Autumn lingered longer than normal this year. New Zealand had its warmest May on record, due mainly to the persistence of anticyclones over the Pacific Ocean to east of New Zealand.
At the same time the Tasman Sea became a breeding ground for low-pressure systems. Fronts and troughs between the highs and lows were sometimes enhanced by extra moisture as they moved on to New Zealand.
This explains the abundance of tornadoes and waterspouts around the country during May and June. The Hawke's Bay flooding soon after Anzac Day was caused by a low to the north and a high to the south combining together like an eggbeater to focus the rain.
La Nina is the name given to the pattern that occurs when the thermostat for the global weather engine, as measured by mid-Pacific sea surface temperatures, is running cooler than normal.
It can take several months after a La Nina fades from the ocean for this change to work its way around the world via the atmospheric weather engine. This lag effect explains why La Nina-related weather systems continued to affect us during May and the first part of June. A parameter that we use to measure La Nina's impact on the atmosphere is called the Southern Oscillation Index (SOI). This dropped dramatically in late May and since then has been hovering just above zero, confirming that the atmosphere has finally moved into neutral territory.
A neutral balance between La Nina and El Nino gives extra variability and variety to our weather, with the Pacific Ocean having only a weak controlling influence on our weather patterns.
Passing high-pressure systems are expected to fade in the Tasman Sea and rebuild over or east of the Chatham Islands. Some of these systems, when east of us, may feed a moist and mild northeasterly flow on to the North Island, enhancing any approaching fronts. The Tasman Sea should continue to be a breeding ground for low-pressure systems. During these colder months we can expect these systems to bring snowfalls, especially to the North Island mountains, in spite of the slow start to winter.
The coldest part of the year is expected to be late July to early August and then things should start to warm, slowly at first. For the next few months, deep lows moving eastwards in the Southern Ocean are likely to bring occasional bursts of polar-chilled southwest wind on to southern New Zealand, along with squally showers. These episodes may well be followed by a few days of frosts and icy fogs for the South Island inland basins.