Seasonal Climate Outlook February - April 2025

Outlook Summary

• A La Niña Advisory is in effect.
• As of January 2025, oceanic and atmospheric conditions in the Pacific meet most of NIWA’s criteria for La Niña.
• While coupling between the ocean and atmosphere is locally strong in the central Pacific, the event is weak and expected to be short-lived.
• International guidance for February – April 2025 indicates about a 50% chance for La Niña conditions to persist.
• Atypical characteristics are evident with this La Niña event, including the location of the coldest sea surface temperature (SST) anomalies in the tropical Pacific being shifted west of what is usually observed during historical La Niña events.
• For New Zealand, this means a blend of typical circulation patterns that are historically associated with La Niña, along with occasional atypical patterns.
• Overall, easterly quarter (NE to SE) flows are expected, along with occasional periods of westerly quarter (SW to NW) flows for the three-month period.
• Seasonal air temperatures are expected to be above average for the north and west of the North Island, with about equal chances for near average or above average seasonal temperatures for all other regions.

• Despite below normal rainfall for most locations early in the three-month outlook period, seasonal rainfall is expected to be near normal for the west of the South Island, with about equal chances for near normal or above normal rainfall for the remainder of New Zealand.
• The risk of occasional heavy rain events remains distinctly possible, particularly for the northern and eastern North Island.
• Late summer through early autumn is considered to be 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.
• Areas not exposed to rain-bearing weather systems may experience extended dry spells, such as in early February, and sub-seasonal projections of rainfall and dryness are updated daily through the NIWA35 forecast.
• Coastal SSTs ranged from about 0.4°C below average to 1.7°C above average around the country at the end of January, but marine heatwave (MHW) conditions are generally confined to coastal waters off the South Island.  Global climate forecasts are for SSTs to remain above average around New Zealand during the rest of summer and into autumn, with MHW conditions possibly returning as the three-month periods unfolds.  NIWA’s SST update will continue to track this situation.
• February – April 2025 soil moisture levels and river flows are forecast to be near normal for the east of the South Island.  For the north of the North Island, near normal river flows are forecast, with about equal chances for near normal or below normal soil moisture levels.  For the rest of New Zealand, river flows and soil moisture levels are about equally likely to be near normal or below normal.

Regional predictions for February – April 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 likely to be above average (50% chance).   
• Rainfall totals are about equally likely to be near normal (35% chance) or above normal (40% chance).  Occasional heavy rainfall events cannot be ruled out as the season progresses.
• Soil moisture levels are about equally likely to be near normal (45% chance) or below normal (45% chance), with near normal (45% chance) river flows likely.

	Temperature	Rainfall	Soil moisture	River flows
Above average	50	40	10	20
Near average	35	35	45	45
Below average	15	25	45	35

Central North Island, Taranaki, Whanganui, Manawatu, Wellington

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

• Temperatures are likely to be above average (50% chance).   
• Rainfall totals are about equally likely to be near normal (40% chance) or above normal (35% chance).  Occasional heavy rainfall events cannot be ruled out as the season progresses.
• Soil moisture levels and river flows are both about equally likely to be near normal (45-40% chance) or below normal (40% chance).

	Temperature	Rainfall	Soil moisture	River flows
Above average	50	35	10	20
Near average	35	40	45	40
Below average	15	25	45	40

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 near average (40% chance) or above average (35% chance).   
• Rainfall totals are about equally likely to be near normal (40% chance) or above normal (35% chance).  Occasional heavy rainfall events cannot be ruled out as the season progresses.
• Soil moisture levels and river flows are both about equally likely to be near normal (40-45% chance) or below normal (35-40% chance).

	Temperature	Rainfall	Soil moisture	River flows
Above average	35	35	25	15
Near average	40	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 about equally likely to be near average (35% chance) or above average (40% chance).

• Rainfall totals are about equally likely to be near normal (40% chance) or above normal (35% chance).  Occasional heavy rainfall events cannot be ruled out.

• Soil moisture levels and river flows are both likely to be near normal (40% chance).

