The Seasonal Cycle of Significant Wave Height in the Ocean: Local Versus Remote Forcing

Abstract

Significant wave height (SWH) stems from a combination of locally generated “wind‐sea” and remotely generated “swell” waves. In the Northern and Southern Hemispheres, wave heights typically undergo a sinusoidal annual cycle, with larger SWH in winter in response to seasonal changes in high‐latitude storm patterns that generate equatorward propagating swell. However, some locations deviate from this hemispheric‐scale seasonal pattern in SWH. For example, in the California coastal region, local wind events occur in boreal spring and summer, leading to a wind speed (WSP) annual cycle with a distinct maximum in boreal spring and a corresponding local response in SWH. Here ocean regions with a WSP annual cycle reaching a maximum in late spring, summer, or early fall are designated as seasonal wind anomaly regions (SWARs). Intra‐annual variability of surface gravity waves is analyzed globally using two decades of satellite‐derived SWH and WSP data. The phasing of the WSP annual cycle is used as a metric to identify SWARs. Global maps of probability of swell based on wave age confirm that during the spring and summer months, locally forced waves are statistically more likely in SWARs than in surrounding regions. The magnitude of the deviation in the SWH annual cycle is determined by the exposure to swell and characteristics of the wave field within the region. Local winds have a more identifiable impact on Northern Hemisphere SWARs than on Southern Hemisphere SWARs due to the larger seasonality of Northern Hemisphere winds.

Improving Wave‐Based Air‐Sea Momentum Flux Parameterization in Mixed Seas

“…T p can be also sensitive to the spectral shape of the wave energy and the chosen filter, while T m can be reliably estimated from observations and WW3 as either an energy-weighted average period or zero-crossing period. A similar argument has been made recently by Colosi et al (2021) as they chose to use a wave age dependent computed with the mean period to construct the seasonal probability of swell over global oceans.…”

“…A similar argument has been made recently by Colosi et al. (2021) as they chose to use a wave age dependent computed with the mean period to construct the seasonal probability of swell over global oceans.…”

Seasonality of the Sub‐Mesoscale to Mesoscale Sea Surface Variability From Multi‐Year Satellite Altimetry

“…We run the 5‐parameter least‐squares fitting with the assumption that the annual and semi‐annual variations contain the majority of SSS variance and they undergo sinusoidal cycles. To evaluate the least‐squares fitting, we adopt the metric of fraction of variance explained (FVE) (Colosi et al., 2021; Draper & Smith, 1998), defined as: $$$\mathrm{F}\mathrm{V}\mathrm{E}=1-\frac{\sum\limits _{i=1}^{N}{\left({y}_{i}-{\hat{y}}_{i}\right)}^{2}}{\sum\limits _{i=1}^{N}{\left({y}_{i}-\overline{y}\right)}^{2}}$$$ where $${y}_{i}$$ is the i th observation, $${\hat{y}}_{i}$$ is the i th estimate, and $$\overline{y}$$ is the mean value. N is the number of total observations, which is 12 in this study.…”

“…We run the 5-parameter least-squares fitting with the assumption that the annual and semi-annual variations contain the majority of SSS variance and they undergo sinusoidal cycles. To evaluate the least-squares fitting, we adopt the metric of fraction of variance explained (FVE) (Colosi et al, 2021;Draper & Smith, 1998), defined as:…”

A Broadband View of the Sea Surface Height Wavenumber Spectrum

“…Winds were much stronger in comparison with the 29 March flights (see Figure S1 in Supporting Information S1 ), with an average speed of 15 m s −1 near the coast and 11–13 m s −1 in the offshore region (Figure 2c , color shading and vectors). These strong northwesterly winds are characteristic of expansion fan wind events (Colosi et al., 2021 ; Villas Bôas et al., 2017 ) and resulted in mixed seas, with the superposition of a remotely generated swell with peak period, wavelength, and direction of 11.3 s, 200 m, and 308°, respectively, and a local wind‐sea with peak period, wavelength, and direction of 6.1 s, 59 m, and 321°, as shown in the MASS directional wave spectrum (Figure 2a ). The overall significant wave height was 3.1 m. The omnidirectional spectrum (integral of the directional spectrum over all directions) computed from the MASS directional spectrum shows good agreement with the spectra from nearby CDIP buoys (Figure 2b ), in particular in comparison to CDIP 094, which is exposed to wind conditions similar to the experiment site.…”