Climat Archives - BULLETIN SCMO

La vague de chaleur sans précédent de juin 2021 dans le nord-ouest du Pacifique

Written on May 25, 2023.

– Par Cuiyi Fei, Rachel H. White, Chris Rodell –

La vague de chaleur extrême qui a eu lieu à la fin du mois de juin 2021 dans la région nord-ouest du Pacifique de l’Amérique du Nord était tellement sans précédent et a eu un tel impact qu’elle continuera probablement d’être étudiée pendant de nombreuses années. Dans le cadre de notre étude interdisciplinaire, dirigée par des chercheurs de l’Université de Colombie-Britannique (White et coll. 2023), nous résumons les conditions qui ont mené à cet événement sans précédent et nous donnons un aperçu des conséquences d’une chaleur inédite. La compréhension de ces événements est particulièrement pertinente dans le contexte de notre monde en réchauffement.

La figure 1 montre à quel point les records locaux ont été battus pendant cette vague de chaleur, avec une comparaison avec deux autres vagues de chaleur notoires : en Europe en 2003 et en Russie en 2010. La vague de chaleur du nord-ouest du Pacifique a battu les records de température des 70 dernières années de plus de 6 degrés, en faisant un événement sans précédent par rapport à la plupart des vagues de chaleur (voir également Thompson, et coll., 2022). En fait, elle est devenue l’un des événements météorologiques extrêmes les plus graves des dernières décennies dans le monde entier et a battu le record national de température du Canada de 4,6 degrés. Bien que la vague de chaleur n’ait duré qu’environ cinq à six jours, soit moins longtemps que les deux autres événements mentionnés dans la figure 1, elle n’en a pas moins été dévastatrice. La température maximale de 49,6°C a été atteinte à Lytton, une petite ville située dans une vallée aride de la Colombie-Britannique, et une grande partie de la ville de Lytton a été tragiquement détruite par un incendie de forêt pendant la vague de chaleur.

Bien qu’aucune théorie globale et quantitative ne puisse être appliquée universellement à tous les événements de températures extrêmes, les vagues de chaleur en été peuvent souvent être attribuées à des anticyclones de blocage – un système de haute pression qui est immobile ou qui se propage lentement. Ces anticyclones de blocage sont souvent associés à la subsidence, au réchauffement radiatif en ciel clair et aux rétroactions terre-atmosphère (Pfahl et Wernli, 2012), qui contribuent tous à l’élévation des températures de surface. La contribution des différents facteurs varie d’un événement à l’autre (Röthlisberger et Papritz, 2023). Dans le cas de cette vague de chaleur, les conditions atmosphériques précédentes sont illustrées à la figure 2, avec une crête et une pression moyenne élevée au niveau de la mer juste au large de la côte ouest de l’Amérique du Nord, qui s’est transformée en une configuration de blocage au cours des quelques jours suivants. L’humidité relative élevée en amont (illustrée par l’ombrage bleu) a fourni un chauffage latent attribuable à la condensation de la vapeur d’eau, contribuant à la température anormalement élevée et soutenant probablement la crête de haute pression (Oertel et coll., 2023). Nos résultats révèlent qu’environ 78% (équivalant à environ 14 degrés) de l’anomalie de température près de la surface est due au chauffage diabatique en amont du blocage.

En ce qui concerne les impacts météorologiques et océanographiques, l’une des conséquences directes de la vague de chaleur a été une hausse marquée des incendies de forêt. Le nombre d’incendies de forêt en Colombie-Britannique est passé de six le 20 juin avant la vague de chaleur à 175 le 3 juillet, juste après la vague de chaleur, la zone touchée par les incendies de forêt ayant été multipliée par un facteur d’environ 640 au cours de cette période. Outre les températures extrêmement élevées, les incendies de forêt ont fourni de la chaleur, des particules de fumée et de la vapeur d’eau pour la formation de nuages convectifs. L’humidité en amont de la troposphère moyenne a par la même occasion apporté une contribution indispensable aux nuages convectifs. Ces nuages convectifs ont à leur tour produit une grande quantité d’éclairs, allumant et renforçant les incendies de forêt. Selon le Centre interservices des feux de forêt du Canada (CIFFC), la foudre a déclenché au moins 127 incendies de forêt entre le 30 juin et le 2 juillet.

Du même coup, cet événement a eu des répercussions importantes sur la vie marine, en particulier sur les espèces intertidales. La figure 3 montre des images thermiques de moules et du littoral intertidal rocheux sur lequel elles se trouvaient. Pendant la vague de chaleur, la température des rochers et de certaines moules a dépassé 50°C, entraînant une mortalité importante (comme en témoignent les moules béantes). Cette mortalité à grande échelle a touché diverses espèces depuis l’extrémité sud de Puget Sound, dans l’État de Washington, jusqu’à la côte centrale de la Colombie-Britannique, le nombre total d’invertébrés marins tués se chiffrant très certainement en milliards (Raymond et coll., 2022).

