Seasonal Climate Summary for the Southern Hemisphere (Winter 2018): Fifteenth-Warmest and Fourteenth-Driest

CSIRO PUBLISHING Journal of Southern Hemisphere Earth Systems Science, 2020, 70, 353–372 Seasonal Climate Summary https://doi.org/10.1071/ES19038

Seasonal climate summary for the southern hemisphere (winter 2018): fifteenth-warmest and fourteenth-driest

Zhi-Weng Chua

Bureau of Meteorology, GPO Box 1289, Melbourne, Vic. 3001, Australia. Email: [email protected]

Abstract. This is a summary of the southern hemisphere atmospheric circulation patterns and meteorological indices for winter 2018; an account of seasonal rainfall and temperature for the Australian region and the broader southern hemisphere is also provided. The climate influences from the El Nin˜o–Southern Oscillation and the Indian Ocean Dipole were weak, with both demonstrating neutral conditions over the season. It was a dry and warm winter for Australia, being the fourteenth-driest and fifteenth-warmest (in terms of mean temperature) in a record of 119 and 109 years respectively. The warm and dry conditions were particularly pronounced over eastern Australia during July. Maximum temperatures were above average while minimum temperatures were below average.

Keywords: Australian climate, ENSO, IOD, seasonal climate summary, seasonal rainfall, seasonal temperature, southern hemisphere climate, southern hemisphere winter, winter climate.

Received 22 April 2020, accepted 7 May 2020, published online 17 September 2020

1 Introduction 2016. Values were largely neutral for the rest of the period, with This summary reviews the southern hemisphere and equatorial the brief presence of weak La-Ninã-like values from late 2017 to climate patterns for winter 2018, with particular attention given early 2018. Over the winter of 2018, the SOI values were –5.5, to the Australasian and equatorial regions of the Pacific and 1.6 and –6.9 in June, July and August respectively, producing a Indian ocean basins (Fig. 1). From hereafter, the use of the term seasonal average of –3.6. winter refers to the austral winter. The main sources of information for this report are analyses prepared by the Australian 2.2 Composite monthly ENSO Index (5VAR) Bureau of Meteorology. The El Ninõ–Southern Oscillation (ENSO) 5VAR Index Unless otherwise stated, anomalies are calculated with (5VAR2) is a composite monthly ENSO index, calculated as the respect to the period 1961–1990, and percentile-based analyses standardised amplitude of the first principal component of the for the period from the start of the relevant dataset to 2018. monthly Darwin and Tahiti mean sea level pressure (MSLP) and monthly indices NINO3, NINO3.4 and NINO4 sea-surface 3 2 Pacific and Indian ocean basin climate indices temperatures (SSTs). Values of the 5VAR that are in excess of one standard deviation are typically associated with El Ninõ 2.1 Southern Oscillation Index (SOI) for positive values, whereas negative 5VAR values of a similar 1 The Troup SOI for the period from October 2014 to September magnitude are indicative of La Ninã. Fig. 3 displays the monthly 2018 is shown in Fig. 2, also shown is a five-month weighted, 5VAR values along with their three-month average for the moving average of the monthly SOI. Sustained negative values of period from October 2014 to September 2018. the SOI below –7 are often indicative of El Ninõepisodeswhile The 5VAR ENSO Index was also indicative of El Ninõ persistently positive values of the SOI above þ7 are typical of a conditions from the start of the period to the middle of 2016. La Ninãepisode. Values then stayed within the neutral range of being within one The SOI values were negative and indicative of El Ninõ from standard deviation of the climatology for the rest of the period. the beginning of the period, only becoming positive in May During the winter of 2018, the values were 0.73, 0.54 and 0.76 for

1The Troup Southern Oscillation Index (Troup, 1965) used in this article is ten times the standardised monthly anomaly of the difference in mean sea level pressure (MSLP) between Tahiti and Darwin. The calculation is based on a sixty-year climatology (1933–1992), with records commencing in 1876. The Darwin MSLP is provided by the Bureau of Meteorology, and the Tahiti MSLP is provided by Me´te´o France inter-regional direction for French Polynesia. 2 ENSO 5VAR was developed by the Bureau of Meteorology and described by Kuleshov et al. (2009). The principal component analysis and standardisation of this ENSO index are performed over the period 1950–1999. 3SST indices obtained from ftp://ftp.cpc.ncep.noaa.gov/wd52dg/data/indices/sstoi.indices.

Journal compilation Ó BoM 2020 Open Access CC BY-NC-ND www.publish.csiro.au/journals/es 354 Journal of Southern Hemisphere Earth Systems Science Z-W. Chua

Fig. 1. Regions used to monitor ENSO and IOD. The NINO regions are used to monitor ENSO, with NINO3 and NINO3.4 typically used to identify El Nin˜o and La Nin˜a events. The IOD index (or Dipole Mode Index, DMI) is used to identify IOD events, by taking the difference between the west and east poles.

20 June, July and August respectively, producing a seasonal average 15 of 0.68. 10 The NINO3.4 index, which measures SSTs in the central Pacific Ocean within 58N–58S and 120–1708W, is used by the 5 Australian Bureau of Meteorology to monitor ENSO; NINO3.4 0 is closely related to the Australian climate (Wang and Hendon

SOI –5 2007). NINO3.4 values were relatively stable over the winter of –10 2018. June 2018 continued the increasing trend that had been –15 occurring for the first half of the year, with the value increasing from 0.218C in May to 0.418C in June. The values then –20 stabilised, with 0.488C and 0.358C being recorded for July and –25 2015 2016 2017 2018 August respectively. All the indices suggested ENSO conditions were neutral over Year the winter of 2018, though there was the indication of a possible Monthly SOI 5-month weighted average trend towards El-Nin˜o-like conditions.

