Seasonal climate outlook March - May 2018

Overview

Weak La Niña conditions continued in the tropical Pacific during February 2018, but the current state of the Ocean – Atmosphere system in the Equatorial Pacific indicates that it is now reaching its decay phase. Below average sea surface temperatures (SSTs) remained present in the central and eastern equatorial Pacific Ocean but weakened compared to January 2018. Moderate cooler than average subsurface ocean waters are still present in the eastern Pacific, but warmer than normal ocean waters have intensified in the western Pacific and spread further eastward. The trade winds have been variable over the past few months, and so has the Southern Oscillation Index (SOI) which is currently on the El Niño side of neutral (preliminary value for February 2018 is – 0.85): both indicate a decoupling between the ocean and the atmosphere.

The consensus from international models is for the tropical Pacific to rapidly transition to an ENSO-neutral state over the next 3 month period (69% chance over March – May 2018). ENSO-neutral remains the most likely outcome over the winter season (June – August 2018), and a transition towards El Niño becomes increasingly likely thereafter (45% chance for El Niño conditions to emerge over the September – November 2018 period).

With La Niña’s influence waning over the next three month period, New Zealand’s regional climate over March – May 2018 is expected to be driven by the warmer than average ocean waters that are present around the country, in the Tasman Sea, and in the Southwest Pacific Ocean, which will influence surface air temperatures and the likelihood of significant rainfall events.

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Attribution, Non-Commercial, No Derivative Work

For March – May 2018, the atmospheric circulation around New Zealand is forecast to be characterised by lower than normal atmospheric pressure northwest of the country, extending over parts of New Zealand, unsettled conditions especially for the North Island, and weak northeasterly quarter flow anomalies. This pressure pattern, in concert with the anomalously warm ocean waters around the country, is expected to be associated with warmer than average temperatures, and above normal rainfall - notably for the North Island.

Outlook Summary

March – May 2018 temperatures are forecast to be above average for all regions of New Zealand with high confidence (60 to 70% chance). As autumn progresses however, frosts may occur from time to time in cooler locations.

For March – May 2018, rainfall totals are forecast to be above normal in the North Island and in the north of the South Island (45 to 50% chance) and about equally likely to be near normal (35 to 40% chance) or above normal (35 to 40% chance) in the west and east of the South Island.

March – May 2018 soil moisture levels and river flows are forecast to be above normal (45 to 55% chance) in the north and west of the North Island and the north and east of the South Island. In the east of the North Island and the west of the South Island, soil moisture levels and river flows for the March – May 2018 period are about equally likely to be near normal (35 to 40% chance) or above normal (35 to 40% chance).

Regional predictions for the March – May 2018 season

Northland, Auckland, Waikato, Bay of Plenty

The table below shows 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 being in any one of the three categories. Forecast information from local and global guidance models is used to indicate the deviation from equal chance expected for the coming three-month period, with the following outcomes the most likely (but not certain) for this region:

• Temperatures are very likely to be above average (70% chance).
• Rainfall totals are most likely to be in the above normal range (50% chance).
• Soil moisture levels and river flows are most likely to be in the above normal range (50 to 55% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	70	50	50	55
Near average	20	35	35	30
Below average	10	15	15	15

Central North Island, Taranaki, Whanganui, Manawatu, Wellington

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

• Temperatures are very likely to be above average (70% chance).
• Rainfall totals, soil moisture levels and river flows are all most likely to be above normal (45 to 50% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	70	45	50	50
Near average	20	35	30	30
Below average	10	20	20	20

Gisborne, Hawke’s Bay, Wairarapa

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

• Temperatures are very likely to be above average (70% chance).
• Rainfall totals are most likely to above normal (45% chance).  
• Soil moisture levels and river flows are equally likely to be in the above normal range (40% chance) or near normal range (40% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	70	45	40	40
Near average	20	35	40	40
Below average	10	20	20	20

Tasman, Nelson, Marlborough, Buller

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

• Temperatures are very likely to be above average (70% chance).
• Rainfall totals, soil moisture levels and river flows are all most likely to be above normal (45 to 50% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	70	45	45	50
Near average	20	35	35	30
Below average	10	20	20	20

West Coast, Alps and foothills, inland Otago, Southland

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

• Temperatures are very likely to be above average (60% chance).
• Rainfall totals, soil moisture levels and river flows are all about equally likely to be near normal (40% chance) or above normal (35% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	60	35	35	35
Near average	30	40	40	40
Below average	10	25	25	25

Coastal Canterbury, east Otago

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

• Temperatures are very likely to be above average (70% chance).
• Rainfall totals are about equally likely to be in the above normal range (40% chance) or near normal range (35% chance).
• Soil moisture levels and river flows are most likely to be in the above normal range (55% chance).

 The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	70	40	55	55
Near average	20	35	35	35
Below average	10	25	10	10

Graphical representation of the regional probabilities

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Background

Weak La Niña conditions continued in the tropical Pacific during February 2018, but the current state of the Ocean-Atmosphere system in the Equatorial Pacific indicates that it is now reaching its decay phase. Below average sea surface temperatures (SSTs) remained present in the central and eastern equatorial Pacific Ocean but weakened compared to January 2018. The NINO3.4 index is negative at – 0.58^oC (was – 0.68^oC last month), and the far eastern Pacific (NINO indices 1 and 2) experienced rapid warming over the course of February.