Outre les effets directs sur la température, la vague de chaleur extrême a également augmenté le débit des cours d’eau en Colombie-Britannique en faisant fondre la glace et la neige. La figure 4 montre des pics importants de débit coïncidant avec la vague de chaleur dans les bassins fortement ou modérément englacés (lignes du haut et du milieu respectivement), tandis que les zones à faible couverture glaciaire (et donc probablement à faible couverture neigeuse) ont observé des changements minimes pendant la vague de chaleur (ligne du bas). Après la vague de chaleur, le débit dans les zones à couverture glaciaire modérée ou faible était généralement inférieur aux valeurs climatologiques (lignes du milieu et du bas). En revanche, le débit des bassins glaciaires est resté proche de la moyenne climatologique (ligne du haut), ce qui a certainement été obtenu au prix d’une perte de masse glaciaire potentiellement irréversible. La gestion des ressources en eau pourrait s’avérer essentielle pendant et après les vagues de chaleur extrêmes à l’avenir, car la capacité des glaciers à soutenir le débit des cours d’eau pourrait diminuer à mesure que les glaciers continuent de fondre en réponse au changement climatique (Clarke et coll., 2015).

– By Cuiyi Fei, Rachel H. White, Chris Rodell –

The extreme heatwave that occurred at the end of June 2021 across the Pacific Northwest region of North America was so unprecedented and impactful that it will likely continue to be studied for many years. In our cross-disciplinary study, led by researchers at the University of British Columbia (White et al. 2023), we summarize the conditions leading up to this record-shattering event, and provide insights into the impacts of such unprecedented heat. Understanding such events is particularly relevant in the context of our warming world.

The degree by which local records were broken during this heatwave is shown in Figure 1, with a comparison to two other notorious heatwaves: in Europe in 2003 and Russia in 2010. The Pacific Northwest heatwave broke 70-year temperature records by more than 6 degrees, making it far more unprecedented than most heatwaves (see also Thompson, et al., 2022). In fact, it became one of the most severe extreme weather events in the past decades around the world and broke the national temperature record of Canada by 4.6 degrees. Although the heatwave lasted only around five to six days, shorter than the other two events included in Figure 1, it was still devastating. The maximum heatwave temperature of 49.6C was reached in Lytton, a small town in an arid valley of BC, and much of the town of Lytton was tragically destroyed in a wildfire during the heatwave.

Figure 1. Exceedance of previous record high temperatures by the June 2021 Pacific Northwest heatwave (a), the July–August European heatwave of 2003 (b), and the July–August Russian heatwave of 2010 (c), Shading is re-analysis data (ERA5) from 1950; markers show individual stations with observational lengths of at least 71 years. Figure from White et al. 2023, Nature Communications.

Whilst no single comprehensive and quantitative theory can be universally applied to all extreme temperature events, heatwaves in summer can often be attributed to blocking highs — a high-pressure system that is stationary or propagating slowly. These blockings highs are often associated with subsidence, clear sky radiative warming and land-atmosphere feedbacks (Pfahl and Wernli, 2012), all contributing to high surface temperatures. The contribution of different factors varies from one event to another (Röthlisberger and Papritz, 2023). In the case of this heatwave, the preceding atmospheric conditions are shown in Figure 2, with a ridge and high mean sea level pressure just off the west coast of North America, which developed into a blocking pattern in the next few days. High upstream relative humidity (shown in the blue shading) provided latent heating from water vapor condensation, contributing to the anomalously high temperature and likely sustaining the high-pressure ridge (Oertel et al., 2023). Our findings demonstrate that approximately 78% (equivalent to around 14 degrees) of the near-surface temperature anomaly was due to the diabatic heating upstream of the blocking.

Figure 2. Meteorological conditions over the North Pacific for 00UTC 23 June. Data from ERA5 reanalysis showing: mean sea-level pressure (MSLP; contoured, hPa), 700 hPa relative humidity (RH; shaded, light blue >70%, dark blue >90%), and 250 hPa wind vectors (m/s, colored by wind speed). Coastlines and country borders are shown in black. Figure from White et al. 2023, Nature Communications.

The high temperatures of this heatwave, as illustrated in Figure 1, had a catastrophic impact on human health. A total of 868 deaths have, so far, been attributed to the heatwave, and emergency visits for heat-related illnesses in some impacted regions were 69 times higher compared to the same period in 2019.

Turning to the meteorological and oceanographic impacts, one direct consequence of the heatwave was a significant increase in wildfires. The number of wildfires in BC increased from six on June 20^th before the heatwave to 175 on July 3^rd, just after the heatwave, with the area affected by wildfires increasing by a factor of about 640 during this period. In addition to extremely high temperatures, the wildfires provided heat, smoke particles and water vapor for convective cloud formation. Upstream mid-troposphere moisture also had an indispensable contribution to the convective clouds. These convective clouds, in turn, produced a large amount of lightning, igniting and strengthening wildfires. According to the Canadian Interagency Forest Fire Centre (CIFFC), lightning triggered at least 127 wildfires from June 30^th to July 2^nd.

