Seasonal Climate Outlook September - November 2026

Outlook Summary

• El Niño conditions persist in the tropical Pacific atmosphere and ocean, strengthening considerably since mid-winter. Impacts on New Zealand’s weather patterns have recently started to be felt and should persist through the outlook period.  
• Air flow patterns are expected to favour a prevailing westerly direction, characteristic of El Niño conditions, with a tendency towards southwesterly flows from October.
• Seasonal air temperatures for September  – November 2026 are most likely to be near average for the west of the South Island. They are about equally likely to be near average or above average for the west of the North Island and the north and east of the South Island. Above average temperatures are most likely for the east and north of the North Island. Despite the lack of cold signals for the season, occasional cold outbreaks are expected to occur throughout spring.
• Rainfall totals for September – November 2026 are about equally likely to be near normal or below normal for the north and east of the South Island, and the west of the North Island. Rainfall is most likely to be below normal for the north and east of the North Island. Meanwhile, rainfall is very likely to be above normal for the west of the South Island.
• Heavy rain events associated with low pressure systems from the north are expected to be relatively less frequent during the outlook period, and potentially beyond. Instead, the highest risk of notable weather systems will originate from the Tasman Sea or Southern Ocean, with heavy rain events more likely for the south and west of the country.
• Below normal rainfall for various regions through winter has led to reduced groundwater recharge. During spring, many of these regions are likely to experience reduced rainfall, more frequent dry spells, and enhanced drying of soils and vegetation, creating challenges for water-reliant sectors.
• During September  – November 2026, soil moisture levels and river flows are expected to be near normal or below normal for the east and north of both islands, the west of the North Island, and the north of the South Island. In the west of the South Island, they are expected to be near normal or above normal.
• Warmer-than-average subsurface ocean temperature anomalies have continued to strengthen and expand across the tropical Pacific, providing clear support for El Niño to intensify further and reach the very strong category early in September.
• Peak El Niño conditions are most likely to occur either in late spring or early summer, with the potential for this event to have significant impacts. It is likely to peak as one of the strongest in recent history, as anticipated over the last few months (see El Niño declared, expected to intensify into one of the strongest on record). The event is already comparable to 1997 and has shown the strongest warming trend on record.
• El Niño is expected to influence spring weather patterns, favouring periods of active westerlies, unusually windy conditions, and greater temperature variability. While these are typical spring features, they are expected to be more pronounced in 2026. The extent and persistence of these conditions will depend on nearby high pressure systems, with the greatest risk of nationwide unsettled weather currently expected during mid-to-late spring.
• Other climate drivers and intraseasonal oscillations, for example, the Indian Ocean Dipole, the Southern Annular Mode and the Madden Julian Oscillation will still contribute to New Zealand’s weather patterns, although El Niño is expected to be most dominant. In general, these other drivers are moving into phases which support El Niño-like impacts.
• Sudden Stratospheric Warmings (SSW) are historically rare in the Southern Hemisphere, but in recent years have become more common. Predictability is limited, but there is justification for enhanced monitoring until the risk reduces in October. If an SSW does occur, its impacts on New Zealand’s weather patterns can be delayed by a month or more. The SSW in 2025 occurred at intervals through September, and had impacts through December.
• Out-of-season Southwest Pacific tropical cyclones (July–October) are extremely rare. When they do occur, it is almost always under El Niño conditions. This remote risk does not currently factor into New Zealand's current Seasonal Climate Outlook, although active monitoring remains in place. The tropical cyclone season formally begins during the outlook period (1 November).

Regional predictions for September  – November 2026

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 August, subsurface ocean temperatures across the equatorial Pacific were markedly above average and intensified further, particularly in the eastern Pacific (east of ~160°W). Temperature anomalies locally exceeded +10°C, occurring between depths of around 150 m (centred near 150°W) and 50 m (close to the South American coast). These subsurface anomalies are also continuing to be expressed at the surface, hence the continuing increases in the Relative Niño 3 Index.

Positive upper-ocean heat content anomalies (0-300 m) remain firmly established along the Equator east of the International Date Line. The magnitude of this subsurface heat reservoir is consistent with other observed indicators and with dynamical and statistical forecast guidance which together suggest that the ongoing El Niño will reach very strong thresholds early in the outlook period, and provide support for further strengthening by the end of spring.

Rainfall and Convection: During August, enhanced convection and rainfall  –  indicated by negative outgoing longwave radiation (OLR) anomalies – was focused around the Intertropical Convergence Zone (ITCZ) through the equatorial Pacific. Suppressed convection and rainfall – indicated by positive OLR anomalies  – persisted across the Indian subcontinent and the Maritime Continent. This pattern of tropical rainfall anomalies has now persisted for the last two months with little change. It was strongly consistent with an El Niño state and closely resembled a canonical El Niño configuration.
Trade winds remained dramatically weakened across the Pacific, and reversed entirely at times, characteristic of El Niño.

In summary, oceanic conditions remain in an El Niño configuration, and are setting the stage for further development during the outlook period, with particularly significant warmth (positive heat content anomalies) stored in the ocean along the Equator in the Pacific. These signals have continually strengthened over the last few months. Meanwhile, the atmosphere is also resembling El Niño. A fully-coupled El Niño has therefore emerged, although stronger ocean and atmospheric responses are anticipated through the outlook period, and likely beyond.    

The Relative Oceanic Niño Index forecasts indicate a 100% chance for El Niño conditions over the next three-month period (September  – November 2026). The IRI/CPC forecasts also support this outlook. The peak of the event itself is expected to occur in 2026, potentially during this outlook period. This still allows for several more months of strengthening, and it remains likely that the event will verify as one of the strongest on record.  

The Southern Annular Mode (SAM) was evenly split between negative and neutral values. Distinctly positive values were not observed, reflecting the lack of slow-moving high pressure systems east of the country. In September, a mixture of neutral and negative values is most likely.

The Indian Ocean Dipole (IOD) value for August is neutral (+0.2°C). Forecasts from the Australian Bureau of Meteorology indicate that the IOD is most likely to become positive over the next three month period, and is forecast to reach thresholds (+0.4°C) required to declare a positive IOD event during the outlook period. However, most forecasts have been reducing the amplitude of this IOD event, due to widespread warming across the Indian Ocean.

Early in August, the Madden Julian Oscillation (MJO) emerged into the western Pacific again; it did not make significant eastwards movement, and was likely interacting with the strong El Niño base state. Typical MJO measurements are likely impacted by this interaction, and potentially by seasonal factors in the Northern Hemisphere tropics and sub-tropics. In comparison to the last 6-9 months, the impact of the MJO on New Zealand’s weather patterns was reduced, and its influence may continue to be reduced during the outlook period.  

Local Sea Surface Temperatures (SSTs) are near average or above average around the majority of the country, and marine heatwave (MHW) conditions are continuing around much of the South Island and Tasman Sea. Forecasts from coupled ocean-atmosphere models for the next three-month period (see Sea Surface Temperature Update | Earth Sciences New Zealand | NIWA) indicate coastal waters should remain warmer than average around the country, especially around the South Island.

[1]The Relative Oceanic Niño 3.4 Index (RONI) is a modern way of measuring oceanic El Niño and La Niña that is complementary to traditional oceanic 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