Fig. 2. Troup Southern Oscillation Index (SOI) values from October 2014 2.3 Indian Ocean Dipole (IOD) to September 2018, with a five-month binomial weighted moving average. The IOD4 is the difference in ocean temperatures between the western node of the tropical Indian Ocean (centred on the 3 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 2 0.48C, neutral when the DMI is sustained between –0.48C and 0.48C and negative when DMI values are less than –0.48C. 1 When under the influence of a strongly negative IOD phase warm maritime air is driven eastwards across the continent, 5VAR 0 leading to a negative IOD typically being associated with an increased chance of a wetter than average spring and/or winter –1 for much of the continent. Negative IOD events often occur in conjunction with La Nin˜a in the Pacific Ocean5, and positive –2 IOD with El Nin˜o. A relationship between ENSO and the IOD is 2015 2016 2017 2018 acknowledged, but it is complicated and continues to be an Year active area of research. An IOD event of positive or negative 5VAR 3-month weighted average phase may have a significant influence on rainfall regimes for Australia. Fig. 4 displays the weekly DMI along with its five- Fig. 3. Anomalies of the composite 5VAR ENSO Index for the period week average from October 2014 to September 2018. from October 2014 to September 2018 with the three-month binomially The IOD index was neutral across most of 2018, with a weak weighted moving average. tendency towards a negative phase. The winter of 2018 had a

4http://www.bom.gov.au/climate/iod/ 5http://www.bom.gov.au/climate/iod/#tabs¼Pacific-Ocean-interaction BoM climate summary: winter 2018 Journal of Southern Hemisphere Earth Systems Science 355

1.5 southern hemisphere often weakening during early autumn, 1.0 before transitioning to the northern hemisphere. A description of 0.5 the real-time multivariate MJO (RMM) index and the associated 0 phases can be found in Wheeler and Hendon (2004). (DMI) –0.5 The phase-space diagram of the RMM for winter 2018 is shown in Fig. 7. The MJO started off strongly in June but

Dipole mode index –1.0 weakened as it progressed eastwards over the Indian Ocean. The –1.5 2015 2016 2017 2018 phase diagram indicates a short pulse over the Maritime Conti- Year nent in the middle of June but otherwise it was weak for the rest of the winter. Weekly DMI 5-week weighted average

Fig. 4. Indian Ocean Dipole; weekly Dipole Mode Index (DMI) and five- 5 Oceanic patterns week running mean from October 2014 to September 2018. 5.1 Sea surface temperatures (SSTs) Fig. 8 shows the SST anomalies globally for winter 2018, relative to 1961–1990. Fig. 9 shows SST deciles, based on the full period seasonal average of –0.25. This suggests the IOD had little of historical observations since 1900. Both figures demonstrate influence on Australia over the winter of 2018. The end of the that SSTs were around average to above average across most of period demonstrates the IOD index quickly increased to positive the globe. The SSTs were particularly warm in the northwest and values during September 2018. the southern Pacific Ocean, the Arctic Ocean and the Atlantic Ocean off the coasts of North and South America, where the 3 Outgoing longwave radiation (OLR) anomaly exceeded 18 above the 1961–1990 average. There was a The OLR in the equatorial Pacific Ocean can be used as an indi- region of significantly below-average SSTs in the North Atlantic cator of enhanced or suppressed tropical convection. Increased Ocean off the southeast coast of Greenland. In the Australian positive OLR anomalies typify a regime of reduced convective region, SSTs were above average especially in the Coral Sea and activity, a reduction in cloudiness and, usually, rainfall. Con- the Tasman Sea. versely, negative OLR anomalies indicate enhanced convection, In terms of globally-averaged values, monthly SSTs were the increased cloudiness and chances of increased rainfall. During La third-warmest on record for June and August (þ0.568C and Ninã (El Ninõ), decreased (increased) cloudiness can be seen near þ0.638C respectively) and second-warmest for July (þ0.638C). the International Date Line. Similarly, when Australia is under the Anomalies for the Australian region were not as strong with influence of a negative IOD event, cloudiness is increased over June being the eleventh-warmest on record (þ0.478C), July the eastern Indian Ocean but decreased over the western Indian fourteenth-warmest on record (þ0.448C) and August thirteenth- Ocean. warmest on record (þ0.398C). The Hovmo¨ller diagram of OLR anomalies along the equator from April 2018 to September 2018 (Fig. 5) indicates a greater 5.2 Equatorial subsurface patterns portion of positive OLR anomalies than negative anomalies over Fig. 10 demonstrates that subsurface waters were around average the winter. Cloudiness was generally below average from mid- in the western equatorial Pacific tending above-average in the June onwards. Averaged over a line along the International Date eastern equatorial Pacific. The shallow subsurface was particu- 8 8 8 8 Line (7.5 S–7.5 N and 170 E–170 E), the monthly OLR anom- larly warm in June but cooled slightly over the winter, with the þ 8 8 8 aly was 1.1 C for June 2018, 0.3 C for July 2018 and 0.4 C for development of a cool anomaly in the eastern equatorial Pacific August 2018. Seasonal spatial patterns of OLR totals and by August. 8 anomalies across the Asia-Pacific region between 40 S and The 208C isotherm depth is generally located close to the 8 40 N for winter 2018 are shown in Fig. 6. equatorial thermocline, which is the region of greatest tempera- Most of Australia experienced a negative OLR anomaly, ture gradient with depth and is the boundary between the warm although toward the southern and eastern coastlines, the anom- near-surface and cold deep-ocean waters. Therefore, measure- 8 aly tended to be positive. Outside of Australia, latitudes 0–20 S ments of the 208C isotherm depth make a good proxy for the except for western Indonesia and Papua New Guinea recorded thermocline depth. Negative anomalies correspond to the 208C 8 mostly positive anomalies and latitudes from 0–20 N recorded isotherm being shallower than average and is indicative of negative anomalies. cooling of subsurface temperatures. If the thermocline anomaly is positive the depth of the thermocline is deeper. A deeper 4 Madden-Julian Oscillation (MJO) thermocline results in less cold water available for upwelling, The MJO is a tropical convective wave anomaly which develops and therefore warming of surface temperatures. in the Indian Ocean and propagates eastwards into the Pacific Fig. 11 indicates the 208C isotherm depth (and by proxy, the Ocean (Madden and Julian, 1971, 1972, 1994). The MJO takes thermocline) was around average in the western equatorial approximately 30–60 days to reach the western Pacific, with a Pacific becoming deeper towards the eastern equatorial Pacific. frequency of 6–12 events per year (Donald et al. 2004). When This positive anomaly was largest during June and July. A the MJO is in an active phase, it is associated with areas of deeper thermocline for the eastern equatorial Pacific can be increased and decreased tropical convection, with effects on the indicative of El Ninõ conditions but given the anomaly was 356 Journal of Southern Hemisphere Earth Systems Science Z-W. Chua

