Seasonal Climate Outlook for December 2023 - February 2024

Outlook Summary

• El Niño continued during November and has around a 100% chance of persisting through summer and an 85% chance of persisting through autumn. Although it will have an important influence on Aotearoa New Zealand’s climate, unusual ocean heat in the western equatorial Pacific and on a global scale has contributed to circulation patterns that are not typically associated with a traditional El Niño.
• Summer air pressure is forecast to be above normal north of New Zealand and below normal to the south of the country. This is expected to cause more northwesterly quarter winds than normal across the country for the season as a whole. However, the effect of a non-traditional El Niño will likely encourage increased variability in circulation patterns and air flows as compared to historical El Niño summers.
• Summer rainfall is most likely to be near normal in the north of the South Island, about equally likely to be near normal or above normal in the west of the South Island, and about equally likely to be near normal or below normal across the remainder of the country.
• Areas of heavy rain are possible in the North Island during the first week of December.
• Despite the non-traditional El Niño impacts, an increased awareness around the risk for dry spells is recommended across several regions, as detailed below. This may contribute to water restrictions, particularly in areas that may not have had them in recent years.
• Strong lows will occasionally impact the western and lower South Island, delivering heavy rainfall and a risk for flooding. Such activity is possible in mid-to-late December.
• Temperatures are most likely to be above average in the east of both islands and the north of the North Island. Temperatures are about equally likely to be near average or above average in all other regions. Spells of hot, humid conditions look likely during December.
• Seasonal wind strength is forecast to be above normal across most of the country.
• Coastal sea surface temperatures (SSTs) ranged from 0.42˚C to 0.62˚C above average during November. Localised marine heatwaves may form in the months ahead.
• Soil moisture and river flows are about equally likely to be near normal or above normal in the west of the South Island and near normal or below normal in all other regions.
• As of late November, fire danger was low across the country. Variable fire danger conditions are expected in December. Property owners are encouraged to keep on top of grass growth, as grass may dry out and become a wildfire fuel source. More info: www.checkitsalright.nz.

Regional predictions for December 2023 – February 2024

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 for the long-term mean, whereas for rainfall the “near normal” category lies between 80 per cent and 120 per cent of the long-term (1991-2020) mean. 

Northland, Auckland, Waikato, Bay of Plenty

• Temperatures are most likely to be above average (55% chance). Spells of warm, humid conditions are likely during December.
• Rainfall totals are about equally likely to be near normal (40% chance) or below normal (35% chance). Periods of heavy rain are possible during the first week of December with drier than normal conditions thereafter.
• Seasonal wind speeds are expected to be stronger than normal.
• Soil moisture levels and river flows are about equally likely to be below normal (45% chance) or near normal (40% chance).

	Temperature	Rainfall	Soil moisture	River flows
Above average	55	25	15	15
Near average	35	40	40	40
Below average	10	35	45	45

Central North Island, Taranaki, Whanganui, Manawatu, Wellington

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

• Temperatures are equally likely to be near average or above average (45% chance each). Spells of warm, humid conditions are likely during December.
• Rainfall totals are about equally likely to be near normal (40% chance) or below normal (35% chance). Periods of heavy rain are possible during the first week of December with drier than normal conditions thereafter.
• Seasonal wind speeds are expected to be stronger than normal.
• Soil moisture levels and river flows are about equally likely to be below normal (45% chance) or near normal (40% chance).

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

Gisborne, Hawke’s Bay, Wairarapa

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

• Temperatures are most likely to be above average (55% chance). Spells of warm-to-hot conditions are possible after the first week of December.
• Rainfall totals are about equally likely to be near normal (40% chance) or below normal (35% chance). Periods of heavy rain are possible during the first week of December with drier than normal conditions thereafter.
• Seasonal wind speeds are expected to be stronger than normal.
• Soil moisture levels and river flows are about equally likely to be below normal (45% chance) or near normal (40% chance).

	Temperature	Rainfall	Soil moisture	River flows
Above average	55	25	15	15
Near average	35	40	40	40
Below average	10	35	45	45

Tasman, Nelson, Marlborough, Buller

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

• Temperatures are equally likely to be above average or near average (45% chance each). Spells of warm-to-hot conditions are possible after the first week of December.
• Rainfall totals are most likely to be near normal (40% chance), though there is an increased risk for dry spells in the eastern part of the region due to more frequent northwest winds.
• Seasonal wind speeds are expected to be stronger than normal.
• Soil moisture levels and river flows are 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	45	40	45	45
Below average	10	30	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 equally likely to be above average or near average (45% chance each).
• Rainfall totals are about equally likely to be near normal (40% chance) or above normal (35% chance). Strong lows will occasionally bring heavy rainfall and a risk for flooding.
• Seasonal wind speeds are expected to be stronger than normal.
• Soil moisture levels and river flows are about equally likely to be near normal (40% chance) or above normal (35% chance).

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

Coastal Canterbury and the nearby plains, east Otago

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

• Temperatures are most likely to be above average (50% chance). Highly variable temperatures are likely in early December before a possible warming trend thereafter.
• Rainfall totals are about equally likely to be near normal (40% chance) or below normal (35% chance). An increased frequency of northwest winds may lead to longer dry spells.
• Periodic wetter than normal conditions may occur when rain bands “spill over” the main divide or during strong southerly changes.
• Seasonal wind speeds are expected to be stronger than normal.
• Soil moisture levels and river flows are about equally likely to be near normal (40% chance) or below normal (45% chance).

