ENSO UPDATE - LA NINA DEVELOPS

The La Nina phase is underway through the ENSO region with all the parameters measuring a La Nina phase now in agreement and the thresholds have been met in my opinion to declare a La Nina.


The take away is


  • more flooding risks/potential,

  • more severe weather potential for the nation's north and east and

  • extended periods of high humidity through Summer.

  • When we reach the peak of the wet season for the east coast between March and June, there could be floods equal to what we saw last season.


ENSO Status - La NIna Declared

It has been well documented here that La Nina was coming for months on end and here we are, it is La Nina and we have been in this phase since October as the IOD waned.


The mechanics of La Nina

Most of you know how La Nina works but I just want to post this image and then marry it against observations in the ENSO region - as you will see, La Nina is here.

OVERVIEW - A QUICK WRAP.


La Nina conditions have strengthened in the last month, with below-average SSTs now observed across most of the equatorial Pacific. In the last week, all of the Niño index values were between -0.7°C and -1.0°C, with the coolest anomalies in the Niño-3.4 region which is where the basis for ENSO events are measured generally. These values support La Nina conditions and strengthening. The other area that screams La Nina has developed is the warmer water that has developed offshore QLD into the NT.

Below-average subsurface temperatures across the basin were roughly the same values as last month, this reflecting the prevalence of below-average temperatures in the eastern basin. Low-level easterly and upper-level westerly wind anomalies were again observed over parts of the equatorial Pacific, although weaker than last month. I would also mention that the dip in the values is lower than where it was last year and this is cause for concern as the tap was turned on full ball as tracked on after Christmas

Tropical convection was suppressed near and west of the International Date Line and was slightly enhanced over Indonesia where the rainfall events of recent weeks of Australia have emanated from and will again in the short term. The convection is expected to increase in coming weeks and creep south, enhancing the early onset rainfall chances. That is another signal of La Nina developing. The IOD waning has resulted in temperatures above average through the tropics.


October featured La Nina atmospheric conditions with more rain than average over Indonesia, less over the central Pacific, and stronger-than-average trade winds over portions of the equatorial Pacific. This fed into the jet stream and brought that heavier moisture content through central and northern Australia. The IOD also helped out on that front as well, as did the very much above average temperatures over the Arafura, Timor and Coral Seas.


Based off these observation - La Nina is here.

The SOI values also support a La Nina phase, being positive for the past 3 months and maintaining that phase.


All these values together suggest that La Nina has formed and the atmospheric observation is not reflecting that across the Pacific Ocean and will eventually spill over Australia.


There is strengthening signals the La Nina will remain in place through the middle of Autumn. This is something that has been on the table for the past 2 months and that signal is also strengthening.


DATA SETS


CPC/IRI ENSO FORECASTS - Probabilistic Model Data

Essentially the La Nina phase has developed and this will now see a wet and cooler Summer unfold for much of the nation, with periods of heavy rainfall, an active monsoon which will pump moisture into the southern and eastern states. Next year through the latter part of Summer and into Autumn, the east coast must be advised that severe flooding is a risk with the wet season ramping up between February and April for eastern NSW and QLD. With the risk of La Nina lingering through to mid Autumn (last event waned in late Summer), this could cause excessive rainfall lasting into May for the eastern states and a lingering monsoon/wet season for the northern tropics. Deterministic and ensemble data sets will not pick up on these specifics just yet.

CPC/IRI ENSO FORECASTS - Curated international forecast guidance

The near certainty of La Nina being present now until Autumn is gathering pace and this may again lead to a lingering wet and cooler phase through Summer into Autumn. The event neutralises in our late Autumn which is normal. Do not get too caught up on the El Nino developing in the middle of next year with the models responding to the oceans shifting as the seasons shift, so neutral conditions expected by next winter. But coming off a low base through winter next year, it could spark another La Nina in 2022/23 which would be the third time in recorded history since 1950 that this has happened and based on statistics we are due for this to occur.

CPC/IRI ENSO FORECASTS - Break down of member forecast bias.

Strong signals for La Nina right through until about April before the event eases. No strong signal for El Nino which may open the door for a third La Nina later next year, so that will be reviewed in December and January. But at this stage it is looking like the ENSO will remain cooler than normal for the coming 4-6 months, which is a 2 month extension on what we experienced earlier in the year.

ENSO - SSTs for the coming 9 months.

The SSTs are expected to remain cooler through to about the end of March or April next year but some of the models do keep the thresholds of La Nina in place until June which would be rare. At this time conditions in the ENSO region should be neutral/cool neutral during Winter.