OLR anomalies; daily – averaged; base period 1979–2010 18–Mar–2019 to 16–Sep–2019

Apr 1

May 1

Jun 1

Aug 1

Sep 1

0° 40°E 80°E 120°E 160°E 160°W 120°W 80°W 40°W 0° W m–2 15.S – 15.N

MW – Bureau of Meteorology –90 –70 –50 –30 –10 10 30 50 70

Fig. 5. Time-longitude plot of daily-averaged outgoing longwave radiation (OLR) anomalies at the equator over the period from April 2018 to September 2018. The OLR anomaly is from daily data with respect to a base period of 1979–2010, using interpolated OLR data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA. short-lived and relatively weak, conditions should be considered –0.2 million km2 for June and July and an insignificant anomaly neutral. for August relative to the 1981–2000 average.

6 Sea ice 7 Atmospheric circulation The Antarctic sea ice extent was 12.9, 15.7 and 17.4 million km2 7.1 Southern Annular Mode (SAM) in June, July and August 2018 respectively (Fetterer et al. 2002). Positive values of the SAM index during spring and early summer This corresponds to a monthly sea ice concentration anomaly of are associated with increased onshore flow in parts of eastern BoM climate summary: winter 2018 Journal of Southern Hemisphere Earth Systems Science 357

OLR Totals : Average of 20180601 – 20180831

60N

30N

30S

60S

0 30E 60E 90E 120E 150E 180 150W 120W 90W 60W 30W 0

W m–2 80 100 120 140 160 180 200 220 240 260 280 300 320

OLR Anomalies : Average of 20180601 – 20180831

60N

30N

30S

60S

0 30E 60E 90E 120E 150E 180 150W 120W 90W 60W 30W 0

W m–2 –45 –35 –25 –15 –5 5 15 25 35 45

Fig. 6. OLR anomalies for winter 2018 over the Asia-Pacific region (Wm2). Anomalies calculated with respect to a base period of 1979–2000.

Australia, which typically increases the likelihood of above-average Bureau of Meteorology’s Australian Community Climate and rainfall in much of New South Wales and parts of southern Aus- Earth System Simulator (ACCESS) model6. MSLP anomalies tralia, and below-average rainfall in western Tasmania (Wang and are shown in Fig. 13, relative to the 1979–2000 climatology Hendon 2007). Conversely, negative SAM has roughly the inverse obtained from the National Center for Environmental Prediction effect. SAM also has an impact on temperatures. In general, in (NCEP) II Reanalysis data (Kanamitsu et al. 2002). The MSLP areas where rainfall is increased, the temperature is decreased but anomaly field is not shown over areas of elevated topography where rainfall is decreased, the temperature is increased. (grey shading). The SAM index was weak during the winter of 2018, The seasonal MSLP analysis chart for winter 2018 (Fig. 13) commencing at a value of –0.012 in June, increasing to 0.377 was zonal in the southern hemisphere mid- to high-latitudes. The in July and decreasing to –0.343 in August. This suggests the subtropical ridge was evident across the mid-latitudes with SAM was unlikely to have much influence on the weather of centres of high pressure being observed over Australia Australia over the winter of 2018. (reaching 1023.1 hPa), the eastern South Pacific (reaching 1023.3 hPa) and over southern Africa and off its east coast 7.2 Surface analyses (reaching 1025.2 and 1024.0 hPa respectively). The polar low The MSLP pattern for winter 2018 is shown in Fig. 12, com- showed a minimum pressure of 978.3 hPa on the coast of 8 puted using data from the 0000 UTC daily analyses of the Antarctica around 90 E.

