Seasonal Climate Outlook March - May 2025

Outlook Summary

• In February 2025, oceanic and atmospheric conditions remained broadly consistent with a weak La Niña.
• The ocean and the atmosphere in the tropical Pacific are not strongly nor consistently coupled, contributing to the general weakness of this event and some of its unusual characteristics.
• La Niña-like signals in both the ocean and the atmosphere have weakened significantly very recently.
• International guidance indicates about a 65% chance for the tropical Pacific to transition to ENSO-neutral conditions during March – May 2025.
• Overall, a mix of anticyclonic flows and easterly quarter flows are expected over New Zealand over the autumn season.
• Seasonal air temperatures are expected to be above average for the north and west of both Islands, with about equal chances for near average or above average seasonal temperatures for the east of the North Island and the east of the South Island. However, as the season progresses, the threat of frosts will increase.
• Autumn rainfall totals, as a whole, are expected to be near normal or below normal for the west of the North Island and the west of the South Island, near normal for the north of the North Island and north of the South Island, and near normal or above normal rainfall in the east of both islands. Unusual dry spells can be expected at times, especially early in the forecast period. Sub-seasonal projections of rainfall and dryness are updated daily through the NIWA35 forecast.
• Late summer through early autumn is the peak of the tropical cyclone season. The risk for an ex-tropical cyclone to pass within 550 km of New Zealand is normal-to-elevated through April.
• Ocean waters remain much warmer than average around the South Island, with extended areas offshore as well as Cook Strait experiencing marine heatwave (MHW) conditions. Global climate forecasts show Sea Surface Temperatures (SSTs) remaining above average in particular around the South Island over the next three months. NIWA’s SST update will continue to track this situation.
• March – May 2025 soil moisture levels and river flows are about equally likely to be near normal or below normal in all regions of New Zealand, except for the east of the South Island, where near normal soil moisture levels and river flows are likely.

Regional predictions for March – May 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 as a whole. There will inevitably be relatively wet and dry days, and hot and cold days, within a season. 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.

Northland, Auckland, Waikato, Bay of Plenty

• Temperatures are most likely to be above average (45% chance).   
• Rainfall totals are most likely to be near normal (45% chance) for March – May 2025 as a whole. Dry spells are likely, including the beginning of the outlook period.

• Soil moisture levels and river flows are about equally likely to be near normal (40% to 45% chance) or below normal (45% chance).

	Temperature	Rainfall	Soil moisture	River flows
Above average	45	25	10	15
Near average	35	45	45	40
Below average	20	30	45	45

Central North Island, Taranaki, Whanganui, Manawatu, Wellington

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are most likely to be above average (50% chance).   
• Rainfall totals are about equally likely to be near normal (40% chance) or below normal (35% chance).  Dry spells are distinctly possibly, including the beginning of the outlook period.
• Soil moisture levels and river flows are both about equally likely to be near normal (40% to 45% chance) or below normal (45% to 50% chance).

	Temperature	Rainfall	Soil moisture	River flows
Above average	50	25	5	15
Near average	35	40	45	40
Below average	15	35	50	45

Gisborne, Hawke’s Bay, Wairarapa

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are about equally likely to be above average (40% chance) or near average (35% chance).   
• Rainfall totals are about equally likely to be near normal (40% chance) or above normal (35% chance). 
• Soil moisture levels and river flows are both about equally likely to be near normal (40% to 45% chance) or below normal (35% to 40% chance).

	Temperature	Rainfall	Soil moisture	River flows
Above average	40	35	25	15
Near average	35	40	40	45
Below average	25	25	35	40

Tasman, Nelson, Marlborough, Buller

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are most likely to be above average (45% chance).
• Rainfall totals are about most likely to be near normal (45% chance).
• Soil moisture levels and river flows are both about equally likely to be near normal (45% chance) or below normal (40% chance).

	Temperature	Rainfall	Soil moisture	River flows
Above average	45	30	15	15
Near average	35	45	45	45
Below average	20	25	40	40

West Coast, Southern Alps and foothills, inland Otago, Southland

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are most likely to be above average (50% chance).
• Rainfall totals are about equally likely to be below normal (40% chance) or near normal (35% chance). Periods of unusual dryness are likely, interspersed with rainfall events.
• Soil moisture levels are about equally likely to be below normal (45% chance) or near normal (40% chance). River flows are equally likely (45% chance) to be below normal or near normal.

