MCV 0032 – Northern Australia/Monsoon Prediction
Overview
The official commencement of this project was 1st May 2011, but related research work
began in late 2010, as did the recruitment search for a new research scientist and a CliMag
interview. Dr. Wasyl Drosdowsky was employed as the primary researcher on the project on
9th June 2011.
The initial research work used the Predictive Ocean-Atmosphere Model for Australia
(POAMA) version 1.5b, for which hindcasts are available for 1982-2006, and which ran
operationally at the Bureau of Meteorology (in forecast mode) from September 2007 to the
end of 2011. This version of POAMA has been used to investigate the mechanisms and limits
to predictability of northern Australia wet season rainfall, and for our initial work on
developing the wet season onset prediction strategy. POAMA version 2, with hindcasts
available from 1960-2011 and run operationally since August 2011, has been used for all
subsequent work, together with the even later "multi-week" version POAMA-M24.
In the predictability work, the early wet season (premonsoon transition) was identified as the
most predictable time of year for seasonal prediction leads, and POAMA was shown to be
able to capture the seasonally varying air-sea interaction that is the primary driver of the loss
of seasonal predictability (but not intraseasonal predictability) during the peak of the
monsoon season. Hence, the focus of this project on wet season onset (on the seasonal
timescale) and on monsoon bursts and breaks (on the intraseasonal timescale) is well
justified.
For the prediction work, the first focus has been on the wet season onset as defined by the
date at which an accumulation of 50mm of rainfall is reached from 1st September. As with
most coupled models, POAMA has a dry bias over northern Australia, something that must
be taken into account in the design of the prediction scheme. We correct for the bias by
computing the forecast probability of an early onset relative to the model onset climatology.
With this correction, POAMA is able to skilfully predict the onset with a "percent correct"
exceeding 70% over about a third of the Northern Territory (and somewhat smaller fractions
of northern Queensland and WA), when tested with the hindcast dataset. These results have
been presented at several scientific conferences, written-up as a journal paper, and discussed
with the operational sections of the Bureau. The hindcasts and forecasts have also been
published in the "Experimental Products" section of the POAMA website. The output product
is a forecast map of the probability that wet season onset will be earlier (or conversely later)
than normal.
The second prediction product that has undergone development is the intraseasonal prediction
of the monsoon bursts and breaks. The variable that is predicted and verified is outgoing
longwave radiation (OLR) area-averaged over a box covering northern Australia and adjacent
seas. OLR was chosen because it is a quantity that has a long observational record from
satellites, and is a good proxy for monsoon convective activity (i.e. monsoon storms). The
latest multi-week version of POAMA (M24) has been used for this work, as it is designed
specifically for intraseasonal prediction. However, as for the seasonal prediction product, the
bias of the model need to be taken into account to generate skillful forecasts. This bias is a
function of forecast lead time and season. Once corrected, the intraseasonal forecasts are
encouraging with the forecasts of the nesemble mean OLR anomaly showing skill over
climatology at all lead times out to at least 35 days, and over persistence at all lead times
beyond 2 days. The useable skill is expected in the range of 3-15 days. These results have
been presented at several different workshops and meetings and are being written up as a
journal paper.
The above covers Outputs 1 and 2, which have followed their delivery schedule. In 2013 the
work has concentrated on finishing the journal papers and the product delivery development
(i.e. Output 3). The latter has bee undertaken by the National Climate Centre and under the
direction of Dr. Andrew Watkins and Dr Andrew Cottrill with the new wet season onset
webpages (discussed under section 12).
Conclusions
In May 2013 the Bureau started using POAMA-2 forecasts for its official seasonal climate
outlook. This is a major step for the Bureau as it shifts from basing its climate outlooks on
statistical models to dynamical models such as POAMA.
POAMA-2 includes the new ocean data assimilation system called PEODAS. Evaluation of
the impact of PEODAS was carried out as part of the MCV 00008 project based on research
on the Indian Ocean. The new ocean data assimilation system in POAMA-2 has led to an
increase in SST forecast skill in the Pacific Ocean, mostly due to significant improvements in
the assimilation of salinity. Currently there are two versions of POAMA-2 forecasts, seasonal
and multi-week forecasts. The skill of POAMA is similar in both versions in regard to the
wet season onset.
The improvements in the initialisation, particularly coupled assimilation has led to skill
improvements and should form the basis for the next version of POAMA. This coupled
assimilation system has now been implemented in the ACCESS model, ready for POAMA-3.
This system is state-of-the-art and paves the way for further developments in future.
The North Australian Wet Season Onset webpages has a brief description of the project
details including observed wet season onset dates, onset hindcasts avaliable for 1960-2009
for lead times 0, 1 and 2 months, real-time wet season onset forecasts (issued in June, July
and August from 2011) and the hindcast skill for lead times 0, 1 and 2 using percent correct
Brier score and reliability diagrams. These can be found at the POAMA Experimental
website from http://poama.bom.gov.au/realtime/wet_season/ws_home.shtml
This is available to registered users via the Registered User Login.
The new wet season onset webpage for real-time forecasts is now available via a username
(wso) and password (ncc2013) at the following address:
http://www.bom.gov.au/climate/ahead/wet-season/
This website shows the seasonal outlook details for the coming wet season onset, maps of the
onset dates for El Nino, La Nina, neutral and All Years, forecast accuracy using different start
dates (June, July, August and September) and information about the outlook. This website
will be officlally launched in June next year, to provide seasonal forecasts for the following
wet season onset.
Recommendations
The dynamical model POAMA-2, both in terms of re-analysis and hindcasts performs similar
to other international models, and there are no obvious features that other models have that
POAMA is missing. Further improvements therefore must come from new developments to
the model and the assimilation system. The skill of POAMA is relatively high in simulating
the climate variability associated with the El Nino-Southern Oscillation and the wet season
onset dates. However, as has been identified in this project, POAMA is unable to replicate the
observed trend to wetter conditions (and earlier wet season onset dates) in north-western
Australia. We therefore recommend more research on this topic.