Monitoring the SAM

The SAM has continued its burst of exuberance into the positive phase this month. Not much is going on in the tropics, so whenever the SAM is positive there is a chance that a rain-bearing system will “pop-up” unannounced through the mid-latitudes. Climatologists expect the SAM to become entrenched in this phase through summer, in line with the La Nina signal.

Antarctic Oscillation or SAM values and 7-day forecast (6 Nov)

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

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. 

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

Analysis - 2020 La Niña: a closer look

Scientists around the world are busy reporting on the strength of the current La Niña. This is generally measured as the extent of the cooling in the Niño SST regions in the Tropical Pacific. Together with the trade winds and Walker Circulation (SOI), the stronger the La Niña, the moisture theoretically shifts towards Australia. A comparison of 2010 (a record strength event) ocean temperatures in Mid-Oct v 2020 shows a few notable differences; a cooler Coral Sea and cooler Arafura Sea (circled in red) to the north of Australia. These are important regions for convective moisture for weather systems and while cool now, will be watched with interest following basin-wide warming of the Indian Ocean at the end of November.

Other global climate researchers also point out the nuances among La Niña events and the similarities between 2020 La Niña and those of 1983-84 and 1988-89 (Nov-Jan rainfall, bottom panel). Check your farm rainfall records for these summers:

• 1983-84 La Niña: cooler waters through the Maritime Continent (Indonesia/Malaysia) saw the mid-latitudes achieve high decile rainfall through this period and less so through the northern tropics.

• 1988-89 La Niña: this La Niña arrived earlier than the current 2020 La Niña and brought fantastic rains in winter/spring. Southern areas of the continent fared much better than Qld through the analysis period.

If 2020-21 La Niña is characterised by more tropical forcing (SSTs driving the climate to change from dry to wet) over the Indian Ocean than the Maritime Continent north of Australia, then some caution for much anticipated wet season rains through Qld would be prudent. Close monitoring of the phase of the SAM (NSW and s Qld) and MJO activity will be critical in the following months.

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 had a burst of exuberance into positive phase in early October and has not settled there as predicted, rather is tracking along in neutral. While 2010 has become a legendary benchmark La Niña year, it must be remembered that a record positive SAM has been found to be a large contributor to onshore flow and increased rainfall in that year (Lim et. al. 2016).

Antarctic Oscillation or SAM observations below (23 Oct)

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

Chart of the Week - water vapour approaching

The water vapor content of the atmosphere is of critical importance to nearly every facet of atmospheric science. Observing the processes that determine the water vapor content of the atmosphere is, therefore, of fundamental importance. As the earth’s surface is the source of most atmospheric water vapor, observations of vertical turbulent transport of water vapor in the planetary boundary layer have been the subject of many research projects. Evapotranspiration can vary significantly across the earth’s surface due, for example, to variability in insolation, vegetation cover, soil properties, and precipitation. Area-averaged water vapor flux measurements are necessary to get a large-scale view of surface–atmosphere interactions. 

The Extreme Forecasting Index (EFI) is an integral measure of the difference between the ensemble forecast (ENS) distribution and the model climate (M-climate) distribution. This allows the abnormality of the forecast weather situation to be assessed without defining specific (space- and time-dependant) thresholds. The EFI takes values from -1 to +1. If all the ensemble members forecast values above the M-climate maximum, EFI = +1; if they all forecast values below the M-climate minimum, EFI = -1. Experience suggests that EFI magnitudes of 0.5 - 0.8 (irrespective of sign) can be generally regarded as signifying that "unusual" weather is likely whilst magnitudes above 0.8 usually signify that "very unusual" weather is likely. Although larger EFI values indicate that an extreme event is more likely, the values do not represent probabilities as such. 

What does this mean? The prediction for excess water vapour and the presence of an “atmospheric river” will grace eastern Australia this weekend, and is classified as an unusual and extreme event, in areas shaded red (image courtesy ECMWF).

https://apps.ecmwf.int/webapps/opencharts/products/efi2web_wvf?area=Australia&base_time=202010220000&day=2&player_dimension=base_time&quantile=1

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/

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

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.

SOI check

The 30-day SOI remains positive and indicating a La Nina, with the 90-day average firming. The latest daily air pressure values suggest the SOI shift is more related to higher pressure over Tahiti rather than Darwin. We would prefer to see the Darwin hPA contribution drop some more to encourage more moisture flow over inland Australia. SOI explained here

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

Key climate indices: ENSO & Indian Ocean

Refreshed global models are showing consensus for the Niño 3.4 region to stay outside the -0.8 SST threshold in La Niña territory. Some of the international analysts are calling for a stronger, more intense event now and one global expert has put himself out there by calling for back-to-back La Niñas in 2021-22. The typically erratic Indian Ocean Dipole is making a run for wetter phase, which will certainly help, but is unlikely to over-ride the main ENSO influence prior to decaying in November.

Bureau of Meteorology modelling 14 September

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

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 looks set to plummet, assisting Southern Ocean Moisture into the Murray and southern NSW areas, while keeping those northern areas dry in the next fortnight.

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 daily air pressure values suggest the SOI shift is more related to higher pressure over Tahiti rather than Darwin. We would prefer to see the Darwin hPA contribution drop some more to encourage more moisture flow over inland Australia.

The good news is, broad-scale patterns are favouring a shift towards more moisture coming our way.

SOI explained here

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