More record heat in Perth

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Perth is eyeing off another summer heat record with temperatures set to reach the high-thirties in the city every day this week. Western Australia’s Lower West Coast has been no stranger to extreme heat this season.  

 Perth has registered 13 days at or above 40ºC so far this summer, which wallops the previous summer record of seven days from 2015/16. Now, the city is on track to break the summer record for the number of days at or above 35ºC in a single summer. 

 As of 9am on Tuesday, February 15, Perth had registered 25 days at or above 35ºC so far this season. The current record is 31 days from 1977/78. 

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Based on current forecasts, Perth could reach or exceed 35ºC on each of the next six to nine days. This should allow the city to match the summer record of 31 days at or above 35ºC by Sunday and possibly break the record early next week. 

This summer’s unrivalled number of 40ºC and 35ºC days is exceptional because Perth has more than 120 years of historical observational records, with data available back to 1897. 

 What has made this season so hot in Perth? 

 While Australia’s warming climate is likely to have played a role in this season’s extreme heat in Perth, there were several local weather factors that caused the record-breaking heat.  

La Niña played a role in this summer’s hot weather in Perth. While La Niña has caused relatively cool and wet weather over many parts of Australia this season, the county’s west coast has sweltered with an abundance of sunshine and record-breaking heat. 

 La Niña promotes extreme heat in WA because it is directly correlated with more periods of positive Southern Annular Mode (SAM). When the SAM is positive in summer, high pressure systems frequently sit further south and become more prominent over Southern Australia, which pushes cold fronts further south than normal for this time of year.  

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 Image: Mean Sea Level Pressure (MSLP) anomalies during January 2021 showing high pressure anomalies over southern Australia. 

 When high pressure systems sit over southern Australia it directs prevailing easterly and northeasterly winds over southern parts of the continent.  For the east coast, these easterly winds are usually moisture-laden and cool, causing days of cloud and rain. While for the west coast, prevailing easterly winds are generally dry and hot, originating from the hot interior of Australia.  

 The graph below shows that the SAM index was has been predominantly positive so far this summer. 

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Image: Observed Southern Annular Mode (SAM) index values during the last few months. Source: NOAA/CPC 

 The SAM should stay positive for the rest of February, as shown by the red lines on the graph above. This could help promote more warm weather in the final two weeks of the season. 

 Another factor that has led to unusually high temperatures in Western Australia this summer, was a slow start to the wet season in northwestern Australia. A lack of monsoonal cloud in December and January helped cause extremely hot airmasses to build up in the Pilbara and Kimberley. 

 The peak of this heat saw Onslow airport reach 50.7C in January, which was the equal highest temperature on record in the Southern Hemisphere. Weatherzone provides our clients with detailed seasonal forecasts, including temperature and heat extremes, for more information, please contact us at business@weatherzone.com.au.