what’s driven this month’s big wet?

New Zealand has experienced an unusually wet start to the summer season, marked by persistent rainfall, diminished sunshine, and a series of deadly storms. For many residents, the typical summer conditions have failed to materialize, prompting questions about the causes behind this prolonged period of inclement weather.

What the Data Shows

Climate observations confirm the experiences of New Zealanders, particularly in northern regions. Sunshine hours have been significantly below average, while rainfall totals have exceeded normal levels. In central Auckland, the Albert Park weather station recorded approximately 244mm of rain by January 27th – nearly three times the average for the month (based on the 1981–2010 average).

Mount Maunganui has also seen exceptionally high rainfall, reaching roughly 385mm by the end of January – more than four times the usual amount. Similar patterns have been observed across the upper North Island, with repeated heavy rain events contributing to soggy soils, swollen rivers, and increased risks of flooding and landslides.

The storms impacting New Zealand this summer share common characteristics, often originating in the tropics, subtropics, or the north Tasman Sea. These systems carry warm, moisture-laden air, creating the potential for intense rainfall. When this air interacts with cooler air or encounters New Zealand’s mountainous terrain, conditions become favorable for heavy precipitation.

Air forced upwards over hills and mountain ranges, especially along the Coromandel Peninsula, Bay of Plenty, East Cape, and Gisborne regions, causes rapid condensation and high rainfall totals. This explains why these areas are particularly vulnerable to subtropical weather events.

Regional Patterns and Climate Change

The lingering influence of La Niña, part of the El Niño–Southern Oscillation (ENSO) system, is a key factor contributing to the wet conditions. During La Niña, lower atmospheric pressure over Australia and the north Tasman Sea, combined with higher pressure to the south and east of New Zealand, alters typical weather patterns. This results in reduced westerly winds and increased easterly and northeasterly flows.

These northeasterly winds draw warm, humid air from the subtropics towards New Zealand. Even small shifts in wind direction can have significant effects on temperatures. La Niña is also associated with warmer-than-average sea surface temperatures around New Zealand, further increasing the potential for heavy rainfall when winds blow across these warmer waters.

Another influential factor is the Southern Annular Mode (SAM), which describes the north–south movement of westerly winds around Antarctica. A positive SAM phase, dominant this summer, brings higher pressure over the South Island, allowing subtropical storms to drift south and linger near the North Island.

Underlying these regional patterns is the broader influence of climate change, which is steadily warming both the atmosphere and the oceans. A warmer atmosphere can hold more moisture – approximately 7% more water vapor for every 1°C of warming – intensifying rainfall potential when storms develop. While climate change doesn’t directly cause individual weather systems or control large-scale patterns like ENSO or SAM, it acts as a powerful intensifier.

Studies in New Zealand indicate that climate change can increase total rainfall from intense storms by 10–20%, and rainfall intensity during the most extreme downpours by as much as 30%. Scientists are continuing to research the potential impact of climate change on the frequency and strength of La Niña and El Niño events, as well as long-term trends in the Southern Annular Mode.

Frequently Asked Questions

What is La Niña?

La Niña is part of the El Niño–Southern Oscillation (ENSO) system, a climate pattern that dominates variability across the Pacific. During La Niña, atmospheric pressure is lower than normal over Australia and the north Tasman Sea, and higher than normal to the south and east of New Zealand.

How does the Southern Annular Mode (SAM) affect New Zealand’s weather?

The SAM describes the north–south movement of westerly winds around Antarctica. A positive SAM phase, which has been dominant this summer, brings higher pressures over the South Island, allowing storms from the subtropics more room to drift south and linger near the North Island.

What role does climate change play in these weather patterns?

Climate change is steadily warming both the atmosphere and the oceans, allowing the atmosphere to hold more moisture. This intensifies rainfall potential when storms develop, increasing the risk of heavy precipitation and flooding.

As global temperatures continue to rise, extreme weather events like those experienced this season are likely to become more common. How quickly greenhouse gas emissions are reduced will ultimately determine the severity of these impacts.