Climate summary for the southern hemisphere, 2019–20: a strong positive Indian Ocean Dipole

3.1. Southern Oscillation and ENSO Indices

The El Niño–Southern Oscillation (ENSO) is monitored through a range of indices, both atmospheric and oceanic. During 2019–20, both the Troup Southern Oscillation Index⁴ (SOI) and the NINO3.4 and 5VAR sea surface temperature indices were generally consistent with conditions in the equatorial central and eastern Pacific, which were mostly warmer than average, but not at a level sufficient to reach El Niño thresholds on a sustained basis. These are the primary indices used to measure variations in sea surface temperatures and atmospheric conditions along the tropical Pacific Ocean; such variations are well known to influence the climate in the Australian region and much of the broader southern hemisphere. When indices are neutral, as for the bulk of 2019−20, other climate influences tend to dominate. For reference, plots of the indices are included below.

Fig. 1 (left) shows the SOI and a 5-month weighted moving average of the monthly SOI. SOI values were negative through the second half of 2019 but were within the neutral range (generally considered to be between +7 and −7) for most of the year, with the 5-month mean falling to −7.5 at its lowest point. From January 2020 onwards, SOI values were generally close to zero.⁵

Fig. 1.

(Left) Troup Southern Oscillation Index (SOI) values from January 2017 to September 2020, with a 5-month binomial weighted moving average. (Right) Anomalies of the composite 5VAR ENSO index for the period January 2016 to September 2020 with the 3-month binomially weighted moving average.

The ENSO 5VAR Index is a composite monthly ENSO index. Values of the 5VAR within 1 s.d. are typically associated with neutral conditions; as evident in Fig. 1 (right), this was the case for most of 2019−20. The NINO3.4 index is used by The Bureau to monitor ENSO as it is closely related to the Australian climate (Wang and Hendon 2007); it measures sea surface temperatures in the central Pacific Ocean between 5°N–5°S and 120–170°W. NINO3.4 also remained in the neutral range (between +0.8 and −0.8°C) for most of 2019–20. It was positive for most of 2019–20 but did not exceed 0.7°C at any stage, before dropping back to near zero in May and June 2020. Overall, The Bureau’s operational ENSO monitoring⁶ assessed ENSO conditions as neutral throughout the 12 months. The NINO4 index, which represents conditions in the central-western equatorial Pacific (between 160°E and 150°W), was warm for most of the period and peaked near +1°C in late 2019 and early 2020. With the tropical Pacific Ocean indices in a neutral phase, the main oceanic climate influence through the second half of 2019 was the Indian Ocean.

3.2. Indian Ocean Dipole (IOD)

The IOD⁷ is the difference in ocean temperatures from the western node of the tropical Indian Ocean (centred on the equator, off the coast of Somalia) and the eastern node (off the coast of Sumatra). The IOD is said to be in a positive phase when values of the Dipole Mode Index (DMI) are greater than +0.4°C, negative when DMI values are less than −0.4°C, and neutral otherwise. The positive and negative phases of the IOD are known to significantly influence Australian rainfall. The IOD is correlated with ENSO, with positive IOD events more likely to occur during El Niño events (Stuecker et al. 2017), but significant positive or negative IOD events can occur in the absence of a corresponding ENSO signal.

The positive IOD is known to have a significant influence over temperature and rainfall.⁸ During positive IOD periods, above average maximum temperatures typically occur in the west and south of Australia, and above average minimum temperatures in the south-west, but below average minimum temperature in the tropics, with below average rainfall in many parts of Australia.

The IOD’s primary influence on climate is during the austral winter and spring. Strong IOD phases are rare during the austral summer as DMI variability is low at this time of year (Yang et al. 2015). This is due to the monsoon trough shifting south over the tropical Indian Ocean and changing the overall circulation, meaning that an IOD ocean temperature pattern cannot be sustained. The influence of the positive IOD event of 2019, however, continued into summer as the monsoon trough remained farther north. The monsoon trough departed India much later than usual, only beginning its retreat on 9 October, compared with the average date of 1 September (India Meteorological Department 2019).