6For more information on the Bureau of Meteorology’s ACCESS model, see http://www.bom.gov.au/nwp/doc/access/NWPData.shtml 358 Journal of Southern Hemisphere Earth Systems Science Z-W. Chua

(RMM1, RMM2) phase space for 22–May–2019 to 19–Aug–2019 4 Western 76Pacific

8 5

0 RMM2 Maritime and Africa Continent West. Hem. West.

–1

1 4

–2

–3

2 Indian 3 Ocean –4 –4–3 –2 –1 0 1234 Labelled dots for each day. RMM1 Wheeler and Hendon (2004) Blue line is for Aug. green line is for Jul. MW – Bureau of Meteorology

Fig. 7. Phase-space representation of the MJO index for winter 2018. Daily values are shown with June in red, July in green and August in blue. The eight phases of the MJO and the corresponding (approximate) locations of the near-equatorial enhanced convective signal are labelled.

ERSSTv5 ANOMMEAN 1 June to 31 August 2018

SSTA (°C)

4.0 3.0 2.0 1.0 0.5 –0.5 –1.0 –2.0 –3.0 –4.0

Fig. 8. Global sea-surface temperature anomaly (SSTA, 8C) from 1961–1990 averages for austral winter 2018 (ERSSTv5). BoM climate summary: winter 2018 Journal of Southern Hemisphere Earth Systems Science 359

ERv5 SST percentiles 1 June to 31 August 2018 Distribution based on gridded data

Highest on record Very much 10 above average 8–9 Above average 4–7 Average 2–3 Below average Very much 1 below average Lowest on record

Fig. 9. Global sea-surface temperature (ERSSTv5) decile map for winter 2018, compared with observations since 1900.

Pacific Ocean eq Anomaly Δ = 0.5°C

May 18

Jun 18

Jul 18 0 m 4 50 m 3 100 m 2 150 m 1 200 m 0 250 m –1 300 m –2 350 m Aug –3 18 –4 400 m 100°E 120°E 140°E 160°E 180°E 160°W 140°W 120°W 100°W 80°W 60°W Analysis done Aug 30 22:33

Fig. 10. Cross-sectional monthly equatorial subsurface temperature analysis for May–August 2018. Red shading indicates positive (warm) anomalies, and blue shading indicates negative (cool) anomalies. 360 Journal of Southern Hemisphere Earth Systems Science Z-W. Chua

Five–day 20°C isotherm depth 2°S to 2°N average 20°C isotherm depth anomalies (m) 20°C isotherm depth anomalies (m)

M M

J J 2018

J J

A A

140°E 160°E 180° 160°W 140°W 120°W 100°W 140°E 160°E 180° 160°W 140°W 120°W 100°W

–30 –20 –100 10 20 30 –30 –20 –100 10 20 30 Global tropical moored buoy array program office, NOAA/PMEL Feb 11 2020

Fig. 11. Hovmo¨ller diagram of the 208C isotherm depth and anomaly along the equator for May–August 2018, obtained from NOAA’s TAO/TRITON data (http://www.pmel.noaa.gov/tao/jsdisplay/).

MSLP 2.5 × 2.5 ACCESS OP, ANAL (hPa) 20180601 0000 20180830 0000

1035 1030 1025 1020 1015 1010 1005 1000 995 990 985 980 975 970 965

Fig. 12. Southern hemisphere mean sea level pressure (MSLP) pattern for winter 2018. BoM climate summary: winter 2018 Journal of Southern Hemisphere Earth Systems Science 361

MSLP 2.5 × 2.5 ACCESS OP. ANAL. -NCEP2 (hPa) 20180601 0000 20180830 0000

20.0 17.5 15.0 12.5 10.0 7.5 5.0 2.5 0.0 –2.5 –5.0 –7.5 –10.0 –12.5 –15.0 –17.5 –20.0

No Data

Fig. 13. Southern hemisphere mean sea level pressure (MSLP) anomalies (hPa) for winter 2018. Anomalies calculated with respect to a base period of 1979–2000.

Z0500 2.5 × 2.5 ACCESS OP. ANAL. (M) 20180601 0000 20180830 0000

6200 6100 6000 5900 5800 5700 5600 5500 5400 5300 5200 5100 5000 4900 4800

Fig. 14. Winter 2018 500 hPa mean geopotential height (gpm), from NCEP2 reanalysis. 362 Journal of Southern Hemisphere Earth Systems Science Z-W. Chua

Z0500 2.5×2.5 ACCESS OP. ANAL.-NCEP2 (M) 20180601 0000 20180830 0000

240 210 180 150 120 90 60 30 0 –30 –60 –90 –120 –150 –180 –210 –240

Fig. 15. 500 hPa geopotential height (gpm) anomalies for winter 2018, from 1979–2000 NCEP2 climatology.

|V|0850 2.5 × 2.5 ACCESS OP. ANAL.-NCEP2 (M/S) 20180601 0000 20180830 0000

25 20 15 10 5 No Data

Fig. 16. Austral winter 2018, 850 hPa vector wind anomalies (m s1).