This event also had significant impacts on marine life, particularly intertidal species. Figure 3 shows thermal images of mussels and the rocky intertidal shoreline they were on. During the heatwave, the temperature of the rocks, as well as some of the mussels, exceeded 50°C, leading to significant deaths (as evidenced by the mussels gaping open). Such large-scale mortality occurred across various species from the southern end of Puget Sound, Washington State, to the BC Central Coast, with the total number of killed marine invertebrates almost certainly in the billions (Raymond et al., 2022).

Figure 3. Thermal images showing surface temperatures exceeding 50°C during low tide on 28 June, 2021, on a rocky intertidal shoreline (left) and within a mussel bed (right) in the vicinity of Vancouver, BC; the mussels in this picture have died and are gaping open. Scale bars indicate the range in temperature from the coolest to warmest parts of the image, while the value at the upper left indicates the temperature in the cross-hairs at the center. Figure from White et al. 2023, Nature Communications.

In addition to the direct temperature impacts, the extreme heatwave also increased streamflow in British Columbia by melting ice and snow. Figure 4 shows substantial peaks in streamflow coinciding with the heatwave in heavily or moderately glaciated basins (top and middle rows respectively), while areas with little glacier coverage (and therefore likely little snow cover) experienced minimal change during the heatwave (bottom row). After the heatwave, streamflow in areas with moderate or little glacier coverage was typically lower than climatological values (middle and bottom row). In contrast, the streamflow from glaciated basins remained close to the climatological average (top row); this was almost certainly achieved at the cost of potentially irreversible glacier mass loss. Managing water resources may be critical during and after extreme heatwaves in the future, as the ability of glaciers to sustain streamflow may diminish as glaciers continue to melt in response to climate change (Clarke et al., 2015).

Figure 4. Streamflow observations at nine stream gauge stations in 2021 (black line) relative to the 1979–2020 median (blue line) and one standard deviation range (shaded). Gauges are organized from top to bottom by basin glacier coverage: highly glaciated basins (a–c), lightly glaciated basins (d–f), and minimally or non-glaciated basins (g–i). See Supplementary Fig. S6 for the locations of these gauges. Figure from White et al. 2023, Nature Communications.

This unprecedented heatwave is a vivid example of a record-shattering mid-latitude heatwave, illustrating some potential effects of extreme heat. There is no doubt that increasing background temperatures due to anthropogenic climate change made this heatwave hotter and, therefore, more extreme; however, the recording-breaking temperatures of recent extreme weather events such as this heatwave are a combination of anthropogenic climate trends and internal variability that has always been able to cause large anomalies in temperature. Understanding of our complex interacting climate system remains incomplete and quantitative estimates of the contribution of anthropogenic factors to this heatwave have relatively large uncertainties due to the many interacting factors that may have played a role, including moisture and land-atmosphere feedbacks and possible anthropogenically-forced changes to atmospheric circulation patterns. Long-term adaptation in response to long-term climate change and the short-term response to extreme events have both overlapping and separate components (Dolan, 2021). To reduce the impacts of future heatwaves, it is imperative that we understand, mitigate, and adapt on both timescales.

While anthropogenic climate change was not the sole cause of this event, this heatwave presents an opportunity to enhance people’s awareness and preparedness. Even though many future extreme weather events may not be as unprecedented as this heatwave, it is undeniable that global warming will result in more record-shattering heatwaves in the future (Fischer et al. 2021). Hence, it is vital to take proactive measures toward climate adaptation, including building infrastructure, improving emergency response, and promoting sustainable strategies such as the use of clean energy in the industry, agriculture, and people’s daily lives. We must build resilience against extreme events and climate change for a safer and more sustainable future.

Cuiyi Fei is a PhD Candidate in the Department of Earth, Ocean and Atmospheric Sciences at University of British Columbia. She is working on understanding the mechanism of quasi-stationary waves, but she has a broad interest in general circulation in the atmosphere and ocean.

Rachel White is an Assistant Professor in Atmospheric Science in the Department of Earth, Ocean and Atmospheric Sciences at University of British Columbia. Her research interests include large-scale climate dynamics, connections between atmospheric circulation patterns and extreme weather events, and the subseasonal to seasonal predictability of extremes.

Chris Rodell is a PhD Candidate in Atmospheric Science at the University of British Colombia. His focus is on Fire Weather, Behaviour and Smoke forecasting.

References

Clarke, G.K., Jarosch, A.H., Anslow, F.S., Radić, V. and Menounos, B., 2015. Projected deglaciation of western Canada in the twenty-first century. Nature Geoscience, 8(5), pp.372-377. https://www.nature.com/articles/ngeo2407

Emerton, R., Brimicombe, C., Magnusson, L., Roberts, C., Di Napoli, C., Cloke, H.L. and Pappenberger, F., 2022. Predicting the unprecedented: forecasting the June 2021 Pacific Northwest heatwave. Weather, 77(8), pp.272-279. https://rmets.onlinelibrary.wiley.com/doi/10.1002/wea.4257

Fischer, E.M., Sippel, S. and Knutti, R., 2021. Increasing probability of record-shattering climate extremes. Nature Climate Change, 11(8), pp.689-695. https://www.nature.com/articles/s41558-021-01092-9

