ClimateWatch: SEPTEMBER & Spring outlook as El Niño conditions grow (+17 Maps & Video)

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ClimateWatch: SEPTEMBER & Spring outlook as El Niño conditions grow (+17 Maps & Video)

8/09/2023 2:35pm

Originally published Friday Sept 1 — As we do each month we take a look at the general air pressure trends and climate drivers moving in and around New Zealand, Australia, the South Pacific and the Southern Ocean.

As we head through September New Zealanders should notice an uptick in high pressure and longer dry spells. Rainfall totals should be slightly lower for many places – although may still be close to normal for some too. Despite a drier trend emerging, we’re still seeing a number of cold fronts interacting with NZ bringing some chances of rain.

Temperatures look to lean warmer than average across NZ over September and Spring by half a degree to a full degree.

September is known for being chaotic but there is some order to the chaos this month with high pressure mostly north of the Roaring Forties (Whanganui northwards) and low pressure dominating the Southern Ocean. Between the belt of highs and lows is often a westerly flow – which is likely to interact with the South Island and lower North Island more.

EL NINO

An El Niño “Alert” remains in place – meaning an El Niño announcement is looking more likely in the coming month.

Latest statement from the Bureau of Meteorology (BoM):

The Bureau’s El Niño Alert continues, with El Niño development likely during spring. When El Niño Alert criteria have been met in the past, an El Niño event has developed around 70% of the time.

Sea surface temperatures (SSTs) in the tropical Pacific are exceeding El Niño thresholds and have continued to warm slightly over the last fortnight. Climate models indicate further warming of the central to eastern Pacific is likely, with SSTs remaining above El Niño thresholds until at least early 2024.

The 90-day Southern Oscillation Index (SOI) is presently just below El Niño thresholds, while trade winds and Pacific cloudiness have not yet demonstrated sustained El Niño patterns. Overall, atmospheric indicators suggest the Pacific Ocean and atmosphere are not yet consistently reinforcing each other, as occurs during El Niño events.

We are currently just north of the red arrow – which means El Niño weather conditions are only starting to be noticed now – and will continue to build for the rest of 2023. El Niño conditions may not ease again until Autumn or perhaps even Winter 2024.

Climate models from various nations (excluding NZ’s Niwa due to their odd commercial reasons) shows El Niño conditions growing.

El Niño looks to be peaking in December/January based on all international modelling (excludes Niwa’s data).

For El Niño to be declared two things are needed. 1) Much warmer than average sea surface temperatures (SST) in the eastern tropical Pacific – that part is done. 2) Have more high pressure stuck over eastern Australia and the Tasman Sea – that part isn’t yet locked in and is why NZ and Australia have no official El Niño announcement yet.

Marine heatwave intensity around NZ has eased a little this week – but many places are still warmer than they should be and this works a little against El Niño weather patterns.

AIR PRESSURE

WEEK 1 — A bit unsettled in Week 1 with low pressure in the sub-tropics and Tasman Sea – still a bit of a hangover from La Niña earlier this year. (Map shows Sept 1st).

WEEK 2 — Despite a chaotic weather pattern there is more order to the chaos we we go through September with high pressure dominating the top half of the maps and low pressure dominating the southerly half/Southern Ocean. This encourages plenty of westerly winds in between. (Map shows Sept 8th).

WEEK 3 — Not 100% locked in this far out but powerful high pressure dominates NZ and more high pressure coming from the Indian Ocean to replace it. (Map shows Sept 15th).

RAINFALL

Much of NZ is in a healthy place soil-moisture-wise – a change from where we were just a month ago. Even Hawke’s Bay is now leaning a little drier than average following the immense rainfall earlier this year.

Upcoming rain with white boxes highlighting the driest areas. For NZ it’s starting to look a lot more like El Niño…but for Australia it still looks like La Nina with potentially up to 200mm falling around Sydney and coastal NSW in the next couple weeks.

*IBM* and *WeatherWatch’s* departure from normal Rain map for SEPTEMBER shows the country close to average but leaning drier for the most part. The lower North Island and eastern South Island may have some wet weather to push those areas closer to normal.

Still leaning drier than usual in many regions in OCTOBER but the hints of white and green shading indicate still some chaos to the weather pattern – bringing some wet weather into the eastern North Island which, again, isn’t always the case with El Niño and may be part of the reason we don’t yet have an official annoucement.

NOVEMBER looks to lean slightly drier for many regions, while the lower South Island is leaning wetter. Over the next 3 months the departure from normal generally is leaning a little drier across all of NZ – but with still some wet weather in the mix.

TEMPERATURES

Temperature trends for September look to be warmer than average by 0.5 to 1C in NZ, but closer to normal in much of Australia.

Looking at all of Spring and NZ leans around 0.5 to 1C warmer than average while parts of Australia are closer to normal in the southern coastline, but inland may be more than 2C above normal.

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Comments

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Peter Thomas Langer on 8/09/2023 1:36pm

why should the el nino behave like a normal one the la nina didnt there was more rain in the east and south than there should of been the la nina was all over the place we only had one year out of 3 but gisborne and HB had the full monty and other odd places

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David on 1/09/2023 8:04pm

At a local level, in Hawkes Bay, I have noticed a return of the old El Niño pattern where forecast rain a week out subsequently pushes out over that week with the amount of forecast rain and it’s percentage chance falling away to nothing – only six weeks ago we were well above soil field capacity and now the limiting factor to grass growth is perversely a lack of moisture – I have no doubt El Niño has arrived!

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Andre Bezuidenhout on 1/09/2023 4:57am

Thank you for the Spring forecast. It looks like this will be the end of any snow on Mount Ruapehu for the rest of the year, or is there still a chance for a Southerly storm?

Reply

jet on 2/09/2023 5:04am

there still southerlys this month im no weather expert tho

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