Background on Extreme winds
In the Statement of Work (SOW) the following opening phrase is stated, “.. extreme wind events occupy an increasing place in the mass media as they have direct societal and economic implications (human loss, material destructions...), and are expected to become more destructive in the future as a consequence of global warming”. Besides global warming, societies and economies are becoming increasingly vulnerable to extremes ( see e.g., https://wisc.climate.copernicus.eu/wisc/#/). Among the 15 climate disasters of 2019 that cost more than $1 billion, half of them were related to tropical cyclones (TCs), including events in all ocean basins with landfall causing human loss. Significant direct and indirect socio-economic cost of wind extremes in the Arctic is identified in the EU Arctic Climate Change, Economy and Society project (Crépin et al., 2017).
1.1.2. Coupled large scale ocean-atmosphere impacts
Since they are considered to be a mechanism which limits the build-up of heat and energy in tropical regions, TCs are necessarily related to large scale circulation patterns which may be global in extent. For instance, a relationship has been previously reported between the solar cycle and the occurrence of Atlantic TCs.
Also known, the El Niño phenomena modulate the frequency, location and occurrence of TCs. El Niño moves the subtropical ridge and the preferred cyclone tracks. Areas west of Japan and Korea tend to experience much fewer September–November tropical cyclone impacts during El nino and neutral years. During El Niño years, the break in the subtropical ridge tends to lie near 130 E which would favor the Japanese archipelago, and Guam’s occurrence of a TC passage is one-third more likely than of the long-term average. At variance, the tropical Atlantic Ocean experiences depressed activity due to increased vertical wind shear across the region during El Niño years.
Finally, Zhang et al. (2020) recently reported cyclonic mesoscale ocean eddies have been invigorated by strong wind storms. This in turn can enforce the transfer of energy to the Gulf Stream and its Pacific counterpart Kuroshio. Indeed, ocean eddies move into the current, providing a feedback between TC statistics (intensity, occurrence, trajectories, motions) and ocean heat transport.
Acknowledging their impacts on the coupled ocean-atmosphere system, marine-atmosphere extremes are also a key integral part of the climate-change question. The number of storms, regional and seasonal distributions, actual status and how this might change in the future, are all challenging statistics to gather. For instance, TCs play a substantial role in the maintenance of the general atmospheric circulation in the Northern Hemisphere. In the second half of the year, TCs transport into mid-latitude about half of the total moisture and angular momentum.
In spite of the World Meteorological Organisation (WMO) Regional Specialized Meteorological Centres (RSMC) for tropical cyclones, massively integrating satellite observations and model outputs to monitor and issue short-term TC forecasts to public authorities, severe forecast errors remain. Of particular importance, rapid intensification (RI, e.g. Smith and Montgomery, 2015) is responsible for the highest forecast errors, and for a disproportionate amount of human and financial losses. Indeed, a forecast error for an extreme event to jump from the Specialized Meteorological Center network organized by the World lowest intensity to the highest within a couple of days can result in very inappropriate notices to evacuate and prepare for the storm arrival. Numerical models still fail to fully answer why different initial tropical cyclone structures can result in different steady-state maximum intensities for what appear the same environmental conditions (Tao et al., 2020).
Intensive storms in high latitudes - polar lows - have small horizontal extent (200 - 300 km) and cannot be fully captured in contemporary weather forecast models. Identifying changes in these differing evolutions and associated risks, determining causal factors and/or significant intensity trends in all available observations are thus all critical and timely elements to be more accurately evaluated, to help develop improved models and to assess a potential increase in the global incidence of explosive intensifications of extremes in a global warming context.
To scientifically prove a trend in extremes, one needs many measurements of extremes, a constant and continuous probability of sampling the extremes and a stable measurement system. Wind extremes are in fact relatively infrequent and therefore for trend detection long time series are required. Hence, satellites, orbiting the Earth in a stable environment and lasting for a decade or more, are essential platforms for studying trends, particularly when consistently integrated to be used as a constellation over several decades.
Atlantic storm activity was historically high in the mid-1990s, and after some decrease, is rising again (Wang et al., 2011). Further north, in the Nordic and Barents Seas, the number of polar lows is also increasing following a relatively quiet period of 2001-2007 (Stoll et al., 2018). Although in the late XXIst century the polar low activity is projected to decrease due to the faster Arctic warming (Zahn and Von Storch, 2010), they are still becoming an important threat to the rapidly expanding economic activity in the Arctic.
Recently Bathia et al., (2019) suggested that the extreme event frequency may not have strongly evolved over recent decades, but growing evidence is accumulated to reveal that a significantly larger proportion is reaching higher intensities. Ocean regions with the largest increase in sea surface temperatures (SSTs) are generally hypothesized to be collocated with the largest positive changes in intensification rates. More specifically, criteria for TCs and polar lows (PLs) developments can, to leading order, be expressed using bulk formulas for fluxes of momentum, sensible and latent heat between the ocean and the atmosphere. Key, the total enthalpy fluxes are very different between TCs and PLs, with a much larger role of the latent heat fluxes for TCs. More stringent geostrophic constraints and larger static stability are dominating factors for PLs to also explain their much smaller sizes. Influential meteorological parameters governing the polar low genesis and development are given in (Bracegirdle and Gray, 2008).