Seasonal climate outlook April - June 2018

Overview

Weak La Niña conditions continued in the tropical Pacific during March 2018, but trends in low-level winds and in sub-surface ocean temperatures during the month indicate that the event is coming to an end. 

The Southern Oscillation Index (SOI) is currently positive at about +1.0 for March. Sea surface temperatures (SSTs) remained colder than average in the eastern equatorial Pacific Ocean but have returned to near normal around the Dateline. However, the layer of colder than average SSTs is now very shallow (75 metres or less, except close to the South American coast), with warmer than average sub-surface water spreading eastwards during March as far as 120°W.

The consensus from international models is for the tropical Pacific to transition to an ENSO-neutral state over the next 3 month period (75% chance over April – June 2018). ENSO-neutral remains the most likely outcome over the late-winter season (July – September 2018). The forecast models predict about an equal chance of the Pacific remaining neutral or transitioning towards El Niño over the spring (September – November 2018).

Even though La Niña’s influence will wane over the next three-month period, New Zealand’s regional climate over April – June 2018 is expected to be driven by persistence of more northeasterly airflow than normal, and by the persistence of warm ocean waters that are present around the country. The northern Tasman Sea will remain unsettled, with lower pressures than normal, bringing the likelihood of significant rainfall events to the North Island and the upper South Island.

NIWA meteorologist Ben Noll describes the seasonal climate outlook for April - June 2018.

Outlook summary

April – June 2018 temperatures are forecast to be above average for all regions of New Zealand with high confidence (60% chance). The warm ocean waters around the country are forecast to persist through the next three months, but by the end of winter are expected to return to near normal west of the country. Even though seasonal-average temperatures are very likely to be above normal, frosts will occur in cooler locations as late autumn progresses.

For April – June 2018, rainfall totals are forecast to be above normal in the north and east of the North Island (45% chance), near normal in the west of the South Island (45% chance), and near normal (40% chance) or above normal (35 to 40% chance) in all other regions. 

April – June 2018 soil moisture levels and river flows are forecast to be above normal in the north and east of the North Island and in the north of the South Island (45% chance), near normal (40% chance) or above normal (40% chance) in the west of the North Island, and near normal (45% chance) in the west and east of the South Island.

Regional predictions for the April – June 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 (60% chance).
• Rainfall totals, soil moisture levels and river flows are all most likely to be in the above normal range (45% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	60	45	45	45
Near average	30	35	35	35
Below average	10	20	20	20

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

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	60	40	40	40
Near average	30	40	40	40
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 (60% chance).
• Rainfall totals, soil moisture levels and river flows are all most likely to be in the above normal range (45% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	60	45	45	45
Near average	30	35	35	35
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 (60% chance).
• Rainfall totals are equally likely to be normal (40%) or above normal (40%).
• Soil moisture levels and river flows are most likely to be above normal (45% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	60	40	45	45
Near average	30	40	35	35
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 most likely to be in the near normal range (45%).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	60	25	25	25
Near average	30	45	45	45
Below average	10	30	30	30

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 (60% chance).
• Rainfall totals are about equally likely to be in the near normal range (40% chance) or above normal range (35% chance).
• Soil moisture levels and river flows are most likely to be in the near normal range (45% chance).

 The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	60	35	35	35
Near average	30	40	45	45
Below average	10	25	20	20

Graphical representation of the regional probabilities

Attribution, Non-Commercial, No Derivative Work

Background

Sea surface temperatures (SSTs) in the central and eastern equatorial Pacific Ocean remained below average during March 2018, but have weakened somewhat relative to February values. The NINO3.4 index (over a box 170-120 ^oW, 5^oS-5^oN) is negative at -0.61^oC, much the same as it was last month (-0.58 ^oC), but temperatures have eased closer to average to the west and east of this central region.

Cooler than average subsurface ocean waters (within the first 150 metres of the ocean) in the central and eastern equatorial Pacific (east of the Dateline) persisted through the month of February 2018. However, during March these cooler waters retracted to the top 75 metres of the ocean, except near the South American coast, as warmer than average subsurface waters (100-200 metres depth) in the western Pacific expanded eastward to reach the central Pacific to about 120^oW. Large positive anomalies (> +3^oC) are now present in the central Pacific basin (180-160^oW), centred at about 150m depth.

The enhanced easterly trade winds that were present in the western Pacific through to mid-January have vanished, and strong westerly wind anomalies replaced them during February 2018, linked in part to westerly wind bursts related to the propagation of one of the strongest Madden-Julian Oscillation convective events ever recorded. The westerly wind burst was particularly noticeable at 850hPa (approximately 1500 metres altitude), and was associated with a strong downwelling Kelvin wave in the ocean, which contributed to an eastward shift in warmer than average sub-surface waters. The SOI has shown considerable variability since the beginning of the year, and is currently on the La Niña side of neutral, with a preliminary value of +1.0 for March 2018 to date.

March 2018 rainfall and convection anomalies in the tropical Pacific remain consistent with La Niña conditions, the pattern being reinvigorated since February: the Intertropical Convergence Zone remains to the north of its climatological position in the central Pacific, whereas the South Pacific Convergence Zone was extremely weak during March. The ENSO Precipitation Index (ESPI) has remained negative for March 2018  (-1.1 for the last 30 days to the 25th of March 2018), also reflecting a continuation of waning La Niña conditions.

In summary, while weak La Niña conditions remain present in the Pacific Ocean, the expansion and intensification of warmer than normal subsurface ocean waters 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 transition to an ENSO-neutral state over the next 3 month period (75% chance over April – June 2018). ENSO-neutral remains the most likely outcome over the late-winter season (July – September 2018). The forecast models predict about an equal chance of the Pacific remaining neutral or transitioning towards El Niño over the spring (September – November 2018).

Coastal waters remain much warmer than average all around New Zealand, in the Tasman Sea, and in the Southwest Pacific. However, the anomalies for coastal New Zealand continue to weaken from the peak reached in December 2017. The latest vertical profiles from the Argo floats in the Tasman Sea show that the surface layer is more well-mixed down to 60m in March (and about +1.0 ^oC warmer than average), as opposed to the strong stratification and extreme warmth at the surface (+2.5 ^oC) in the January 2018 profiles.

The warmest coastal anomalies (for the past three months) are around the west of the South Island, where they remain at least +2.0^oC above normal. According to the dynamical models’ forecasts, warmer than average SSTs are likely to persist for at least part of the next 3 months (April – June 2018).

Contact

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

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

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.