Seasonal Climate Outlook May -July 2025

Outlook Summary

• Neutral El Niño – Southern Oscillation (ENSO) conditions are now present in the tropical Pacific.  
• The Southern Oscillation Index (SOI), and sea surface temperatures (SSTs) in the central Pacific (in the Niño 3.4 region), which are standard indices for monitoring ENSO, are in the neutral range.
• International guidance indicates about an 80% chance for the tropical Pacific to remain in an ENSO-neutral state over the forecast period.
• However, intermittent La Niña-like atmospheric patterns may still occur over the next three months.  
• For New Zealand, higher than normal atmospheric pressure is expected to the east of the country, leading to northeasterly flow anomalies that may shift to a more northerly direction towards the end of the three-month period.
• Low pressure systems forming north of the country are expected to affect New Zealand and may lead to heightened risks for heavy rainfall events.
• Seasonal air temperatures are expected to be above average across all New Zealand regions, with particularly warm overnight temperatures driving this trend.

• May – July rainfall totals are expected to be above normal for the north of the North Island. For the western regions of both the North and South Islands, rainfall is forecast to be near normal or below normal. For the rest of New Zealand, near normal or above normal seasonal rainfall totals are about equally likely. Sub-seasonal, or monthly, projections of rainfall and dryness are updated daily through the NIWA35 forecast.
• Ocean waters around New Zealand are currently much warmer than average, with extended offshore areas experiencing Marine Heat Wave (MHW) conditions. Global climate forecasts indicate that SSTs in the New Zealand region are likely to remain overall higher than average for the next three-month period, although the intensity may decrease.
• During May – July 2025, near normal soil moisture levels and river flows are expected in the north and east of the North Island. In the north and east of the South Island, near normal or above normal soil moisture and river flows are forecast. In the west of both islands, normal or below normal soil moisture are forecast, while river flows are anticipated to be in the near normal range in the west of the South Island, and below normal in the west of the North Island.

Regional predictions for May – July 2025

The tables below show the probabilities (or percent chances) for each of three categories: above average, near average, and below average. In the absence of any forecast guidance there would be an equal likelihood (33% chance) of the outcome for any of the three categories. Forecast information from local and global guidance models is used to indicate the deviation from equal chance that is expected for the coming three-month period. All outlooks are for the three months averaged as a whole. During these three months, there will inevitably be relatively wet and dry periods, as well as hot and cold periods. The exact range in temperature and rainfall within each of the three categories varies with location and season. However, as a guide, the “near average” or middle category for the temperature predictions includes deviations up to ±0.5°C relative to the long-term mean, whereas for rainfall the “near normal” category lies between 80 percent and 120 percent of the long-term (1991-2020) mean.

During April 2025, upper-oceanic heat content (temperature anomalies in the upper 300 meters of the ocean) was above average in the far western tropical Pacific. Anomalies exceeding +2°C persist south of the equator from the Maritime Continent to the International Date Line. However, east of the International Date Line, below average heat content anomalies observed last month have significantly weakened, indicating a likely dissipation of the 'dipole' in heat content anomalies (enhanced east-west temperature gradient) that was present over the past few months across the equatorial Pacific, consistent with a transition to ENSO-neutral conditions.

Consequently, while the subsurface equatorial Pacific remains warmer than normal in the West Pacific Warm Pool region between approximately 50 and 150 meters depth (with values exceeding +2°C at around 150 meters depth), the cooler than normal ocean waters east of the International Date Line that were present last month have now mostly dissipated.

Based on the consensus of dynamical and statistical models monitored by NIWA, ENSO-neutral conditions are very likely (about an 80% chance) to persist over the next three-month period (May – July 2025) and remain the most likely outcome for the remainder of the calendar year, but intermittent La Niña-like atmospheric patterns may still occur over the next three-month period. Note however that long-range ENSO forecasts issued during the autumn season tend to be less reliable than at other times of the year.

The Southern Annular Mode (SAM) was positive during the month of April (92% of days positive) and the monthly average anomaly was +1.2std. Forecasts indicate that the SAM will likely become neutral next week and remain neutral until at least mid-May.

The Indian Ocean Dipole (IOD) has remained in a neutral phase over the past few months, with the average anomaly for the month of April 2025 being +0.19˚C. The guidance from the Australian Bureau of Meteorology is for the IOD to remain neutral over the forecast period.

During April 2025, intra-seasonal convective activity associated with the Madden-Julian Oscillation (MJO) was generally weak. The MJO is forecast to remain weak in the coming weeks and is not expected to significantly affect either the tropical Pacific or New Zealand.

Ocean waters around New Zealand are currently much warmer than average, with extended offshore areas experiencing Marine Heat Wave (MHW) conditions (SSTs exceeding the climatological 90th percentile for 5 or more consecutive days). Coupled Ocean-Atmosphere Guidance indicates that sea surface temperatures in the New Zealand region will likely remain above average over the next three months, although the intensity of these anomalies may decrease.

In the absence of strong large-scale climate drivers, local and regional influences – inherently less predictable – are likely to dominate New Zealand’s climate over the forecast period.

The Relative Oceanic Niño 3.4 Index (RONI) is a modern way of measuring oceanic El Niño and La Niña that is complementary to oceanic traditional indices. While traditional oceanic indices like the Niño 3.4 Index monitor SSTs in one region, the RONI compares the average SST in the central equatorial Pacific with the average SST across the global tropics. Since tropical rainfall patterns respond to changes in ocean temperatures, this new relative index can help forecasters better determine if the equatorial Pacific is warmer or cooler than the rest of the global tropics, which has become more challenging to discern as seas warm because of climate change.

Forecast Confidence