The full probability breakdown is:

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

Graphical representation of the regional probabilities

Background

The NINO3.4 Index anomaly (which covers the west-central equatorial Pacific) over the last month (through 28 November 2023) was +1.90˚C, within the range of a strong El Niño (classified when the NINO3.4 Index is greater than +1.5˚C). In terms of 30-day anomalies ending 28 November, the NINO3.4 Index is exceeded only by 2015 and 1997 considering data back to 1981. Recent daily values of the NINO3.4 Index reached as high as 2.18˚C above average. From an oceanic perspective, this El Niño continues to rank with the most significant events in recent decades.

The Southern Oscillation Index (SOI) was in the El Niño range during November (-1.3) and September-November (-1.0).

Of the models monitored by NIWA, there’s around a 100% chance of El Niño continuing through February 2024 and an 85% chance that the event persists through autumn 2024.

In the subsurface equatorial Pacific, temperature anomalies of +3˚C to +6˚C were occurring in the upper 100 metres in the central and east as of late November. Anomalies intensified at depth around the NINO3.4 region, resulting from a Westerly Wind Burst (WWB) during the second half of November. Peak oceanic El Niño strength is most likely in January, but sustained peak intensity into February cannot be ruled out.

Trade wind strength was below normal or well below normal in the Pacific during November, particularly just north of the equator and in the central and west. In parts of the region, this event qualified as a WWB. This WWB will be responsible for the eastward propagation of warm sea water through January. Another meaningful reduction or reversal in trade winds is possible in mid-December.

During November, convective forcing was focused in the western tropical Pacific and over Africa. Notably, activity was reduced over the eastern tropical Pacific, uncharacteristic of a strong oceanic El Niño. Forcing was greatly reduced over the eastern tropical Indian Ocean and Australia, in association with the strongly positive Indian Ocean Dipole (IOD).

In the Pacific Ocean, convective forcing has been commonly located west of what is typical during strong El Niño events, likely owing to the basin-wide signal of above average sea surface temperatures and anomalously high ocean heat on a global scale. This has contributed to atypical weather impacts in New Zealand during the El Niño event to-date. At some level, this is expected to continue into the summer season, likely contributing to variable weather patterns.

During early December, the Madden-Julian Oscillation (MJO) is expected to be active over the eastern Indian Ocean and Maritime Continent (phases 3-5) before moving into the western Pacific (phases 6-7). This will lead to an increased likelihood of tropical cyclone development in the South Pacific during this time.

During December, phase 3 has historically been associated with near normal or above normal rainfall in many regions, with phase 4 tending drier in most areas aside from the eastern coastal fringes of both islands. Phases 5 and 6 typically had a drier theme before the North Island and northern South Island turned wetter in phase 7.

In terms of temperatures, phases 3-4 have historically featured changeability before a decided turn toward warmer conditions in phases 5-6. This may imply building heat and humidity during December.

The combination of an atypical El Niño and an active MJO bring an increased chance of a more variable-than-normal start to the summer season.

As the summer season progresses, the eastward propagation of the warmest water in the tropical Pacific may see circulation patterns become more El Niño-like. This will influence the weather patterns that New Zealand experiences. In a general sense, this is expected to favour high pressure near and north of the North Island and low pressure to the south of the South Island. The associated pressure gradient (difference in air pressure over distance) is expected to drive stronger than normal and frequent northwesterly winds and occasionally intense lows and fronts into the western and lower South Island, which may cause heavy rainfall and increase the risk for flooding. More frequent high pressure to the north of the country will also reduce, but not eliminate, the chance for ex-tropical cyclones.

The impacts from the 2023-24 El Niño will likely be distinctly different from 2015-16, 1997-98, and 1982-83 for New Zealand. Climate-sensitive sectors are encouraged to make use of the New Zealand drought dashboard, which provides 35-day outlooks of rainfall and drought, updated once daily: https://niwa.co.nz/climate/seasonal-climate-outlook.

As of late November, a strongly positive IOD continued. During October, convective patterns associated with the IOD were most anomalous in the tropics, with connections to weather patterns across Australasia appearing muted. The IOD event will continue to influence the region through the first half of summer, at times amplifying the circulation patterns of El Niño.

The Southern Annular Mode (SAM) was variable during November. The SAM is forecast to continue to display variability during December.

New Zealand’s coastal water temperatures were slightly above average at the end of November. In the months ahead, the expectation for stronger winds, in association with a fully developed El Niño, means that the risk for the development of widespread marine heatwave conditions in New Zealand’s coastal waters is low; however, during periods of unusually warm conditions, localised-to-regional marine heatwaves may continue or develop, particularly near the north and east of both islands. For more information, see the NIWA Sea Surface Temperature Update.

Figure 1: 30-day SST anomalies and marine heatwave conditions (stippled), calculated with respect to the 1991-2020 climatological period.

NZ 30-day coastal SST anomalies (to 28 November)

North NI	+0.43˚C
West NI	+0.44˚C
East NI	+0.60˚C
North SI	+0.44˚C
West SI	+0.42˚C
East SI	+0.62˚C

Forecast confidence

Temperature: medium-high confidence

Forecast confidence for temperatures is medium-high. Seasonal modelling expects more northwesterly quarter winds during summer, which will transport warm-to-hot and humid air masses from Australia and/or the tropics at times. Warmer air masses will likely be interspersed with periods of variable or cooler temperatures.

Rainfall: medium confidence

Climate model skill was mixed across New Zealand during spring, which has an influence on confidence for summer. The effect of a non-traditional El Niño will likely result in more variable rainfall patterns than experienced during strong El Niño summers in the past. This is reflected by a slight reduction in the probabilities for a drier than normal season. Despite this, an increased awareness around the risk for dry spells is recommended for several regions.