On the influence of ENSO complexity on Pan-Pacific coastal wave extremes

Wind-generated waves are dominant drivers of coastal dynamics and vulnerability, which have considerable impacts on littoral ecosystems and socioeconomic activities. It is therefore paramount to improve coastal hazards predictions through the better understanding of connections between wave activity and climate variability. In the Pacific, the dominant climate mode is El Niño Southern Oscillation (ENSO), which has known a renaissance of scientific interest leading to great theoretical advances in the past decade. Yet studies on ENSO’s coastal impacts still rely on the oversimplified picture of the canonical dipole across the Pacific. Here, we consider the full ENSO variety to delineate its essential teleconnection pathways to tropical and extratropical storminess. These robust seasonally modulated relationships allow us to develop a mathematical model of coastal wave modulation essentially driven by ENSO’s complex temporal and spatial behavior. Accounting for this nonlinear climate control on Pan-Pacific wave activity leads to a much better characterization of waves’ seasonal to interannual variability (+25% in explained variance) and intensity of extremes (+60% for strong ENSO events), therefore paving the way for significantly more accurate forecasts than formerly possible with the previous baseline understanding of ENSO’s influence on coastal hazards.

As coastal breaking waves represent the ultimate dissipation of the energy generated by local and remote storms through large increases in surface wind over the ocean, their activity is modulated by the large-scale ocean–atmosphere coupled variability. This emphasizes the importance of better understanding the connections between coastal dynamics and modes of climate variability in order to improve their prediction at subseasonal timescales and beyond (1, 2). 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 (3). Recently, the littoral community has started to identify the role of ENSO as a major driver of coastal vulnerability across the Pacific (4, 5). The alternating coastal conditions, with shifts in wave activity and water-level anomalies between the northeastern and northwestern Pacific, were noted to mimic the well-known oscillations of ENSO phases. However, the Pacific wave climate and coastal variability associated with ENSO remains only understood at a basic reconnaissance level (6). As a matter of fact, even the most recent studies on the connection between wave climate and coastal extremes have only relied on a simplified view of ENSO (7, 8), omitting the existence of different regimes with distinct teleconnections and dynamics, recently coined “ENSO diversity and/or complexity” (9, 10).“ENSO diversity” originates from the concept that Sea Surface Temperature (SST) anomaly patterns exhibit wide variations. In particular, the repeated occurrence of SST patterns in the central Pacific (CP) in the 2000s suggested that ENSO events may be grouped into two flavors: the conventional El Niño, with SST anomalies concentrated in the eastern Pacific (EP El Niño), and the CP El Niño, with SST anomalies located around the dateline (11, 12). The “complexity” or sometimes “diversity and complexity” further refers to ENSO’s irregular temporal behavior characterized by large variations in amplitude and duration. In particular, recent progresses demonstrated that ENSO’s seasonal phase locking (i.e., its tendency to peak in boreal winter) (13) can produce a low-frequency instability known as the “Annual cycle-ENSO combination mode” and generate a deterministic variability at near-annual timescales, which significantly broadens the ENSO continuum (14). Such diversity and complexity translate to pronounced differences in remote ENSO climate impacts on the climate system through atmospheric and oceanic teleconnections (15, 16). In particular, one of ENSO’s most significant influences is its modulation of Tropical Cyclone (TC) activity, one of the most severe natural hazards (17). Because the large-scale air–sea environment mostly drives these storms, TC activity is substantially modified by ENSO through atmospheric and oceanic pathways (18). Similarly, ENSO also strongly affects extratropical storms and related coastal wave activity (19, 20).A variety of oceanic and atmospheric wave reanalysis products and TC databases were examined to capture comprehensively the regional and large-scale climate variability in the Pacific associated with ENSO diversity and complexity and how it affects coastal waves’ variability. More specifically and unlike any previous studies, the focus is not solely directed toward the direct influence on extratropical wave patterns of El Niño at its winter peak but also on its delayed and early effects on summer TC storminess considered as an integral wave regime potentially affecting coastlines far from the storms’ generation (21). In particular, since ENSO’s influences on tropical and extratropical storm activity are subject to a strong seasonal synchronization, the combined ENSO–Annual cycle influence on teleconnections patterns and coastal wave variability is considered. Insights from these unraveled seasonally modulated ENSO teleconnections allow us to develop a simple mathematical model that points toward a strong predictability of the Pacific coastal wave variability over a range of timescales much broader than just the interannual band and therefore opens up the door for predictions of coastal hazards significantly more accurate than current state-of-the-art seasonal forecasts.