Oertel, A., Pickl, M., Quinting, J.F., Hauser, S., Wandel, J., Magnusson, L., Balmaseda, M., Vitart, F. and Grams, C.M., 2023. Everything hits at once: How remote rainfall matters for the prediction of the 2021 North American heat wave. Geophysical Research Letters, 50(3), p.e2022GL100958. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2022GL100958

Pfahl, S. and Wernli, H., 2012. Quantifying the relevance of atmospheric blocking for co‐located temperature extremes in the Northern Hemisphere on (sub‐) daily time scales. Geophysical Research Letters, 39(12). https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2012GL052261

Raymond, W.W., Barber, J.S., Dethier, M.N., Hayford, H.A., Harley, C.D., King, T.L., Paul, B., Speck, C.A., Tobin, E.D., Raymond, A.E. and McDonald, P.S., 2022. Assessment of the impacts of an unprecedented heatwave on intertidal shellfish of the Salish Sea. Ecology, 103(10). https://esajournals.onlinelibrary.wiley.com/doi/10.1002/ecy.3798

Röthlisberger, M. and Papritz, L., 2023. Quantifying the physical processes leading to atmospheric hot extremes at a global scale. Nature Geoscience, pp.1-7. https://www.nature.com/articles/s41561-023-01126-1

Thompson, V., Kennedy-Asser, A.T., Vosper, E., Lo, Y.E., Huntingford, C., Andrews, O., Collins, M., Hegerl, G.C. and Mitchell, D., 2022. The 2021 western North America heat wave among the most extreme events ever recorded globally. Science Advances, 8(18), p.eabm6860. https://www.science.org/doi/full/10.1126/sciadv.abm6860

White, R.H., Anderson, S., Booth, J.F., Braich, G., Draeger, C., Fei, C., Harley, C.D., Henderson, S.B., Jakob, M., Lau, C.A., Mareshet Admasu, L., Narinesingh, V., Rodell, C., Roocroft, E., Weinberger, K.R., and West, G. 2023. The unprecedented Pacific Northwest heatwave of June 2021. Nature Communications, 14(1), p.727. https://doi.org/10.1038/s41467-023-36289-3

SSM par satellite : Un outil de surveillance pour le golfe du Saint-Laurent?

Written on April 2, 2023.

– Par Jacqueline Dumas, Julien Laliberté et Denis Gilbert –

L’utilisation de la technologie de télédétection par satellite pour surveiller le golfe du Saint-Laurent a été démontrée pour la première fois en 1961 lorsque TIROS-2 a capté la débâcle des glaces printannières. À la suite de cette percée, les satellites ont été de plus en plus utilisés pour améliorer notre compréhension de la dynamique des océans.

Planification de l’adaptation : Une approche interdisciplinaire de la réduction des risques liés au changement climatique

Written on February 28, 2023.

– Par Sarah Kehler et S. Jeff Birchall –
Le changement climatique pose un problème complexe et sans précédent. Avec la hausse des températures, le climat mondial devient de plus en plus instable. Les incidences sur le climat, telles que l’élévation du niveau de la mer et la fréquence des phénomènes météorologiques extrêmes, ont de graves répercussions sur les systèmes écologiques et humains. Bien que le changement climatique soit un phénomène mondial, des conséquences uniques et graves se produisent à l’échelle locale.

Surveiller le littoral de l’Île-du-Prince-Édouard

Written on February 21, 2023.

– L’équipe du laboratoire climatique de l’UPEI est dirigée par Adam Fenech et composée de Xander Wang, Don Jardine, Ross Dwyer, Andy MacDonald, Luke Meloche et Catherine Kennedy –

L’érosion côtière est le principal défi que le changement climatique pose à l’Île-du-PrinceÉdouard en raison des ondes de tempête, de l’élévation du niveau de la mer et des niveaux d’eau élevés. Les fragiles rivages de sable et de grès de l’Île-du-Prince-Édouard subissent souvent l’usure de l’eau, des vagues, de la glace et du vent. On a mesuré que l’élévation du niveau de la mer à Charlottetown, à l’Île-du-Prince-Édouard, a augmenté de 36 centimètres au cours du dernier siècle (1911-2011 d’après Daigle, 2012), et on prévoit qu’elle augmentera encore de 100 centimètres au cours des 100 prochaines années (GIEC, 2021).

Le Groenland sera-t-il vraiment « vert » après la perte de sa masse glaciaire?

Written on January 30, 2023.

-Par Xander Wang, Pelin Kinay, Aminur Shah et Quan Dau-

Nombreux sont ceux qui pensent qu’en raison du réchauffement climatique, un mythe de longue date concernant le Groenland pourrait devenir réalité : une terre « verte », comme son nom l’indique, au lieu de la terre blanche couverte de glace qui existe actuellement. Des preuves scientifiques récentes donnent à penser que les couches de glace du Groenland fondent rapidement en raison de la hausse de la température de l’air et du réchauffement des eaux océaniques,

Correction des biais dans les estimations de l’équivalent en eau de la neige de surface au moyen de l’apprentissage automatique

Written on January 22, 2023.

