Rapid summer Russian Arctic sea-ice loss enhances the risk of recent Eastern Siberian wildfires.

In recent decades boreal wildfires have occurred frequently over eastern Siberia, leading to increased emissions of carbon dioxide and pollutants. However, it is unclear what factors have contributed to recent increases in these wildfires. Here, using the data we show that background eastern Siberian Arctic warming (BAW) related to summer Russian Arctic sea-ice decline accounts for ~79% of the increase in summer vapor pressure deficit (VPD) that controls wildfires over eastern Siberia over 2004-2021 with the remaining ~21% related to internal atmospheric variability associated with changes in Siberian blocking events. We further demonstrate that Siberian blocking events are occurring at higher latitudes, are more persistent and have larger zonal scales and slower decay due to smaller meridional potential vorticity gradients caused by stronger BAW under lower sea-ice. These changes lead to more persistent, widespread and intense high-latitude warming and VPD, thus contributing to recent increases in eastern Siberian high-latitude wildfires.

Origins of Barents-Kara sea-ice interannual variability modulated by the Atlantic pathway of El Niño-Southern Oscillation.

Winter Arctic sea-ice concentration (SIC) decline plays an important role in Arctic amplification which, in turn, influences Arctic ecosystems, midlatitude weather and climate. SIC over the Barents-Kara Seas (BKS) shows large interannual variations, whose origin is still unclear. Here we find that interannual variations in winter BKS SIC have significantly strengthened in recent decades likely due to increased amplitudes of the El Niño-Southern Oscillation (ENSO) in a warming climate. La Niña leads to enhanced Atlantic Hadley cell and a positive phase North Atlantic Oscillation-like anomaly pattern, together with concurring Ural blocking, that transports Atlantic ocean heat and atmospheric moisture toward the BKS and promotes sea-ice melting via intensified surface warming. The reverse is seen during El Niño which leads to weakened Atlantic poleward transport and an increase in the BKS SIC. Thus, interannual variability of the BKS SIC partly originates from ENSO via the Atlantic pathway.

Trends and variability in polar sea ice, global atmospheric circulations, and baroclinicity.

We analyze the polar sea ice distribution and the global sea level pressure (SLP) and baroclinicity distributions over the "satellite" period of 1979-2020. In the Arctic, there are statistically significant sea ice extent (SIE) decreases in all calendar months, and the annual mean has lost 2.22 million km2 over the four decades. The Antarctic SIE, in marked contrast, increased up to 2014, then commenced a remarkable retreat (the annual mean ice extent decreased by 2.03 million km2 in the 3 years to 2017), and subsequently increased to near its long-term average value in 2020. The shifts in seasonal-mean SLP patterns are consistent with a warming planet. At the synoptic scale, we diagnose the changes in the baroclinicity, the mechanism by which cyclones, fronts, and other weather systems are generated. Through a novel presentation, we give an overview of the relative roles of changes in the vertical shear and static stability in influencing the global trends in baroclinicity. In both the Arctic and Antarctic regions, baroclinicity is shown to have increased in each season (with the sole exception of the Arctic in summer). This increase, coupled with midlatitude decreases in baroclinicity, results in poleward shifts of the storm tracks.

Antarctic Peninsula warm winters influenced by Tasman Sea temperatures.

The Antarctic Peninsula of West Antarctica was one of the most rapidly warming regions on the Earth during the second half of the 20th century. Changes in the atmospheric circulation associated with remote tropical climate variabilities have been considered as leading drivers of the change in surface conditions in the region. However, the impacts of climate variabilities over the mid-latitudes of the Southern Hemisphere on this Antarctic warming have yet to be quantified. Here, through observation analysis and model experiments, we reveal that increases in winter sea surface temperature (SST) in the Tasman Sea modify Southern Ocean storm tracks. This, in turn, induces warming over the Antarctic Peninsula via planetary waves triggered in the Tasman Sea. We show that atmospheric response to SST warming over the Tasman Sea, even in the absence of anomalous tropical SST forcing, deepens the Amundsen Sea Low, leading to warm advection over the Antarctic Peninsula.

On the use of composite analyses to form physical hypotheses: An example from heat wave - SST associations.

This paper highlights some caveats in using composite analyses to form physical hypotheses on the associations between environmental variables. This is illustrated using a specific example, namely the apparent links between heat waves (HWs) and sea surface temperatures (SSTs). In this case study, a composite analysis is performed to show the large-scale and regional SST conditions observed during summer HWs in Perth, southwest Australia. Composite results initially point to the importance of the subtropical South Indian Ocean, where physically coherent SST dipole anomalies appear to form a necessary condition for HWs to develop across southwest Australia. However, sensitivity tests based on pattern correlation analyses indicate that the vast majority of days when the identified SST pattern appears are overwhelmingly not associated with observed HWs, which suggests that this is definitely not a sufficient condition for HW development. Very similar findings are obtained from the analyses of 15 coupled climate model simulations. The results presented here have pertinent implications and applications for other climate case studies, and highlight the importance of applying comprehensive statistical approaches before making physical inferences on apparent climate associations.

The central role of diminishing sea ice in recent Arctic temperature amplification.

