If you’re a resident of south-eastern Australia, particularly in Sydney and other parts of the New South Wales coast, the weather might feel like a never-ending loop of rainy days. Since the beginning of May, the region has been inundated with uninterrupted rain, resulting in accumulated falls ranging from 50 to 100 millimeters over the course of the week. The Illawarra and Hunter regions were hit particularly hard with heavy downpours over the weekend.
According to current forecasts from the Bureau of Meteorology (BOM), this pattern is far from over. In fact, there is a possibility that Sydney’s main weather station could break the record for the most consecutive days of rain by the time this event concludes. BOM meteorologist Edward Medlock stated, “This pattern has been stagnant for six days, and is expected to remain there for at least another seven days, continuing these drizzly coastal conditions. The thing we’re looking at right now is whether this will lead to a record of number of consecutive rain days in a row [in Sydney]. The record so far is 16 days, which was when Lismore flooded back [in] February and March 2022 … so we’re not there yet but it could happen if this high decides to stick around.”
Meanwhile, other parts of Australia, including Tasmania, inland regions of New South Wales, Victoria, and South Australia, have been experiencing a consistent pattern of clear skies, mild days, and cool nights. The forecast for these areas remains largely unchanged for the upcoming week.
So, what’s causing this sudden halt in Australia’s normally dynamic weather patterns? Climate scientists attribute this phenomenon to a “blocking high” – a crucial component of Australia’s weather system that holds significant implications for future rainfall patterns in the context of climate change.
A “blocking high” refers to a robust high-pressure system that remains nearly stationary for an extended period, ranging from several days to several weeks. In Australia’s case, these blocking highs typically form just south of the continent, obstructing the path of rain-bearing weather systems that generally flow from east to west. CSIRO climate scientist James Risby likened blocking highs to “a rock in a stream,” explaining that “the flow of storms, which normally moves along the stream, has to go around that block.”
The impacts of blocking highs can vary depending on their strength and location. The current setup primarily affects rainfall, with the high directing a constant stream of onshore winds over the east coast, favoring showery weather in the region. However, blocking highs can also drive bouts of extreme heat and dry conditions. Earlier this year, a blocking high located in the Tasman Sea contributed to a four-day autumn heatwave over south-eastern Australia, with record-breaking overnight minimum temperatures close to 30 degrees Celsius in some locations.
While blocking systems are a common occurrence in Australia, Mr. Medlock noted that the current event stands out due to the strength of the high-pressure system and the time it is taking to move on. “There’s always slow highs and fast highs, but something like this [is] fairly unique,” he said. “If they stick around for you know, a week long, it’s probably uncommon.”
Blocking highs play a critical role in shaping Australia’s weather patterns, particularly in their contribution to rainfall through their association with “cut-off lows.” Cut-off lows are low-pressure systems that have broken away from the main belt of low pressure and are often associated with sustained, heavy rainfall, strong winds, and high seas. Dr. Risby emphasized the importance of cut-off lows, stating that “those are really important in bringing rainfall to southern Australia, both in the south-western and south-eastern Australia.”
On the other hand, blocking highs can also set the stage for dangerous heatwaves and fire conditions by fanning hot winds from the north into south-eastern Australia. “They also generate extreme temperatures blocking highs,” Dr. Risby explained. “For example, in south-east Australia the typical weather set up for a heatwave is that there would be a high in the Tasman Sea region which would be directing a strong northerly flow into south-eastern Australia.”
As climate change progresses, understanding the future behavior of blocking systems becomes increasingly important. Scientists are working to unravel questions about the future frequency of blocking highs in regions like Australia and their preferred positions. Small differences in where blocks tend to form under climate change could have significant impacts on regional rainfall patterns.
While there is still uncertainty surrounding the precise changes in blocking high frequency and location, scientists are confident that the heat impacts associated with these systems will be exacerbated in a warming climate. Dr. Risby warned that “blocking highs in future could lead to more intense heat waves,” as the continent, atmosphere, and ocean become warmer due to global warming.
As Australia continues to grapple with the monotonous rain cycle caused by the current blocking high, it serves as a reminder of the complex interplay between weather systems and the potential impacts of climate change on regional weather patterns. Understanding and adapting to these changes will be crucial for Australia’s future resilience in the face of extreme weather events.