	Temperature	Rainfall	Soil moisture	River flows
Above average	50	25	15	10
Near average	35	35	40	45
Below average	15	40	45	45

Coastal Canterbury and the nearby plains, east Otago

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are about equally likely to be above average (40% chance) or near average (35% chance).
• Rainfall totals are about equally likely to be above normal (40% chance) or near normal (35% chance).
• Soil moisture levels and river flows are most likely to be near normal (45% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	40	40	25	25
Near average	35	35	45	45
Below average	25	25	30	30

Graphical representation of the regional probabilities

The Southern Oscillation Index (SOI) was indicative of neutral conditions in February, with a value of +0.29, it also showed significant intra-seasonal variability over the course of the month, indicating that the tropical Pacific ocean – atmosphere system is not strongly nor consistently coupled.

During February, upper-oceanic heat content (temperature anomalies in the upper 300 m of the ocean) was above average in the western tropical Pacific. Anomalies exceeding +2^oC are extending south of the Equator from the Maritime Continent to the International Date Line, an eastward expansion compared to last month. Significant negative anomalies were present east of the International Date Line in the central and eastern Pacific, reaching -2^oC in places.

The subsurface equatorial Pacific remained cooler than normal between 50m and 150m depth east of the International Date Line, but the cooler waters (i.e. anomalies exceeding -5^oC) have shifted eastward and towards the surface and are confined above 100m depth east of ~ 120^oW. The West Pacific Warm Pool remained warmer than normal in February and positive anomalies exceeding +3˚C are present just west of the International Date Line between about 50 m and 150 m depth. Sub-surface temperatures remained much higher than normal in the eastern Indian Ocean, exceeding +5˚C anomalies at about 90˚E.

Weak and atypical La Niña-like conditions are currently still present in the Pacific, but some indicators have recently shown a significant weakening. As was the case last month, the coupling between the ocean and the atmosphere is not strongly and consistently sustained, and other climate drivers such as the Indian Ocean Dipole (IOD) and the Southern Annular Mode (SAM) are in the neutral range. Inconsistent extra-tropical teleconnections are therefore to be expected. For New Zealand, higher mean sea level pressure than normal is expected to the south and east of the country, extending at times over the country, with occasional periods of easterly quarter flow anomalies.

Based on the dynamical and statistical models monitored by NIWA, the current La Niña conditions are expected to wane over the forecast period, and a transition to ENSO-neutral conditions is most likely (about a 65% chance) during March-May. While long-range ENSO forecasts tend to be less reliable as one enters the autumn season, the guidance available suggests the possibility of re-emerging La Niña conditions next summer.

The Southern Annular Mode (SAM) was slightly negative overall during the month of February (65% of days negative) but remained in the neutral range, with the monthly average anomaly being -0.26std. The GEFS forecast is for the SAM to remain in the neutral range over the next two weeks.

The Indian Ocean Dipole (IOD) index was also in the neutral range in February 2025, with the average anomaly for the month of February 2025 being -0.07^oC. The guidance from the BoM is for the IOD to remain neutral throughout the forecast period.

During February, convective forcing associated with the MJO propagated into the western Pacific (phases 6 -7) and then weakened considerably. This MJO signal temporarily counteracted the general La Niña pattern (i.e. enhanced convection over the Maritime Continent, decreased convection around the International Date Line, and enhanced trade winds) and was partly responsible for the intraseasonal variations recorded in the SOI.

A weak MJO pulse is forecast to initiate in the western hemisphere at the beginning of March and weaken thereafter, and is not forecast to significantly affect the Maritime Continent and the western Pacific.

Ocean waters remained much warmer than normal in February 2025 around the South Island, and Marine Heat Wave conditions (five or more consecutive days above the climatological 90^th percentile) are present over extended areas off the west and east coasts of the South Island, as well as in Cook Strait. Model guidance generally suggests that near normal to abovenormal ocean temperatures are likely, especially around the South Island, during the March – May 2025 period, i.e. the current SST anomaly pattern is expected to persist.  NIWA will continue to monitor the situation in the SST update.

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