NP
Nonlinear Processes in Geosciences
Davide Faranda

Davide Faranda

I’m researcher in physics at the French National Center for Scientific Research (CNRS). My main interests are devoted to the construction of a statistical mechanical and dynamical systems framework for the study of atmospheric motions. In particular, my works aim at the characterization of the metastable states of the atmospheric circulation with simple mathematical and statistical tools, and the characterization of rare atmospheric events, such as storms, heatwaves and cold spells. In my research, I collaborate both with mathematicians and physicists working on the characterization of highly turbulent flows, adapting the techniques used in that field to climate science.

After Lorenzo and Ophelia, should we prepare European coasts for tropical storms and hurricanes?

After Lorenzo and Ophelia, should we prepare European coasts for tropical storms and hurricanes?

Autumn is hurricane season in the north tropics and indeed 2019 does not make exception from this point of view. After Dorian hitting Bahamas and North Carolina, the American National Hurricane Center named Lorenzo a tropical depression originating near Capo Verde. On September 25th Lorenzo became a category 1 hurricane, according to the Saffir-Simpson scale. This scale categorizes the hurricanes by their maximum sustained winds over a minute time. Lorenzo kept traveling northward and intensified to category 4 with 230 km/h winds on September 27th. Than the NHC forecasted a continuous decrease in the intensity of the hurricane caused by the cooler temperature and low heat content of the ocean at higher latitudes. However, after an initial weakening, Lorenzo gained power again become the strongest easternmost category 5 hurricane recorded in the Atlantic basin, surpassing Hugo in 1989. Travelling further northward, Lorenzo rapidly lost power and was sucked by the mid-latitude flow, transforming into an extra-tropical cyclone. Extratropical-Cyclones (commonly referred to as “storms” in English or “tempetes” in French) have completely different structures than tropical cyclones or hurricanes. They extend on a larger region and are associated to a warm and a cold front which produce respectively extended but moderate precipitations (warm front) or heavy but very localized rain showers (cold front). They can still be associated to intense winds and produce storm surge. When a hurricane such Lorenzo makes its extratropical transition near the European coasts it comes with a hybrid structure and can still be very damaging. Despite the high waves (up to 12.5 meters) and the strong winds, the damages caused by Lorenzo were minimal in Ireland, where the storm made land-fall. The effective warnings provided by the national meteorological service and based on the excellent quality of weather forecasts contributed to avoid the damages.
The question for climatologists and stakeholders is whether hurricanes could, in a future climate, reach the European coasts with a tropical structure. So far, we have only be able to observe few cases of hurricanes making their extra-tropical transition near the European coasts. As further recent examples we cite Leslie that, in 2018, almost made it to Portugal with a tropical structure and Debbie, that in 1961, made land-fall to Ireland, although the latter case is disputed due to the lack of satellite observations. Projection of changes in frequency, position and intensity of hurricanes in future climate is very difficult. The resolution of current climate models does not allow to simulate correctly the intensity of tropical cyclones. A recent ad-hoc study performed with a relatively high resolution model, suggests that future tropical cyclones wil be more prone to hit western Europe increasing the frequency and impact of hurricane force winds. This is confirmed also by theoretical arguments suggesting that the global temperature increase due to greenhouse gases emission will cause the extension of the tropical regions to higher latitudes and a larger availability of moisture. The combination of these two ingredients is key for hurricanes development. Adapting European coasts for this type of events might be necessary but very costly and challenging: hurricanes come with stronger maximum winds and heavier rainfall than the extratropical storms that these regions are used to face.
The difficulties in the forecasts and projections of future hurricanes are due to the highly non-linear behavior of these phenomena: their genesis depends on the aggregation of convective structures, their trajectory on very small variations of sea-surface temperature. The non-linear geophysics community helps forecast and projections tasks by studying the underlying convective phenomena by tracking the energy transfers in the turbulent cascades and proposing ad-hoc mathematical models.

