5. MONITORING CLIMATE AND WEATHER SYSTEMS

5.1 The variable climate of Australia

Use AUSTRALIAN RAINMAN to look at average rainfall for your location. Do you get the same rainfall amounts and distribution each year? The answer is '…hardly ever…' If you did, your crop yields would seldom vary (which of course they do) and your pasture growth would almost always be the same (apart from problems with soil nutrients, pests or diseases).

As we saw in chapter 1, climate and weather are a function of differential heating of the earth's atmosphere and oceans. Water and air are in continuous circulation to balance out the heated and cooled areas. In the process of balancing out these heated and cooled areas, there is a transfer of energy, which leads to the climate and weather received at any part of the earth. The effect of the earth's rotation every day, as well as the orbit of the Earth around the Sun being completed every 365 days, also complicates our climate and weather patterns (see Will it Rain? for more information).

Another cause of this variable climate in Australia is due to the fact Australia is a huge island sitting in the middle of even larger oceans. The influence of air masses firstly interacting with warm and cool ocean currents and then travelling over Australia is never the same from year to year. This creates and maintains the variable climate and weather received.

In summary the reasons giving rise to Australia's variable climate are:

• A big country leads to non uniform rainfall or increased variability
• The Southern Ocean contributes to winter rainfall in the south
• Influences of the monsoonal north in summer
• The sea surface temperature 'hot spot' to the north of Australia
• The Walker circulation influence in the Pacific Ocean (where Australia sits on the western edge)

5.2 Major weather influences in Australia (From Climate and Your Farm NSW Agriculture)

The major interest in agriculture is rain. You may have noticed that most significant rainfall events in eastern Australia involve air that has been brought over the State from northern waters (the Indian or occasionally, the Pacific Ocean), the north-east, the Coral Sea say, or from the north-west- typically from near the Indonesian archipelago.

The reason for this is that warm air can hold more moisture (in the form of water vapour) than can cold air. Evaporation from the warmer ocean waters provides a ready source of moisture. Cooler air originating from the Southern Ocean will have less moisture available for precipitation, although good falls may still occur from these systems from time to time. Unfortunately, having a good source of moisture alone is not sufficient for rain to develop. A mechanism to force the air aloft and allow precipitation to form, is also necessary. This might be a front, a low pressure system, or a cut-off cold pool.

Some features are quite easy to identify. Cloud bands with frontal systems, cloud associated with low pressure systems, cloud spiralling into and out of tropical cyclones are just a few examples.

Some major systems which influence the weather includes:

Northwest Cloud Bands The Northwest Cloud Band (NWCB) is a long, narrow cloud band, extending from the north-west of Australia to mid-latitudes. It is sometimes, but not always, associated with deep convection. Its equatorial extremity is usually to the west of Australia and its orientation northwest/southeast. Most cloud bands occur from mid-autumn to early spring. They sometimes bring extensive rain to inland, southern and eastern Australia.

Cold Pools Pools of cold air in the upper atmosphere, cut off from southern systems, can move north over SA / Vic / NSW where they may interact with tropical maritime air from the north-west or over the Coral Sea.

Cold fronts A moving cold air mass will force warmer air to rise ahead of it. As this air rises there is often precipitation.

Tropical Depressions In summertime tropical depressions can move southwards from Queensland into NSW dragging air south from the Coral Sea.

Upper level troughs Wind flow around an upper level trough indicates air usually on the east, is forced to converge with humid air originating from the tropical seas. This north east air is then squashed into a narrow area and forced to rise. The cloud band that results has a well defined back edge that can then be tracked by satellite (Regano and Woods 1998).

5.2.1 Airmasses of the Australian region (Adapted from Climate and your farm - NSW Ag.)

There are eight major airmass types that affect various parts of Australia
(see figure 17).

Fig 17. Airmasses of the Australian region

Source: NSW Agriculture

In weather reports, you won't hear them labelled, but more likely just described as '…a cold, south-west air flow onto the coast…' or '… warm, moist, unstable air from the Coral Sea…'.

1. Modified polar maritime. This very cold, moist and unstable airmass arises in the Southern Ocean on the margin of the Antarctic (55°S). It only affects southern Australia occasionally in winter (during strong southerly flow after the passage of a vigorous cold front) and is often accompanied by snow and sleet (in alpine areas).

2. Southern maritime. Arising in the Southern Ocean (35-55°S), this cool airmass is moist and unstable at low levels, but is stable above. It brings cool, moist, cloudy weather and drizzle to southern Australia at any time of the year, but little rain unless orographically uplifted.

3. Tropical maritime Tasman. This warm airmass, sourced in the north Tasman Sea, is unstable and moist to high levels. The airmass brings warm, cloudy and drizzly weather to coastal regions of eastern Australia, with heavier rain if some means of lifting is available. This airmass is influential along the central coastal regions most of the year, but its influence diminishes towards the south, especially in winter.

