Weather news

In this issue, we are delighted to present a final invited contribution from prizewinners at the 2023 RMetS Early Career and Student Conference. Ruth Chapman (University of Exeter; now at Niels Bohr Institute) et al. present a detailed model analysis of mechanisms by which the Atlantic Meridional Overturning Circulation (AMOC) can tip between different states – critical to understand given the importance of the AMOC on global climate and its sensitivity to climate change. Dan Skinner and Simon Lee We start in California, with the Park Fire, the largest fire on record in the state attributed to arson. Ignited on 24 July and still not fully contained by early September, this wildfire has consumed 1739km2 of land and destroyed over 700 dwellings. While the size (fourth biggest in Californian history) and longevity of the fire are notable, the behaviour of the smoke plume is also of interest. Pyrocumulonimbus clouds, towering to more than 30 000ft, were observed, making containment even more difficult, as the blaze became self-sustaining. Strong updrafts draw in oxygen, increasing the temperature of the fire, thus increasing the rising motion. Lightning can occur, as with a regular cumulonimbus, and sparks from this can ignite subsequent fires. Wind shear in the lowest layers of the atmosphere can create fire tornadoes, as was suspected in the Park Fire, though assessing the strength, or even presence of a fire tornado is very difficult due to the already damaged environment. Meanwhile, in neighbouring Canada, a wildfire forced the evacuation of more than 20 000 people between 22 and 25 July from the town of Jasper, Alberta and the surrounding national park. More than 350 structures were destroyed by the flames, which was reported to have been started by a lightning strike. As a major tourist destination, this disaster will surely have major economic impacts for some time to come. Much of southeast Europe has experienced a particularly hot and dry summer, and once again, parts of Greece have been badly affected by wildfires, following its hottest June and July on record. The area northeast of Athens saw numerous fires erupt between 11 and 15 August, threatening the northeastern suburbs of the Greek capital. The Olympic stadium in northern Athens was opened to accommodate those fleeing their homes. Given the high population density, it is perhaps surprising that only one fatality was reported following this spate of wildfires. Mid-August saw wildfire smoke, originating from North America, carried across the North Atlantic on a strong jet stream towards the United Kingdom (this also transported the remnants of Hurricane Ernesto and helped spawn Storm Lillian). This led to a few days of hazy skies (when it was sunny!) and some spectacular sunrises and sunsets. (Sources: CALFire, San Francisco Chronicle, BBC News, The Guardian.) While the exceptionally warm Atlantic basin has already this year shown what it is capable of in terms of major hurricanes (Hurricane Beryl, see August 2024 Weather news), meteorologists have been somewhat puzzled by the overall lack of activity observed thus far, including through the climatologically more active late August and early September part of the season. The finger of blame is currently being pointed towards the robust, and anomalously northward-shifted, West African monsoon (WAM). The WAM, a source of African easterly waves (AEWs) which can form a ‘seed’ from which Atlantic hurricanes grow, has been so strong that it has generated anomalously strong easterly shear in the eastern part of the basin, suppressing named storm formation. Meanwhile, being so far north, the AEWs have been emerging over cooler waters than normal, accompanied by drier air, also hindering tropical cyclone development. Having brought historic rainfall to parts of the Sahara Desert in Mali and Mauritania, the WAM, as measured by the dekadal mean position of the intertropical front (ITF), is now (mid-September) beginning its steady seasonal march southwards. And with ocean temperatures and heat content remaining high, potential is there for a significant uptick in activity during the second half of the season. Indeed, at the time of writing, Tropical Storm Francine is on the board… we will be watching closely! (Sources: NHC, Phil Klotzbach (CSU), CPC, The Watchers.) Inspired by Professor Ed Hawkins' ‘climate stripes’, and resulting from a collaboration between the Universities of Edinburgh and Leeds, North Carolina State University, the Software Sustainability Institute and the Met Office, ‘air quality stripes’ (see https://airqualitystripes.info) have been created to visualise the trends in PM2.5 air pollution from 1850 to 2021 for a selection of cities around the globe (Figure 1). Using a blend of data from the UK Earth System Modelling Project climate model combined with satellite observations, the stripes clearly highlight the pattern of industrialisation around the world. European cities display much improved air quality in recent decades, in contrast to cities in newly industrialised or developing nations where greener technologies and government policy are not yet sufficiently established to bring about cleaner air. On a semi-related note, a plume of sulphur dioxide passed over the UK during the final week of August following another surge in volcanic activity on the Reykjanes peninsular, south of Reykjavik. Despite fears from some quarters that this could cause eye irritation and flu-like symptoms, the cloud of gas remained elevated with mostly little change detected in concentrations of SO2 at the