Perth is being hit by record-breaking heat this summer, with the city already registering an unprecedented eight 40ºC days so far this season.

While much of eastern Australia has been kept relatively mild by La Niña and a positive Southern Annular Mode this summer, the west coast has been baking.

Image: Forecast 850 hPa air temperature on Saturday afternoon, according to the ECMWF model. This is a pattern we have seen a lot this summer, with intense heat over WA and milder air over eastern Australia.

In December, the city registered five days at or above 40ºC and four of these were consecutive days around Christmas.

In January, Perth has already had three days over 40ºC (as of 9am Friday) and could see another two on Friday and Saturday.

Perth has never recorded this many 40ºC days during summer in records dating back to 1897.

Here are some of the standout records that have been matched or broken in Perth so far this summer, as of 9am on Friday:

• Five days at or above 40ºC in December, a new record for the month.
• Four consecutive days at or above 40ºC in December, equal record for any month.
• 42.8ºC on December 25, hottest Christmas Day on record at any Australian capital city's main weather station.
• Eight days at or above 40ºC so far this summer, a new record for the season.

If Perth reaches at least 40.0ºC on Friday and Saturday, which looks likely, this will become the city’s first five-day run at or above 40ºC on record.

This month’s record-breaking heat in Perth has been caused by several factors:

• A lack of monsoon cloud cover in Australia's western tropics has allowed extremely hot air to build up over the Pilbara and Kimberley regions of WA.
• Regular bursts of offshore winds along the state’s west coast have caused the mercury to soar in Perth in recent weeks.
• Climate change has also made extreme heat more likely, with maximum temperatures in southwestern Australia increasing by 1.4ºC since 1910.

Image: Summer maximum temperature anomalies in southwestern Australia between 1910 and 2021. Source: Bureau of Meteorology