-Par Fraser King-

Pendant les hivers froids du Canada, les accumulations de neige qui ne sont pas constamment déneigées ou pelletées augmentent lentement en taille et en densité. Du point de vue du bilan hydrique, ces accumulations de neige agissent comme des châteaux d’eau éphémères, attendant que les températures printanières les réchauffent suffisamment pour les faire fondre en masse.

L’importance des avant-dunes côtiers en tant que solution naturelle pour la protection du littoral : ce que nous apprend l’ouragan Fiona

Written on December 16, 2022.

Par Jeff Ollerhead, Robin Davidson-Arnott et Bernard O. Bauer
Les avant-dunes sont souvent présentés comme une solution naturelle pour empêcher l’inondation du littoral pendant les grosses tempêtes, ce qui permet d’atténuer les dommages potentiels aux précieuses infrastructures côtières et de réduire l’impact de l’érosion des vagues et des ondes de tempête. La destruction récente causée par l’ouragan Fiona (septembre 2022) sur la côte nord de l’Île-du-Prince-Édouard (Î.-P.-É.) offre une occasion idéale de valider cette affirmation.

Le changement climatique et le réchauffement des océans dans un avenir proche entraîneront trois conséquences prévues pour de nombreuses zones côtières canadiennes : i) une hausse des taux d’élévation du niveau relatif de la mer; ii) une augmentation de la fréquence et de l’ampleur des tempêtes majeures; et iii) une diminution de la couverture de glace de mer en hiver. Ces trois facteurs entraîneront une augmentation des taux d’érosion du littoral. Le sixième rapport d’évaluation du Groupe d’experts intergouvernemental sur l’évolution du climat (GIEC), l’organe des Nations Unies chargé d’évaluer les données scientifiques relatives au changement climatique, rend ces conséquences très claires, et Fiona illustre bien la tendance (figure 1).

Davidson-Arnott et Bauer (2021) ont récemment publié un article examinant les contrôles de la réponse géomorphique des systèmes plage-dune à l’élévation progressive du niveau de l’eau en lien avec d’autres contrôles à long terme de la réponse côtière (p`. ex. la climatologie du vent, la croissance de la végétation, le contexte géologique). La plupart des données donnent à penser que la plage et l’avant-dune (ainsi que le profil du littoral) peuvent migrer vers l’intérieur des terres de manière intacte, en suivant le rythme actuel de l’élévation relative du niveau de la mer (Fox-Kemper et coll. 2021). Cette situation peut toutefois être perturbée si la fréquence et/ou la gravité des tempêtes érosives augmentent. Cela réduira ou empêchera le système plage-dune de se rétablir dans une position vers l’intérieur des terres par le biais de l’apport de sable du littoral aux dunes dans des conditions de vagues de beau temps et de processus éoliens.

Un programme de recherche lancé il y a plus de 20 ans aux dunes de Greenwich (qui font partie du parc national de l’Î.-P.-É.; figure 2A) démontre que les dunes évoluent et migrent généralement vers l’intérieur des terres, comme l’ont laissé entendre Davidson-Arnott et Bauer (2021; figure 2B). Notre compréhension collective des processus en cause est éclairée par de multiples études sur le site, menées à diverses échelles spatiales et temporelles, du transport de sédiments vers les dunes pendant des événements éoliens individuels, à la variabilité saisonnière du transport de sédiments sur l’avant-dune, à l’évolution décennale du système d’avant-dunes au cours des 100 dernières années (p. ex. Walker et coll. 2017; Mathew et coll. 2010).

L’impact de Fiona sur le système plage-dune a été prononcé, avec l’érosion d’une partie importante du talus de la pente de l’avant-dune. L’intégrité des dunes est cependant restée largement intacte, et la crête des dunes n’a pas été rompue. La question qui se pose maintenant consiste à déterminer si l’avant-dune de Greenwich, et probablement d’autres endroits le long de la côte nord de l’Î.-P.-É., est susceptible de rester intacte dans un climat changeant. Il y a de l’espoir, car nous savons, grâce à des documents d’archives, que toute l’avant-dune de Greenwich a été érodée lors d’une importante tempête en 1923 et que le système d’avant-dune s’est complètement rétabli, bien que sur plusieurs décennies (Mathew et coll. 2010). Une question connexe est de savoir comment le système d’avant-dunes peut être géré pour avoir le plus de chances de « survivre » au changement climatique. Fiona donne un aperçu de ces deux questions.

-By Jeff Ollerhead, Robin Davidson-Arnott, and Bernard O. Bauer-

Figure 1: A) Foredune erosion at the Greenwich Dunes eastern beach access post-Fiona (photo taken west to east). The inset shows the boardwalk in May 2022, prior to the stairs being attached (photo taken east to west). B) Overwash and scarped foredune at Greenwich post-Fiona (photo taken east to west).

Introduction

Foredunes are oft-touted as a nature-based solution to preventing shoreline inundation during major storms, serving to mitigate potential damage to valuable coastal infrastructure and reducing the erosional impact of waves and storm surge. The recent destruction imparted by Hurricane Fiona (September 2022) on the north coast of Prince Edward Island (PEI) provides an ideal opportunity to validate this assertion.