MSLP was close to average across most of the globe with the The associated anomalies from 1979–2000 climatology are exception of high-pressure anomalies around southern South shown in Fig. 15. America and low-pressure anomalies south of Australia and in Geopotential height is valuable for identifying and locating the south-eastern Pacific Ocean. features like troughs and ridges which are the upper-level equivalents of surface low- and high-pressure systems respec- 7.3 Mid-tropospheric analyses tively. An upper low was evident off the Antarctic coast around The 500 hPa geopotential height, an indicator of the steering of 1808E. A positive geopotential height anomaly was observed surface synoptic systems across the southern hemisphere, is over Antarctica and a weaker negative anomaly was observed shown for winter 2018 in Fig. 14 based on the NCEP2 reanalysis. off the Antarctic coast around 1208E. Elsewhere across the BoM climate summary: winter 2018 Journal of Southern Hemisphere Earth Systems Science 363

|V|0200 2.5 × 2.5 ACCESS OP. ANAL.-NCEP2 (M/S) 20180601 0000 20180830 0000

25 20 15 10 5

Fig. 17. Austral winter 2018, 200 hPa vector wind anomalies (m s1).

(a)

Rainfall (mm)

1200 mm

800 mm

600 mm

400 mm

300 mm

200 mm

100 mm

50 mm

25 mm

10 mm

2 mm

Australian rainfall analysis (mm) 0 mm 1 June to 31 August 2018 Australian Bureau of Meteorology

http://www.bom.gov.au © Commonwealth of Australia 2018, Australian Bureau of Meteorology ID code: AWAP Issued: 20/02/2020

(b)

Rainfall decile ranges

Highest in period Very much 10 above average

8–9 Above average

4–7 Average

2–3 Below average

Very much 1 below average Lowest in period

Rainfall deciles (1900–2018 clim.) 1 June to 31 August 2018 Distribution based on gridded data Australian Bureau of Meteorology

http://www.bom.gov.au © Commonwealth of Australia 2020, Australian Bureau of Meteorology ID code: Analyser Issued: 04/09/2020

Fig. 18. (a) Rainfall totals for winter 2018. (b) Rainfall deciles for winter 2018; decile ranges based on grid-point data with respect to data from 1900–2018. 364 Journal of Southern Hemisphere Earth Systems Science Z-W. Chua southern hemisphere, geopotential height anomalies were driest winter in 119 years of records, with seasonal rainfall close to average. 32.9% below the 1961–1990 average (Table 1). In particular, New South Wales recorded its ninth-driest winter on record. The dry conditions over eastern Australia could be in part due to the 8 Winds weak El-Nin˜o-like state observed over the season. Figs 16 and 17 show winter 2018 low-level (850 hPa) and upper- June rainfall was below average across central Australia, the level (200 hPa) wind anomalies respectively (winds computed Northern Tablelands in New South Wales and along the Queens- from ACCESS and anomalies with respect to the 22-year 1979– land coast, above average around the Gascoyne region of 2000 NCEP climatology). Isotach contours are at an interval of Western Australia and mostly around average elsewhere. 5ms1. July was a drier month, with rainfall below average across The 850 hPa winds were close to average over Australia and most of the southern half of Australia, especially in New South globally. There was an area of stronger anomalies over the Wales which recorded its fifth-driest July on record with some Antarctic coast between 1708W and 1208E corresponding to the central parts of the state experiencing their driest July on record. high- and low-pressure anomalies discussed in Section 7. Tasmania was an exception, recording significantly above- At the 200 hPa level, wind anomalies were around average average rainfall resulting in its eighth-wettest July on record. for most of the globe except for several regions. There was a August saw a reversal of the trend, with rainfall being above couple of strong anomalies associated with anticyclonic patterns average across the southwest of Australia and the northern parts off the west coast of Australia and the southwest coast of South of Northern Territory. Rainfall was mostly average to above America. Additionally, there was also a strong anomaly associ- average elsewhere, being particularly above average in south- ated with a cyclonic pattern over north-eastern China. ern New South Wales and the northern Queensland coast. Parts of the Arnhem region in Northern Territory and the Lower Eyre Peninsula in South Australia had their wettest 9 Australian region August on record. 9.1 Rainfall Rainfall for winter 2018 was below average across most of 9.2 Rainfall deficiencies Australia (Fig. 18). The exceptions were towards the north coast The first half of 2018 resulted in rainfall deficiencies over large of the Northern Territory, the Pilbara region of Western Aus- parts of New South Wales along with border regions in the adja- tralia and the west coast of Tasmania, where rainfall was above cent states (Fig. 19). The winter of 2018 also saw below-average average, and around south-western Australia, where rainfall was rainfall across these areas, worsening these deficiencies around average. Averaged nationally, it was the fourteenth- (commencing from January 2018) and resulting in widespread

Table 1. Summary of the seasonal rainfall ranks and extremes on a national and state basis for winter 2018. The rank refers to 1 (lowest) to 119 (highest) and is calculated over the years 1900–2018 inclusive

Region Highest seasonal Lowest seasonal Highest daily Area-averaged Rank of Difference total (mm) total (mm) total (mm) total (mm) area-averaged from mean (%) total (mm)

