Analysis - MJO: what is it and why do we watch it?

The Madden-Julian Oscillation (MJO) is the major fluctuation in tropical weather on weekly to monthly timescales. The MJO can be characterised as an eastward moving 'pulse' of cloud and rainfall near the equator that typically recurs every 30 to 60 days.

MJO Explainer

Click on the image to play this 3-minute MJO explainer

Predicting the MJO and where is it now?

Due to its slowly evolving nature, accurate prediction of the MJO is fundamentally related to scientists ability to monitor the feature and to assess its relative position and strength. Dynamical models generally do not predict the MJO well, partly because of the inherent difficulties that still remain regarding the correct mathematical treatment of tropical convective (rainfall) processes. Meteorologists use a variety of data and analysis techniques to monitor, study, and predict the formation and evolution of the MJO.

In the chart below;

• The black line is observations with dates (where it is now is largest black dot #21)

• The yellow lines are potential modelled outcomes (bracketed by shading)

• The green line is the median prediction (where to next)

• The phase is its location: Australia generally 5-6

• The strength is indicated by where it is plotted: inside the circle = weak. Outside the circle = stronger. The closer to the outside, the stronger the pulse.

https://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/mjo.shtml

So what?

Dynamical models favour little to no MJO activity during Week-1 (22-29 Dec) but suggest potential renewed MJO activity over the Indian Ocean during Week-2 (30 Dec-6 Jan).

Given the current weak signal, the MJO is unlikely to substantially influence the tropical convective pattern or subsequent extratropical response. The ongoing La Niña remains the primary tropical convective driver (see the latest NCEP conditions report).

The weaker MJO may reduce tropical moisture but it has a silver lining - a strong MJO in phases 8-1 can bring on heat waves which we can do without!

Analysis - Sea Surface Temperature Distributions 2010 v 2020 La Niña's

December 2010 Sea Surface Temperatures and rainfall deciles

Record warm sea surface temps around Australia’s north and the Maritime Continent

With the exception of a small strip, record rainfall through eastern Australia in that month

December 2020 Sea Surface Temperatures and rainfall outlook (21 Dec 2020 - 3 Jan 2021)

SSTs as of 11 December 2020 (tropicaltidbits.com)

Fortnightly precipitation forecast 21 Dec-3 January 2020 (courtesy BOM 10 December).

So what?

The La Niña signature can be clearly seen in both images, with cool water extending into the western Pacific Ocean. Models are anticipating the Coral Sea to warm and deliver rainfall to eastern Qld areas (although the latest 7-day change maps on tropicaltidbits.com would suggest otherwise). The waters around the Maritime Continent are not near as warm as 2010. The next research article suggests that we need these waters, and those to the east of Australia to be much warmer to supply rainfall events this summer - this warming may well happen in the next month or so.

Seasonal forecast: December 2020 - February 2021

Here are four of the major seasonal rainfall models showing for December, January, February.

The general circulation models (GCMs) are not exactly showing a conventional La Niña signature through central inland areas, instead favouring coastal Queensland, Cape York and SE corner of NSW. The cooler waters to Australia’s north and ever-persistent high-pressure systems in the Tasman Sea are perceived as having a negative influence on rainfall through central NSW through to the Darling Downs/Southern Qld.

Despite forecast rain for the first week in December, some caution is needed for northern NSW and southern Qld areas looking to plant summer crops or purchase livestock. The Southern Annular Mode has not progressed into a positive phase (in keeping with La Niña), which is likely flowing through into these model outputs and requires careful monitoring during the next few weeks.

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IRI MME (US)

BOM (AU)

ECMWF (EU)

CMCC (ITA)

Analysis - stratospheric cooling over Antarctica: a closer look

The Southern Annular Mode (SAM) index has finally broken through into positive phase over the last week, which could affect weather patterns in eastern Australia during the coming months.

The SAM is an index that measures the north-south displacement of a band of westerly winds flowing between Australia and Antarctica. 

When the SAM index is negative, these westerly winds move towards Australia and when it is positive, they contract towards Antarctica.

The positive phase of the SAM is linked to the stratospheric cooling over Antarctica, which made headlines last year for extreme warming (Left graph) relating to a negative SAM, not seen since the horrible drought year of 2002. The circle showed “çoupling” with the troposphere, which is where the SAM is measured and consequently, spent the rest of the year in a negative phase.

The second image (Right graph) shows the current - and very opposite situation. Stratospheric cooling has just “coupled” with the SAM, showing the cooling (blue) colours right through different heights of the atmosphere.

What does this mean? Stratospheric cooling (or warming) was found by Hendon et al (2020) to have a connection with the SAM. It’s anticipated that the SAM will be positive during La Niña, but this analysis does provide added confidence. A positive SAM occurs when the winds in the southern ocean contract (last image), allowing more tropical moisture to move south into the mid-latitudes increasing the chance of late spring and summer rain. SAM moving back into positive territory is a good sign for rainfall on summer pastures and crops from NSW through to the northern Darling Downs.

Want to learn more about SAM - watch this 2 minute youTube video

Spring 2019 - sudden stratospheric warming (top) and SAM (below). Last years event was the most extreme event not seen since 2002.

Current (Nov 2020) chart showing stratospheric cooling (top), coupling with a positive SAM (below).

The winds in the Southern Ocean contract further south, enabling tropical La Niña moisture to reach the mid-latitudes.

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.

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.

Seasonal forecast: October - December 2020

Form: Coming out of a dreadful run of form from a wet winter that didn’t eventuate for central/northern areas, GCMs are keeping in-step with the developing La Niña signal with cool, wet conditions throughout the forecast area. The only model a bit less bullish on a solid wet signal was UK Met, whereas the IRI was heavy on the tropics copping a wet time in the coming three months.

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

Seasonal forecast: October - December 2020 (Copy)

Model form: Coming out of a dreadful run of form from a wet winter that didn’t eventuate for central/northern areas, GCMs are keeping in-step with the developing La Niña signal with cool, wet conditions throughout the forecast area. It should be noted, inputs to the forecast are now settled. This was not the case in May when the IOD and ENSO are still finding their feet. The only model a bit less bullish on a solid wet signal was UK Met, whereas the IRI was heavy on the tropics copping a wet time in the coming three months.

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