Climate change and ocean warming in the near future will lead to three anticipated consequences for many Canadian coastal areas: i) an acceleration in rates of relative sea level rise, ii) an increase in the frequency and magnitude of major storms, and iii) a decrease in winter sea ice coverage. All three factors will drive increases in rates of shoreline erosion. The Sixth Assessment Report from the Intergovernmental Panel on Climate Change (IPCC), the United Nations body for assessing the science related to climate change, makes these consequences abundantly clear, and Fiona illustrates the trend nicely (Figure 1).

Davidson-Arnott and Bauer (2021) published a paper recently examining controls on the geomorphic response of beach-dune systems to progressive water level rise in relation to otherlong-term controls on coastal response (e.g., wind climatology, vegetation growth, geological context). Most of the evidence suggests that the beach and foredune (together with the nearshore profile) can migrate landward intact, keeping pace with current rates of relative sea- level rise (Fox-Kemper et al. 2021). This situation can be disrupted, however, if the frequency and/or severity of erosive storms increases. This will reduce or prevent the beach-dune system from re-establishing itself in a landward position via the delivery of sand from the nearshore to the dunes under fair weather wave conditions and aeolian processes.

A research program initiated more than 20 years ago at Greenwich Dunes (part of PEI National Park; Figure 2A) demonstrates that the dunes are generally evolving and migrating inland as suggested by Davidson-Arnott and Bauer (2021; Figure 2B). Our collective understanding of the processes involved is informed by multiple studies at the site, conducted at various spatial and temporal scales, from sediment transport to the dunes during individual wind events, to seasonal variability in sediment transport over the foredune, to the decadal evolution of the foredune system over the past 100 years (e.g., Walker et al. 2017; Mathew et al. 2010).

Figure 2: A) Field site at Greenwich Dunes, PEI, and locations of profile lines (Ln 1 to Ln 8). Projection is UTM (Zone 20N). B) Profile changes at line 5 over a 20-year period (2002-2022) not including the impact of Fiona. A vertical exaggeration (VE) of 2 times is used.The impact of Fiona on the beach-dune system was pronounced, with a substantial portion of the stoss slope of the foredune system being eroded. The integrity of the dunes remained largely intact, however, and the dune crest was not breached. The question now is whether the foredune at Greenwich, and likely at other locations along the north coast of PEI, is likely to remain intact with a changing climate? There is hope, because we know from archival records that all of the foredune at Greenwich was eroded away during a major storm in 1923, and that complete recovery of the foredune system occurred, albeit over several decades (Mathew et al. 2010). An associated question is how the foredune system can be managed to provide the greatest likelihood of ‘surviving’ climate change? Fiona provides insight into both questions.

Hurricane Fiona – The Storm

Although Atlantic hurricanes pass over or close to PEI every few years (Table 1), they are relatively infrequent events, with periods of several years without notable impacts (e.g., 2015-2018). By the time these warm-cored storms reach PEI, they have typically transitioned from their ‘hurricane’ or ‘tropical storm’ designation into an extra-tropical configuration with significantly reduced wind speeds, unless merging with mid-latitude cyclonic disturbances travelling through the region. Hurricane Fiona was not unusual in terms of its trajectory, extra- tropical status, and reduced wind speeds in the Maritimes, but its impact on the north coast of PEI was considerably more significant than any other tropical-origin storm in recent times.

Figure 3A shows the evolution of 4 (of 5) hurricanes to impinge on PEI since 2013, which is the period of record for the Stanhope weather station (ECCC ID#8300590-6545) which is representative of conditions on the north coast of PEI. Hurricane Ida is not included because it had slower windspeeds than the other four storms and tracked across PEI twice—once from the south and then from the north on the following day—after doing a loop in the Gulf and losing energy. It is evident that Fiona was the most significant storm during this decade, with winds peaking at 89 km h^-1 and sustained winds above 60 km h^-1 for 7 consecutive hours. A peak wind gust was recorded at 131 km h^-1 just before midnight on September 23. An important factor for coastal impacts is that the wind was consistently out of the north for most of the storm, which lasted approximately 24 hours.

Figure 3: A) Mean hourly wind speed and direction at the Stanhope, PEI station for 4 hurricanes to impinge on PEI since 2013. B) Mean hourly wind speed and direction for Fiona relative to two of the most intense winter storms recorded at the Stanhope, PEI station since 2013. Nomenclature follows the Beaufort Scale.

In contrast, the other three hurricanes shown in Figure 3A (Teddy, Dorian, and Arthur) were much less effective in their potential to cause shoreline erosion on the north shore of PEI. Dorian, for example, never reached ‘gale’ status (Beaufort Scale) and at its peak, had speeds of only 55 km h^-1. Unlike Fiona, which tracked to the east of PEI, Dorian tracked almost directly over our field site. This meant that the wind in advance of the eye was generally from the east in the alongshore direction and then transitioned rapidly to a westerly direction (also alongshore but from the opposite direction) after the eye passed. Neither direction is conducive to wave generation or significant storm surge at Greenwich. Arthur and Teddy had greatly reduced wind speeds, never quite attaining 40 km h^-1, and in the case of Arthur, the wind direction was dominantly from the south, consistent with its track across the western tip of PEI.

