With the current La Niña strongly in place at the onset of the Australian monsoon and reaching predicted optimum strength at the height of the rainy season the risk of experiencing a repeat of the Queensland floods of 2010-2011 is theoretically possible, but in practice will depend on the actual position of the ITCZ and related storms and depressions escaping to the south, possibly dragging weather fronts over the Australian mainland. Models show the current 7 day period (December 14-21) is still quite dry for SE Queensland and much of coastal New South Wales – whereas some North Queensland coastal areas can already expect up to a maximum of 250 mm (10 inches) of rain.

Christmas is still of the charts but judging by current sea surface temperature anomalies, which for Northeast Queensland are close (+2, +3 degrees Celsius) to resembling those around the same time last year, very likely the year will end wetter than normal in Northwest, North, Northeast and perhaps East Australia.

The current La Niña will stay in place for the rest of the Australian rainy season, but as several models show is likely to weaken by February.

Currently observed sea surface temperature anomalies and La Niña forecast for January-March 2012, according to NOAA. La Niña is strongest over December 2011 and January 2012 – and is forecast to weaken considerably over February. Currently the ITCZ is still slightly north of the drawn zone. [See full resolution image to better interpret La Niña forecast.]

Somewhere in July 2011 the La Niña episode that started about a year before – one of the strongest in decades – faded out [and temporarily showed some inclination towards an El Niño state]. But over recent months it has grown back and both current sea surface temperature anomalies and model forecasts show it is strongly in place during the southern hemisphere summer months – with a probable peak in December/January.

For Australia this means and a possible repeat of the 2010-2011 La Niña-related floods. The big uncertainty is not the amount of rain, but where exactly it will fall, which is down to specific meteorological conditions over the remainder of December and January – and hard to predict in advance.

ITCZ: what brings the annual monsoon

The Intertropical Convergence Zone (ITCZ) is a trough of connected low pressure systems encircling the planet at the point where average temperatures are highest. Here warm and moist air rises and cools down, forming thunder storms and creating a zone of high precipitation – and which all tropical rainforests depend.

The ITCZ is not fixed on the equator, but moves up and down to the northern and southern hemisphere to ‘follow the summer.’ If Earth where a featureless sphere, with only land and no oceans, the ITCZ would simply follow the solar zenith, travelling from the solstice over the Tropic of Cancer (which runs just north of the cities of Calcutta and Hong Kong) around June 21 towards the solstice over the Tropic of Capricorn (which runs close to Rockhampton and Gladstone through southern Queensland) around December 21 – a week from now – thereby crossing the equator twice a year (March 21/September 21).

The ITCZ isn’t really tied to astronomy though; it is tied to the climate, the zone with the highest average (day and night) temperatures. That means it stays within the tropics. As water masses have a high thermal inertia over the open Indian Ocean and Pacific Ocean the ITCZ tends to move little, as in normal situation the highest water temperatures and thus the highest convection remain close to the equator. Over land temperature differences between summer and winter are much greater, so for instance in Africa the ITCZ travels thousands of kilometers to reach Eqypt in the north and Madagaskar in the south. Over the Indian Ocean the annual migration is also very large as the during the northern hemisphere summer the ITCZ is drawn far up towards the large and fast-warming land masses of Eurasia [where it is now blocked by the ‘Asian brown cloud‘].

In the case of Australia the ITCZ usually remains locked over the north coast, but sometimes the solar-driven heat generation can draw it further in land during December and January. As the ITCZ is also a source area for tropical storms it can send off depressions further south, which can bring large amounts of rain, far from the actual monsoon climate region.

From theory to practice: Queensland forecast

The current SST anomalies and La Niña model outcomes show there is clearly above average energy available to create a strengthened North Australian rainy season for 2011-2012. With positive sea surface temperature especially in the month of December approaching those of last year, there is good reason to anticipate again above average amounts of total rain. In terms of expected damage much depends on where the rains will fall – and especially whether the ITCZ will make swings to the south – or send cyclones down the coast.

The above chart shows cumulative precipitation for the 7 day period of December 14 up to December 21, according to the NCEP Global Forecasting System (GFS) [forecast plot by BSch.au.com].

In the next 7 days the brunt of the Australian monsoon will hit a zone stretching over the Coral Sea from the northern coastal areas of Queensland towards New Caledonia. In the current 7 day period GFS cumulative rain forecasts are up to 325 mm offshore to the east of northern Queensland and locally between 100-250 mm in coastal areas in northern Queensland, including Weipa, Cooktown and possibly Cairns. Southern Queensland, including the areas around Rockhampton and Brisbane can enjoy a relatively dry week, with cumulative precipitation at no more than 10-25 mm. A stretching NW-SE weather front can affect inland areas of western and southwestern Queensland, and inland New South Wales, with storms bringing very local rains, at up to 100-200 mm over the current 7 day period, as predicted by GFS.

© Rolf Schuttenhelm | www.bitsofscience.org