Seasonal Model Survey

This is the last survey of seasonal models for the year, as accuracy is greatly reduced during the wet season. The general consensus throughout the global models is for wetter conditions although probabilities have been slightly reduced. The cooler waters north of Australia may be flowing through in the models to issue a more cautious approach to the Big Rivers and Kununurra areas. This La Niña has a few tricks left in it. One cannot assume broadscale and plentiful moisture for all.

Seasonal Model survey 23 October 2020

Climate in-brief

This weeks chart shows humidity well above normal down the centre of Australia.

• Conditions in the tropics, our major source of moisture, continue to be “disorganised” according to the latest NCEP update this week, as the atmosphere continues to realign with warmer tropical waters;

• The SAM index is finally weakly positive, assisting the formation of moist air into eastern Australian agricultural areas;

• 90-day SOI has exceeded La Niña thresholds (+8), and the air pressure at Darwin is now below normal for the first time this season;

• Multi-week models are favouring rain across southern areas and the ranges in the next 10-days. All other areas look to week 3-4 of October for moisture to arrive;

• Summer cropping analysis for La Nina years; and

• What does this years La Nina mean for our climate in 2021? - the latest research.

Australian weekly rainfall and relative landscape water balance

The air pressure at Darwin is finally below normal. Moisture has streamed down the inland over the last week.

Rainfall maps courtesy http://www.bom.gov.au/jsp/awap/rain/

The high pressure system in the Tasman Sea keeping eastern Australia dry, but the interior has been in the sweet spot.

Australian relative landscape water balance ‘upper soil’ courtesy BOM

Climate drivers

Not too much change from last fortnight. The SAM is finally back in positive territory after a month below zero. Research would suggest our climate drivers are in a good state for rainfall across eastern Australia in the coming weeks and months. The 90-day SOI is now in La Nina territory at +8. For those in central-eastern cotton areas looking for rain, a more emphatic positive shift in the SAM and SOI would help prospects.

What to know about the climate drivers and how they are measured? Click here

30-day air pressure anomaly

Why hasn’t it rained? The 30-day mean sea level air pressure shows an intense high pressure ridge sitting in the Tasman Sea, with moisture in recent days streaming down through the red centre into SA and Western Victoria.

https://psl.noaa.gov/map/

Multi-week model survey (8-28 days)

The next genuine chance for rain appears to be in the eastern half of NSW mid-October with northern areas coming online at the end of October through to early November. The Niño 4 SST region has finally warmed up. This index has quite a strong correlation with rain in CQ and Gulf areas in November, so it’s plausible to expect November will be more favourable than October.

Model form: the BOM multi-week model continues the ‘blue kiss of death’ with over-exuberant wet forecasts. Since May if this model were remotely close Keepit and Copeton Dams would be overflowing. Seasonal and multi-week models struggle with systems originating in the Tasman Sea and assume a simplistic east-west moisture airflow. The NCEP/GFS continues to jump around and is showing a colourful system with some strength eventuating across NSW mid-Oct. The IRI model is normally super-cautious and rain across NSW this month adds to confidence levels. NT and Ord cropping zones will have to wait for the end of the month, as the air pressure at Darwin has finally moved below zero.

9-day ECMWF rainfall outlook

Fresh this morning, this European model shows rainfall for the next week in patches around southern areas, northern NSW ranges with storms developing through SE Qld.

Access courtesy https://meteologix.com/au

Monitoring the SAM

Recent research by Hendon et al, (2014) at the Centre for Australian Weather and Climate Research found the Southern Annular Mode to be the leading mode of circulation variability through parts of inland eastern Australia where cotton is grown; particularly in the spring planting period.
 
A positive SAM has been proven to increase moisture supply into fronts and troughs forming in eastern Australia. The chart provided by Climate Prediction Center (US) shows the most recent observations of the SAM or (AAO). 

A negative SAM traditionally leads to a dry westerly circulation pattern through eastern Australian cropping areas.
 
BOM researchers Lim and Hendon (2013) found the SAM has the strongest connection with El-Niño Southern Oscillation in October and November – whereby the SAM is influenced most by El Niño or La Niña conditions.

Growers and advisors are encouraged to monitor the phases of the SAM together with seasonal forecasting, multi-week and short term weather models during planting. A short cartoon explaining the SAM can be found HERE. 