An assessment of all Atlantic hurricanes since 1953, using data from the Charlottetown Airport (YYG) weather station (ECCC ID#8300300-6526 and ID#8300301-50621; in continuous operation since 1953) indicates that Fiona was among the most intense hurricanes to hit PEI in three decades. The barometric pressure during Fiona was the lowest recorded at 95.85 kPa and the maximum hourly windspeed of 73 km h^-1 ranks it third in intensity behind Hurricanes Juan (Sept 29, 2003; 82 km h^-1; 98.55 kPa) and Arthur (Jul 5, 2014; 76 km h^-1; 98.1 kPa). However, the inland location of Charlottetown airport means the windspeeds experienced on the north shore of PEI are often different. As shown in Figure 4A, the records from the Stanhope station show that the windspeeds during Fiona peaked at 89 km h^-1 rather than the 73 km h^-1 experienced at the airport. Moreover, the Stanhope station recorded a peak of only 39 km h^-1 during Hurricane Arthur, rather than the 76 km h^-1 experienced at the airport, which is due to the southerly wind direction during Arthur.

Even though Fiona stands out as one of the most intense Atlantic hurricanes to impact the north shore of PEI, there are other mid-latitude frontal systems that rival its intensity. A storm on March 12/13, 2022 had barometric pressures almost as low (96.24 kPa) as during Fiona, but windspeeds were below a ‘strong breeze.’ Figure 4B shows the evolution of Fiona relative to two of the most intense storms recorded at the Stanhope station since 2013. Storm A (Feb 15/16, 2015) attained peak windspeeds of 61 km h^-1, which makes it the second-most intense wind event after Fiona. Storm C (March 26/27, 2014) was of similar intensity with peak windspeeds of 60 km h^-1 and near gale conditions for 9 hours continuously. In the period 2014-2022, there were a total of thirteen storms that had peak windspeeds of 61 km h^-1 or greater (based on Charlottetown data), only two of which were hurricanes (Fiona and Arthur). The other eleven storms had a mid-latitude origin and all occurred in the winter or early spring (i.e., December 15 through March 31). This is of considerable importance when assessing beach-dune interaction, because beach-dune systems on PEI are often covered by snow, and the north coast is protected from wave erosion by shore-fast ice, during this period (Figure 4). Thus, many of the most significant storms to hit PEI are incapable of forcing substantial shoreline change, despite their intensity, due to the presence of snow and shore-fast ice.

Figure 4: Photo of the beach-dune system at Line 7 taken February 15, 2008, showing snow on the foredune and shore-fast ice in the nearshore (view to east).

Hurricane Fiona – Beach-Dune Impacts

As noted above, an impact of Fiona was the erosion of a substantial portion of the stoss slope of the foredune along the Greenwich shoreline (Figures 5 and 2B). Our data also show, however, that landward retreat was less and preservation of the foredune greater at the western end of the study site, the downdrift end of the local littoral system. At the eastern end of the study site, the foredune is losing volume and retreating more rapidly.

Figure 5: Profiles across the foredune at lines 2, 5, and 8 and comparative photos from May and October 2022 (before and after Fiona; taken from east to west). The dashed grey line shows mean sea level and the blue line is the estimated maximum water elevation during Fiona (storm surge + wave runup) based on measurements of wrack lines. A vertical exaggeration (VE) of 2 times is used.

Much of the eroded sand during Fiona was lost to the nearshore, but a proportion was transported over the crest and onto the lee slope. Although it cannot be seen on Figure 5 given the scale, the data show a small increase in elevation on the lee slope from May to October 2022, and freshly deposited sand was evident there when surveying post-Fiona. Where the foredune was low, some sand moved inland by overwash (Figure 1B). Much of the sand that was moved to the nearshore zone will gradually make its way back to the beach-dune system driven by fair-weather waves and aeolian processes.

One challenge we faced in assessing Fiona and its impacts, is a lack of marine data for the north shore of PEI. Fisheries and Oceans Canada does not maintain any real-time tide stations on the north shore of PEI. Water level gauges operated by the Canadian Centre for Climate Change and Adaptation (CCCCA) at the University of Prince Edward Island at North Rustico and North Lake were not working properly during Fiona. The only estimate we could obtain was from CCCCA’s gauge at Red Head, which captured a peak storm surge of at least 2 m during Fiona. ECCC has no operational buoys in the Gulf of St. Lawrence to report on wind speed, wave height and period, atmospheric pressure, etc., so we are left to hindcast likely wave characteristics using terrestrial wind records. This situation is far from ideal, as researchers attempt to quantify the impacts of climate change in general, and storms in particular, in Maritime Canada.

The Future

As Figure 2 shows, over the past 20 years the foredune crest at line 5 has translated landward and grown vertically. Sand has been transported to the lee side, maintaining total dune volume. In short, the system is evolving in a manner consistent with the Davidson-Arnott and Bauer (2021) model, with the differences in erosion along the shoreline caused by Fiona explained by differing amounts of sediment available in the littoral cell. This observation highlights the need to assess beach-dune sediment balance in three dimensions along a stretch of coastal foredune (i.e., both on-offshore and alongshore) when assessing the robustness of a foredune to provide natural protection.

