By James Renwick
New Zealand sits astride the middle latitudes in western Pacific Ocean, exposed to wind and weather from all quarters. Yet we do not usually receive our rain and storms from all quarters. Because the flow of the winds is normally from the west, most of the rain New Zealand experiences arrives from somewhere to the west or the southwest of us. Combine that with our mountainous topography (which acts as a barrier to the westerly winds) and you have a recipe for lots of rain in western regions and relatively dry conditions in the east.
The great Southern Oceans are the home to most of the storms in the Southern Hemisphere, as seen in Figure 1, and the majority of the rain-bearing storms that affect New Zealand start life in the Southern Hemisphere “storm track” southwest of Australia. As weather forecaster and poet Erick Brenstrum eloquently put it in his poem “Houghton Bay” –
White surf breaking
on a south coast
driving against the wind’s flat hand.
I look away to sea
the south the south
the mother of storms
her long silence
touches my heart.
Storms generated over the ocean south of Australia often sweep up into the Tasman Sea and run across New Zealand, bringing much of the rain we receive through the year. The tracks of storms vary on a weekly basis, sometimes lying farther south, closer to 60°S, and sometimes lying farther north, closer to 40°S, crossing New Zealand. This variation is what’s known as the “Southern Annular Mode” or the SAM. When the SAM is “positive”, the storms and strong westerly winds tend to lie far to the south of New Zealand and we bask under high pressure systems and generally dry conditions (NIWA’s Water and Atmosphere (PDF); Nature). When the SAM is “negative”, the storms and strong westerly winds tend to lie over New Zealand and bring windy, unsettled and wet conditions to much of the country, as shown in the figure below. While rain-bearing storms can swing by New Zealand from the southwest and west at any time, they tend to come much more frequently during the negative SAM, but they usually stay away during the positive SAM.
The other main area where storms and rainfall over New Zealand come from is the north Tasman Sea and the subtropical western Pacific. Though less frequent than storms coming from the southern oceans, those that develop over the north Tasman Sea or in the subtropics can pack a major punch when they come, since they tend to bring warm subtropical air laden with moisture. These northern visitors tend to affect mostly the north of the country, from Northland through to Gisborne, as they often run to the east of the country. Northland in particular is exposed to such storm systems, and has felt their effects many times in the past few years.
In the winter, the area over the north Tasman just east of Australia is where a lot of “east coast lows” form, many of which move across or near to the North Island, sometimes bringing large amounts of rain. In the summer and autumn, tropical cyclones forming to the north of New Zealand can transform into mid-latitude storms and can pass close enough to New Zealand to bring heavy rain and strong winds, especially to the north and east of the North Island. Some of the country’s most damaging and memorable storms, such as the “Wahine” storm and cyclone “Bola” have been the result of these former tropical cyclones.
The tropical El Niño/La Niña cycle also has an effect on the track of storms near New Zealand, both the storms from the southern oceans and those from the subtropics. During El Niño, especially in summer, the storm track is dragged north to lie near New Zealand and we tend to have cool, windy summers with increased rain in the west. El Niño summers tend to coincide with negative SAM episodes too. During La Niña, the storm track tends to be pushed south and the positive SAM is often in evidence, especially during summer. Looking to the north, during La Niña years subtropical storms tend to concentrate over the western Pacific and La Niña summers often see increased rainfall over the north and east of the North Island. In El Niño years, subtropical storms range more widely across the South Pacific and relatively fewer visit our northern shores.
Variations of El Niño and the SAM aside, storms continue to cross our shores, most often from the Tasman Sea to the west. The topography of New Zealand ensures that the west coast will stay the wet coast and Canterbury and Hawkes Bay will remain the most drought-prone parts of the country.
Dr James Renwick is Professor of Physical Geography, Victoria University of Wellington.