There was a strong positive IOD event in the second half of 2019. DMI values exceeded +0.4°C from late July 2019 onwards and remained above that threshold until late December (Fig. 2). The event peaked in October with DMI values near +2°C. Although there are differences between different data sets in the assessment of the strength of historical IOD events, the 2019 IOD event was clearly the strongest since 1997, and the strongest known event to occur in the absence of a clearly defined El Niño. DMI values then remained close to zero through the first half of 2020.

Fig. 2.

Dipole Mode Index timeseries from March 2016 to November 2020.

Widespread dry conditions in Australia in the second half of 2019, as well as anomalies elsewhere in the hemisphere, including dry conditions in much of Indonesia and the Australian Indian Ocean islands, and wet conditions in equatorial eastern Africa, were consistent with the conditions that would be expected with the positive IOD, in conjunction with other influences, such as the negative Southern Annular Mode phase late in 2019.

6.1. Sea surface temperatures (SSTs)

The average SST in the southern hemisphere for 2019–20 was 0.63°C above the 1901–2000 average.¹² This was the equal third-highest value on record, with the record highest value set during the strong El Niño event of 2015–16. Warm anomalies were strongest later in 2019–20, with an anomaly of +0.68°C for January to June 2020 (third highest on record), compared with +0.58°C for July to December 2019 (fourth highest). In both cases, the record high values were also in 2015–16.

The most prominent warm areas of the southern hemisphere were in the tropics (Fig. 5), in the western half of the Pacific and the western and central Indian Ocean. A large part of the Indian Ocean had SSTs for the 12-month period more than 1°C above the 1961–90 average. Similar anomalies were found in the equatorial Pacific near the Date Line between 160°E and 170°W, and in parts of the eastern tropical South Atlantic. Outside the tropics, a prominent area of warm conditions was east of New Zealand, with anomalies exceeding +1°C over a broad area. Most tropical areas were at least 0.5°C above the 1961–90 average, except for the eastern half of the Pacific Ocean and an area stretching east from the eastern tropical Indian Ocean through most of the northern waters of Australia. SSTs were within 0.5°C of average over most areas south of 30°S, except for the region east of New Zealand, and smaller areas off the coast of Argentina.

Fig. 5.

ERSSTv5 sea surface temperature (SST) analysis for the period from July 2019 to June 2020: anomalies from 1961 to 1990 baseline (top) and percentiles from all years of record (bottom).

In the period from July to December 2019, SSTs were more than 0.5°C below the 1961–90 average in the eastern equatorial Indian Ocean west of Sumatra (Fig. 6), the eastern node of the IOD, but by the January to June 2020 period, they had returned to above average in this region. In the January–June period, almost all tropical areas apart from the eastern half of the Pacific were at least 0.5°C above average.

Fig. 6.

Global ERRSTv5 sea surface temperature (SST) anomalies from 1961 to 1990 baseline for July to December 2019 (top) and January to June 2020 (bottom).

6.2. Subsurface

Ocean heat content, an indicator of heat through the ocean subsurface, was at record or near-record levels for most of the period. Averaged over the southern hemisphere, values for the 0–2000-m layer reached 15.34 × 10²² J in the quarter January–March 2020, the highest value on record for any quarter to that time. October–December 2019 (15.10 × 10²² J) was the highest October–December value on record, with April–June 2020 ranking second and July–September 2019 third.¹³ The warmth was especially pronounced in the South Atlantic, where ocean heat content was at record levels in all four quarters; the strongest warm anomalies here were in a band stretching across the ocean from west to east near 30°S. Anomalies were less extreme in the South Pacific and Indian Oceans, where ocean heat content was generally lower than it had been the previous year. Values for the South Indian Ocean for the April–June 2020 quarter were the lowest for that quarter since 2013.