Australia 1426.0 at Mt Read Zero at several 153.0 at Lake Echo Power 42.93 14 –32.9 locations Station on 26 July Queensland 784.8 at Bellenden Zero at several 128.4 at Cape Tribulation 18.17 16 –64.6 Ker (Top) locations Store on 2 July New South Wales 542.0 at Murwillumbah 15.9 at Mogil Mogil 142.6 at Ourimbah 53.76 8 –53.6 (Dungay) (Benimora) (Dog Trap Road) on 28 March Victoria 1108.7 at Falls Creek 29.6 at Irymple 100.6 at Mt Baw Baw on 170.36 30 –16.2 (Rocky Valley) (Arlington) 18 June Tasmania 1426.0 at Mt Read 72.8 at Grindstone Pt 153.0 at Lake Echo Power 539.67 95 23.1 Station on 26 July South Australia 484.0 at Uraidla 1.0 at Mt Dare 65.2 at Mt Lofty (Cleland 42.92 36 –22.8 Conservation Park) and Lenswood on 6 August Western Australia 823.4 at Karnet Zero at several 102.0 at Mt Augustus 53.74 55 –11.6 locations on 6 June Northern Territory 60 at Cape Wessel Zero at several 22.2 at Pirlangimpi Airport 1.84 16 –89.9% locations on 13 August BoM climate summary: winter 2018 Journal of Southern Hemisphere Earth Systems Science 365 severe deficiencies across most of New South Wales, with some which included severe and lowest on record deficiencies, were areas in western and central New South Wales experiencing lowest eased. The below-average seasonal rainfall for winter also resulted on record deficiencies as well (Table 2). Conversely, rainfall in severe five-month rainfall deficiencies over southern Northern deficiencies along the south-western coast of Western Australia, Territory and south-western and northern parts of Queensland.

(a)(b)

Rainfall percentile ranking Rainfall percentile ranking Serious Serious deficiency deficiency Severe Severe deficiency deficiency Lowest on Lowest on record record

Rainfall deciles: (1900–2018 clim.) Rainfall deciles (1900–2018 clim.) 1 January to 30 June 2018 1 January to 31 August 2018 Distribution based on gridded data Distribution based on gridded data Australian Bureau of Meteorology Australian Bureau of Meteorology http://www.bom.gov.au http://www.bom.gov.au © Commonwealth of Australia 2020, Australian Bureau of Meteorology ID code: Analyser Issued: 04/09/2020 © Commonwealth of Australia 2020, Australian Bureau of Meteorology ID code: Analyser Issued: 04/09/2020 (c)

Rainfall percentile ranking

Serious deficiency Severe deficiency Lowest on record

Rainfall deciles (1900–2018 clim.) 1 April to 31 August 2018 Distribution based on gridded data Australian Bureau of Meteorology http://www.bom.gov.au © Commonwealth of Australia 2020, Australian Bureau of Meteorology ID code: Analyser Issued: 25/02/2020

Fig. 19. (a) Rainfall deficiencies for the 6-month period January–June 2018. Climatology for 1900–2018 inclusive. (b) Rainfall deficiencies for the 8-month period of January–August 2018. Climatology for 1900–2018 inclusive. (c) Rainfall deficiencies for the 5-month period of April–August 2018. Climatology for 1900–2018 inclusive.

Table 2. Percentage areas in different categories for winter 2018 rainfall. ‘Severe deficiency’ denotes rainfall at or below the fifth percentile. Areas in decile 1 include those in ‘severe deficiency’, which in turn includes areas which are ‘lowest on record’. Areas in decile 10 include areas which are ‘highest on record’. Percentage areas of highest and lowest on record are given to two decimal places because of the small quantities involved; other percentage areas are to one decimal place. Climatology for 1900–2018 inclusive

Region Lowest on Severe Decile Decile Highest on record (%) deficiency (%) 1 (%) 10 (%) record (%)

Australia 0.16 5.6 17.5 0.2 0.00 Queensland 0.33 7.6 22.6 0.0 0.00 New South Wales 0.82 13.9 47.3 0.0 0.00 Victoria 0.00 0.0 2.2 0.3 0.00 Tasmania 0.00 0.0 0.0 12.6 0.00 South Australia 0.00 0.5 4.3 0.0 0.00 Western Australia 0.26 3.9 7.8 0.1 0.00 Northern Territory 0.00 6.2 23.9 0.5 0.00 366 Journal of Southern Hemisphere Earth Systems Science Z-W. Chua

(a)

6°C

5°C

4°C

3°C

2°C

1°C

0°C

–1°C

–2°C

–3°C

–4°C

–5°C

–6°C

Mean temp. anoms. (1961–2018 clim.) 1 June to 31 August 2018 Australian Bureau of Meteorology

http://www.bom.gov.au

(b)

Temperature decile ranges

Highest in period

10 Very much above average

8–9 Above average

4–7 Average

2–3 Below average

1 Very much below average Lowest in period

Mean temp. deciles (1910–2018 clim.) 1 June to 31 August 2018 Distribution based on gridded data Australian Bureau of Meteorology http://www.bom.gov.au © Commonwealth of Australia 2020, Australian Bureau of Meteorology ID code: Analyser Issued: 16/03/2020

Fig. 20. (a) Mean temperature anomalies (8C) for winter 2018; based on 1961–1990 climatology from AWAP data. (b) Mean temperature deciles for winter 2018 from analysis of AWAP data: decile ranges based on grid-point values over the winters 1910–2018. BoM climate summary: winter 2018 Journal of Southern Hemisphere Earth Systems Science 367

Table 3. Summary of the mean seasonal maximum temperatures, extremes and rank for Australia and regions for winter 2018. Rank given is 1 (lowest) to 109 (highest) calculated, over the years 1910–2018 inclusive7

Region Highest seasonal Lowest seasonal Highest daily Lowest daily Area-averaged Rank of mean maximum (8C) mean maximum (8C) maximum maximum temperature area-averaged temperature (8C) temperature (8C) anomaly (8C) temperature anomaly