The rise in Arctic near-surface air temperatures has been almost twice as large as the global average in recent decades-a feature known as 'Arctic amplification'. Increased concentrations of atmospheric greenhouse gases have driven Arctic and global average warming; however, the underlying causes of Arctic amplification remain uncertain. The roles of reductions in snow and sea ice cover and changes in atmospheric and oceanic circulation, cloud cover and water vapour are still matters of debate. A better understanding of the processes responsible for the recent amplified warming is essential for assessing the likelihood, and impacts, of future rapid Arctic warming and sea ice loss. Here we show that the Arctic warming is strongest at the surface during most of the year and is primarily consistent with reductions in sea ice cover. Changes in cloud cover, in contrast, have not contributed strongly to recent warming. Increases in atmospheric water vapour content, partly in response to reduced sea ice cover, may have enhanced warming in the lower part of the atmosphere during summer and early autumn. We conclude that diminishing sea ice has had a leading role in recent Arctic temperature amplification. The findings reinforce suggestions that strong positive ice-temperature feedbacks have emerged in the Arctic, increasing the chances of further rapid warming and sea ice loss, and will probably affect polar ecosystems, ice-sheet mass balance and human activities in the Arctic.

Large-scale factors in tropical and extratropical cyclone transition and extreme weather events.

Transition mechanisms characterizing changes from hurricanes to midlatitude cyclones and vice-versa (extratropical and tropical transition) have become a topic of increasing interest, partially because of their association with recent unusual storms that have developed in different ocean basins of both hemispheres. The aim of this work is to discuss some recent cases of transition and highly unusual hurricane developments and to address some of their wider implications for climate science. Frequently those dramatic cyclones are responsible for severe weather, potentially causing significant damage to property and infrastructure. An additional manifestation discussed here is their association with cold surges, a topic that has been very little explored in the literature. In the Southern Hemisphere, the first South Atlantic hurricane, Catarina, developed in March 2004 under very unusual large-scale conditions. That exceptional cyclone is viewed as a case of tropical transition facilitated by a well-developed blocking structure. A new index for monitoring tropical transition in the subtropical South Atlantic is discussed. This "South Atlantic index" is used to show that the unusual flow during and prior to Catarina's genesis can be attributed to tropical/extratropical interaction mechanisms. The "Donald Duck" case in Australia and Vince in the North Atlantic have also been examined and shown to belong to a category of hybrid-transitioning systems that will achieve at least partial tropical transition. While clearly more research is needed on the topic of transition, as we gain further insight, it is becoming increasingly apparent that features of large-scale circulation do play a fundamental role. A complex interaction between an extratropical transition case and an extreme summer cold surge affecting southeastern Australia is discussed as an example of wider climate implications.

Road accidents and rainfall in a large Australian city.

We investigate the impact of rainfall on daily road accidents in the metropolitan area of Melbourne, Australia, over 1987-2002. Our analysis from several viewpoints of the accident count, which has been normalised for variation in traffic volume, indicated that the effect of rainfall is multifaceted. Owing to a large non-linear trend a subdivision into three epochs (1987-1991, 1992-1996 and 1997-2002) was made. Nominal daytime and nighttime as well as 3h raw counts were available for the first two epochs only. Generally, the effect of rainfall across the epochs shows a tendency for larger values in autumn with smaller values in spring. For the daily, daytime and nighttime cases there is an approximate 40% decrease in both the volume-normalised dry and wet means from the first to second epoch. Since the second epoch is wetter than the first, and both dry and wet cases are affected in a similar way, then it appears that a non-weather influence is at work. It is suggested that law enforcement measures may be largely responsible. We obtained a conservative estimate of relative risk of an accident in wet conditions based on a matched-pair analysis of 3h dry and wet periods over the first two epochs (1987-1996). As with other studies we find that the risk is greater than unity in almost all cases suggesting that the presence of rainfall consistently represents a driving hazard. Rainfall occurring after a dry spell has an enhanced effect on the volume-normalised accident count as the spell duration increases. The effect of dry spells is more clearly described when broken down by rain class. Generally, there is an increase in the impact of a dry spell when it first rains as the spell duration and rainfall amount increase.

The association of rainfall and other weather variables with road traffic volume in Melbourne, Australia.

An investigation into the effect of weather variables on traffic flow at a site in Melbourne, Australia, for the period 1989-1996 was performed. Rainfall was the strongest correlated weather parameter and it had the greatest impact in winter and spring, when traffic volume is reduced on wet days. There are statistically significant decreases of 1.35 and 2.11% in traffic volume on wet days in winter and spring. The reduction increases to 2-3% over the 2-10mm range, the largest being 3.43% for the 2-5mm class in spring. For the first time, our study considers separately daytime and nighttime periods. We found a reduction of 1.86% in winter and 2.16% in spring during daytime rainfall. The reduction at nighttime is significant over all seasons, ranging from 0.87% in winter to 2.91% in spring. We have explored an application where the traffic volume was used to normalise the road accident count and found the rain effect to increase by 2.4, 1.9 and 5.2% relative to the daily, daytime and nighttime dry mean accident count. Generally, the normalised count is greater than the raw count, with a larger increase for the higher rainfall classes.