Abstract
At first order, wind-generated ocean surface waves represent the dominant forcing of open-coast morpho-dynamics and associated vulnerability over a wide range of time scales. It is therefore paramount to improve our understanding of the regional coastal wave variability, particularly the occurrence of extremes, and to evaluate how they are connected to large-scale atmospheric regimes. Here, we propose a new “2-ways wave tracking algorithm” to evaluate and quantify the open-ocean origins and associated atmospheric forcing patterns of coastal wave extremes all around the Pacific basin for the 1979–2020 period. Interestingly, the results showed that while extreme coastal events tend to originate mostly from their closest wind-forcing regime, the combined influence from all other remote atmospheric drivers is similar (55% local vs. 45% remote) with, in particular, ~22% coming from waves generated remotely in the opposite hemisphere. We found a strong interconnection between the tropical and extratropical regions with around 30% of coastal extremes in the tropics originating at higher latitudes and vice-versa. This occurs mostly in the boreal summer through the increased seasonal activity of the southern jet-stream and the northern tropical cyclone basins. At interannual timescales, we evidenced alternatingly increased coastal wave extremes between the western and eastern Pacific that emerge from the distinct seasonal influence of ENSO in the Northern and SAM in the Southern Hemisphere on their respective paired wind-wave regimes. Together these results pave the way for a better understanding of the climate connection to wave extremes, which represents the preliminary step toward better regional projections and forecasts of coastal waves.
Highlights
Introduction published maps and institutional affilWind-generated ocean surface waves represent the principal driver of open littoral vulnerability over a wide range of scales including long interannual timescales [1]
To characterize El Niño Southern Oscillation (ENSO) variations, we used the classic monthly Niño3.4 index calculated as the interannual anomalies related to the monthly mean climatology of sea surface temperatures (SST) averaged in the region
These spatial patterns reveal that coastlines are mostly under the influence of wave extremes generated in the neighboring open-ocean region by their associated paired large-scale atmospheric forcing
Summary
Surface wave data [significant height Hs , peak period Tp , and direction Dp ) and. 10-m winds were extracted from the fifth generation of the European Centre for MediumRange Weather Forecasts (ECMWF) Re-Analysis (ERA5) at a 1-day temporal resolution between 1979 and 2020 and a 0.5◦ × 0.5◦ horizontal resolution (30 km × 30 km for the Climate 2021, 9, 168 atmospheric variables, i.e., wind speed and direction) over the Pacific basin (100◦ E–300◦ E; 90◦ S–90◦ N). To illustrate how large-scale atmospheric regimes are seasonally paired with openocean wave patterns, Figure 1 presents climatology maps of 10-meters wind speeds and significant wave heights (shading) and their direction (arrows) averaged in the boreal winter (December–February, DJF, Figures 1a and 1c respectively) and summer (June–August, JJA, Figures 1b and 1d respectively). We can observe the strong westerlies between 30 and 50◦ N, a signature of the strengthened northern hemisphere polar front and easterlies or trade winds (10–30◦ N) associated with the intensified Aleutian low and subtropical high-pressure system, respectively, in the boreal winter (Figure 1a). Both regimes coincide with stronger ocean surface wave patterns located directly underneath (Figure 1c). Panel (b), displays the spatial origin of all extreme events occurring in this region
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