4. Tropical maritime Pacific. This airmass is similar to the tropical maritime Tasman airmass but is warmer, coming from further north in the Coral Sea and tropical western Pacific Ocean. This airmass affects the Northern Queensland coast most of the year and can bring heavy rainfall if associated with tropical cyclones.

5. Tropical maritime Indian. With very similar characteristics to tropical maritime Pacific, this airmass, arising in the eastern Indian Ocean, affects the north-western coastal area of Australia.

6. Equatorial. Coming from the ocean area to the north and west of Australia, this very warm, moist and unstable airmass only effects north and west of Australia, in summer in association with the monsoon. It brings extremely heavy rainfall and high humidity to this area, but during very active monsoon seasons can affect areas further south.

7. Tropical continental. This airmass arises over Central Australia, is very hot, dry and unstable in summer, but is cooler in winter. Cloud and rainfall are restricted by a lack of moisture in the airmass. The airmass affects north-central Australia for most of the year and may bring heatwave conditions to southern Australia in summer under strong northerly flow.

8. Subtropical continental. A warm and dry airmass coming from over south-central Australia, it dominates inland southern Australia, especially in winter.

5.2.2 Ocean currents and their affect on climate
The main ocean currents which influence Australia are shown in Figure 18.

Fig. 18. Simplified map of main ocean currents affecting Australia

Source: NSW Agriculture

If these ocean currents are warm (see Fig. 19), there is more evaporation. Moisture-laden air carried over the mainland brings a greater chance of rain.

Ocean currents don't affect climate directly - it requires winds (which are linked with pressure systems) moving in the right direction to bring these moisture-bearing clouds over the land. And yet, ocean currents are extremely influential - as we will see when we consider the effects of El Niño - it all begins with the temperature of an ocean current. With cold ocean currents, there is less evaporation, and less chance of rainfall over Australia.

Figure 19. A simple model of dry conditions influenced by cold ocean currents. The model is affected by other influences apart from high or low evaporation from water, so it doesn't always work like this.

How do ocean temperatures affect rainfall?

As an island, Australia's climate is dominated by the effects of the large stretches of ocean around it. As a very large and flat island, distance from the oceans affects how much rain we get. The key influences appear to be:

the Southern Oscillation, in the Pacific, particularly in the winter and spring

the Southern oceans, noticeably in autumn and winter

the Indian Ocean plays an important part in the weather of the western part of the continent, and perhaps as far as western NSW.

the tropical seas to the north of Australia, with their monsoons

the Indonesian islands, with their tremendous heat, humidity, and complex patterning of sea and land leading to atmospheric instability

the Australian continent itself perhaps, with it large size; its dry, hot spaces, and sparse vegetative cover

the effects of latitude, with the southern half of the continent being influenced by frontal systems (high and low pressure fronts).

SYSTEMS AFFECTING AUSTRALIA - THE PACIFIC OCEAN AND EL NIÑO
Explanation of El Niño and La Niña (from the BoM SCO)

El Niño - a sustained warming, in excess of 1° C above normal, of the central and eastern tropical Pacific ocean, typically centred around the NIÑO 3 region (see Fig. 20). This warming is usually accompanied by negative values of the SOI, a decrease in the strength of the Pacific Trade Winds, and a reduction in rainfall over eastern and northern Australia which often results in drought. The most recent strong El Niño began in Autumn 1997 and ended Autumn 1998.

La Niña - a sustained cooling, in excess of 1° C below normal, of the central and eastern tropical Pacific Ocean, typically centred around the NIÑO 3 region (see Fig. 20). This cooling is usually accompanied by positive values of the SOI, an increase in the strength of the Pacific Trade Winds, and higher than normal rainfall over eastern and northern Australia, sometimes with serious flooding. The most recent strong La Niña was in 1988/89; a fairly weak event occurred in late 1995 and through much of 1996.

Fig. 20. The sea surface temperature monitoring site indicating El Niño or La Niña occurrence.

Niño areas in tropical Pacific

Source: QDPI

Exercise Idea.
Investigate the effects of El Niño and La Niña in your area by using AUSTRALIAN RAINMAN and library resources.

El Niño's Global Effects:
The South American El Niño current is caused by large-scale interactions between the ocean and atmosphere.
Nowadays, the term El Niño refers to a sequence of changes in circulations across the Pacific Ocean and Indonesian archipelago when warming is particularly strong (on average every three to eight years). Characteristic changes in the atmosphere accompany those in the ocean, resulting in altered weather patterns across the globe (see Fig. 21).

Fig. 21. Areas most consistently affected by El Niño

Source: BoM

The Walker Circulation
The walker circulation is named after Sir Gilbert Walker, a Director-General of British observatories in India who, early this century, identified a number of relationships between seasonal climate variations in Asia and the Pacific region. The Walker Circulation and how it changes with El Niño or La Niña is best shown by the following diagram. (see Fig. 22).