Subsurface temperatures in the equatorial Pacific (Fig. 7) were generally fairly close to average for most of the period, consistent with the neutral ENSO conditions. Subsurface waters were warmer than average near the Date Line during the summer, with peak anomalies exceeding 2°C between 100- and 150-m depth. Cool subsurface waters developed in the western equatorial Pacific in January and February and then propagated to most of the remainder of the Pacific east of 160°E from March onwards, with anomalies in May locally exceeding −3°C in parts of the central and eastern Pacific.

Fig. 7.

Subsurface ocean temperature anomalies¹⁴ (°C) along the equator from November 2019 to February 2020 (left) and March to June 2020 (right), obtained from NOAA’s TAO/TRITON data.

8.1. Southern Annular Mode (SAM)

The Southern Annular Mode (SAM), also known as the Antarctic Oscillation (AAO), describes north–south shifts of the belt of strong westerly winds in the middle to high latitudes of the southern hemisphere,¹⁶ with positive SAM indicating a poleward shift of these winds and associated frontal systems, and negative SAM an equatorward shift. The SAM is known to influence temperature and rainfall patterns across Australia with varying influences across the seasons (Hendon et al. 2007), with negative SAM associated with below-average rainfall on the east coast south of the tropics in spring and summer. It varies on shorter timescales, typically of a few weeks, than do ENSO and the IOD. A number of SAM data sets based on hemispheric analyses extend back to 1979, with SAM indices based on station data covering the whole post-1900 period. In this analysis, the data set of the NOAA Climate Prediction Center (CPC) is used.

SAM was predominantly in its negative phase during the second half of 2019 (Fig. 9), bringing stronger westerly winds over the southern Australian continent. The negative values were especially persistent and strong from October to December. Daily values of the SAM index in the CPC data set were continuously negative from 18 October to 23 December. The strongest monthly negative value of the index was −1.84 in November, the fourth-lowest of the post-1979 period and the lowest for November since 2009. The October–December mean was −1.37, the lowest of the post-1979 period for an October–December timespan (although the mean value for September–November 2002 was −1.45). Both the 2019 and 2002 events were associated with sudden stratospheric warmings in the Antarctic (Section 8.3).

Fig. 9.

Monthly Southern Annular Mode (SAM) index values from January 2016 to June 2020 (data: Climate Prediction Center).

SAM returned to more neutral levels from January 2020 onwards, and leaned positive for most of the first half of 2020. Monthly values reached +1.43 in March and +1.07 in June, although both were lower than those observed in the corresponding months of 2016.

8.2. Surface analyses

Mean sea level pressure (MSLP) anomalies for the four quarters of the period are shown in Fig. 10, relative to the 1979–2000 climatology obtained from the NCEP II Reanalysis data (Kanamitsu et al. 2002). The MSLP pattern is computed using data from the 0000 hours UTC daily analyses of The Bureau’s ACCESS model.¹⁷ The MSLP anomaly field is not shown over areas of elevated topography (grey shading).

Fig. 10.

Southern hemisphere mean sea level pressure (MSLP) anomaly pattern for July to September 2019 (top left), October to December 2019 (top right), January to March 2020 (bottom left) and April to June 2020 (bottom right). Anomalies (hPa) calculated with respect to base period of 1979–2000. Areas of elevated topography are shown in grey.

The most striking MSLP anomalies were in the October–December period, during the time with the most strongly negative SAM, with positive MSLP anomalies over most of Australia but strongly negative anomalies south of the continent, consistent with typical patterns during a negative SAM phase and resulting in anomalous westerly flow over southern Australia. The anomalies were especially strong in November (not shown), with monthly anomalies of +5 hPa over South Australia and −12 hPa near 50°S south of Tasmania. December showed similar spatial patterns but with weaker anomalies. There was also strong ridging over the South Pacific centred near 150°W during this period. Negative anomalies persisted over much of the July–December period in the Indian Ocean between 30 and 60°S, and there were weak negative anomalies over the western tropical Indian Ocean, which had anomalously warm SSTs during the positive phase of the IOD.