Abrupt Warming could bring our planet a “Hothouse Earth” with catastrophic consequences for our economy and society

Abrupt Warming could bring our planet a “Hothouse Earth” with catastrophic consequences for our economy and society

Most of us have enjoyed swings in childhood. Some have even tried to swing faster and make a full 360 degrees’ loop. Those who succeeded had a very strange feeling of not being able to predict whether, increasing the energy of the swing, the transition from normal oscillations and 360 loops would happen. Indeed, there is an energy threshold such that the swing goes from oscillations to full loops and the change in the behavior is abrupt. Say now that the swing is our planet and the energy pumped in the Earth system are the anthropogenic emissions, in a recently published paper in the Proceedings of the National Academy of Sciences (https://doi.org/10.1073/pnas.1810141115) Will Steffen and co-authors found that increasing the emissions would push the earth towards an abrupt change in trajectory, leading in a very short time span to a 5 degrees’ warmer climate.

Up to now, scientists have predicted a fast but smooth increase of the planet temperature with increasing anthropogenic emissions. Although catastrophic, this scenario would leave enough time to adapt our society to a warmer climate and the associated consequences such as sea-level rise. This study has however identified a series of interconnected factors which could cause a chain reaction and push the Earth towards a “hothouse” state. Deforestation, permafrost thawing, relative weakening of land and ocean physiological CO2 sinks can drive further warming – even if we stop emitting greenhouse gases. Like going through the full loop could cause injuries, this process is likely to be unstoppable and irreversible and will lead to devastating consequences. The authors say: “a Hothouse Earth trajectory will likely exceed the limits of adaptation and result in a substantial overall decrease in agricultural production, increased prices, and even more disparity between wealthy and poor countries”. A Hothouse Earth trajectory would almost certainly flood deltaic environments, increase the risk of damage from coastal storms, and eliminate coral reefs (and all of the benefits that they provide for societies) by the end of this century or earlier.

The results of this study have animated a debate in the climate change community and there is actually a substantial disagreement about the possibility of crossing the tipping point described in the article. The non-linear geophysics community is working hard to understand these critical phenomena in simple systems which represents idealized climate.

June Heatwave 2019: can we attribute the event to anthropogenic emission?

June Heatwave 2019: can we attribute the event to anthropogenic emission?

If August Rodin had lived nowadays, he would have placed his gates of hell (la Porte de l’Enfer) in Gallargues-le-Montueux, where the absolute French temperature record (45.9 °C) was set on June 28th this year.

The last week of June has been very hot, not only in the south of France, but overall central Europe: in the Alps, some locations such as Chamonix (France) and Aosta (Italy) experienced temperatures close or above 40°C, setting absolute records. The atmospheric circulation responsible for this event consisted of a marked low pressure system centered on the Atlantic Ocean between France and Spain and a high pressure ridge extending from Morocco and Algeria up to central France and the Alpine regions. The strong Meridional wind caused exceptionally warm air advection from Africa and loads of Saharan dusts.

June heatwaves can have a large impact on professional and educational activities compared to July and August heatwaves, where most of the people are on holiday. It is therefore important to understand if greenhouse emissions have affected the intensity of such event and if similar heatwaves will be more likely in the future. A branch of climate science, termed attribution, tries to provide these answers via European collaborative projects (e.g. EUPHEME). The June 2019 heatwave is very interesting from this point of view because a quick attribution study has been published, at a record time, just few days after the events.

The first step for attributing the event to climate change is to provide its definition: which variable/indicator? Which spatial area? Which period of time? In their study the authors considered the three-day average of daily mean temperatures, as they are relevant for health impacts, and two spatial scales: the whole of France and one city, Toulouse. Only June heatwaves were analyzed because the atmospheric circulation is generally different in June than in July and August and the impacts are also different.

The study used a combination of high resolution observations and state-of-the-art climate models to highlight that the probability of this event has largely increased compared to the past and that similar events will be more likely in the future. The authors found that this event is about 4°C warmer than a century ago. They have however also recognized that the quantification of the increase in probability of this event under climate change strongly depends on the models/observations considered. They pointed out to the inadequacy of current models at simulating these events. This poses serious challenges both for the scientific community as well as for the decision makers: on one side, new techniques should be developed to improve the representation of extreme events in climate models. The non-linear geophysics community strongly contributes to this task by proposing bias correction algorithms to improve models output, and techniques issued from dynamical systems theory or statistical mechanics to simulate heatwaves with large deviations. On the other side, decision makers should be aware that uncertainty is a key feature of these studies and take action to prevent the worst possible scenarios both with adaption strategies (heat plans) and by reducing greenhouse gases emissions.