The SAM has been more negative than positive, which stifles East Coast Low (ECL) events along the Tasman Sea. Its been slow to align with the La Nina and wet Modoki phase. Its good to see ECLs popping up off SE Qld in the forecasts, as these events can be game-changers. This is also a key driver for Namoi and Gwydir Valleys, so a shift north of the black line will direct more moisture into those regions.

Antarctic Oscillation or SAM forecast below (25 Sep)

http://www.bom.gov.au/climate/model-summary/#tabs=Indian-Ocean

SOI check

The 30-day SOI remains positive and indicating a La Nina, with the 90-day average firming. The latest technical report from NCEP (US) this week reports a strengthening Walker Circulation (tropical air flow in the troposphere), which acts as a large-scale conveyor belt for moisture reaching Australia. The atmosphere is gradually responding to the warm ocean temperatures.. Darwin hPA is now below 1010, which is a good sign for moisture transport over inland Australia. SOI explained here

Click on the image to see the latest values courtesy longpaddock.com.au

Global tropical hazards

(from U.S Climate Prediction Center)

 A typically stable period for our northern tropics, there are no cyclones or hazards forecast. In contrast the Bay of Bengal and South China Sea is very active. Click on the image to go through to the CPC website.

Cyclone season 2020-21: almost double last years cyclones anticipated

With the Australian cyclone season less than one month away, AccuWeather meteorologists say Australia may have not only a more active tropical season than the last but also one with more storms making landfall on the nation's shores.

During the 2019-2020 hurricane season, the Australian basin produced nine cyclones, slightly below the average of 11, with the first cyclone developing in a late start to the season. Cyclone Blake, which formed off of northwestern Australia on Jan. 4, 2020, came two months after the official start of the season, which runs from November to April.

The first cyclone of this season is expected to form before the end of December, kicking off a potentially more active season for the basin.

For the full article, click here: https://www.accuweather.com/en/hurricane/australian-tropical-season-to-pull-a-topsy-turvy-from-last-year/823466

Summer crop GSR analysis - Lower Namoi/Gwydir

This weeks chart is from CLIMATEAPP. It shows MOREE, NSW YTD rainfall (RED line) ahead of median (Blue line). Click the chart to go through to www.climateapp.net.au and choose your location in ‘hows the season?’

ENSO and rainfall

With so much emphasis on forecast models its prudent to look at statistics under this scenario;

• When the oceans are in this state (RHS image) there is a high probability that above median rainfall will occur in the next 4 month period at Burren Junction;

• A La Nina condition has profound affects on the Rowena growing Season Rainfall at Rowena. The botton chart shows La Nina (dark blue), neutral (light blue) and El Nino (red) state, based on a fairly couse assumption on the Nino3.4 index at years end;

• With LT average for that location being around 288mm, the worst case scenario over the analysis period looks to be average, with many La Nina totals well exceeding average;

• Post-January, the correlation with ENSO decays and other factors such as the MJO take over.

Chart courtesy AgEcon and BOM climate data online

Different flavours of La Niña - a word of caution

While scientists monitor the magnitude of cooling SSTs across the ENSO regions for La Niña strength, one could easily assume a moderate-to-strong event equates to “lots of rain, just on what scale”. Commentary from international experts this week helps to explain the nuances among La Niña events, and how each occurrence differs in terms of rainfall.

2010 v 2020 SST distribution (top image, below) shows cooler waters in the Coral and Arafura Sea regions now (red circles), when compared with 2010. These are key zones for convective moisture, feeding into northern area agricultural areas.

The 2020 La Niña, characterised by cooler waters through the Maritime continent north of Australia, are believed to be more similar to events occurring in the 1980s rather than signature events of 1998 or 2010. Whilst analogue year and statistical analysis has its critics, its useful to revisit Nov-Jan rainfall in those years.

• 1983-84 La Niña rainfall (Nov-Jan) shows NSW, SA and Southern Qld in the sweet spot with a north-west moisture flow from a warmer Indian Ocean, and drier in the tropics.

• 1988-89 La Niña rainfall (Nov-Jan) shows the main beneficiaries of the event occurring in NSW and SA, with much of Qld not achieving average 3-monthly rainfall.

At this stage, guidance suggests from a tropical forcing perspective, the 2020-21 La Niña is expected to have more activity through the Eastern Indian Ocean and less over the Maritime Continent (north of Australia). Under this scenario, summer rainfall potentially favours the mid-latitudes rather than the tropics and northern Australia.