surface. (Sources: https://airqualitystripes.info, UKESM, Evening Standard.) It is well known that the low-lying atolls and island states of the South Pacific are particularly vulnerable to the effects of climate change, despite contributing a mere 0.02% of global emissions, and a new report from the WMO brings this into sharp relief. Sea level rise in this region is accelerating beyond the global average, having risen 10–15cm (nearly twice the global rate) since 1993. In 1980, Pago Pago on American Samoa did not experience a single incidence of coastal flooding; in 2021, there were 102 reports. Sea-surface temperatures are also increasing at around three times the global warming rate (around 0.4 degC per decade), with a marked increase in marine heatwave duration since 2010. During 2023, a 6-month marine heatwave, classified as extreme by the WMO, was observed in a large area around New Zealand, leading to mass bleaching of the coral reefs in the region. The ocean absorbs around 25% of CO2 emissions, with a recording station in Hawaii showing a 12% increase in acidity between 1988 and 2020. Increasing acidity has profound consequences on the marine food chain. The NOAA Annual 2023 Global Climate Report (https://www.ncei.noaa.gov/access/monitoring/monthly-report/global/202313) also paints a vivid picture of a warming planet. NOAA ranks 2023 as the warmest year in its global record (dating back to 1850), mirrored by upper ocean (highest 2000m of ocean) heat content. This has been tracked globally since 1958 and is a key climate indicator because oceans store 90% of the excess heat in the Earth system. In addition, Antarctic sea ice was the lowest on record. (Sources: WMO, NOAA.) As one can clearly see from this month's Weather log, August 2024 saw a very mobile weather pattern across the North Atlantic, with numerous low pressure systems moving towards Iceland, and bands of cloud and rain sweeping across the UK. According to provisional analysis from World Climate Service, the North Atlantic Oscillation (NAO) index for August 2024 was the most positive on record since 1900 for any calendar month (relative to their respective climatologies). It is worth noting that there are numerous ways to calculate the NAO index, but in all cases, it attempts to characterise the difference in surface pressure or geopotential height between the Icelandic Low and Azores High, and by extension, the strength of the interposing westerly flow. This strengthened August westerly flow appears to be consistent with recent results published by Dunstone et al1. (2023), who found a relationship between the state of the stratosphere in late spring and the summer NAO (SNAO) index. An anomalously strong stratospheric polar vortex (SPV) in May, such as this year, was shown to be correlated with a positive SNAO index and its influence can be identified as late as August. One potential mechanism proposed to favour a stronger May SPV is a major sudden stratospheric warming in late winter, as observed this year. This generates conditions in the lower stratosphere that shield the regrowing vortex from further disruptive wave activity from the troposphere. In a positive SNAO summer, the UK should expect higher pressure bringing a more settled, warmer and drier summer. However, early indications are that despite a positive SNAO index this year, the ‘UK node’ was weaker than elsewhere over the North Atlantic domain, highlighting the difficulties in characterising the weather over our small islands by a single number! Conditions over Iceland, typically more unsettled with an enhanced Icelandic Low, were very much to type with unusually high precipitation and the lowest August average mean sea-level pressure since 1820 recorded at Reykjavik. (Sources: World Climate Service, Met Office, Icelandic Meteorological Office.) Apropos the previous story, the succession of rain bands and showers affecting UK brought particularly soggy conditions to Allt Dearg House, on the Isle of Skye. Its rainfall total of 719.4mm for August 2024 smashed the previous August high here, which was ‘only’ 558.8mm set in August 2019. It was also the 14th wettest month on record at this site. On the other side of the Atlantic, another extreme rainfall event unfolded in August, but over a much shorter period. On 18/19 August, a slow-moving frontal system accompanied by a succession of thunderstorms generated exceptional 24h totals across a section of northeast USA, generating severe flash flooding in what is estimated to have been a 1-in-1000 year event. Provisional figures of 376.7mm and 342.9mm reported near Oxford, Connecticut, are both being subject to further QC to see if either location has officially broken the previous state record of 324.4mm. (Sources: SEPA, Met Office, NOAA (climate.gov).) We would like to take this opportunity to mark the recent 50th anniversary of GATE, which was at the time one of the most complex international scientific experiments ever undertaken. Ten primary countries, of which the UK was one, alongside over 60 other collaborating countries embarked on a 3-month campaign to intensively observe meteorological conditions at scales from tropical waves right down to cumulus convection over the eastern tropical Atlantic. GATE was successfully completed in September 1974, with the knowledge acquired and datasets produced stimulating improvements to models and forecasts for many, many years thenceforth. For further details see https://www.nature.com/articles/255017a0. (Sources: John Nicholls FRMetS, Nature.)

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