Australia 33.9 at Kalumburu –0.3 at Mt Hotham 36.9 at Kangaroo Flats on –5.4 at Mt Hotham 1.29 105 25 August on 29 August Queensland 32.2 at Coconut Island 16.7 at Applethorpe 35.2 at Urandangi 8.5 at Applethorpe 1.56 105 Aerodrome on 30 August on 18 June New South 23 at Casino Airport 2.5 at Mt Ginini AWS 30.3 at Casino Airport –4.7 at Thredbo 1.39 104 Wales AWS AWS on 15 August AWS on 19 August Victoria 17.2 at Mildura –0.3 at Mt Hotham 24.2 at Mildura Airport on –5.4 at Mt Hotham 0.52 89 Airport 5 July on 29 August Tasmania 14.4 at Friendly 3.2 at Kunanyi 19.8 at Friendly Beaches –3.1 at Kunanyi on 0.33 89.5 Beaches on 10 August 27 August South Australia 22.1 at Oodnadatta 10.4 at Mt Lofty 31.5 at Moomba Airport 5.5 at Mt Lofty on 1.42 102 Airport on 30 August 6 August Western 33.9 at Kalumburu 15.1 at Shannon 36.8 at Kalumburu on 27 8.4 at Shannon on 1.09 99 Australia August 9 August Northern 33.6 at Oenpelli 22.7 at Alice Springs 36.9 at Kangaroo Flats on 15.1 at Arltunga 1.36 105 Territory Airport Airport 25 August on 18 June

9.3 Temperature record (Table 4). New South Wales, central Australia and north- The mean temperature for winter 2018 was above average eastern Western Australia were the regions where the cool across southern Western Australia and South Australia, Tas- anomaly was the greatest (Fig. 22). Temporally, the cool mania, inland parts of Queensland, northern parts of Northern anomaly was most pronounced in August where parts of the Territory and along parts of the eastern coastline (Fig. 20). Pilbara, Kimberley and North Interior districts of Western Elsewhere, they were mostly around average. The nationally Australia along with south-western Northern Territory, experi- averaged mean temperature anomaly was þ0.598C above the enced minimum temperatures in decile 1 with some areas in 1961–1990 average, the 15th-highest in a record of 109 years. these parts also being the lowest on record (Table 5). The The positive mean temperature was mostly driven by above- coastal regions along the Great Australian Bight experienced a average maximum temperatures with almost the entirety of the warm anomaly while the rest of the country was mostly around continent experiencing above-average maximum temperatures. average. The nationally averaged maximum temperature anomaly was þ1.298C, the fifth-highest on record (Table 3). Most of the Northern Territory, South Australia, Queensland and New South 10 Southern hemisphere Wales had mean maximum temperatures in decile 10. Fig. 23 depicts the global rainfall as a percentage of the cli- Although each of the months was warmer than the average in matological average from 1951 to 2000, for June to August terms of maximum temperatures, July was significantly more 2018. The map is a product of the Global Precipitation Cli- anomalous with an anomaly of þ2.328C, making it the second- matology Centre (GPCC) gauge-based analysis data. For the warmest July on record. Most of the country had mean maxi- hemisphere, the driest region for the period was over central mum temperatures in decile 10 with Queensland, New South and northern Australia. Rainfall was also notably below aver- Wales, South Australia, Western Australia and the Northern age over the southern and eastern parts of South America and Territory observing temperatures among the four warmest on southern Africa. record (Fig. 21). June saw the daytime warmth focused on the Fig. 24 shows the global land and surface temperature anom- northern half of the mainland while in August, the warmth was aly with respect to the climatological period of 1981–2010, for towards the Queensland coast. June to August 2018. The map is a product of land surface In contrast, the mean minimum temperature was below temperatures from the Global Historical Climatology Network- average. The anomaly was –0.138C, the equal 51st coolest on Monthly (GHCN-M) dataset and SSTs from the Extended 368 Journal of Southern Hemisphere Earth Systems Science Z-W. Chua

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http://www.bom.gov.au © Commonwealth of Australia 2020, Australian Bureau of Meteorology ID code: Analyser Issued: 20/02/2020

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Max_temp.deciles. (1910–2018 clim.) 1 June to 31 August 2018 Australian Bureau of Meteorology

http://www.bom.gov.au © Commonwealth of Australia 2020, Australian Bureau of Meteorology ID code: Analyser Issued: 25/02/2020

Fig. 21. (a) Maximum temperature anomalies (8C) for winter 2018; based on 1961–1990 climatology from AWAP data. (b) Maximum temperature deciles for winter 2018 from analysis of AWAP data: decile ranges based on grid-point values over the winters 1910–2018. BoM climate summary: winter 2018 Journal of Southern Hemisphere Earth Systems Science 369

Table 4. Summary of the mean seasonal minimum temperatures, extremes and rank for Australia and regions for winter 2018. Rank refers to 1 (lowest) to 109 (highest) calculated, over the years 1910–2018 inclusive7

Region Highest seasonal Lowest seasonal Highest daily Lowest daily Area-averaged Rank of mean minimum (8C) mean minimum (8C) minimum minimum temperature area-averaged temperature (8C) temperature (8C) anomaly (8C) temperature anomaly

