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 Ninõ–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 infor- mation 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´teó 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 Ninõ and La Ninã 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-Ninõ-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 Ninã in the Pacific Ocean5, and positive –2 IOD with El Ninõ. 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.

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