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Wednesday 1 March 2017

Odds shorten on an El Niño developing this year

The BoM has upgraded its ENSO outlook to an El Niño watch after observing changes in the tropical Pacific Ocean and the atmosphere, and the eight climate models it monitors.

As the ENSO Wrap-Up released yesterday reveals, the Southern Oscillation Index keeps moving downwards, sea surface temperatures in the eastern Pacific have warmed above average for the first time since June 2016, and seven of the eight climate models the Bureau watches say the warming trend will spread to the central Pacific by July 2017. This means that the probability of El Niño forming in 2017 has risen to about 50%, though the BoM again cautions that model accuracy in autumn is lower than during the rest of the year.

A word of explanation about that 50%. It doesn't mean there's a 50/50 chance of El Niño vs La Niña. A Bureau spokesman explained on Facebook, "Overall, the ENSO cycle spends most years (more than 50%) in its neutral phase. About 25% of years have an El Niño event and about 25% see La Niña. This year, based on our most recent outlooks, we have double the normal chance of El Niño, and very little chance of La Niña."

Put another way, the Bureau's monitoring of a changing situation indicates that the probability of us moving into an El Niño has risen to about 50%, or double the normal 25%. The chances of a La Niña event are now very small. El Niños often, but not always, result in below average winter-spring rainfall over eastern Australia and above average maximum temperatures during the same period over the southern half of the country.

Summer report cards show a continent of extremes

It's the first day of autumn, and time to assess the sizzler, or washout, of departing summer depending on which side of the country you live.

The remarkable aspect of the season's rainfall was how consistently wet it was in WA, SA and the NT while being consistently dry in eastern QLD, NSW and VIC over almost the whole season. TAS had a bit each way with slightly above average rain in the north and below average in the south. In WA, much of the Kimberley and northern Interior, a large area straddling the WA/SA border and many pockets across southern WA all had record summer rainfall resulting in widespread and damaging flooding. Almost half of the remainder of WA, and well over half of SA and the NT recorded rain in the top 10% of all summer totals on record. However in the eastern mainland states south of Townsville, most rainfall was in at best the bottom 30% of recorded totals for the season, with large areas in the driest 10% and some with record low summer rainfall.

Despite the dry weather in the east, the rainfall averaged over the whole country came in at 49% above average, the fourth wettest summer in the reliable record since 1900. There were no surprises when the tally showed that the WA average total was the highest on record for the state, the new figure of 339.8mm comprehensively dismissing the old record of 306.8mm set in 2011. It was the NT's third wettest summer on record with 579.2mm and SA's sixth highest on 143.7mm.

Temperatures just reflected the constant wet, cloudy weather in the west and dry, sunny conditions in the east. In the northwestern two-thirds of the country, warm nights and cool days helped cancelled one another out in many areas to give mean temperatures for the season close to or below average, but a large area of northern WA had summer temperatures in the lowest 10% of all readings. However, in the southeastern two-thirds, warm nights and almost continuously hot days conspired to push mean temperatures across almost all of NSW and the southern half of QLD into the hottest 10%, with an enormous area of NSW and southern QLD experiencing its hottest summer on record. Reliable temperature records go back to 1910.

Not surprisingly, then, NSW had its hottest summer on record, with a whole-of-state mean temperature 2.57° above average; the old record was 2.44° above in 2006. The average maximum temperature across the state was 3.01° above the norm, the hottest on record, just breaking the 1939 high of 2.96°. In Sydney, most stations, including the 157-year old Observatory Hill station next to the CBD, set new records for the trifecta of maximum, minimum and mean temperature records.

Queensland's mean and average minimum temperatures for summer were the second highest on record. Brisbane had its hottest mean summer temperature on record, its equal highest average minimum temperature and its third highest average maximum temperature.

Canberra Airport recorded the city's highest summer average maximum temperature in a record stretching back to 1939 at 30.5°, 3.4° above normal. The airport had 53 days above 30°, over double the average of 26 days.

Full details of the season for Australia are here, and there are links to season summaries for each state and the main capital cities here. There are AWN stories on major events during the season with links to detailed Special Climate Statements by the BoM: the major floods through Central Australia and the WA flooding and eastern states heatwaves.

Find places and weather terms more easily

There's been an improvement to the links at the top left of Daily Weather Summaries that help you find where places are and explain that weather word or term you don't understand.

There are many maps on the internet but the ones I find I use most often are Google Maps, Geonames and the Australian Gazetteer. Each has its strengths and weaknesses. Google Maps has the most up-to-date road information, different views including contours (use the hamburger in the search box), and the unmatched Street View. Geonames has a huge range of data sources and a good crowd sourcing community, so it lists many useful places you won't find elsewhere. The Australian Gazetteer gives a map interface for over 370,000 official place and feature names across Australia - you won't find 20 Ounce Gully WA in other online maps but you will in the Gazetteer. Avoid the trap of using the big "Search" box - use the smaller one just above the map.

Search for "weather glossary" in Google and you'll find there are hundreds. Many are just copies or edited versions of others, and some created for a US audience are inappropriate to Australia. The five I have chosen are original works from impeccable authorities. You should find the information you're after in at least one of them.

Of the two Australian ones, the Weatherzone glossary is more detailed and comprehensive than the Bureau of Meteorology's, and covers the slang of weather enthusiasts as well (e.g. "Flang: [Slang] FLash bANG, a very close lightning strike followed immediately by thunder.") The BoM glossary is concise with some useful illustrations, and a good reference for Bureau terms used in forecasting. It is all the average person is likely to need.

The two glossaries from the USA are both much more detailed than the Australian ones. That from NOAA (National Oceanic and Atmospheric Administration) contains some terms and definitions that are very much US oriented, but the level of detail, especially in a day-to-day weather-watching sense, makes it very useful. The American Meteorological Society's glossary is by far the most comprehensive I have found on the web, and includes many terms that are related to the weather but not specific weather terms.

The final glossary, from the uk.sci.weather newsgroup puts together definitions constantly submitted or updated by contributors in Britain and around the world and so has the advantages of being a living, international document. For example it has a definition of the recently-discovered "sting jet", found to be responsible for many highly damaging wind events. None of the other glossaries do (yet). Many definitions have links to longer articles on the item, and the writing is very easy to read. The other links below "Glossary" on the left have a heap of additional useful information accumulated during this group's 21 years' existence.