There were fewer strong MSLP anomalies in the first half of 2020 than there had been in the previous 6 months, consistent with the return of key climate influences to more neutral levels. Positive MSLP anomalies were centred near New Zealand in the January–March period, and to the east of the country in April–June. There was also an anomalous trough west of South America in the April–June period.

8.3. Upper atmosphere

The most notable event in the upper atmosphere during 2019–20 was a significant sudden stratospheric warming (SSW) over high latitudes of the southern hemisphere (Lim et al. 2021). Starting in the fourth week of August, the polar cap warmed by 35°C at 30 hPa over 3 weeks, whereas the polar westerly jet at 10 hPa weakened by 80 m s⁻¹. This major weakening of the polar vortex then propagated downward and was a trigger for the strongly negative SAM from October onwards. Although SSW events are fairly common in the Arctic, they are rare in the Antarctic, with the only previous event comparable with the 2019 event occurring in 2002 (although the 2019 event surpassed that of 2002 on a number of key metrics).

The strongest 500-hPa geopotential height anomalies (Fig. 11) were observed in the October–December quarter, consistent with the SSW and the negative SAM (Section 8.1). Strong positive anomalies were observed over Antarctica, with almost the entire continent having 500-hPa geopotential heights 60 geopotential metres (gpm) or more above the 1979–2000 average. By contrast, negative anomalies were observed in most areas between 40 and 50°S, consistent with a northward displacement of the normal zonal circulation in these latitudes and mostly coincident with negative pressure anomalies at the surface (Fig. 11). This followed a July–September quarter when there was strong anomalous ridging over the Antarctic Peninsula (with a peak anomaly of +132 gpm), and anomalous troughs centred near 60°E in the Indian Ocean and south of New Zealand.

Fig. 11.

The mean 500-hPa geopotential metre (gpm) height anomalies for July to September 2019 (top left), October to December 2019 (top right), January to March 2020 (bottom left) and April to June 2020 (bottom right). Anomalies calculated with respect to base period of 1979−2000.

As with many other features of the climate system, conditions reverted to a situation closer to long-term averages in the first half of 2020, particularly in the January–March quarter where the only strong feature was ridging east of New Zealand. This strengthened further in the April–June period, in which an anomalous trough developed in the Indian Ocean centred near 40 to 60°E. Weak to moderate positive anomalies prevailed throughout the year over most areas beneath or north of the subtropical ridge.

Fig. 12 and 13 show 2019–20 low-level (850 hPa) and upper-level (200 hPa) wind anomalies respectively (winds computed from ACCESS and anomalies with respect to the 22-year 1979–2000 NCEP climatology). Isotach contours are at an interval of 2 m s⁻¹ (850 hPa) and 5 m s⁻¹ (200 hPa).

Fig. 12.

The 850-hPa vector wind anomalies (m s⁻¹) for July to September 2019 (top left), October to December 2019 (top right), January to March 2020 (bottom left) and April to June 2020 (bottom right).

Fig. 13.

The 200-hPa vector wind anomalies (m s⁻¹) for July to September 2019 (top left), October to December 2019 (top right), January to March 2020 (bottom left) and April to June 2020 (bottom right).

The strongest 850-hPa wind anomalies were mostly in the October–December period. During this period, there were strong westerly anomalies over and south of southern Australia, consistent with the negative SAM phase, and easterly anomalies further north. Easterly anomalies had also been evident over northern Australia between July and September. There were also strong easterly anomalies (reaching 6 m s⁻¹) in the central and eastern Indian Ocean between 70 and 100°E, during the peak of the positive IOD phase. Anomalies at this level were fairly weak in most tropical and subtropical areas in the first half of 2020, although there were some anticyclonic anomalies associated with areas of above-average surface pressure in the South Pacific. One noteworthy anomaly was a strong westerly anomaly in the April–June 2020 period over equatorial eastern Africa, aligning with the active wet season at that time in the region (Section 11).

