts.

The pattern seen on 23 Jan may recur, with an anticyclone lingering to south of New Zealand as a low pressure system forms or deepens to the north.  These two systems may combine to focus a day or two of strong wind and heavy rain onto parts of the northeast of the North Island.

The cyclone risk should peak in February and then start to ease away.