December Climate Watch: La Niña Forms, Southern Ocean Storms Persist

La Niña Update

• 💡 La Niña has been officially declared, with conditions present since September, but it's expected to be weak and short-lived, potentially returning to neutral by early next year.
• 🎯 The declaration does not mean sudden dramatic changes as the conditions have been on weather maps for months, but low-pressure zones northeast of Australia and north of New Zealand will be monitored.
• 📌 For New Zealand, typical La Niña effects (humidity) require high pressure to the east, but currently, high pressure to the north is blocking tropical weather.

Other Climate Drivers

• 📉 The Indian Ocean Dipole (IOD) is still negative but has significantly weakened in November and is rapidly returning to neutral, reducing the likelihood of tropical lows and rain in northern and western Australia.
• 🌊 The Southern Annular Mode (SAM) indicates an unsettled and stormy pattern over the Southern Ocean, frequently bringing cold blasts to southeast Australia and southern New Zealand.
• 🔥 Sea surface temperatures are warmer than average around much of Australia and the top of New Zealand's North Island and Chatham Islands, creating strong marine heatwaves and increasing the potential for heavier rain from low-pressure systems.

December Weather Outlook for NZ & Australia

• 🌬️ For New Zealand, the weather pattern resembles November's spring-like conditions—windy and changeable—but with many regions experiencing summer-like temperatures.
• ☀️ Australia will see more high pressure, leading to an initial reduction in wet weather for early December, though changes may occur mid-to-late month in the tropical central and western Pacific and Australia's north.
• 🚧 A high-pressure belt acts as an "invisible brick wall," separating stormy Southern Ocean systems from tropical energy, which means tropical La Niña rain often stays north of New Zealand.

Rainfall & Soil Moisture

• 🌧️ Rainfall forecasts for the next 16 days show large dry areas in Western and South Australia, while New Zealand's West Coast will receive rain, and the North Island will experience spring-like showers.
• 💧 Soil moisture is wetter in the top of New Zealand's North Island, but eastern parts of the North Island, Canterbury, and the Taranaki/Witomo zone are becoming drier, with the drought index highlighting more eastern areas.