The most consistent feature of the 200-hPa wind anomalies was a high frequency of westerly anomalies between latitudes 30 and 45°S, marking a southward extension of the climatological maximum of westerly winds at this level. These were most prominent in the central South Pacific in the October–December quarter, which also saw a marked southward displacement of the climatological peak winds in the Australian sector, with easterly anomalies over the continent. In early 2020, when mid-latitude anomalies were weaker than in other seasons, there was an area of very strong westerly anomalies (exceeding 20 m s⁻¹) in the equatorial central and eastern Pacific, a region where 200-hPa winds are typically fairly weak.

10.1. Rainfall

The overall 12-month period from July 2019 to June 2020 was substantially drier than average over most of Australia (Table 1, Fig. 16). This combined an exceptionally dry second half of 2019, during the period when the IOD was strongly positive, with near-average conditions in the first half of 2020. The 12-month period as a whole was the third-driest on record for Australia, with national area-averaged rainfall of 346.5 mm, 26% below the 1961–90 average. Only 1901–02 (298.2 mm) and 1951–52 (325.5 mm) were drier, although 2019–20 was only slightly drier than 2018–19 (346.9 mm). All states and territories were drier than average, with South Australia (fourth driest on record) 44% below the 1961–90 average. The only substantial areas with significantly above-average rainfall were western Tasmania and an area of southern Victoria east of Melbourne. There were large areas with rainfall below the 10th percentile for the 12-month period, including most of South Australia from Adelaide northwards and westwards, much of the southern half of Western Australia, the Top End of the Northern Territory, much of the south-east quarter of Queensland, and the far south-east of New South Wales and far eastern Victoria.

Fig. 16.

Rainfall deciles for July 2019 to June 2020 (left), July to December 2019 (centre) and January to June 2020 (right); decile ranges based on grid-point data with respect to all available data 1900–2020.

The second half of 2019 was exceptionally dry. It was the driest July–December period on record for Australia, with area-averaged rainfall of 63.4 mm, 61% below the 1961–90 average. All states and territories other than Victoria and Tasmania also had their driest July–December on record, with Queensland, New South Wales, South Australia and the Northern Territory all between 65 and 75% below the 1961–90 average. Rainfall was below average almost everywhere in mainland Australia apart from locally in southern Victoria, although it was well above average in western Tasmania, associated with the occurrence of negative SAM phases. Many areas in the eastern two-thirds of Australia had their driest July–December on record, especially in the northern half of New South Wales and southern inland Queensland. Most areas west of the ranges in New South Wales and away from the east coast in Queensland had less than 100 mm for the period, and a substantial area of the eastern interior received less than 10 mm. Every month of the period was significantly drier than average, with all 6 months ranking in the 12 driest on record for Australia for their respective month, but November and December were particularly extreme, both ranking as the driest on record nationally.

By contrast, rainfall in the first half of 2020 was generally close to average. Nationally, rainfall was 6% below the 1961–90 average, with all states and territories being within 25% of average and none ranking in the top or bottom 20 years. Rainfall was above average in most of New South Wales and Victoria, although there was a remaining dry area in far south-eastern New South Wales and eastern Victoria. Seasonal rainfall was above the 90th percentile in central Victoria, and parts of inland New South Wales. Outside these two states, rainfall for the 6-month period was generally fairly close to average, although somewhat below average in western parts of Western Australia. January to March was wet in most eastern areas south of the tropics, including a major rain event in February that produced some flooding in the Sydney area; April to June was dry along the New South Wales coast and in southern Queensland, but wet in most of Victoria and in New South Wales west of the ranges. April to June was also dry in most of the southern half of Western Australia.