Cooler than average subsurface ocean waters (within the first 150 metres of the ocean) in the central and eastern equatorial Pacific (east of about 150^°W) persisted during February 2018, but warmer than normal subsurface ocean waters (~ 150 metres depth) in the western Pacific increased significantly and expanded eastward to reach the central Pacific to about 160^oW. Large positive (exceeding + 2^oC) upper ocean heat content anomalies now dominate the western half the equatorial Pacific basin up to 160^oW, and the heat content anomalies are only slightly negative (about – 0.5^oC) in the eastern Pacific.

The enhanced trade winds that were present in the western Pacific during January 2018 have vanished, and strong positive zonal wind anomalies (weakened trade winds) replaced them during February, linked in part to westerly wind bursts related to the propagation of one of the strongest Madden-Julian-Oscillation (MJO) convective envelopes ever recorded (which was partly responsible for the initiation of Tropical Cyclone ‘Gita’). The Southern Oscillation Index (SOI) has shown considerable variability since the beginning of the year, and is currently on the El Niño side of neutral, with a preliminary value of – 0.85 for February 2018 as a whole.

February 2018 has also seen the breakdown of the rainfall and convection anomalies in the tropical Pacific which were previously consistent with La Niña conditions: while the Intertropical Convergence Zone remains shifted north of its climatological position in the central Pacific, discernible La Niña–like signals are no longer present in the western Pacific and within the South Pacific Convergence Zone. The ENSO Precipitation Index (ESPI) has weakened considerably, and while it remains negative for February 2018 (– 0.37 for the last 30 days to the 27^th of February 2018), it no longer reflects La Niña conditions.

In summary, while weak La Niña conditions remain present in the Pacific Ocean, the breakdown of the atmospheric patterns, along with expansion and intensification of warmer than normal subsurface ocean waters towards the central Pacific, signal that La Niña has reached its decay phase. This tendency is expected to continue over the next few months: the international consensus is for a rapid transition to an ENSO-neutral state over the next three month period (69% chance over March – May 2018). ENSO-neutral remains the most likely outcome over the winter season (June – August 2018), but the models indicate that a transition towards El Niño becomes increasingly likely thereafter (45% chance for El Niño conditions to emerge over the September – November 2018 period).

Ocean waters remain much warmer than average all around New Zealand, in the Tasman Sea, and in the Southwest Pacific. While the anomalies for coastal New Zealand weakened compared to the peak reached at the end of January 2018, they remain close to or exceed + 1^oC. According to the dynamical models’ forecasts, warmer than average SSTs are likely to persist for at least part of the next 3 months (March – May 2018).

Contact

Chris Brandolino, Principal Scientist – Forecasting, NIWA National Climate Centre Tel (09) 375 6335, Mobile (027) 886 0014

Dr Brett Mullan, Principal Scientist, NIWA National Climate Centre Tel (04) 386 0508, Mobile (027) 294 1169

Notes to reporters and editors

1. NIWA’s outlooks indicate the likelihood of climate conditions being at, above, or below average for the season as a whole. They are not ‘weather forecasts’. It is not possible to forecast precise weather conditions three months ahead of time.
2. The outlooks are the result of the expert judgment of NIWA’s climate scientists. They take into account observations of atmospheric and ocean conditions and output from global and local climate models. The presence of El Niño or La Niña conditions and the sea surface temperatures around New Zealand can be a useful indicator of likely overall climate conditions for a season.
3. The outlooks state the probability for above average conditions, near average conditions, and below average conditions for rainfall, temperature, soil moisture, and river flows. For example, for winter (June–July–August) 2007, for all the North Island, we assigned the following probabilities for temperature: ·  Above average: 60 per cent ·  Near average: 30 per cent ·  Below average: 10 per cent We therefore concluded that above average temperatures were very likely.
4. This three-way probability means that a random choice would be correct only 33 per cent (or one-third) of the time. It would be like randomly throwing a dart at a board divided into three equal parts, or throwing a dice with three numbers on it. An analogy with coin tossing (a two-way probability) is not correct.
5. A 50 per cent ‘hit rate’ is substantially better than guesswork, and comparable with the skill level of the best overseas climate outlooks. See, for example, analysis of global outlooks issued by the International Research Institute for Climate and Society based in the US published in the Bulletin of the American Meteorological Society (Goddard, L., A. G. Barnston, and S. J. Mason, 2003: Evaluation of the IRI’s “net assessment” seasonal climate forecasts 1997–2001. Bull. Amer. Meteor. Soc., 84, 1761–1781).
6. Each month, NIWA publishes an analysis of how well its outlooks perform. This is available online and is sent to about 3500 recipients of NIWA’s newsletters, including many farmers.
7. All outlooks are for the three months as a whole. There will inevitably be 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 approximately 80 per cent and 115 per cent of the long-term mean.
8. The seasonal climate outlooks are an output of a scientific research programme, supplemented by NIWA’s Capability Funding. NIWA does not have a government contract to produce these outlooks.
9. Where probabilities are within 5% of one another, the term “about equally” is used.