Fig. 22. Walker circulation changes during normal and El Niño times

Source: BoM

The Southern Oscillation Index
The SOI is a measure of the pressure difference between Tahiti and Darwin. When the SOI is negative, it reflects higher pressure over Darwin compared to Tahiti. This usually means drier than normal times in Australia. When the SOI is positive (+5 up to +20), it reflects lower pressure over Darwin compared to Tahiti. This usually means wetter than normal times in Australia.

The areas affected by the SOI in Australia

There is a relationship between the SOI and rainfall in Australia. The strength of this relationship varies. The relationship between the SOI and rainfall is strongest over eastern Australia. (see Fig. 23.) The Southern Oscillation also has a strong influence on cloud cover, temperature, humidity and evaporation, number of wet days and frequency of cyclones.

Fig. 23. - Probability of exceeding median rainfall for Sep./Nov. based on either the positive or negative SOI phase in July-Aug

Source: QDPI

HOW DOES THE SOI AFFECT AGRICULTURAL PRODUCTIVITY?

Rural productivity, especially in Queensland and New South Wales, is linked to the behaviour of the Southern Oscillation. The graph below (Fig. 24.) shows how Australia's wheat yield has fluctuated with variations in the Southern Oscillation. Negative phases in the oscillation (lower than normal rainfall) tend to have been linked with reduced wheat crops, and vice versa while positive phases (higher than normal rainfall) are linked to high wheat yields.

Fig. 24.

Source: BoM

SYSTEMS AFFECTING AUSTRALIA - THE INDIAN OCEAN
Sea surface temperatures (SSTs) off the west Australian coast may also be useful in estimating the probability of rain over the continent. SSTs in summer and autumn have an effect on the probability of rainfall in early winter over parts of southern and eastern Australia, through the formation of north-west cloudbands.

Fig. 25. Monitoring the SST off the coast of WA

Source: QDPI

There is greater probability of higher rainfall when the SST is warmer than normal. Conversely, there is greater probability of lower rainfall when the SST is cooler than normal. This is another example of why farmers monitor the sea surface temperatures (see Table 5). See AUSTRALIAN RAINMAN to see if SST's affect your area.

Table 5. Rainfall screen grab from AUSTRALIAN RAINMAN. Ravensthorpe WA ±0.3°c SST: Jun.-Jul. Rainfall: Aug.-Sep.

Source: QDPI

Up to date sea surface temperatures are available from the BoM and QDPI internet sites (see references).

The software package Australian Rainman analyses rainfall data. Sea surface temperatures in the Indian Ocean may be used in the forecast system in Rainman.

Exercise:
Using Australian Rainman, analyse the rainfall data for your location using
a) the SOI when consistently positive, neutral or consistently negative
b) the Indian Ocean when warmer, cooler than normal, or average
How can the seasonal climate forecast help minimise risks and losses in agriculture?

Extension idea: In groups, visit and interview farmers to see how better understanding the variable climate and using seasonal climate forecasts in agriculture can help to improve decisions, and minimise losses. Present the findings to the rest of the class.

5.3.1 4 day outlook maps

Fig. 26. 4 day outlook maps.

Source: BoM

Farmweather Services (on poll-fax)
Provided by the Bureau of Meteorology (as at July 1998)

National directory of FARMWEATHER services is available on
Freepoll 1800 061 404

As well as reacting to the climate and weather farmers receive, farmers may also be pro-active in their management to climate and weather they may receive. That is why four day, 10 day, and 90 day outlooks are important for farmers. For instance, knowing when a cold change is due, stock can be moved to sheltered paddocks. If a 90 day outlook indicates drier than normal times are expected, management can be modified to adjust for this scenario .

5.3.2 The 10 day outlook

Fig. 27. The 10 day outlook available on the internet.

Source: COLA

5.3.3 The three month outlook

Fig. 28a. The three month outlook available on the internet.

Source: QDPI

Fig. 28b. The three month outlook available on the internet.

Source: BoM

Exercises
As a group exercise, monitor how accurately the four day, 10 day, and possibly 90 day outlooks are. Discuss the results in class presentations.

Examples

Day 1 Day 2 Day 3 Day 4 Temp R’fall Temp R’fall Temp R’fall Temp R’fall 4 day forecast 4 day actual   Days 1-5 Days 6-10 10 day outlook                     10 day actual                       Rainfall   Month 1 Month 2 Month 3 90 day outlook (source BoM)       90 day probability forecast (source QCCA)       90 day actual

Explain why forecasts are so important for local agricultural enterprises?
What have you learnt about probability based forecasts? What are the advantages and disadvantages of using a probability based forecast?