The heaviest daily rainfall in Australia during 2019–20 was 562.0 mm on 11 January 2020 at Dum In Mirrie, south-west of Darwin, a record daily rainfall for the Northern Territory. This was associated with a tropical low that subsequently became Tropical Cyclone Claudia.

10.2. Rainfall deficiencies

The very dry conditions in the second half of 2019 resulted in extensive rainfall deficiencies at a range of timescales. These then progressively eased through the first half of 2020 as rainfall returned to near- to above-average levels in many areas. Nationally, the proportion of Australia experiencing serious rainfall deficiencies (below the 10th percentile) at the 12-month timescale (Fig. 17) increased from 34.3% at the end of June 2019 to 69.2% in December. It then fell to 20.6% by April 2020, before increasing to 27.6% at the end of June 2020 as rainfall deficiencies expanded in southern Western Australia with the dry start to the 2020 cool season there. The area of severe 12-month rainfall deficiencies (below the 5th percentile) increased from 19.3% of Australia at the end of June 2019 to 55.2% in December, before decreasing to 9.8% by April 2020.

Fig. 17.

Rainfall deficiencies for the 12 months ending June 2019 (left), December 2019 (centre) and June 2020 (right).

Rainfall deficiencies were even more widespread at shorter timescales, with 81.6% of Australia showing serious deficiencies for the 6 months to December 2019. At the state level as of December 2019, 97.1% of New South Wales had serious 6-month deficiencies, whereas 94.7% of South Australia showed serious deficiencies at the 6-month timescale, and 94.0% at the 12-month timescales.

10.3. Temperature

Overall, the period from July 2019 to June 2020 was the equal third-warmest such period on record for Australia (Table 2), with temperatures 1.22°C above the 1961–90 mean, 0.17°C below the record set in 2015−16. Mean maximum temperatures were 1.66°C above the 1961–90 mean (second highest) and minimum temperature 0.77°C above the 1961–90 mean (eighth highest). Mean temperatures were above the 1961–90 average almost everywhere except for some small areas in eastern South Australia and south-western Victoria (Fig. 18). Much of interior Western Australia had mean temperatures 1.5–2.5°C above average; the state as a whole was 1.56°C above average, 0.29°C above the previous record from 2015 to 2016. It was the warmest July to June period on record for most of Western Australia south of 20°S except for the immediate coastal fringe, as well as locally in inland parts of south-east Queensland.

Fig. 18.

Mean temperature anomalies (1961−90 climatology) for July 2019 to June 2020 (left), July to December 2019 (centre) and January to June 2020 (right). Data from version 2.4 of the ACORN-SAT dataset.

As with rainfall, there was a contrast between the second half of 2019 and the first half of 2020. July to December 2019 was the warmest such period on record, with mean temperatures 1.59°C above average. Daytime maximum temperatures were particularly extreme, 2.38°C above average and 0.39°C above the previous record, set in 2013. Maximum temperatures for the July to December 2019 period were the highest on record for New South Wales, the Northern Territory, South Australia and Western Australia, and Western Australia also had its highest mean temperatures on record, 0.53°C above the previous record from 2015.

January to June 2020 was also significantly warmer than average, but anomalies were much weaker than they had been in the preceding 6 months, with a national mean temperature anomaly of +0.84°C. Much of the northern half of Australia remained warm during this period, with temperatures widely 1–2°C above the 1961–90 average, but temperatures were below average over a broad area of the south encompassing much of South Australia and Victoria, and parts of western New South Wales. The large positive diurnal temperature range anomalies of late 2019 also reverted to near-average in early 2020 as rainfall returned to near-average; the mean national diurnal temperature range was 1.57°C above average for July to December 2019, but only 0.21°C above average for January to June 2020.