Summer Rainfall Outlook 2021/22

% chance of exceeding the rainfall for Summer of 2021/22.

The development of a La Nina in the Pacific Ocean and the waning of the negative IOD with a persistent positive SAM appears to be the main drivers for a wet and cooler summer for much of the nation. As per last Summer, the impact of a La Nina developing has a higher impact from after Christmas as there is a lag time from the event developing, peaking and waning. That is usually around 2-4 weeks. And so I suspect as per the Spring we have been experiencing, wetter as we go once again seems to be a fair forecast nationally. The monsoon will be very much above average and early this year for the north. That will fuel a deeper moisture profile over much of the nation with the higher chance of rainfall being above the average again, the further north and east you go. For southern coastal areas of SA and WA, conditions could be seasonal with a slight above/below average rainfall bias but from there, as you fan out north through the nation, you find the moisture, the better rainfall and thunderstorms coverage and cooler weather. This has been a significant signal now since I have started this page back in July, that the Spring would be wet, a La Nina Summer would develop and the potential for a very wet end to Summer and start of Autumn is likely.

Summer Temperature Outlook 2021/22

% chance of exceeding the rainfall for Summer of 2021/22.

The cooler bias across the nation does not mean COLD, it means that there is a lower than average chance of exceeding the maximum temperatures. But due to persistent cloud cover, higher humidity and rainfall/thunderstorm chances, the weather is set to be more tropical meaning a humid and sultry summer for many areas is likely. The higher humidity will make it to the southern and eastern coast line regularly with thicker cloud cover, widespread rainfall chances and if that is in combination with onshore winds over the south and east, it could be quite cool. The tropics under enhanced rainfall signal is expected to be marginally cooler than normal overall, but if you live there, you know that the monsoonal breaks can be unbearable hot. And over the western interior, we are already seeing evidence of what is to come, persistent cloud cover and widespread showers and thunderstorms and you will see below average temperatures there and that will spread east over the next fortnight. This is something of a preview of our Summer.

Risk of riverine flooding - December 2021 through February 2022

This chart represents the chance of flooding and potential disruption for outback areas in particular. Now you don't have to see rainfall falling overhead to be impacted by flooding, with the Darling River for example flooding upstream but can impact communities deep into NSW as we have seen throughout this year. The higher values exist over northern and southeast Australia, as has been the expectations since July. The weather over the southeast carries a high risk of disruption, with major river systems at 100% capacity and saturated catchments. These areas are sitting ducks to flooding, especially along the Murray River, even though the catchments have had a break in the past 2-3 weeks which has been a godsend. Over the north, the flooding could start early through the wet season, from December onwards. Elsewhere, excessive convection and persistent unsettled weather could see the flood risk increasing in the next two weeks, especially with near record rainfall for parts of the outback of the NT and WA.

Rainfall Outlook - Autumn 2022

% chance of exceeding the rainfall for Autumn 2022

The residual impacts of La Nina that forms over Summer will persist into March of 2022 and as we saw with the last event, the rainfall was heaviest through February to April on the east coast and the above average rainfall continued over the northern parts of the nation. Now not one event is ever the same, but the current data sets below support the rainfall remaining heightened over these regions through Autumn, especially March and April. The weather will start to dry out with some confidence over southern and western parts of the nation as the rainfall focus moves north and east. The Indian Ocean is expected to remain neutral through next year with the positive forecast from the BoM model indicating the impact of the monsoonal season. Some climate agencies support another negative IOD developing through winter but there is little skill in forecasting that driver ahead of time.

Temperature Outlook Autumn

% chance of exceeding the median temperatures for Autumn 2022

The temperatures will continue to be seasonal to below seasonal for much of the nation, again with the ongoing impacts of the active monsoon, higher than average SSTs over the northern and eastern parts of the nation with prevailing winds pumping moisture inland, producing more cloud and rainfall chances. The southwest of the nation may start to see a warmer end to Summer with elevated temperatures continuing into March and April. With the higher than average humidity forecast over the northern and eastern areas of the nation, the day time temperatures may be marginally warmer than normal. Nights will likely be above average for much of the nation with above average rainfall, humidity and cloud coverage.

Another look at this in a couple of weeks. In that time the BoM should have declared a La Nina and it will be across all your social media feeds, but really we have been in La Nina for a while now and I have been robust in that stance and I fully expect it to last until the end of Summer into early Autumn at the earliest, late Autumn at the latest.



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