Australia 24.1 at Cape Wessel –4.2 at Mt Hotham 26.4 at Browse Island –14.2 at Perisher –0.13 51.5 on 13 June Valley on 29 August Queensland 24.0 at Coconut Island 0.5 at Stanthorpe 25.6 at Horn Island on –7.4 at Stanthorpe on 0.14 63 1 and 2 July 21 August New South Wales 13.0 at Byron Bay –4.1 at Perisher Valley 18.4 at Byron Bay on –14.2 at Perisher –0.02 56.5 6 July Valley on 29 August Victoria 9.3 at Wilsons Promontory –4.2 at Mt Hotham 14.9 at Lake Entrance –9.8 at Mt Hotham on 0.09 65.5 Lighthouse (Eastern Beach 29 August Road) on 11 August Tasmania 8.8 at Hogan Island –1.6 at Liawenee 13.7 at Hogan Island –8.4 at Liawenee on 0.25 69.5 on 8 June 23 August South Australia 11.8 at Neptune Island 3.1 at Yunta Airstrip 20.9 at Port Pirie –5.5 at Yunta Airstrip –0.11 57 Aerodrome AWS on on 21 June 30 August Western Australia 22.7 at Troughton Island 4.3 at Collie East 26.4 at Browse Island –4.0 at Eyre on 11 July –0.16 49.5 on 13 June Northern Territory 24.1 at Cape Wessel 3.3 at Alice Springs 25.9 at Oenpelli –5.0 at Arltunga on –0.54 32 Airport Airport on 1 July 21 August

Table 5. Percentage areas in different categories for winter 2018. Areas in decile 1 include those which are ‘lowest on record’. Areas in decile 10 include areas which are ‘highest on record’. Percentage areas of highest and lowest on record are given to two decimal places because of the small quantities involved; other percentage areas are to one decimal place. Climatology for 1910–2018 inclusive7

Region Maximum temperature Minimum temperature Lowest Decile 1 Decile 10 Highest Lowest Decile 1 Decile 10 Highest on record on record on record on record

Australia 0.00 0.0 43.3 0.14 0.20 11.4 1.0 0.0 Queensland 0.00 0.0 59.7 0.08 0.0 0.4 0.0 0.0 New South Wales 0.00 0.0 57.9 1.16 0.90 15.2 0.0 0.0 Victoria 0.00 0.0 1.1 0.0 0.0 1.4 0.0 0.0 Tasmania 0.00 0.0 0.0 0.0 0.0 0.0 0.0 0.0 South Australia 0.00 0.0 61.6 0.0 0.0 5.2 0.2 0.0 Western Australia 0.00 0.0 15.0 0.0 0.0 13.9 3.0 0.0 Northern Territory 0.00 0.0 67.7 0.0 0.60 25.8 0.0 0.0

Reconstructed Sea Surface Temperature (ERSST) dataset. temperatures were observed over southern South America and off For the hemisphere, the area-averaged temperature was the equal its western coast, off the coast of western Australia and around the second-highest in the GISS Surface Temperature Analysis data- Maritime Continent. set, the sixth-highest in the NOAA Global Surface Temperature The dry conditions in Argentina were following the country dataset and the tenth-highest in the Hadley Centre/Climate coming out of its worst drought in at least 50 years with an Research Unit dataset. Warmer than average temperatures were estimated economic loss of around US$6 billion (Herring et al. observed over Brazil, Africa and Australia. Lower than average 2020; Stella et al. 2019).

7A subset of the full temperature network is used to calculate the spatial averages and rankings shown in Table 3 (maximum temperature), Table 4 (minimum temperature) and Table 5. This dataset is known as ACORN-SAT (see http://www.bom.gov.au/climate/change/acorn-sat/ for details). These averages are available from 1910 to the present. As the anomaly averages in the tables are only retained to two decimal places, tied rankings are possible. 370 Journal of Southern Hemisphere Earth Systems Science Z-W. Chua

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Min.temp.anoms. (1961–1990 clim.) 1 June to 31 August 2018 Australian Bureau of Meteorology

http://www.bom.gov.au © Commonwealth of Australia 2020, Australian Bureau of Meteorology ID code: Analyser Issued: 20/12/2020

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Temperature decile ranges

Highest in period Very much 10 above average

8–9 Above average

4–7 Average

2–3 Below average

Very much 1 below average Lowest in period

Min_temp.deciles. (1910–2018 clim.) 1 June to 31 August 2018 Distribution based on gridded data Australian Bureau of Meteorology http://www.bom.gov.au © Commonwealth of Australia 2020, Australian Bureau of Meteorology ID code: Analyser Issued: 25/02/2020

Fig. 22. (a) Minimum temperature anomalies (8C) for winter 2018; based on 1961–1990 climatology from AWAP data. (b) Minimum temperature deciles for winter 2018 from analysis of AWAP data: decile ranges based on grid-point values over the winters 1910–2018. BoM climate summary: winter 2018 Journal of Southern Hemisphere Earth Systems Science 371

GPCC Monitoring Product Version 6 Gauge–Baded analysis 1.0 degree precipitation percentage of normals 1951/2000 for Season (Jun, Jul, Aug) 2018 (grid based) 90N

60N

30N

30S

60S

20 40 60 80 100 125 167 250 500

Fig. 23. Global precipitation as a percentage of normal for June–August 2018 (%). Source: Global Precipitation Climatology Centre, Deutscher Wetterdienst, Germany.

Land & Ocean Temperature Departure from Average Jun 2018–Aug 2018 (with respect to a 1981–2010 base period) Date Source: GHCN–M version 3.3.0 & ERSST version 4.0.0

–5 –4 –3 –2 –1 012345 Degrees Celsius Please Note: Gray areas represent missing data Map Projection: Robinson

Fig. 24. Global land and ocean temperature anomalies for June–August 2018. Source: National Centers for Environmental Information. 372 Journal of Southern Hemisphere Earth Systems Science Z-W. Chua

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