The most extreme individual month was December 2019. National mean temperatures for the month were 3.20°C above the 1961–90 average and more than 1°C above the previous December record. This was also the largest monthly anomaly observed for any month, surpassing October 2015 (+3.03°C). It was the hottest December on record for all states and territories except Victoria and Tasmania. There was a particularly extreme and extensive heatwave in the second half of the month. Australia’s five hottest area-averaged days on record for mean temperature, and seven hottest for maximum temperature, all occurred between 17 and 29 December. On 18 December, the nationally averaged maximum temperature was 41.88, 1.58°C above the record prior to December 2019. In total, there were 11 days during the month when the nationally averaged maximum temperature reached 40°C; there had only been 11 previously in total between 1910 and November 2019, 7 of which occurred during the summer of 2018–19 (Bureau of Meteorology 2020). The highest temperature during this period, and during 2019–20 as a whole, was 49.9°C at Nullarbor (SA), an Australian record for December and the highest in any month since 1998, and Eucla (WA) reached 49.8°C on the same day. Keith (SA) reached 49.2°C on 20 December, the highest ever in Australia at such a high latitude (36.1°S). Further extreme heat occurred in early January, including 48.9°C at Penrith on 4 January, a record for metropolitan Sydney (and for any site east of the ranges in New South Wales), and 44.0°C at Canberra on the same day, its highest on record. Although temperatures moderated in February, it was still Australia’s second-hottest summer on record behind 2018–19.

Western Australia had its warmest spring on record. Autumn 2020 was quite cool in south-eastern Australia, particularly by day; most of inland New South Wales had mean autumn maximum temperatures below the 10th percentile, and the statewide mean maximum for the season was the lowest since 1995. May was the only 1 of the 12 months during the period with national temperatures below the 1961–90 average.

The lowest temperature in Australia during 2019–20 was −12.3°C at Glen Innes Airport on 19 July 2019, a rare instance of the year’s lowest temperature occurring outside the Australian Alps.

10.4. Fire weather

Summer 2019–20 is now widely referred to as ‘Black Summer’ owing to the unprecedented extent and severity of fire activity. Major fire activity in eastern Australia began in early September and continued until February, particularly affecting eastern parts of New South Wales, Victoria and southern Queensland (Royal Commission into National Natural Disaster Arrangements 2020). There were also major fires in South Australia around Adelaide and on Kangaroo Island. In addition to direct losses of life, property and biodiversity, there was also extensive and long-lived smoke pollution in many areas, which had major health impacts, with over 400 excess deaths attributed to smoke exposure (Borchers Arriagada et al. 2020).

A metric of the incidence of significant fire weather is the aggregated Forest Fire Danger Index (FFDI) (McArthur 1967), which is an indicator of fire danger in forested areas, and combines temperature, humidity, wind speed and antecedent dryness information. It is particularly relevant in forested areas of eastern and south-western Australia. It is less applicable to grassland areas, particularly in arid and semi-arid regions of interior Australia where the occurrence of fire is heavily influenced by fuel availability.

Monthly aggregated FFDI was well above average over most of Australia from September onwards (Fig. 19). In eastern Australia, September values were the highest on record over much of the north-east quarter of New South Wales away from the coast, along with parts of eastern Queensland. Although easing slightly in October, record high monthly FFDI values expanded again in November, covering most of the south-east quarter of Queensland and most areas on and east of the ranges in New South Wales. There was a dramatic expansion in December, covering almost all of New South Wales, Queensland except for the far north, eastern and northern Victoria, and north-east Tasmania. Over the September to December period as a whole, almost all of the area that experienced major fires during the summer had record high FFDI values (the only major exception being parts of the Australian Alps).

Fig. 19.

Monthly Forest Fire Danger Index (FFDI) deciles (1950−2019 climatology) for each of the months from September 2019 to February 2020.

Fire weather conditions generally moderated in January, particularly in Queensland, northern New South Wales and Victoria, although there were still areas of record high monthly FFDI in the southern ranges of New South Wales. By February, monthly FFDI had mostly returned to near- to below-average in eastern Australia, influenced by heavy rain during the middle of the month in many areas.