The seasonally modulated influence of ENSO and surface gravity waves on tropical vs. extra-tropical storms and associated impact on the coastal sea level

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Start date : October 1, 2021

Deadline : June 9, 2021

Supervisor : Lionel Renault

Co-supervision of thesis : Julien Boucharel

Funding: Competition for a doctoral contract

Summary

Sea level rise represents one of the most dramatic and tangible consequences of global warming and will increasingly affect a dominantly seafront worldwide population (Nicholls et al., 2011; 2014). At the century scale and without any mitigation effort, this could lead to the displacement of up to 200 million people (Nicholls et al., 2011). Nonetheless, these numbers might lay on the lower-end of even the most pessimistic projections as most global and regional studies do not incorporate all the sea-level components of flooding levels, most notably waves (Nicholls et al., 2014; Menéndez and Woolworth, 2010). Yet, by transferring considerable amounts of energy to the coast, waves have been shown to contribute significantly to extreme sea level episodes, sometimes by adding meters during the most severe storms, and related coastal hazards (Serafin et al., 2017; Rueda et al., 2017; Vitousek et al., 2017; Voudouskas et al., 2021a, 2018; Kirezci et al., 2020). This emphasizes the importance of better understanding the connections between coastal dynamics and modes of climate variability.

In particular, the Pacific basin is under the siege of El Niño Southern Oscillation (ENSO), the strongest interannual climate fluctuation, which has widespread effects on weather, climate and societies (McPhaden et al., 2006). Recently, the morphodynamics community has started to identify the role of ENSO as a driver of coastal vulnerability around the Pacific (Barnard et al., 2015, 2017). One of ENSO’s most significant influences is its modulation of Tropical Cyclone (TC) activity, one of the most severe natural hazards, which threaten 1-2 billion people each year and shatter damages worldwide (Peduzzi et al., 2012). Because the large-scale air-sea environment mostly drives these storms, TC activity and related coastal hazards are substantially modified by ENSO though atmospheric and oceanic pathways (Lin et al., 2020). Moreover, ENSO also strongly affect extratropical storms and coastal wave activity (Eichler and Higgins, 2006). The local air-sea coupling can also have a large influence on the oceanic and atmospheric circulation (see e.g., Renault et al., 2019). In particular, surface gravity waves induced roughness can lead to a modulation of the air-sea fluxes (Renault et al., 2012) and thus of the TC intensity.

The Eastern Pacific is the second most active TC basin in the world threatening coastal populations in Mexico, the Southwest U.S and Hawaii. Additionally, these areas are under the influence of some of the biggest swells generated at higher latitudes, by in particular the Aleutian low-pressure systems. A recent study (Boucharel et al., 2021, in revision) has evidenced increased coastal impacts from both tropical and extra-tropical storms in the Eastern Pacific that emerge from the deterministic nonlinear interaction between El Niño frequency and the annual cycle. Boucharel et al. (2021) formulated a model for the modulation of wave energy activity (based on the mathematical model derived in Boucharel and Jin, 2020). Besides the model’s relative simplicity, its analytical solutions yield remarkable correlations with monthly reanalysis data (0.55 on average across the Pacific) and allow accounting explicitly for the ENSO seasonal influence on summer TCs and winter ETCs and thus for the cascading effects through which ENSO’s alteration of background climate conditions leads to predictable pan-Pacific seasonal wave activity. This conceptual model offers a new robust mathematical framework for understanding ENSO-driven coastal wave activity and for guiding process studies based on more complex models.

This PhD can be divided into 2 parts:

1/ Based on an existing wave-ocean-atmosphere (WW3-CROCO-WRF) configuration of the Eastern Pacific region, the PhD candidate will first carry out a simulation that consider the full coupling between the ocean, the atmosphere and the waves over a 20-year period. The synergetic use of the CFOSAT and future SWOT data (wave, surface stress, SSH) along with other existing satellite data (SSH, SST, surface stress and wave), in situ data (wave buoys) and ocean and wave reanalysis products will allow evaluating the representation of the TCs, the waves and the oceanic circulation in the simulation. To assess the effect of the sea surface roughness induced by the waves on the TCs intensity, the PhD candidate will perform an additional simulation in which the roughness induced by the waves will not be sent to the atmospheric model. A statistics analysis will allow us to determine the extent to which waves can modulate the seasonal TCs characteristics.

2/ The second part of this PhD strives to test the aforementioned theory in particular the ENSO-annual cycle nonlinear interactions, by designing sensitivity experiments (oceanic nudging towards different key ENSO timescales in the equatorial band, seasonal vs. interannual open boundary conditions…) in particular to quantify the seasonally modulated effect of ENSO on the respective contribution of wave-induced sea level (i.e. “run-up”) originating from tropical and extra-tropical storms. The run-up will be computed from the classic parameterization by Stockdon et al. (2006) as a function of deep-water significant wave height, peak wavelength and foreshore slope. Bridging the gap between climate variability and coastal impacts necessitates not only the use of Wave-Ocean coupled modeling to assess the different components of coastal water level but also of new-generation satellite products of coastal bathymetry and shoreline position to refine the wave run-up parameterizations and assess the probability of overtopping episodes. Such products are in particular developed at LEGOS in the “Littoral” research team, in partnership with the Centre National d’Études Spatiales (CNES).

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