Latest Bureau Of Meteorology Climate Driver Update, Australian Daily Extreme Temperatures Jan-Aug 2021 & Spring rainfall predictions

Issued 31 August 2021

Tropical Pacific Ocean likely to cool, but remain ENSO-neutral

The El Niño–Southern Oscillation (ENSO) is currently neutral. Sea surface temperatures (SSTs) in the central tropical Pacific Ocean are near average, despite slowly cooling over the past two months. Waters beneath the surface are slightly cooler than average.

While most models indicate SSTs in the central tropical Pacific are likely to continue cooling over the coming months, ENSO-neutral is favoured to persist. Only two of the seven models surveyed suggest SSTs will remain cool for long enough to be considered a La Niña event. This cooling of the tropical Pacific may be contributing to the wetter than median climate outlooks in Australia.

Most climate models predict the negative Indian Ocean Dipole (IOD) event will persist through the southern hemisphere spring, despite current values rising marginally above the −0.4 °C threshold. Stronger-than-usual easterly winds across the tropical eastern Indian Ocean over the last three weeks have weakened the IOD event, however, this is most likely to be temporary. A majority of the five climate models surveyed by the Bureau predict the negative IOD will persist into November. A negative IOD increases the chance of above average winter–spring rainfall for much of southern and eastern Australia.

The Madden–Julian Oscillation (MJO) is currently in the western Indian Ocean, where it has remained since early August. This phase of the MJO typically sees increased cloudiness over the Southeast Asian parts of the Maritime Continent and eastern Indian Ocean, along with enhanced easterly wind anomalies, which may have acted to weaken the negative IOD. Forecasts generally indicate a weak MJO will progress eastwards across the Indian Ocean in the coming week.

The Southern Annular Mode (SAM) index has been positive over the past week and is forecast to remain so for the next fortnight. A positive SAM typically has a drying influence on south-west and south-east Australia during winter, and enhances chances of above average rainfall over much of New South Wales during spring.

Climate change continues to influence Australian and global climate. Australia's climate has warmed by 1.44 ± 0.24 °C over 1910–2019, while southern Australia has seen a reduction of 10–20% in cool season (April–October) rainfall in recent decades.

Next Climate Driver Update expected on 14 September 2021.Climate Driver

Daily maximum temperature extremes graph for Australia: 1 January to 31 August 2021

Above median spring rainfall likely for eastern two-thirds of Australia

Issued: 26 August 2021

• Rainfall for spring (September to November) is likely to be above median for south-east WA, the NT, SA, Queensland, NSW, Victoria and eastern Tasmania (chance of exceeding median is greater than 60%). Much of the eastern States show a greater than 80% chance, as does eastern parts of the NT and SA. Conversely, parts of western WA and south-west Tasmania are likely to have below median rainfall (chance of exceeding the median is less than 40%).

• The pattern for the months of September and October is similar to the spring outlook, though the likelihood of exceeding the median rainfall is not as high in the eastern two-thirds of the country, and extends further west into eastern WA for September.

• Rainfall for October to December is likely to be above median for the eastern half of Australia, and below median for north-west WA.

• Past accuracy (Opens in new window) for September to November rainfall is high to very high for most of the eastern two-thirds of Australia, with moderate accuracy for central and eastern Victoria, most of WA and the central NT shifting to low accuracy for central areas of WA.

BOM September 2021 rainfall outlook: Taree to the NSW-Qld border

Water Security State of Play: NSW Northern Rivers December 2020

 

As of 26 November 2020 an estimated 80.5 per cent of the NSW Northern Rivers region was not in drought. However, 1 per cent of the region was in full-blown drought, another 10 per cent drought affected and 8.2 per cent recovering, according to the NSW Dept. of Primary Industries.

By 2 December urban water supply in the region was coping but beginning to fall markedly. 

Clarence Valley Council sources its urban water from the Shannon Creek Dam and the Nymboida River Weir. Currently Shannon Creek Dam is at 84 per cent capacity and water flow at Nymboida Weir is well below safe extraction level. Level One water restrictions are in place across the valley.

Ballina Shire Council, Byron Shire Council, Lismore City Council and Richmond Valley Council all principally source their urban water through Rous County Council. The two dams in this cluster are currently at 90 per cent (Rocky Creek) and 89 per cent (Emigrant Creek). With Mullumbimby in Byron Shire sourcing its water from council’s own Laverty’s Weir. Everyday water restrictions apply in all four local government area ie., recommended water use of 160 litres per person per day.

