NSW has registered its first 40ºC of the year this week, although the heatwave will be short-lived.

A mass of hot air is currently sweeping across NSW as a low pressure trough drifts over southeastern Australia. This summery air has produced the first heatwave of the year to feature days over 40ºC in NSW.

On Tuesday, eight weather stations in the state's north and west reached or exceeded 40ºC, including Griffith, Tibooburra and White Cliffs. Other stations are going to reach the forties today and a few more will join the list on Thursday.

This week's heat will be classified as a low-intensity to severe heatwave in some parts of NSW. It's also helping to elevate the fire risk in some parts of the state, with Very High fire danger ratings affecting multiple districts.

But while this brief spell of hot weather is a typical taste of summer in NSW, the start of January has been much cooler than last year's infamous Black Summer.

During the first 13 days of 2021, only two days have seen temperatures reaching 40ºC in NSW. In 2020, the first 10 days of January saw somewhere in the state reach 43°C or higher.

The contrast between this summer and last summer is largely due to the influence of sea surface temperature patterns in the Pacific and Indian Oceans.

La Niña has been active in the Pacific Ocean throughout spring and is likely to persist until at least the end of this summer. La Niña typically causes above-average rain and below-average daytime temperatures over eastern Australia at this time of year.

On the other side of Australia, the Indian Ocean Dipole (IOD) has been in a neutral phase for the past year. In 2019, a strong positive IOD caused below-average rain and above-average daytime temperatures over large parts of Australia. That was Australia's hottest and driest year on record. By contrast, the current neutral IOD has not been suppressing rainfall or promoting intense daytime heat over Australia in recent weeks and months.

Fortunately, the influence of these climate drivers has meant fewer extreme heatwaves and less bushfire activity in NSW so far this summer.