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

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

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

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

	Temperature	Rainfall	Soil moisture	River flows
Above average	45	15	20	20
Near average	40	50	40	40
Below average	15	35	40	40

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 near average (40% chance) or above average (35% chance).
• Rainfall totals are most likely to be near normal (40% chance) or above 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	35	35	25	25
Near average	40	40	45	45
Below average	25	25	30	30

Graphical representation of the regional probabilities

Background

Sea surface temperature(SSTs) anomalies in the central equatorial Pacific (Niño 3.4 Index) during January (-0.72°C ) were just over NIWA’s conventional threshold to define La Niña (-0.7°C).  SSTs are currently cooler than average in the central and east-central equatorial Pacific, while warmer than average ocean temperatures are present in the western equatorial Pacific. Meanwhile, near average or above average SSTs persisted in the eastern equatorial Pacific. These current oceanic conditions depart from the typical pattern associated during most La Niña events and align with so-called ‘Modoki’ conditions, where the coldest SST anomalies during ENSO events are shifted to the central, rather than eastern, Pacific.  The relative Niño 3.4 Index (RONI) was -1.28°C  for the past 30 days (to 26 January 2025), reflective of the central equatorial Pacific being significantly cooler than the average of the global tropics and being in La Niña territory.

The Southern Oscillation Index (SOI) is technically in the neutral range (+0.07 as of 27 January 2025)  However, over the past two months, the SOI crossed the La Niña threshold occasionally but failed to

remain consistently in La Niña territory.  The source of  this is likely due to local or regional response to the SST anomalies from the atmosphere, rather than a basin-wide response.

During January, upper-oceanic heat content was above average to the west of 160°E, with anomalies exceeding +2°C  in the westernmost parts of the Maritime Continent.  Significant negative anomalies (exceeding -2°C ) intensified between 150°W and 120°W.

The sub-surface continued to cool between 50m and 150m depth east of the International Date Line, with anomalies exceeding -5°C  extending between 150°W and 120°W.  The West Pacific Warm Pool remains warmer than average and positive anomalies are more profound and extensive than last month, having expanded eastward towards the International Date Line below ~ 100m depth.  Sub-surface temperatures remained much warmer than normal in the eastern Indian Ocean, exceeding +5°C  anomalies at about 90°E.

In summary, weak and atypical La Niña conditions are present in the Pacific Oceanic and atmospheric conditions in the Pacific meet most of NIWA’s criteria for La Niña. Consequently, a La Nina Advisory is in place. While the coupling between the ocean and atmosphere is locally strong in the central Pacific, as a whole the event is weak and expected to be short-lived.  It also presents atypical characteristics, with the location of the maximum SST anomalies in the Pacific shifted west of what is usually observed during La Niña events.  Inconsistent extra-tropical teleconnections, or linkages from far distances, are therefore to be expected.  Over New Zealand, atmospheric circulation patterns are expected to depart at times from those typically associated with La Niña.

The Southern Annular Mode (SAM) was slightly negative overall during the month of January (56% of days negative) but remained in the neutral range, with the monthly average anomaly being -0.13.  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 neutral in January 2025, with the average anomaly for the month being -0.50°C.  The guidance from the Australian Bureau of Meteorology is for the IOD to remain neutral throughout the forecast period.

During January, convective forcing associated with the MJO reached the eastern Indian Ocean (phase 3) and enhanced convective activity affected most of the Maritime Continent, while decreased convection was present further east to the International Date Line, associated with the enhanced trade winds that are currently present in the central Pacific.

TheMJO is forecast to propagate in the western Pacific and weaken slightly, while stronger than normal trade winds are forecast to intensify in the central Pacific.

Ocean temperatures have cooled significantly since December 2024 around the North Island, and pockets of below average SSTs have appeared along the west coast of the North Island. Conversely, much warmer than average SSTs persist along and off the west coast of the South Island, with Marine Heatwave conditions (five or more consecutive days above the 90^th percentile) affecting large areas offshore.  Model guidance generally suggests that near normal to warmer than normal ocean temperatures are likely over the February – April 2025 period.  NIWA will continue to monitor the situation in the SST update.

Forecast Confidence

Temperature

Rainfall