2020 officially saw a record number of $1 billion weather and climate disasters.

The preinstrumental change in the frequency of RF emergence may not precisely reflect the variation in the occurrence of drought over the last 1,200 years because the observations and documentation of the RFs may have varied throughout history due to the succession of dynasties and corresponding changes to society. However, the RFs do show a relatively high emergence frequency during AD 1051–1250, a unique paleoclimatic stage known as the Medieval Climate Anomaly (MCA) or Medieval Warm Period (MWP) in the Northern Hemisphere, and a relatively low emergence frequency during the Little Ice Age (LIA) (AD 1501–1900). During AD 1051–1250, the average frequency of RF emergence is 10 times per 50 years. In comparison, the LIA registered an average frequency of only 1.8 times. The highest frequency of low water level events, 14 times, occurred in the 50-year period of AD 1101–50, and the lowest frequency with none occurring during AD 1601–50. It is also interesting to note that there is a new increasing trend of low-water-level events since the mid-nineteenth century.

Three extremely low water levels occurred in 1107, 1129, and 1796; among them, 1796 witnessed the lowest water level among all years of RF emergence of approximately 0.65 m below the reference RF corresponding to water levels of approximately 137.4 m MSL at WCR and approximately 136.1 m MSL at QHS.

Conclusion

It was shown that the droughts occurred more frequently during the Medieval Climate Anomaly (MCA) of Northern Hemisphere, relatively less frequently during the Little Ice Age (LIA), and once again more frequently under the background of modern global warming. It also suggested that a warmer Euro-Asian continent during the MCA was in favor of the stronger East Asian summer monsoon (EASM), and the resulting less precipitation, lower water level, and more frequent and more severe droughts of the Yangtze River, on the centennial scale; on the other hand, a cooler Euro-Asian continent during the LIA would help develop a weaker EASM, and the resulting more precipitation, higher water level, and less frequent and less severe droughts in the Yangtze basin.

23 August 2022

Key Climate Drivers for northern Australia

The Bureau's ENSO (El Niño–Southern Oscillation) Outlook has been raised to La Niña ALERT, meaning there is about a 70% chance (about triple the normal likelihood) of La Niña forming later in 2022. A La Niña event typically increases the chance of above-average spring and summer rainfall across much of northern Australia. La Niña years are also associated with an average to above-average number of tropical cyclones in a season—the average number across the Australian region for all years since 2000 is about 9 per season. There is also an increased likelihood of tropical cyclone activity occurring in November, compared to non-La Niña years. The monsoon onset date at Darwin occurs about a week or so earlier than usual during a La Niña event, typically around mid-December.

A negative Indian Ocean Dipole (IOD) event continues. All climate model outlooks surveyed indicate the negative IOD state is likely to persist until late spring. A negative IOD event is associated with above-average spring rainfall for the Northern Territory, Queensland and much of southern Australia. It also typically increases the chances of warmer days and nights for much of the tropical northern regions.

During negative IOD years, the chance of observing a tropical cyclone across the Australian region in November increases, compared to positive or neutral IOD years.

Warmer-than-average sea surface temperatures (La Niña) around much of Australia also favour above-average rainfall in much of Australia in the coming months. Climate outlooks indicate warmer waters are likely to persist through spring.

The influence of the above climate drivers is clearly depicted in the Bureau's latest climate outlooks which show a very high likelihood of above-median rainfall across the Northern Territory and Queensland during this year's early wet-season period. Likewise, the Northern Rainfall Onset outlook indicates an early rainfall onset (that is, the first rainfall accumulation of 50 mm after 1 September) is likely for virtually all of Queensland and the Northern Territory.

Read more about climate drivers in the Climate Driver Update

Madden–Julian Oscillation strengthens over the Indian Ocean

A pulse of the Madden-Julian Oscillation (MJO) recently strengthened over the tropical western Indian Ocean. There is good agreement amongst climate models that this pulse will further strengthen and track in an easterly direction in the coming week. However, the majority of models indicate it will weaken before the end of August, prior to reaching Maritime Continent longitudes. As a result, the MJO is not expected to exert a significant influence on rainfall patterns across northern Australia or further north, across the Maritime Continent.

For Australia, a typical outcome associated with an MJO pulse in this region at this time of the year is a strengthened easterly trade wind flow affecting north-eastern Australia, leading to an increased chance of above-average rainfall for northern Queensland in the coming week.

Read more about the Madden–Julian Oscillation (MJO)