Based on our studies, it is anticipated that where there is sufficient sediment supply, sand ramp emplacement will take place relatively quickly and dune healing will occur. The increasing frequency and intensity of storms brought on by a warming climate may, however, disrupt the critical equilibrium between dune scarping and healing processes that have characterized this system for the last 50 years. As ocean temperatures warm, hurricanes and tropical storms will retain their intensity longer as they move north. This will likely be exacerbated by that fact that as sea ice (and particularly shore-fast ice) declines in the coming decades, winter events like Storm A (Feb 15/16, 2015) will become more effective in eroding the dunes. So, hurricanes reaching PEI will likely become more frequent and stronger (as suggested by Fiona) but the system will also be subjected to additional erosion from strong winter storms (i.e., Nor’easters) in the near future.

As Mathew et al.’s (2010) work demonstrates, the Greenwich beach-dune system can recover from even catastrophic erosion. Recovery from the storm of 1923 happened at a decadal scale, rather than at an annual scale, but it did happen. The management imperative is therefore to facilitate dune healing along this coast after major storms via natural processes by: 1) preventing human disturbance of the natural vegetation through activities such as trampling, driving of all-terrain vehicles, and construction; and 2) by providing substantial accommodation space for the foredune to migrate inland and grow upward as relative sea level rises. Management of the Greenwich Dunes by Parks Canada has employed this strategy, and our monitoring program indicates that it played an important role in the ability of the foredune there to withstand the impact of storm surge and erosion by large waves during the passage of Fiona.

Table 1: History of Atlantic Hurricanes impinging on PEI since 1999

Jeff Ollerhead is a Professor in the Geography and Environment Department at Mount Allison University in Sackville, NB. He is a coastal geomorphologist who studies beaches and salt marshes. In recent years, he has been particularly involved in designing and monitoring salt marsh restorations in the upper Bay of Fundy.

Robin Davidson-Arnott retired from the Department of Geography, Environment and Geomatics in 2009 and is now continuing research as Professor Emeritus. In addition to work on beach and nearshore sedimentation, he has carried out research on coastal salt marshes, erosion of cohesive coasts, particularly underwater erosion, beach/dune interaction, and the dynamics of coastal sand dunes. He has published extensively in book chapters and refereed journals, and his book Introduction to Coastal Processes and Geomorphology was published by Cambridge University Press in 2010 and a second edition in 2019.

Bernard Bauer is a process geomorphologist specializing in sediment transport dynamics in aeolian, nearshore, and fluvial systems. His research is primarily directed at advancing fundamental scientific understanding of Earth systems, but increasingly he is interested in ensuring that the latest scientific knowledge is used by coastal managers and environmental decision makers to inform policy development. Bauer is Emeritus Dean of Arts & Sciences at the University of British Columbia Okanagan.

References

Davidson-Arnott, R.G.D. and B.O. Bauer, 2021. Controls on the geomorphic response of beachdune systems to water level rise. Journal of Great Lakes Research, Vol. 47(6), p. 1594-1612. https://doi.org/10.1016/j.jglr.2021.05.006

Fox-Kemper, B., H.T. Hewitt, C. Xiao, G. Aðalgeirsdóttir, S.S. Drijfhout, T.L. Edwards, N.R. Golledge, M. Hemer, R.E. Kopp, G. Krinner, A. Mix, D. Notz, S. Nowicki, I.S. Nurhati, L. Ruiz, J.-B. Sallée, A.B.A. Slangen, and Y. Yu, 2021. Ocean, Cryosphere and Sea Level Change. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1211–1362. https://www.ipcc.ch/report/ar6/wg1/

Mathew, S., R.G.D. Davidson-Arnott and J. Ollerhead, 2010. Evolution of a beach-dune system following a catastrophic storm overwash event: Greenwich Dunes, Prince Edward Island, 19362005. Canadian Journal of Earth Sciences, Vol. 47, p. 273-290. https://doi.org/10.1139/E09-078

Ollerhead, J. and R. Davidson-Arnott, 2022. Evolution and management of Atlantic Canadian coastal dunes over the next century. Physical Geography, Vol. 43(1), p. 98-121. https://doi.org/10.1080/02723646.2021.1936790

Walker, I.J., R.G.D. Davidson-Arnott, B.O. Bauer, P.A. Hesp, I. Delgado-Fernandez, J. Ollerhead and T.A.G. Smyth, 2017. Scale-dependent perspectives on the geomorphology and evolution of beach-dune systems. Earth-Science Reviews, Vol. 171, p. 220-253. https://doi.org/10.1016/j.earscirev.2017.04.011

Acknowledgements

Drs. I. Delgado-Fernandez, P.A. Hesp and I.J. Walker are thanked for their many contributions to this work over many years. The list of additional colleagues, students, and funding agencies who have contributed over 20 years is long and they too are thanked (see our published papers for a complete list).

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