Increasing coastal exposure to extreme wave events in the Alaskan Arctic as the open water season expands

The Arctic Ocean's wave climate is intensifying due to the dramatic decline in sea ice extent in recent decades. Sea ice mitigates wave energy by dissipating swells, limiting fetch, and preventing wind-wave generation. As sea ice retreats, increased fetch and longer open water periods lead to higher wave energy and increased coastal erosion and flooding risks. These trends are projected to continue, with ice-free summers expected by mid-century. In the Alaskan Arctic, increased wave climate is linked to coastal hazards associated with extreme weather events like extratropical cyclones, Arctic cyclones, and strong winds. Historically, the most impactful storms occur in the fall when open water persists, coinciding with intense storm events. However, as sea ice decline delays ice formation, Arctic shorelines face increased exposure to storm-induced wave hazards, especially as winter storm intensity equals or surpasses fall intensity, and extratropical cyclones trend northward. This study addresses the lack of robust numerical simulations of coastal wave hazards coupled with projected ice fields. It assesses future sea ice decline and simulates how waves from a representative historical storm event would change under future sea ice conditions, establishing the sensitivity of coastal wave response to regional sea ice extent and predicting future seasonal exposure to wave hazards from similar storm events. Unlike existing wave projections that analyze mean or extreme wave parameters over long timescales and broad spatial scales, this study analyzes a specific extreme event using a high-resolution, region-specific wave and storm surge model, providing more accurate simulations of coastal wave heights.

Numerous studies have projected and analyzed the growth in open water area and season along Alaskan Arctic coasts and inferred growing coastal wave heights. However, before this study, robust numerical simulations of coastal wave hazards in conjunction with projected ice fields had not been undertaken. Existing research highlights the increasing wave climate in the Arctic, driven by sea ice decline, and its contribution to coastal hazards in Alaska, particularly during the fall season. However, there's uncertainty in projecting how future sea ice decline will affect seasonal exposure to coastal hazards.

This study utilizes future sea ice projections from the SSP2-4.5 and SSP5-8.5 climate pathways (moderate and high future emissions) for the decades 2050-2059 and 2070-2079. A multi-model ensemble (MME) was used to create robust projections of future sea ice and assess trends in regional sea ice extent (SIE). The historical record shows dramatic decreases in pan-Arctic SIE in future scenarios. Analysis focuses on the Alaskan Arctic's Beaufort and Chukchi Seas, showing projected future fall and winter SIE departing substantially from the historical average, with the extent of departure depending on the climate pathway. The historical median September SIE is used as a benchmark representing peak exposure to wave hazards due to maximum seasonal fetch. Future scenarios show dramatic decreases in seasonal SIE minimum, with most scenarios reaching zero sea ice coverage. The study simulates the wave and water level impacts of a historical extratropical cyclone that occurred on December 31, 2016, considered representative of typical autumn and winter cyclones impacting the region. The ADCIRC+SWAN coupled model, with newly implemented sea ice parameterizations for both storm surge and wave dissipation (Joyce et al. and IC4M2, respectively), was used to simulate waves and water levels on a scale encompassing Alaska with sufficient resolution to resolve coastal areas. Simulations were run from December 20th to January 9th, using either observed sea ice conditions (baseline) or climate model-projected daily sea ice concentration fields for future decades. The Arctic Coastal Dynamics (ACD) database was used to sample nearshore wave heights along US Chukchi/Bering Sea shorelines. Points within coastal segments with wave heights exceeding 0.5m were analyzed. To study how storm-induced coastal wave heights vary with sea ice coverage, the full ensemble of storm simulations was analyzed, relating the coastal wave response and sea ice coverage to projected future seasonal monthly sea ice extent. Logistic regression was used to model the relationship between coastal wave height and sea ice extent.

Both future scenarios show substantial expansions in open-water area and increased regional sea states compared to the baseline simulation. In the baseline, maximum significant wave heights (Hsmax) were near-zero in ice-covered regions. Future simulations show substantial growth in Hsmax, particularly along western Alaskan coastlines and northward beyond the Bering Strait. The SSP5-8.5 2070 simulation projects massive sea ice reductions, leaving much of the Chukchi and Beaufort Seas ice-free, leading to widespread wave growth with Hsmax increases exceeding four meters throughout much of the Arctic region. Analysis of coastal segments shows similar increases in median Hsmax values along Chukchi Sea coastlines in future scenarios compared to the baseline. In the baseline, median Hsmax is below 0.5m for most segments, but increases rapidly with sea ice diminishment. The SSP5-8.5 2070 simulation generates considerable coastal wave activity along Beaufort shorelines, with barrier island segments exhibiting Hsmax in the 1–2 m range, unlike other simulations. Analysis shows clear upper and lower thresholds of SIE for both Chukchi and Beaufort coasts, beyond which regions reach maximum protection or saturation, respectively. The Chukchi coast rapidly reaches a saturation limit of ~1.5 m, showing high sensitivity to initial SIE reductions. The Beaufort coast is more resilient, with a more gradual response and saturation limit reached only when ice-free. Using MME SIE projections as inputs, projected median coastal Hsmax values for future months were derived. For the Chukchi coast, near-total wave suppression is typically achieved by December in the historical data, but this is delayed by a month or more in future scenarios, with SSP5-8.5 2070 never fully reaching the protection threshold until March, representing a 3-month extension in seasonal exposure. For the Beaufort coast, future scenarios predict a similar seasonal exposure expansion, with historical SIE limiting coastal wave heights even during the September minimum. Future sea ice loss corresponds to both an expanded exposure window and increased wave magnitudes during fall months. The SSP5-8.5 2070 scenario shows the saturation limit reached in September and potentially extending to November.

Both climate pathways show similar dramatic SIE reductions by 2050, extending the fall and winter coastal exposure for both Chukchi and Beaufort regions. By 2070, the pathways diverge, with SSP5-8.5 projecting further sea ice losses than SSP2-4.5. The SSP5-8.5 2070 scenario shows a stark increase in coastal wave exposure. The Chukchi coast reaches saturation with relatively little SIE loss, suggesting the maximum coastal response is primarily dependent on regional Chukchi sea ice coverage rather than increasing open-water areas at higher latitudes. While increased fetch doesn't always directly translate to increased wave heights during extreme events, an extended open-water duration leaves coasts exposed to winter storms of higher intensity and northward trend. This study focused on a representative extratropical cyclone, but future research should explore the impact of various storm events and other synoptic events (Arctic cyclones, anticyclones, extreme wind events) on coastal wave responses. The spatial distribution and seasonality of sea ice would differently affect the coastal wave response for these events. Future changes in storm climatology may also affect the seasonality and magnitude of coastal storm hazards due to greater cyclogenesis and wind speeds. The presence of coastal landfast sea ice, not considered in this study due to projection limitations, represents an area of uncertainty in the results.

This study demonstrates the significant increase in coastal wave exposure in the Alaskan Arctic due to sea ice decline. Both SSP2-4.5 and SSP5-8.5 scenarios predict extended seasons of open water, leading to increased coastal hazards. The high-emission scenario (SSP5-8.5) for 2070 presents a particularly severe risk. Future research should investigate the impact of various storm types, incorporate landfast ice projections, and assess the potential for increased storminess under climate change.

This study simulates a single historical storm event under varying ice conditions. The results, including saturation limits, might differ for storms of varying intensity and tracks. Furthermore, the study does not account for the presence of coastal landfast sea ice, which could affect the estimations of open water wave exposure, particularly during spring and early summer. Projections of landfast sea ice formation and breakup are currently unavailable.