Kyogle Shire Council draws its urban water from a weir on the Richmond River, a natural weir pool on Tooloom Creek and the small Petrochilos Dam with under gravel pump extraction from nearby Peacock Creek. Level One water restrictions are in place across the shire.

Toonumbar embankment dam on Iron Pot Creek about 30 kilometres west of Kyogle was at 55.7 percent of capacity on 30 November 2020.

Tweed Shire Council draws its urban water from the freshwater section of the Tweed River at Bray Park Weir, Clarrie Hall Dam and the weir pool on Tyalgum Creek (Oxley River). Due to low rainfall the Tyalgum district is now on Level Two water restrictions.

The Australian Bureau of Meteorology rainfall and temperature predictions for January to March 2021 show that though the rest of summer may be hot across much of the Northern Rivers region there is reason to hope that rainfall will be adequate to meet our needs.

Ever wondered why you feel much hotter or colder than the temperature gauge indicates?

Australian Bureau of Meteorology, Thermal Comfort observations, January 2018:

We often use the air temperature as an indicator of how comfortable we will feel when involved in sports or other physical activities. However, the air temperature is only one factor in the assessment of thermal stress. In climates where other important factors, principally humidity, can vary widely from day to day, we need more than just the temperature for a more realistic assessment of comfort. However it is useful to be able to condense all the extra effects into a single number and use it in a similar way to the way we used the temperature. The Wet Bulb Globe Temperature (WBGT) and the Apparent Temperature are indices which attempt to do this….

Human thermal comfort depends on environmental and personal factors. The four environmental factors are airflow (wind), air temperature, air humidity, and radiation from the sun and nearby hot surfaces. The personal factors are the clothing being worn and the person's level of physical activity. Thermal sensation is also significantly affected by acclimatisation/adaptation: people living in hot climates have been shown to be comfortable at higher temperatures than those living in cooler climates.

In hotter conditions the body must shed heat to maintain thermal equilibrium. The cooling effect of evaporation of sweat from the skin becomes an important factor. The efficiency of this cooling depends on the humidity of the air. A high humidity reduces the effectiveness of evaporative cooling significantly. The amount of clothing will also affect this cooling efficiency due to its restriction of the air flow over the skin. Fabrics with low vapour permeability (those that don't "breathe") will increase the humidity of air near the skin.

In colder conditions, the body must either reduce heat loss (eg by taking shelter from the wind) or increase heat production, for example, by greater physical activity. In these conditions evaporation and air humidity are relatively unimportant factors. The cooling of the exposed parts of the body by the wind now becomes the most important external factor affecting thermal balance.

The effect of radiation is important under all temperature conditions. Excess radiation always acts to increase the heat load on a person. This can be of assistance under cold conditions, but under hot conditions it's an extra heat load that must be shed.

Of the four environmental factors, wind and radiation are very much influenced by the immediate surroundings. For example, wind speed is reduced by the sheltering effect of belts of trees and solar radiation is affected by short term localised phenomena such as cloudiness. If these factors are to be used as inputs, they are best measured on location, as values can vary significantly over relatively short distances. The remaining two factors (temperature and humidity) are less spatially variable and can be used to give an indication of the general comfort level of a region.

In order to make comparisons between areas, it is convenient to combine the effect of temperature and humidity into one index. This does not mean we can ignore the other environmental and non-environmental factors, but adjustments to the index value, either up or down, can be made to take them into account.

Most people use the temperature alone to provide some guide to the level of comfort. Generally this is quite reasonable because humidity doesn't often vary a lot, particularly in the tropics. However people moving from a less humid to more humid environment will immediately notice the effect of the greater humidity. In many sub-tropical regions of Australia the humidity is usually quite low, but occasionally can become quite high, again reducing comfort to those people not acclimatised.

The Wet Bulb Globe Temperature (WBGT) and Apparent Temperature (AT) are just two methods of combining temperature and humidity into a single number. In fact the real WBGT is also affected by wind and radiation, but the WBGT provided by the Bureau is only an approximation, which ignores variations of wind and radiation (light winds and fairly sunny conditions assumed). The AT can also be extended to take wind and solar radiation into account as well, though generally this is not done. In the AT values provided by the Bureau, wind is taken into account, but not solar radiation. Other indices such as the Physiologically Equivalent Temperature (PET) and the Predicted Mean Vote (PMV) can also be used.

An example of how this works on the ground:

To check thermal stress in your area on any given day go to Thermal Comfort observations index for each State or go directly to Thermal Comfort observations in each State NSW & ACT, Vic, Qld, WA, SA, Tas, NT.