Wave climatology in the Apostle Islands, Lake Superior

Abstract

The wave climate of the Apostle Islands in Lake Superior for 35 year was hindcast and examined using a third-generation spectral wave model. Wave measurements within the Apostle Islands and offshore NOAA buoys were used to validate the model. Statistics of the significant wave height, peak wave period, and mean wave direction were computed to reveal the spatial variability of wave properties within the archipelago for average and extreme events. Extreme value analysis was performed to estimate the significant wave height at the 1, 10, and 100 year return periods. Significant wave heights in the interior areas of the islands vary spatially but are approximately half those immediately offshore of the islands. Due to reduced winter ice cover and a clockwise shift in wind direction over the hindcast period, long-term trend analysis indicates an increasing trend of significant wave heights statistics by as much as 2% per year, which is approximately an order of magnitude greater than similar analysis performed in the global ocean for areas unaffected by ice. Two scientific questions related to wave climate are addressed. First, the wave climate change due to the relative role of changing wind fields or ice covers over the past 35 years was revealed. Second, potential bluff erosion affected by the change of wave climate and the trend of lower water levels in the Apostle Islands, Lake Superior was examined.

Satellite altimetry reveals spatial patterns of variations in the Baltic Sea wave climate

“…This effect is comparatively strong in regional seas and large lakes with an elongated shape (Niu and Xia, 2016) where veering of the prevailing winds may lead to large changes in the fetch length. The presented results suggest that changes in wind direction may serve as the predominant driver of regional climate changes in such water bodies (Anderson et al, 2015).…”

“…The wave climate of relatively small semi-sheltered and seasonally ice-covered water bodies serves as a convenient indicator of changes in the atmospheric circulation and the related response of the water masses (Anderson et al, 2015). The associated changes in the wave fields are to a large extent controlled by the interplay of the geometry of the water body, changing winds, and the overall reaction of the water masses.…”

Response to Reviewer 1

“…This effect is comparatively strong in regional seas and large lakes with an elongated shape (Niu and Xia, 2016) where veering of the prevailing winds may lead to large changes in the fetch length. The presented results suggest that changes in wind direction may serve as the predominant driver of regional climate changes in such water bodies (Anderson et al, 2015). Table 1.…”

Wave climate in the Arctic 1992–2014: seasonality and trends

“…A ±0.2 m bounds of the associated H s index is used to average the wave periods and directions. This approach gives a more accurate physical description of the events (Anderson et al, 2015). We compute trends using Sen's method and test for statistical significance with the Mann-Kendall test.…”